# How to diagnose gas valve problems



## Nestor_Kelebay

In the "Resources for understanding gas valves" thread in the HVAC forum, someone wanted to know how a gas valve worked. (supposedly so they could fix them :yes: )

The only response he got was to be told to replace them, and NOT to fix them. :huh: (can't win, can ya)

I understood the guy just wanted to know how they worked out of curiosity and so that he could better diagnose problems with water heaters, boiler and furnaces not going on or pilot lights not staying lit.

And, of course, the best way to diagnose a problem with something is to understand how that something works. Then every problem with it is easy to diagnose.

Since this is something EVERY DIY'er should understand, I thought I'd also post the response here as well as in the HVAC forum. Read on...

Darsunt:

Gas valves are actually very simple to understand. Once you understand how they work, it'll almost seem embarrassing to have to explain how they work cuz they're so simple.

When you look at a gas valve, imagine two valves in series.

The gas port where the tube to the pilot light connects is between the two valves, so that gas can flow to the pilot light as long as the first valve, or "safety" valve is open. If the second valve, or "main" valve is open, it can also flow to the burner trays.

Both valves are operated by electromagnets. The electric power to hold the first valve open, and therefore allow gas flow to the pilot light comes from the thermocouple or thermopile immersed in the pilot light flame. The thermocouple or thermopile generates electricity from the heat of the pilot light flame.

So, if the pilot light flame goes out, then the voltage to hold the safety electromagnetic valve open is lost and the safety valve closes, thereby shutting off gas flow completely, including the gas flow to the pilot light.

The second electromagnetic valve in the gas valve, the "main" valve, allows gas flow through a much larger tube to the burner trays. This second electromagnetic valve gets power from either a 24 VAC transformer or the thermopile. Up until recently flue dampers were not required on water heaters so 120 VAC power wasn't needed to operate hot water heaters. As a result hot water heaters typcially used thermopiles to generate more electricity than a thermocouple could, and used that electricity to operate both electromagnetic valves in the water heater's gas valve.

A thermopile is nothing more than about 25 thermocouples all connected in series, so that a thermopile generates about 750 millivolts instead of the 30 millivolts that a thermocouple produces.

On a boiler or furnace, there will be 120 VAC power available to operate the circulating pump or blower, and so by using a 24 volt transformer this 120 volt power can be converted into the same 24 VAC power that the safety controls on a boiler or furnace operate on. By simply putting the second electromagnetic valve in series with all of the safety controls, the second electromagnetic valve will open and allow gas to flow to the burner trays as long as every safety control in that "control circuit" allows power to flow through it. If any safety control detects a problem, it interrupts the circuit to that second electromagnetic valve, thereby shutting off gas flow to the burner trays (but not the pilot light).

This is where I fly off on a tangent:
Typically, the thermostat in a house won't be connected directly to that 24 VAC control circuit to the main valve in the gas valve. That's because to heat up a house you also need the boiler's circulating pump or furnace blower to come on to deliver the heat to the house. I'm much more familiar with hot water heating systems, so I'll describe the typical arrangement for a boiler. Typically the thermostat will be connected to a double pole 24 volt relay, and when that relay gets energized, it completes two circuits; a 120 VAC circuit to the boiler circulating pump or furnace blower, and a 24 VAC circuit through to the temperature control (or "aquastat") on the boiler. When power flows through that aquastat circuit, it energizes a second relay that completes a 24 VAC circuit through all the safety controls to the "main valve" in the gas valve. So, as long as the thermostat is calling for heat, the aquastat is controlling the boiler temperature setting (from 160 to 190 deg. F) and turning on and shutting off the gas valve to maintain the boiler water temperature at that setting. If the first relay completed the circuit to the gas valve directly, then the boiler would keep firing until it kicked itself out on high limit. By having that first relay control a circuit through the aquastat that controls a second relay which operates the gas valve, then the aquastat controls the gas valve to, in turn, control the water temperature in the heating system. A boiler will typically have a "ladder diagram" which shows the start up sequence as sequence of "loops" (kinda like a ladder).
Newer boilers or furnaces will simply have a "controller" that the thermostat connects to that does all this and more.
Back to the chase...

So, to diagnose a gas valve, first check to see if the pilot light is on. If so, then the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If the appliance is a hot water heater but it's not firing up, the thermopile might be weak. It may be producing enough power to open the safety valve, but not enough to open both the safety and main valves together. If replacing the thermopile doesn't work, then you need a new gas valve.

If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.

The "button" you have to push on a gas valve when lighting the pilot light simply overrides the first electromagnetic valve to allow gas to flow to the pilot light. Once the thermocouple or thermopile is hot enough, the electricity it generates will be sufficient to hold that electromagnet open and you can release that "button". This can often take a few seconds.

Also, thermocouples gradually lose their ability to generate sufficient voltage to operate that safety electromagnet, and when they do, gas flow to the pilot light stops. So, a pilot light that won't stay lit is often the result of an old and weak thermocouple. Different thermocouples get replaced differently. Most of the time you can just pull down on them to pull them out of a spring clip that holds them in place. Other thermocouples will be held in with a thumb screw you loosen from below to pull the thermocouple out. Basically, if it doesn't have a thumb screw on the bottom, pull down on it.

Some gas appliances, like natural gas burning fireplaces will have a "dual" pilot light that has two pilot light flames; one for a thermocouple and the other that continuously heats a thermopile. The thermocouple will produce the voltage to hold the safety valve open in the gas valve, and the thermopile will produce the voltage that goes through the control circuit to the second electromagnetic valve. In this way, the fire place can be turned on, turned up, turned down and shut off with just the turn of a switch even without external electric power being supplied to it.

This is what a thermoCOUPLE looks like:








The copper "tube" is really a coaxial cable. The outside of the cable is copper and there's an insulated copper wire that runs inside it. When the end of the thermocouple is heated, a voltage is generated between the two copper conductors.

This is what a thermoPILE looks like:








The distinguishing characteristic of a thermopile is that it connects to the gas valve with two electric wires (each with a terminal crimped onto it) rather than with a single nut like the thermocouple does.

And, finally, since both a thermocouple and a thermopile simply create a voltage difference between two wires when their ends are heated, different lengths of thermocouples will all generate the same voltage, just as thermopiles of different length. So you can use a longer thermocouple to replace a shorter one, and use a longer thermopile to replace a shorter one. They're like extension cords or garden hoses or battery booster cables in that respect. That's important to know when you're water heater's pilot light goes out and the hardware store doesn't have a thermopile of the right length. (Just buy a longer one.)


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## Reilley

Cool. I was just wondering about this the other day. Thanks


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## jackpease

*testing of the magnetic valve*

I found this thread really useful but cannot get my pilot to stay on on my 'millivolt' honeywell system.

I've tested the thermocouple, and it sends out 20-30mv, but the pilot keeps going out when l let go of the white button however long i hold it, 

I've undone the spring valve into which the thermocouple goes, and tried to test it, its a very strong spring, how on earth does 20mv hold that spring back - when i've tested it with it makes the spring 'stickier' but not holding it up.

Any hints on testing welcome?

getting an entire new gas valve for a mv system is not straightforward!
thx
jack


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## HoosierJayBee

*Thank you for this hard to find information*

I consider myself to be an above average searcher on the internet. However, recently when I tried to learn about the internal workings of a furnace gas valve, the only place I found any information was in this forum, at this particular thread. Thanks to Nestor_Kelebay, I finally understand how the gas valve separately controls the pilot light and the main burners.

My furnace gas valve is a Honeywell, and all I could find at their web site was installation instructions and wire drawings.

Thank you Nestor_Kelebay. You might want to consider posting this hard-to-find information at popular web sites such as HowStuffWorks.


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## avro1

Good info, thanks! I'm wondering though if 750 mv is for all thermopiles.....even my old one dating back to about the mid 50s???

The reason I ask is now and then(mostly now lately) the thermostat won't open the gas valve. If I jump the thermostat terminals on the gas valve with the thermostat closed the valve opens fine and stays open until the thermostat reaches temp and cuts out. I've tried jumping up at the thermostat but the valve still won't open.

It did this once a few years ago so I cleaned the connections and it was good for a few years. Last fall it started acting up again so I cleaned them again but didn't make much difference this time. I just kept jumping those two terminals for a few days and then for some reason it kept working as it should for the winter(got lucky I guess). I'm working on having the furnace replaced but now it's back at it. I checked the thermopile and it puts out about 310 mv with the burner on and about 240 with just the pilot light. 

It always seemed to me that the circuit was getting weak and the thermopile was putting out enough voltage to hold the gas valve open but not enough to actually move it due to high circuit resistance(which I fixed) low thermopile output or a gas valve that was just getting old and harder to move.

If ALL thermopiles should put out 750 mv then I see where my problem is but since the furnace is so old I'm not sure if this applies to me. That would be an easy temporary fix until I get the furnace replaced.

Any thoughts?


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## MagicalHome

Thanks for that well-descriptive tips. To check for gas pipe leaks, put soap on the pipe connecting to the valve, if it bubbles, it means there's leakage.


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## Jeanric

*What about Mechanical thermostats?*

Thank you for a very informative description.

Does anyone know where I could get a description (and possibly diagram) of a purely MECHANICAL water heater thermostat/gas valve?

Also, does anyone know if there is an alternative to replacing the whole thing (perhaps replacing a spring or some such small part) when hysteresis (differential) becomes too high? I'm already at my third valve on this water heater, and each time it's a $300 part...

Thank you
jeanric


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## physicsepv

Nestor_Kelebay said:


> In the "Resources for understanding gas valves" thread in the HVAC forum, someone wanted to know how a gas valve worked. (supposedly so they could fix them :yes: )
> 
> The only response he got was to be told to replace them, and NOT to fix them. :huh: (can't win, can ya)
> 
> I understood the guy just wanted to know how they worked out of curiosity and so that he could better diagnose problems with water heaters, boiler and furnaces not going on or pilot lights not staying lit.
> 
> And, of course, the best way to diagnose a problem with something is to understand how that something works. Then every problem with it is easy to diagnose.
> 
> Since this is something EVERY DIY'er should understand, I thought I'd also post the response here as well as in the HVAC forum. Read on...
> 
> Darsunt:
> 
> Gas valves are actually very simple to understand. Once you understand how they work, it'll almost seem embarrassing to have to explain how they work cuz they're so simple.
> 
> When you look at a gas valve, imagine two valves in series.
> 
> The gas port where the tube to the pilot light connects is between the two valves, so that gas can flow to the pilot light as long as the first valve, or "safety" valve is open. If the second valve, or "main" valve is open, it can also flow to the burner trays.
> 
> Both valves are operated by electromagnets. The electric power to hold the first valve open, and therefore allow gas flow to the pilot light comes from the thermocouple or thermopile immersed in the pilot light flame. The thermocouple or thermopile generates electricity from the heat of the pilot light flame.
> 
> So, if the pilot light flame goes out, then the voltage to hold the safety electromagnetic valve open is lost and the safety valve closes, thereby shutting off gas flow completely, including the gas flow to the pilot light.
> 
> The second electromagnetic valve in the gas valve, the "main" valve, allows gas flow through a much larger tube to the burner trays. This second electromagnetic valve gets power from either a 24 VAC transformer or the thermopile. Up until recently flue dampers were not required on water heaters so 120 VAC power wasn't needed to operate hot water heaters. As a result hot water heaters typcially used thermopiles to generate more electricity than a thermocouple could, and used that electricity to operate both electromagnetic valves in the water heater's gas valve.
> 
> A thermopile is nothing more than about 25 thermocouples all connected in series, so that a thermopile generates about 750 millivolts instead of the 30 millivolts that a thermocouple produces.
> 
> On a boiler or furnace, there will be 120 VAC power available to operate the circulating pump or blower, and so by using a 24 volt transformer this 120 volt power can be converted into the same 24 VAC power that the safety controls on a boiler or furnace operate on. By simply putting the second electromagnetic valve in series with all of the safety controls, the second electromagnetic valve will open and allow gas to flow to the burner trays as long as every safety control in that "control circuit" allows power to flow through it. If any safety control detects a problem, it interrupts the circuit to that second electromagnetic valve, thereby shutting off gas flow to the burner trays (but not the pilot light).
> 
> This is where I fly off on a tangent:
> Typically, the thermostat in a house won't be connected directly to that 24 VAC control circuit to the main valve in the gas valve. That's because to heat up a house you also need the boiler's circulating pump or furnace blower to come on to deliver the heat to the house. I'm much more familiar with hot water heating systems, so I'll describe the typical arrangement for a boiler. Typically the thermostat will be connected to a double pole 24 volt relay, and when that relay gets energized, it completes two circuits; a 120 VAC circuit to the boiler circulating pump or furnace blower, and a 24 VAC circuit through to the temperature control (or "aquastat") on the boiler. When power flows through that aquastat circuit, it energizes a second relay that completes a 24 VAC circuit through all the safety controls to the "main valve" in the gas valve. So, as long as the thermostat is calling for heat, the aquastat is controlling the boiler temperature setting (from 160 to 190 deg. F) and turning on and shutting off the gas valve to maintain the boiler water temperature at that setting. If the first relay completed the circuit to the gas valve directly, then the boiler would keep firing until it kicked itself out on high limit. By having that first relay control a circuit through the aquastat that controls a second relay which operates the gas valve, then the aquastat controls the gas valve to, in turn, control the water temperature in the heating system. A boiler will typically have a "ladder diagram" which shows the start up sequence as sequence of "loops" (kinda like a ladder).
> Newer boilers or furnaces will simply have a "controller" that the thermostat connects to that does all this and more.
> Back to the chase...
> 
> So, to diagnose a gas valve, first check to see if the pilot light is on. If so, then the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If the appliance is a hot water heater but it's not firing up, the thermopile might be weak. It may be producing enough power to open the safety valve, but not enough to open both the safety and main valves together. If replacing the thermopile doesn't work, then you need a new gas valve.
> 
> If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.
> 
> The "button" you have to push on a gas valve when lighting the pilot light simply overrides the first electromagnetic valve to allow gas to flow to the pilot light. Once the thermocouple or thermopile is hot enough, the electricity it generates will be sufficient to hold that electromagnet open and you can release that "button". This can often take a few seconds.
> 
> Also, thermocouples gradually lose their ability to generate sufficient voltage to operate that safety electromagnet, and when they do, gas flow to the pilot light stops. So, a pilot light that won't stay lit is often the result of an old and weak thermocouple. Different thermocouples get replaced differently. Most of the time you can just pull down on them to pull them out of a spring clip that holds them in place. Other thermocouples will be held in with a thumb screw you loosen from below to pull the thermocouple out. Basically, if it doesn't have a thumb screw on the bottom, pull down on it.
> 
> Some gas appliances, like natural gas burning fireplaces will have a "dual" pilot light that has two pilot light flames; one for a thermocouple and the other that continuously heats a thermopile. The thermocouple will produce the voltage to hold the safety valve open in the gas valve, and the thermopile will produce the voltage that goes through the control circuit to the second electromagnetic valve. In this way, the fire place can be turned on, turned up, turned down and shut off with just the turn of a switch even without external electric power being supplied to it.
> 
> This is what a thermoCOUPLE looks like:
> 
> 
> 
> 
> 
> 
> 
> 
> The copper "tube" is really a coaxial cable. The outside of the cable is copper and there's an insulated copper wire that runs inside it. When the end of the thermocouple is heated, a voltage is generated between the two copper conductors.
> 
> This is what a thermoPILE looks like:
> 
> 
> 
> 
> 
> 
> 
> 
> The distinguishing characteristic of a thermopile is that it connects to the gas valve with two electric wires (each with a terminal crimped onto it) rather than with a single nut like the thermocouple does.
> 
> And, finally, since both a thermocouple and a thermopile simply create a voltage difference between two wires when their ends are heated, different lengths of thermocouples will all generate the same voltage, just as thermopiles of different length. So you can use a longer thermocouple to replace a shorter one, and use a longer thermopile to replace a shorter one. They're like extension cords or garden hoses or battery booster cables in that respect. That's important to know when you're water heater's pilot light goes out and the hardware store doesn't have a thermopile of the right length. (Just buy a longer one.)


Great article thanks for taking the time to put this together, it clarified what I had intuitively figured out.


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## wouldy

Outstanding! Now I'm sure my problem's the thermopile.

I have a Hearthstone floor unit which shuts down sometimes -- and evermore frequently. When that happens the pilot light always remains lit. If I stomp on the floor near the unit the burners will relight. From your excellent explanation above, which includes the fact that TWO valves must be open, I surmise that I need to replace the thermopile.

I spent many hours last winter searching (between stomps) for plausible diagnosis of the problem, and I thank you for that.


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## guyinsb

*750mv thermopile, gas valve*

Stomping has worked for me as well. The measured output of my thermopile is currently around 350mv; sadly, I failed to get a calibration reading when it was installed (new) a few years ago. So I'm thinking the cause for the valve occasionally failing to open is one of the following:
- pilot light has become too faint (easy to adjust and find out)
- faulty thermopile (a little more work to replace)
- faulty valve (a major pain - my plumber is reluctant to do this)

The expensive option is to replace the boiler and all the controls with a modern system. Certainly the new system will be more efficient,
but the efficiency comes at the cost of complexity and possibly shorter life.
My boiler heats radiator water for a 14 unit apartment in Santa Barbara; it ain't that cold here, and natural gas prices seem pretty reasonable of late.
Can anybody comment on the sanity of just replacing my old General Controls 750mv valve with some equivalent? Any recommendations appreciated.


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## wouldy

Your problem seems to involve large -- really large -- "industrial size" issues. At least having yours done professionally would appear to put you in a position of a tax deduction for repairing it.

Sorry I can't help.

Good luck.

wouldy


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## wouldy

Ever notice how civil DIY'ers are, generally?''

wouldy


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## doxland

Thanks for the excellent tutorial Nestor
I've got a Honeywell Controller in a slide in fireplace.
Pilot lights fine and latches( stays on with flashing LED)
My problem is that switching to main the pilot drops out no matter how long it's been on. I've waited an hour thinking perhaps the thermopile was slow building sufficient voltage.
Any comment on what's going on here? 
The pilot's been off all summer and just trying out now.
Pilot color is just plain blue and about 1.5 inches long.
Burning Nat Gas.
False alarm and problem solved. 
It was finger trouble


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## oh'mike

doxland said:


> Thanks for the excellent tutorial Nestor
> I've got a Honeywell Controller in a slide in fireplace.
> Pilot lights fine and latches( stays on with flashing LED)
> My problem is that switching to main the pilot drops out no matter how long it's been on. I've waited an hour thinking perhaps the thermopile was slow building sufficient voltage.
> Any comment on what's going on here?
> The pilot's been off all summer and just trying out now.
> Pilot color is just plain blue and about 1.5 inches long.
> Burning Nat Gas.


Please start a new thread in HVAC--Members don't look in to the HOW TO section for questions.--Moderator


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## davedavidson

*hvac tech*

valves" thread in the HVAC forum, someone wanted to know how a gas valve worked. (supposedly so they could fix them :yes: )

The only response he got was to be told to replace them, and NOT to fix them. :huh: (can't win, can ya)

I understood the guy just wanted to know how they worked out of curiosity and so that he could better diagnose problems with water heaters, boiler and furnaces not going on or pilot lights not staying lit.

And, of course, the best way to diagnose a problem with something is to understand how that something works. Then every problem with it is easy to diagnose.

Since this is something EVERY DIY'er should understand, I thought I'd also post the response here as well as in the HVAC forum. Read on...

Darsunt:

Gas valves are actually very simple to understand. Once you understand how they work, it'll almost seem embarrassing to have to explain how they work cuz they're so simple.

When you look at a gas valve, imagine two valves in series.

The gas port where the tube to the pilot light connects is between the two valves, so that gas can flow to the pilot light as long as the first valve, or "safety" valve is open. If the second valve, or "main" valve is open, it can also flow to the burner trays.

Both valves are operated by electromagnets. The electric power to hold the first valve open, and therefore allow gas flow to the pilot light comes from the thermocouple or thermopile immersed in the pilot light flame. The thermocouple or thermopile generates electricity from the heat of the pilot light flame.

So, if the pilot light flame goes out, then the voltage to hold the safety electromagnetic valve open is lost and the safety valve closes, thereby shutting off gas flow completely, including the gas flow to the pilot light.

The second electromagnetic valve in the gas valve, the "main" valve, allows gas flow through a much larger tube to the burner trays. This second electromagnetic valve gets power from either a 24 VAC transformer or the thermopile. Up until recently flue dampers were not required on water heaters so 120 VAC power wasn't needed to operate hot water heaters. As a result hot water heaters typcially used thermopiles to generate more electricity than a thermocouple could, and used that electricity to operate both electromagnetic valves in the water heater's gas valve.

A thermopile is nothing more than about 25 thermocouples all connected in series, so that a thermopile generates about 750 millivolts instead of the 30 millivolts that a thermocouple produces.

On a boiler or furnace, there will be 120 VAC power available to operate the circulating pump or blower, and so by using a 24 volt transformer this 120 volt power can be converted into the same 24 VAC power that the safety controls on a boiler or furnace operate on. By simply putting the second electromagnetic valve in series with all of the safety controls, the second electromagnetic valve will open and allow gas to flow to the burner trays as long as every safety control in that "control circuit" allows power to flow through it. If any safety control detects a problem, it interrupts the circuit to that second electromagnetic valve, thereby shutting off gas flow to the burner trays (but not the pilot light).

This is where I fly off on a tangent:
Typically, the thermostat in a house won't be connected directly to that 24 VAC control circuit to the main valve in the gas valve. That's because to heat up a house you also need the boiler's circulating pump or furnace blower to come on to deliver the heat to the house. I'm much more familiar with hot water heating systems, so I'll describe the typical arrangement for a boiler. Typically the thermostat will be connected to a double pole 24 volt relay, and when that relay gets energized, it completes two circuits; a 120 VAC circuit to the boiler circulating pump or furnace blower, and a 24 VAC circuit through to the temperature control (or "aquastat") on the boiler. When power flows through that aquastat circuit, it energizes a second relay that completes a 24 VAC circuit through all the safety controls to the "main valve" in the gas valve. So, as long as the thermostat is calling for heat, the aquastat is controlling the boiler temperature setting (from 160 to 190 deg. F) and turning on and shutting off the gas valve to maintain the boiler water temperature at that setting. If the first relay completed the circuit to the gas valve directly, then the boiler would keep firing until it kicked itself out on high limit. By having that first relay control a circuit through the aquastat that controls a second relay which operates the gas valve, then the aquastat controls the gas valve to, in turn, control the water temperature in the heating system. A boiler will typically have a "ladder diagram" which shows the start up sequence as sequence of "loops" (kinda like a ladder).
Newer boilers or furnaces will simply have a "controller" that the thermostat connects to that does all this and more.
Back to the chase...

So, to diagnose a gas valve, first check to see if the pilot light is on. If so, then the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If the appliance is a hot water heater but it's not firing up, the thermopile might be weak. It may be producing enough power to open the safety valve, but not enough to open both the safety and main valves together. If replacing the thermopile doesn't work, then you need a new gas valve.

If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.

The "button" you have to push on a gas valve when lighting the pilot light simply overrides the first electromagnetic valve to allow gas to flow to the pilot light. Once the thermocouple or thermopile is hot enough, the electricity it generates will be sufficient to hold that electromagnet open and you can release that "button". This can often take a few seconds.

Also, thermocouples gradually lose their ability to generate sufficient voltage to operate that safety electromagnet, and when they do, gas flow to the pilot light stops. So, a pilot light that won't stay lit is often the result of an old and weak thermocouple. Different thermocouples get replaced differently. Most of the time you can just pull down on them to pull them out of a spring clip that holds them in place. Other thermocouples will be held in with a thumb screw you loosen from below to pull the thermocouple out. Basically, if it doesn't have a thumb screw on the bottom, pull down on it.

Some gas appliances, like natural gas burning fireplaces will have a "dual" pilot light that has two pilot light flames; one for a thermocouple and the other that continuously heats a thermopile. The thermocouple will produce the voltage to hold the safety valve open in the gas valve, and the thermopile will produce the voltage that goes through the control circuit to the second electromagnetic valve. In this way, the fire place can be turned on, turned up, turned down and shut off with just the turn of a switch even without external electric power being supplied to it.

This is what a thermoCOUPLE looks like:








The copper "tube" is really a coaxial cable. The outside of the cable is copper and there's an insulated copper wire that runs inside it. When the end of the thermocouple is heated, a voltage is generated between the two copper conductors.

This is what a thermoPILE looks like:








The distinguishing characteristic of a thermopile is that it connects to the gas valve with two electric wires (each with a terminal crimped onto it) rather than with a single nut like the thermocouple does.

And, finally, since both a thermocouple and a thermopile simply create a voltage difference between two wires when their ends are heated, different lengths of thermocouples will all generate the same voltage, just as thermopiles of different length. So you can use a longer thermocouple to replace a shorter one, and use a longer thermopile to replace a shorter one. They're like extension cords or garden hoses or battery booster cables in that respect. That's important to know when you're water heater's pilot light goes out and the hardware store doesn't have a thermopile of the right length. (Just buy a longer one.)[/quote]
How in the h&** do i print this page


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## del schisler

davedavidson said:


> valves" thread in the HVAC forum, someone wanted to know how a gas valve worked. (supposedly so they could fix them :yes: )
> 
> The only response he got was to be told to replace them, and NOT to fix them. :huh: (can't win, can ya)
> 
> I understood the guy just wanted to know how they worked out of curiosity and so that he could better diagnose problems with water heaters, boiler and furnaces not going on or pilot lights not staying lit.
> 
> And, of course, the best way to diagnose a problem with something is to understand how that something works. Then every problem with it is easy to diagnose.
> 
> Since this is something EVERY DIY'er should understand, I thought I'd also post the response here as well as in the HVAC forum. Read on...
> 
> Darsunt:
> 
> Gas valves are actually very simple to understand. Once you understand how they work, it'll almost seem embarrassing to have to explain how they work cuz they're so simple.
> 
> When you look at a gas valve, imagine two valves in series.
> 
> The gas port where the tube to the pilot light connects is between the two valves, so that gas can flow to the pilot light as long as the first valve, or "safety" valve is open. If the second valve, or "main" valve is open, it can also flow to the burner trays.
> 
> Both valves are operated by electromagnets. The electric power to hold the first valve open, and therefore allow gas flow to the pilot light comes from the thermocouple or thermopile immersed in the pilot light flame. The thermocouple or thermopile generates electricity from the heat of the pilot light flame.
> 
> So, if the pilot light flame goes out, then the voltage to hold the safety electromagnetic valve open is lost and the safety valve closes, thereby shutting off gas flow completely, including the gas flow to the pilot light.
> 
> The second electromagnetic valve in the gas valve, the "main" valve, allows gas flow through a much larger tube to the burner trays. This second electromagnetic valve gets power from either a 24 VAC transformer or the thermopile. Up until recently flue dampers were not required on water heaters so 120 VAC power wasn't needed to operate hot water heaters. As a result hot water heaters typcially used thermopiles to generate more electricity than a thermocouple could, and used that electricity to operate both electromagnetic valves in the water heater's gas valve.
> 
> A thermopile is nothing more than about 25 thermocouples all connected in series, so that a thermopile generates about 750 millivolts instead of the 30 millivolts that a thermocouple produces.
> 
> On a boiler or furnace, there will be 120 VAC power available to operate the circulating pump or blower, and so by using a 24 volt transformer this 120 volt power can be converted into the same 24 VAC power that the safety controls on a boiler or furnace operate on. By simply putting the second electromagnetic valve in series with all of the safety controls, the second electromagnetic valve will open and allow gas to flow to the burner trays as long as every safety control in that "control circuit" allows power to flow through it. If any safety control detects a problem, it interrupts the circuit to that second electromagnetic valve, thereby shutting off gas flow to the burner trays (but not the pilot light).
> 
> This is where I fly off on a tangent:
> Typically, the thermostat in a house won't be connected directly to that 24 VAC control circuit to the main valve in the gas valve. That's because to heat up a house you also need the boiler's circulating pump or furnace blower to come on to deliver the heat to the house. I'm much more familiar with hot water heating systems, so I'll describe the typical arrangement for a boiler. Typically the thermostat will be connected to a double pole 24 volt relay, and when that relay gets energized, it completes two circuits; a 120 VAC circuit to the boiler circulating pump or furnace blower, and a 24 VAC circuit through to the temperature control (or "aquastat") on the boiler. When power flows through that aquastat circuit, it energizes a second relay that completes a 24 VAC circuit through all the safety controls to the "main valve" in the gas valve. So, as long as the thermostat is calling for heat, the aquastat is controlling the boiler temperature setting (from 160 to 190 deg. F) and turning on and shutting off the gas valve to maintain the boiler water temperature at that setting. If the first relay completed the circuit to the gas valve directly, then the boiler would keep firing until it kicked itself out on high limit. By having that first relay control a circuit through the aquastat that controls a second relay which operates the gas valve, then the aquastat controls the gas valve to, in turn, control the water temperature in the heating system. A boiler will typically have a "ladder diagram" which shows the start up sequence as sequence of "loops" (kinda like a ladder).
> Newer boilers or furnaces will simply have a "controller" that the thermostat connects to that does all this and more.
> Back to the chase...
> 
> So, to diagnose a gas valve, first check to see if the pilot light is on. If so, then the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If the appliance is a hot water heater but it's not firing up, the thermopile might be weak. It may be producing enough power to open the safety valve, but not enough to open both the safety and main valves together. If replacing the thermopile doesn't work, then you need a new gas valve.
> 
> If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.
> 
> The "button" you have to push on a gas valve when lighting the pilot light simply overrides the first electromagnetic valve to allow gas to flow to the pilot light. Once the thermocouple or thermopile is hot enough, the electricity it generates will be sufficient to hold that electromagnet open and you can release that "button". This can often take a few seconds.
> 
> Also, thermocouples gradually lose their ability to generate sufficient voltage to operate that safety electromagnet, and when they do, gas flow to the pilot light stops. So, a pilot light that won't stay lit is often the result of an old and weak thermocouple. Different thermocouples get replaced differently. Most of the time you can just pull down on them to pull them out of a spring clip that holds them in place. Other thermocouples will be held in with a thumb screw you loosen from below to pull the thermocouple out. Basically, if it doesn't have a thumb screw on the bottom, pull down on it.
> 
> Some gas appliances, like natural gas burning fireplaces will have a "dual" pilot light that has two pilot light flames; one for a thermocouple and the other that continuously heats a thermopile. The thermocouple will produce the voltage to hold the safety valve open in the gas valve, and the thermopile will produce the voltage that goes through the control circuit to the second electromagnetic valve. In this way, the fire place can be turned on, turned up, turned down and shut off with just the turn of a switch even without external electric power being supplied to it.
> 
> This is what a thermoCOUPLE looks like:
> 
> 
> 
> 
> 
> 
> 
> 
> The copper "tube" is really a coaxial cable. The outside of the cable is copper and there's an insulated copper wire that runs inside it. When the end of the thermocouple is heated, a voltage is generated between the two copper conductors.
> 
> This is what a thermoPILE looks like:
> 
> 
> 
> 
> 
> 
> 
> 
> The distinguishing characteristic of a thermopile is that it connects to the gas valve with two electric wires (each with a terminal crimped onto it) rather than with a single nut like the thermocouple does.
> 
> And, finally, since both a thermocouple and a thermopile simply create a voltage difference between two wires when their ends are heated, different lengths of thermocouples will all generate the same voltage, just as thermopiles of different length. So you can use a longer thermocouple to replace a shorter one, and use a longer thermopile to replace a shorter one. They're like extension cords or garden hoses or battery booster cables in that respect. That's important to know when you're water heater's pilot light goes out and the hardware store doesn't have a thermopile of the right length. (Just buy a longer one.)


How in the h&** do i print this page[/quote]

easy high light the text and copy and past to your word pad than their is a little icon at the top of word pad click on print. It will load and print. that is the way i do it . Or you can do a right mouse click and select all that will high light the text . Now go up to file and select print or control P another way


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## tman77xx

This is a very useful thread and am learning a lot from it partly because of my situation with my current Weil McClain boiler. The problem I'm having is very simple. The boiler comes on twice a day - morning and night. The pilot light is auto so when the thermostat calls for heat, a spark is sent to the ignition lighting up the pilot. And after two or so minutes, the gas chambers will be lit. It does so for the very first instance both in the morning and night but thereafter, the pilot just stays lit and the chambers aren't lit or heating the house. Several troubleshooting methods applied to and including bypassing the thermostat but failed to yield any results. A contractor came by and gave it a crack at it and said it was the gas valve but I'm not confident that that could be the problem as from reading this post, the thermoCOUPLE should be swapped out first before assuming it is the gas valve as it is a more pricey unit to replace. So in short, I get heat both times of the day for say an hour and it just sits idle with the pilot lit. Any help and/or comment would be greatly appreciated.

My Weil McClain is a CGM-5 Series 10 ~ 10 yrs old running gas with baseboard and radiators.


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## bbo

How in the h&** do i print this page

file

print


or control + p


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## Doc Holliday

One more troubleshooting tip, on a residential gas furnace if a gas valve is not "clicking" meaning it's apparently not receiving 24 volts you can jump directly off of the control board using the R and the C terminals, R to one terminal on gas valve and C to the other terminal on gas valve (or wherever else you can retain 24 volts from) to the gas valve. If it then "clicks" meaning it is opening with voltage supplied to it than most likely it's not the gas valve which is bad.

You would need to pull the original control wires from the gas valve prior to putting the jumped 24 volts to it. Grounds can stay.


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## intoodeep

*intoodeep*

Nice description of gas valve operation. Something I must be missing is that most residential water heaters I have worked on don't have thermopiles or external power (24v) for the main valve...only a thermocouple (which I assume is for the first valve only?). So how is the main valve opened when heat is required? I think it must be the water temp. is sensed where the gas control is attached to the tank and somehow (mechanically?) controls the main gas valve feeding the burner. Sorry if I missed this somewhere.


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## G1Tech

jackpease said:


> I found this thread really useful but cannot get my pilot to stay on on my 'millivolt' honeywell system.
> 
> I've tested the thermocouple, and it sends out 20-30mv, but the pilot keeps going out when l let go of the white button however long i hold it,
> 
> I've undone the spring valve into which the thermocouple goes, and tried to test it, its a very strong spring, how on earth does 20mv hold that spring back - when i've tested it with it makes the spring 'stickier' but not holding it up.
> 
> Any hints on testing welcome?
> 
> getting an entire new gas valve for a mv system is not straightforward!
> thx
> jack


Jack, you should test the "coil" of the gas valve. To do this you need a multimeter, and put it on 'ohms'. You're going to check continuity. On your Honeywell valve there are three terminal connections, they are labeled as 'TH', 'TH TP', and "TP'. To check the 'coil' you need to check continuity between 'TH' and 'TH TP'. You should get a reading between 1.1 and 3.7. any higher, the valve is toast(sorry). You cannot replace this coil, it's an integral part of the valve. Let me know how you make out.


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## Yoyizit

You can use any 1.5v battery and two resistors to mimic the thermocouple when testing the gas valve. Do not use the battery without the resistors.


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## G1Tech

intoodeep said:


> Nice description of gas valve operation. Something I must be missing is that most residential water heaters I have worked on don't have thermopiles or external power (24v) for the main valve...only a thermocouple (which I assume is for the first valve only?). So how is the main valve opened when heat is required? I think it must be the water temp. is sensed where the gas control is attached to the tank and somehow (mechanically?) controls the main gas valve feeding the burner. Sorry if I missed this somewhere.


A water heater valve is a different animal. You're right when you say it's the water temp that opens the 'main' valve.The valve is threaded into the water tank and there is a probe into the water. The probe is filled with wax, or some other heat sensitive material, and as it heats up it expands, which shuts the valve, AND, as it cools it contracts, opening the valve.
Hope this helps.
By the way, it's a 'special' type of wax, not like candle wax, or bee's wax. Therefore it can be 're-used' even if heat has 'melted' it.


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## G1Tech

Yoyizit said:


> You can use any 1.5v battery and two resistors to mimic the thermocouple when testing the gas valve. Do not use the battery without the resistors.


You can use this method, even without the resistors(just don't hold it on any longer than 5 seconds), but if you need 1500 millivolts to hold open the magnet, your valve is pretty much screwed. At one time you could get replacement magnets but it's pretty much impossible now. It's against the law. Or at least where I live it is.


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## Yoyizit

G1Tech said:


> You can use this method, even without the resistors(just don't hold it on any longer than 5 seconds), but if you need 1500 millivolts to hold open the magnet, your valve is pretty much screwed. At one time you could get replacement magnets but it's pretty much impossible now. It's against the law. Or at least where I live it is.


That's why I use the two equal valued 5.6 ohm resistors to put out 750 mV. 1.5v is so high I'd think it would give false positive results some of the time.
You can also do 28 mV with resistors.


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## Stig

WOW! This thread is bang on with regards to my problem with my Vulcan gas hydronic boiler heater. The prognosis is pilot is good, have replaced the thermocouple. Heater main burner fires up when gas valve is turned around from pilot to ON. The problem arises during an hour or two or even a bit longer when the main burner has not ignited and the pilot is out. There is no thermopile on this unit. I have adjusted the pilot to give a stronger flame thinking that it is getting blown out at some stage by the main burner igniting. Also sometimes when I turn gas valve to ON the main burner ignites rather aggressively. Nestors gas valve explanation helps me to understand the workings however this problem has thrown me. Any help would be appreciated.


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## appledude

*symptoms of faulty gas valve?*

Stepdad got home from hospital, and Coleman 7663A mobile home furnace has a lit pilot, but does not come on. 

1st repairman said knob was not in full indent position, so pilot would stay lit, but not turn the burners on - 

2nd said need a new furnace because of possible invisible hairline cracks in the "drum" up high, that will allow hot/cold air to mix and blow pilot out. 

Heat exchanger looks good according to #1, and from what I saw on his camera. 

Would a faulty gas valve allow the pilot to stay lit, and not deliver gas to the burners? Couldn't quite tell from the posts I read here - also, is this a job for an average do-it-yourselfer?


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## oh'mike

Apple Dude---I suggest you start a new thread---you will get better attention that way----Mike----


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## walterhein

*How to replace a pilot assembly to an old gas boiler?*

Hello Nestor,
I have read your explaination how a gas valve works and found it very enlightening.
My problem is that on the BEESTON gas boiler of our communal heating system the pilot light is lost intermittantly and I am looking for reasons.
After your article I am convinced it is not the gas valve!
The boiler is 21 years old and BEESTON has gone out of service.
The thermocouple has been replaced numerous times but doesn't clear the problem.
I believe the flame does not permanently cover the tc and once the mV are reduced momentarily, the gas valve trips.
I now want to replace the existing pilot assembly with one from a different boiler manufacturer like Potterton, Baxi, Ideal etc. in order to ascertain a perfect geometry of flame and position to thermocouple.
You strike me a an expert who could probably give some advise which available pilot assembly would be suitable for a Beeston boiler.
I thank you in adavance for any information provided.
I also thank other readers of my message who could assist me.
Kind regards,
walterhein


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## fishycomics

@Nestor_Kelebay Big thank you searching the net and very little info at all again a big thank you


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