# How do I test Millivolt valve system when Pilot is on, but main burner not lighting



## mnalep (Oct 24, 2010)

I am trying to diagnose a problem with an old furnace. I have some pictures to help clarify what I have, a ThermoPILE system. 

The problem is that the PILOT flame is on, but the main burner does not always light.

I cleaned all electrical contacts today with sandpaper, and made sure they were all tight. 

I also made sure the PILOT line and BLEED line were not blocked. (Took them off and blew through them).

I also took out the THERMOPILE sensor, and cleaned it off with steel wool.














































And this is a repeat , but I am trying to show that there is a steel connection(steel plate) between RED(Thermopile,pos) to TH1(thermostat,pos).

Also a steel connection(steel plate) between TH2(thermostat,neg) to L2(LIMIT switch, pos):









I was trying to use the "*MILLIVOLT TROUBLESHOOTING CHART*", on page 2 of this diagram:
http://www.robertshawtstats.com/spaw2/SiteContent/Files/faq/ControlTips-MillivoltSystems.pdf

But I'm not sure what connections to put my meter across.

For example, the chart says in *TEST-A*, to test across TP and TH for more than 100mV, with the thermostat contacts CLOSED. (On my gas valve, would that be RED to TH1, or RED to TH2, or something else?)

For example, the chart also says in *TEST-B *to test from TH/TP to TP for greater than 325mV, with the thermostat contacts OPEN. (On my gas valve, would that be TH1 to WHITE, or TH1 to RED, or something else?)

Also, when it says thermostat contacts CLOSED - does that mean with the thermostat temperature set to higher than the room temperature?

I attempted to do *TEST-A*, with thermostat set to turn furnace on(I assume that what what they meant by _thermostat connections CLOSED_?) and measured:
RED to TH2 = 123mV
WHITE to TH1 = 387mV
TH1 to TH2 = 20mV
RED to WHITE = 400mV
L1 to L2 = 26mV

For some reason I did not measure RED to TH1, or WHITE to TH2. Should I have?

Also, those readings were what actually displayed on my meter, when the dial was on the 2000mV range, and the 200mV range. 

So I think I'm reporting the voltages correctly? That is, if the meter displayed 123 on either of the ranges(2000mV, or 200mV) - that means I measured 123 Millivolts?

If the gas valve is sticking, can this be disassembled and cleaned? Can it just be replaced?


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## yuri (Nov 29, 2008)

Take the 2 leads from the thermopile/aka pilot generator off the valve and connect them with alligator clips to your meter. Do not hold them in your hands or juice will flow thru you and give wrong reading. Should have at least 500 mv for the system to work. A healthy generator will produce over 600 and up to 750 mv. A furnace that old has a HUGE chance of a cracked heat exchanger and is not safe to use IMO.


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## kenmac (Feb 26, 2009)

As Yuri said. A good burning pilot & generator will produce between 600-750 mv. That's an old furnace. Back when they use real steel to make the heat exchangers. It still could be cracked.. If that's an old generator assy. I would buy a new 1. Any corrosion in the wiring ( like the green stuff on the limit wires) will eat up your mv's..You need good clean connections. 

How old is that pilot assy.?


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## mnalep (Oct 24, 2010)

ken,

There was very little dirt or anything on the leads when I cleaned them. I was just doing it for completeness.

I'd think the pilot tube is original, the pile generator is maybe 4 -5 years old.

As I recall the limit switch is still original, and I had it out about 4 years ago and it looked a bit cruddy. But that would only affect when the main burners are turned off - right?


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## mnalep (Oct 24, 2010)

Yuri,

I was looking over my notes, and I did disconnect the pilot leads with the pilot light on the pile generator. And I think that was around 700 on the 2000mV scale. So if I had 400 with the leads attached, what does that mean?


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## mnalep (Oct 24, 2010)

I forgot to mention that I got the main burner to light on 2 occassions by tapping on either the gas pipe before the main shutoff valve, or on the honeywell gas millivolt valve itself.

Does that indicate more a gas valve problem or a thermopile generator/pilot flame problem?


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## yuri (Nov 29, 2008)

The gas valve ( which came over with Christopher Columbus :laughing is finally seizing up/kaput. It has been obsolete for over 30 yrs now. Gas valves nowadays are one piece with 100% shutoff for the pilot flame gas and shutoff valve etc all in one valve.


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## kenmac (Feb 26, 2009)

mnalep said:


> ken,
> 
> But that would only affect when the main burners are turned off - right?


 

the millivolts should still travel through the limit wires back to the valve. I see green (corrosion ?)on 1 of the limit wires in the pic. Take off the connections & clean then see if the valve opens. You can also loose mv's at the t.stat. Jump those wires out & see if valve will open


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## mnalep (Oct 24, 2010)

Yuri,

I searched for that gas valve, and found this one as the replacement:

http://americanhvacparts.com/Mercha...OD&Product_Code=GSV820A1047A&Category_Code=GV

How would I convert the 6 wires I have now, to that (looks like 3 terminals) gas valve?


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## mnalep (Oct 24, 2010)

kenmac, those pictures are from befor I cleaned up the connections, and what looks green really was not.


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## yuri (Nov 29, 2008)

I don't give hands on advice how to alter gas lines, manifolds etc. Have to be a licensed gas fitter to do that here. Pilot tubing and gas line fittings need altering and I doubt you can access those fittings. Seriously, that furnace is unsafe to use. Doubt that a 40 yr old limit control will be accurate or work safely when required. You have to live there so it is your call. VERY inefficient unit. Less than 5% of those units left here and mostly in slummy rental properties where the owner doesn't care.


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## mnalep (Oct 24, 2010)

I found a place that has a replacement limit control. I'm going to go see if it is the same, but the guy says it is. Around $30.


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## hvaclover (Oct 2, 2008)

Well there's thirty loonies down the crapper....could have gone for a new furnace.

I have not seen one of those in twenty years...


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## mnalep (Oct 24, 2010)

hvaclover, Thanks for your opinion, but that $30 would not even cover the taxes on a new furnace. Probably not even the permit. 

I may end up with a new furnace, eventually. But if I do, I will make my choice - based on my needs.


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## hvaclover (Oct 2, 2008)

mnalep said:


> hvaclover, Thanks for your opinion, but that $30 would not even cover the taxes on a new furnace. Probably not even the permit.
> 
> I may end up with a new furnace, eventually. But if I do, I will make my choice - based on my needs.



You are obviously an intelligent person. But I think you fail to understand the nature of that ticking time bomb of a killer that heats your home. 
You came to this board for advice which we give freely. But what we also do is point out raging unsafe conditions when we come a cross them.

Brother yours is one of them.

I have nothing to gain whether you buy a furnace or not.

It's obvious from your pics that the furnace burners are flashing back into the vestibule due to the scorching on the burner cover. Fire hazard there.

The age of your furnace makes it a candidate for replacement on age alone. Carbon Monoxide level there. You will probably think " My monoxide detector will sound if the furnace starts pouring monoxide into my home,". Wrong again, by the time your detector sounds (which I bet even if you have one is over five years and the battery is dead or the sensor is no longer effective).

You do not maintain the furnace wall at at all hence the discoloration of the burners.

Penny wise and Pound foolish....you must be very frugal not to have changed that unit by now.

If you have family leaving with you than you are a very selfish person for not taking a more proactive approach to ensuring their safety from monoxide poisoning.

I don't know what the state of your finances are nor is it my business. But i do know that since there are so many aid programs that can arrange a new furnace for you you have no excuse not to replace it. So you cannot plead poverty as an excuse for poor safety judgment.

I hope noting happens to you or yours but I don't think you will be swayed to action by my words either. Folks who hold on to stuff a old as your furnace are a stubborn lot and if a tech came to your home and tested your furnace and found a monoxide you would still keep using it.

It happened to me on the same model of furnace you have.

I monoxide tested the furnace for a gentleman who lived in my town and found a dangerous leak. When I attempted to shut down the unit the customer became belligerent and would have struck me with a hammer he picked up. I backed off on shutting it down. I did not stick around to collect my service call but did manage to get a sign off on the leak.
I filed a police report to cover my self.


The next day in the late after noon they carried the customer and his three kids out of the home to the coroner's

You want to risk that, then I can't prevent it. But at least this post has your attention. Maybe you'll make the smart choice.


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## yuri (Nov 29, 2008)

When the heat exchangers break on those old units they can break/fracture like glass and split wide open. Can happen overnight at any time due to metal fatigue. CO detectors are primarily meant for catastrophic failures/blocked chimneys etc not to give advice when to replace the furnace, Your average HDepot detector is under $50. My UEI one is several $100s in cost and far more accurate. The one the utility co has can ACCURATELY detect down to 1 ppm. Doubt your WMart special is going to give you the same accuracy for that few $$. To each his own.


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## hvaclover (Oct 2, 2008)

[_*quote=yuri;523300]When the heat exchangers break on those old units they can break/fracture like glass and split wide open.*_ Can happen overnight at any time due to metal fatigue. CO detectors are primarily meant for catastrophic failures/blocked chimneys etc not to give advice when to replace the furnace, Your average HDepot detector is under $50. My UEI one is several $100s in cost and far more accurate. The one the utility co has can ACCURATELY detect down to 1 ppm. Doubt your WMart special is going to give you the same accuracy for that few $$. To each his own.[/quote]

Yuri, that is ironic. I used a UEI-Kane Mae co detector (made in the UK. Damn fine instrument) when i was testing the furnace in my last post.

What you say about these very old clam shell ht ex cells is true. I pulled the back off the one I was testing in my last post and it was cracked along the top front to rear dimension.

During the investigation I was shown some pics the mechanical authority took off the heat exchanger after the sad occurrence. The crack on the top had opened up all the way down the back like an open clam shell.

Thank God all if not most of those death traps have been replaced around here.

Hoooo boy...


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## yuri (Nov 29, 2008)

I had a 1968 Furnaz*man old solid steel local made unit once split wide open at the back. Customer's father used to be the fire chief in area station. Once the fan came on ALL the flames shot out the front, burnt the wiring and finally game over. She smelled SOMETHING for a week and knowing something about fires and all the horror stories from her Father you would think would have called sooner? Metal turns purple/brittle/gets fatigued with age and WILL split. But When?? Read about em in the paper we will. Real pretty ain't it.


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## hvaclover (Oct 2, 2008)

One thing that gets my back up as a pro hvac tech is the guy who gives you crap when you locate and show him a crack and he still wants to use the furnace.

I used to come unglued on these guys because of their apathetic nature to the safety of their children.

I went on a a clean and check and found an old 70% Bryant with flame rolling out of the furnace. The HO built a basement kids bed room with bunk beds and the furnace access was right in the kids room. Well local code forbids the access to heating units thru a bed room. 

I took one look at that abortion and when I saw the flames come out of the furnace I went for the power switch. Idiot HO knew I was going to shut it down. I body checked him so hard (Go Wings lol) he bounced off the wall.

He'd had two previous guys tell him he had a crack in the exchanger and he was looking for that one idiot service man who would just clean it.

SOB called the cops and tried to get me busted for assault. I Waited in my truck. When they came they talked to him and then me.

LOLOLOL the cops called the city inspector he and red tagged the furnace and busted the HO for renovations with out a permit.

He ended up having to correct building violations and mechanical violations concerning the furnace.


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## yuri (Nov 29, 2008)

I get that a lot. SOBs just want to pay $95 for a tuneup and figure your stamp of approval will make it last another year. Then if it breaks down they phone and say "your guy said it was OK" and want free repairs or to say you are negligent. Had to call the utility to shut one down on a guy with a 2 yr old breathing that garbage. Pleads poverty while holding Starbucks coffee in his hand. I document everything, call the utility co to shut it down. They record all phone calls for liability issues in our area and I cut them NO slack even if it is cold out. There are lots of good customers but it is the snakes who ruin it for everyone and make you suspicious all the time.


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## hvaclover (Oct 2, 2008)

yuri said:


> I get that a lot. SOBs just want to pay $95 for a tuneup and figure your stamp of approval will make it last another year. Then if it breaks down they phone and say "your guy said it was OK" and want free repairs or to say you are negligent. Had to call the utility to shut one down on a guy with a 2 yr old breathing that garbage. Pleads poverty while holding Starbucks coffee in his hand. I document everything, call the utility co who records all phone calls for liability issues in our area and cut them NO slack. There are lots of good customers but it is the snakes who ruin it for everyone and make you suspicious all the time.[/quote\\\
> 
> 
> Well I am glad I don't have to deal with them any more. I only do my regulars and don't accept the calls my free phone book listing generate.
> ...


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## mnalep (Oct 24, 2010)

hvaclover,


Thank you for that your concern! 

That is indeed a sad story. I cannot imagine how awful that would have been. It is obvious that you wish you could have had this gentlemen stop the furnace. 

I will take proper precautions.


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## hvaclover (Oct 2, 2008)

yeah, that's what he said too...


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