# Goodman Furnace Pressure Switch Stuck Open



## HvacSteve (Sep 4, 2017)

I would start with checking the heat exchangers. There's absolutely nobody in your area that can't come out before Tuesday?


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## Coors (Mar 8, 2010)

Found 1 person, but I am third on the list for tomorrow and they only work half the day...so it looks like Monday is the best I can do.
I don't really want to tear it apart for the heat exchangers. The furnace is only 13 years old, but I guess it could be that. 

I am still wondering why I have the water leak in the image I posted earlier...seems like a blockage somewhere.

Anyone else have any ideas?


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## HvacSteve (Sep 4, 2017)

The water leaking could be from the secondary heat exchanger, which is why I recommended having them looked at. Could also be causing your pressure switch problem.


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## user_12345a (Nov 23, 2014)

When it thows a stuck pressure switch error, you should check the voltage drop across it or disconnect it and ohm out.

If it's truly stuck closed, you need to replace the pressure switch. 

It needs to be open until the draft inducer comes on. 


The leakage based on location could be a failed gasket or even the drain line connection.

If the secondary heat exchanger was leaking, you would see water in the blower compartment.


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## York v45 (Feb 23, 2014)

I fought condensate leaks on my GMNT for two seasons until I finally changed out the collector box and sealed it up with high temp RTV. That fixed the leaks.


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## Coors (Mar 8, 2010)

user_12345a said:


> When it thows a stuck pressure switch error, you should check the voltage drop across it or disconnect it and ohm out.
> 
> If it's truly stuck closed, you need to replace the pressure switch.
> 
> ...


Thanks, for the options to look at. I do not see water in the blower compartment, just at the spot in the picture. Once it throws a code again I will check the voltage across the pressure switches. I assume the one connected to the gas valve is the one that matters the most in my case? I turned it on and it ran fine and both had 25 VAC.


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## user_12345a (Nov 23, 2014)

There should be a separate pressure switch that the board monitors.

I haven't seen the schematic, though if a pressure switch is connected in series with the gas valve it's a secondary safety device.

And shouldn't throw the same error code.

This is a timed only 2-stage furnace with a single speed inducer, so it shouldn't be overly complicated. most Others have separate pressure switches for each stage.

With power applied and no inducer running, if it's stuck closed it will be be 0 vac and when open 24 vac, +/-.

You can confirm 100% by shutting power, disconnecting one lead and checking resistance. 

Pressure switch should be open without the inducer running.


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## user_12345a (Nov 23, 2014)

are you sure about the error code?

this manual says 3 flashes = pressure switch open with inducer running.

now 2 flashes = pressure switch closed with inducer not operating.

HUGE difference.


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## Coors (Mar 8, 2010)

100% positive on the 3 flashes. I attached a few images. The first one showas the error codes on my furnace and I am getting 3 flashes.
The second image shows the two switches. The one on the left is by the gas valve but I was wrong...it just runs a vacuum line down by where I see a leak. That sensor shows 0 volts until the burners kick on then it jumps to 25 VAC. The one on the right goes to the vacuum motor and jumps to 28 VAC and STAYS at 28 volts when the furnace gives up trying to fire. I did what u said and disconnectEd the right switch and powered off the unit at main electricAL switch and the resistance went to OL (3rd image)? I was on the positive terminal and touching a ground on the furnace.

Thank u for the help.


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## Coors (Mar 8, 2010)

And when I take the vacuum line off and suck on the line I hear it switch and it goes to 2.3 k ohms


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## user_12345a (Nov 23, 2014)

The schematic says 3 flashes is pressure switch stuck open, not closed.

Your reading also confirms that it's not closing.

The reason why one of the switches reads 0v until the burners kick in is that it's not energized at all until power is sent to the gas valve.

No voltage = no voltage drop.

I guess it's intermittently working or something because the primary pressure switch must close before it tries to turn on the burners yet you got volt drop across it.



> And when I take the vacuum line off and suck on the line I hear it switch and it goes to 2.3 k ohms


Is that kilo-ohms?

It should be very near 0 ohms closed.

If you got a good connection when checking resistance, I'm betting it's a bad pressure switch. Contacts failed.

I'm not sure your reading is valid, because it wouldn't have read 28v across if closing but with a high resistance.

Now, if the pressure switches are identical* with the same pressure ratings*, to be 100% sure you can swap them.

Hoses, brackets stay in the same place, just do a swap.

Leave the pressure switch now in series with the gas valve with one wire disconnected, tape the connection to make sure it doesn't short anything. (leary of feeding power to gas valve through 2.3k ohms - it should either be just about 0 ohms or open)

Restore power and turn on the heat.

It should get to the point where the ignitor warms up.

If it does, order a new pressure switch, model numbers must match.

A technician would check with a manometer, see if it's a bad switch or there's an actual problem causing it not to close.

(problems could range from blocked condensate lines/trap, blocked exhaust/intake pipes, inducer problem, etc)


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## beenthere (Oct 11, 2008)

You don't ohm out a pressure switch to ground to see if its open or closed. You ohm it out by its terminals directly, with the wires removed from it.


PS: Try cleaning out the condensate trap. Also make sure the exhaust el drain line is not clogged.


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## user_12345a (Nov 23, 2014)

I see no mention of ohming it to ground.


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## beenthere (Oct 11, 2008)

user_12345a said:


> I see no mention of ohming it to ground.


Reread post #10. 



> I was on the positive terminal and touching a ground on the furnace.


And then look at the pics again.


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## Coors (Mar 8, 2010)

Well I turned the thermostat to off last night and left it alone for about an hour and I was siting in the other room and it kicked on (started to warm up) for about 20 seconds and shut off. I ran to the thermostat and it was off. It may be possesed!

I powered everything down over night so I could sleep. In the morning the igniter would glow then shut off...after a few times it worked and brought the house from 67 to 72. Turned it off for 4 hours to leave the house and came back aND turned it on. It has sense run perfect for the last 5 hours! I hate this darn thing. I don't get it. I will never buy a Goodman that's for sure.


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## user_12345a (Nov 23, 2014)

This can happen to any brand of unit, they all use similar internal components.

It sounds like your pressure switch is being erratic. It could be due to the switch itself or a problem in the furnace's heat exchanger/venting or drain hoses/trap.

Have you tried making sure the drain lines and trap aren't blocked?

The turning on with no call for heat thing? If that continues, disconnect the wire connected to W on the board and see if it persists. likely a thermostat or low voltage wiring issue.


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## Coors (Mar 8, 2010)

The start up with no call for heat has not happened again. Who knows?

And yes. I spent a good hor cleaning all lines and took off the drain catch and washed them out yesterday. I also took a shopvac to the exhaust line and sucked and blew it out. Had good flow threw it.

I did notice this morning it was running fine then for 1 second the flame would shut off and come right back on (total time of under a second) it was like it coughed. It also was running from 67 to 72 degrees and the whole furnace did a shut down cycle when it was at 68 degrees so the call for heat should of been on the entire time and then started up 
again and didn't shut off until it was at 72 degrees. Could my thermostat be bad? Only like 3 years old.
Thanks again for all the help


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## user_12345a (Nov 23, 2014)

If it throws an error code when it cycles, *it's not the thermostat causing it to shut down when the call for heat should be continuous.*

You can bypass the stat and monitor it be disconnecting the wire connected to W on the board and jumpering R and W on the board. This is not recommended for an extended time - *ie no unattended operation like this.*

To diagnose the pressure switch issue properly, need a manometer to see what pressure the switch is seeing.

see:
https://www.youtube.com/watch?time_continue=367&v=qQ2b-DZ82Bs

If the pressure is fine and the switch is dropping out (confirm with voltage drop test), you know to replace the pressure switch.

Otherwise you have to do more testing and figure out why it's dropping out. There are so many things it could be and it could be tough to narrow down.


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## Coors (Mar 8, 2010)

Well the technician came today and checked out the furnace. He said 95% of the time it is not the pressure switch, so he checked basically everything I did and it was the pressure switch. He put a new one in and it started up a lot more consistently.

He did a CO check on the exhaust and said it read 257ppm, and normal should be around 130pppm. He said that means my heat exchangers may have a problem, but said that it would be fine over this winter and to call him next fall for a checkup. He said if it gets over 300ppm then I will need to look into replacing it or a new unit. He recommended I buy some CO detectors which I have, but that makes me nervous.

Does his explanation sound reasonable or should I get another opinion? 

Thanks,
Cody


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## user_12345a (Nov 23, 2014)

Non-sense.

If a heat exchanger is bad on a unit this new, it gets replaced under warranty. You don't replace the whole furnace.

Now, many different things can cause high monoxide in the exhaust and it's exceedingly unlikely that the heat exchanger is bad in such a new unit.

Can't diagnose it based on monoxide - more reliable diagnostics: seeing defects during visual inspection, flames changing when fan comes on, doing combustion analysis and seeing excess air spike when fan comes on.

Poor combustion from being really underfired or a bit over-fired will cause it. Wrong fuel to air mix.

He should have clocked the meter and/or checked gas pressure.

Something is wrong even if it's producing 100 ppm in the exhaust.

With good combustion, there should only be a small trace like 20 ppm for example.

Natural gas burns clean.

He shouldn't have left it like that or recommended furnace replacement.


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## Coors (Mar 8, 2010)

Ok...to be fair he did say that the heat exchanger should be under warranty, but the labor would not be. He said that the labor was to the point that you need to decide if you want to replace or get a new unit.

But the immediate issue is what to do with the higher PPM values. The original company I called couldn't get here until tomorrow, so I may just keep that appt. and see what they say. 

Thanks for the help and honest replies...I don't mind paying a bill if I get an honest repair...but they are hard to come by.

Thanks,
Cody


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## user_12345a (Nov 23, 2014)

The labour should be a fraction of the cost of a new furnace and absolutely worth it.

The higher ppm is a concern and I think that fuel input should be the first thing that should be checked before jumping to something more complicated.

You can actually clock the meter yourself. This is not a substitute for having a pro check it.

http://hvactechhangout.com/home/system-measurements/how-to-clock-a-gas-meter/

You would have to do it on low fire and high fire with all other gas appliances off.

If your unit is set to run on first stage, it's for the first 12 minutes -> so you clock it on low and wait for it to switch to high.

Do you know if the monoxide reading was checked on low, high or both?


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## Coors (Mar 8, 2010)

He checked the reading after the furnace was running. So basically he put it all back together and the furnace kicked on...then he went outside after it has been running for a few minutes and tested it.


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## user_12345a (Nov 23, 2014)

That's not the most accurate test, better to let it warm up and stabilize.

This furnace with it's single speed inducer gives the wrong fuel to air mix on low as well (the better ones run at a lower speed), which may or may not result in elevated readings.

But not 250+ ppm.

I'm skeptical. Clock the meter* when you have a chance and get a second opinion if u don't mind spending the money.

*If you find it significantly below or above btu rating, u know u need to call a technician to check gas pressure.


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## beenthere (Oct 11, 2008)

Did he say what the O2 reading was?

If you milk the heat exchanger swap out, it could take 6 hours.


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## Coors (Mar 8, 2010)

Well I called another company that is well known and they guy did a few tests and did not physically see a hole, but said the I have a hole in the heat exchanger. He put a device to read pressure and turned on the fan and said if the heat exchanger was working properly, the pressure should be zero. Well the pressure increased.

He is going to give me a quote to install a new heat exchanger and fix the collection box (has a leak) and also said that he wouldn't rule out putting in a new furnace.

Thoughts?


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## user_12345a (Nov 23, 2014)

It's crazy to have a heat exchanger fail that early.

I would go ahead and change it.

However, before doing that....

Anything obvious that restricts airflow should be corrected. Would be wise to have a static pressure test done.

For example, having a smaller than recommended filter, undersized return drop.

If ducts are undersized, should investigate putting a smaller furnace which needs less airflow if sufficient to heat the house.

Could also have the opposite where the heat exchanger runs too cool due to too much airflow and/or being underfired.

Your model may be more likely to have that problem, being 2-stage with a 1-speed inducer. The extra combustion air on low may cool the flame/exhaust, making that more likely.

You'll know if that's the problem if it looks really corroded.

If you get it changed, after that they should check temperature rise and gas pressure, make any necessary adjustments or it will happen again.

Fan speed/gas pressure can be adjusted.


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## Coors (Mar 8, 2010)

Guy noted a few things the original installer did wrong 11 years ago (before we owned the home) the exhaust pipes have a few plumbing elbow (tight 90 bend not sweeping 90). Also I guess it has ran full blast all the time. He said that it should run at a low speed for 10 min then kick on the higher speed after that. Ours was always on high. Simple switch no one noticed. I have had 4 repair guys look at it in the 8 years I have owned the home. Could explain some of what u mentioned above. 

He quoted me $700 to repair it and around $2500-$3000 for a new furnace. As much problems as I have had I am leaning towards new furnace. They are coming Monday to give a better quote.

Thanks


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## user_12345a (Nov 23, 2014)

What size is the return air drop and filter?

What size/type of house is it heating?

I would takge advantage of warranty unless the furnace is oversized for ducts and house. Any unit will give trouble and have premature failure if not installed correctly.

$3000 is a lot of money when u could get another 10+ years out of it.

At the end of the day, goodman owes u a heat exchanger. 

What you should know though in making a decision - is that the gmh is a 'not so good furnace", at least as a 2-stage furnace. When set up as a 1-stage, compared to other 1-stage units it's probably decent.

1. Low is only 75% capacity, most others are around 60 to 66%

2. As previously mentioned, the inducer is 1-speed, so on low the fuel to air ratio is wrong and the efficiency falls below 95%

I would be tempted to keep this unit locked into high fire should u choose to keep it - *it will burn more fuel overall on low.*

3. It's exclusively time delay controlled. The better 2-stage ones give the option of using a 2-stage thermostat, so it only hits high when low can't keep up alone and can switch from high to low.

Even if you look at goodman again, the GMVC rather than GMH is a vastly superior product. Has other advantages. (variable speed ecm blower comes to mind)

If what you have is not a good fit (again, size, etc), I would be very tempted to see if it's possible for your contractor to cash out the warranty and put it towards a GMVC96.

I have no idea how flexible goodman is, it may depend on the dealer.


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## Coors (Mar 8, 2010)

Our house is almost 1400 sqft. It is an L shaped ranch with a 80% finished basement. The filter size is 20x16x1 and is a merv 7 paper filter from menards. As u can see it tapers down from 13 inches to 10 inches. A repair guy also put in a vent 2 years ago because it was restricted and whining.


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## user_12345a (Nov 23, 2014)

A 90 000 btu furnace on 1400 sq ft?

While L shaped house needs a lot of heat per sq ft, this sounds out of line. Unless perhaps it's a insulated house.

These units need lots of air.

And your air ducts are most likely too small and this may have contributed to killing the heat exchanger after 11 years instead of 20.

Ask your dealer about evaluating the duct system. Ask about load calculations.

Putting return air in the utility room is a big no no,* especially when taking combustion air from inside.*

Puts the room under negative pressure and can affect combustion. 

Can cause water heater to backdraft.

Don't blame goodman for these problems.

Temperature rise, better yet static pressure* should be checked before repair.

*Use manufacturer's chart to check cfm.


There's no point of putting a new heat exchanger in if it's going to overheat and fail again.



> The filter size is 20x16x1 and is a merv 7 paper filter from menards.


While not horribly small, you should probably be using fiberglass on 90k furnace.

16x25 better, media filter even better.

Good luck with this. Bottom line is ask for more testing before making a decision.


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