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Tim H.
Tim H., HVAC Professional
Category: HVAC
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Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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Payne furnace showing code #31. Pressure switch (HK06NB123)

Customer Question

Payne furnace showing code #31. Pressure switch (HK06NB123) failure (or open too long.) Here's what I have done -- removed both lines from the pressure switch and blown air through them. The lower one makes a gurlgle sound but air passes through the trap. The upper line blows into the housing without restriction.

I have tested the pressure switch in 2 ways -- one while measuring resistance across the two terminals with vacuum lines attached (and induction motor running) and one while applying vacuum to the line (via a mityvac) and measuring resistance. Both tests resulted in the multimeter showing infinite resistance.

I have gone outside and the supply vent and return vent are both moving quite a bit of air (so the induction motor) is running.

Furnace will return code 31 immediately if pressure switch is hooked up.

If either wires or vacuum lines are removed from the pressure switch, furnace will ignite and provide heat until this switch is determined to be failing. Once this happens, the furnace will lock out until it decides to restart and then it will work fine again. Long cycles but it's keeping the house from freezing.

What's next??
Submitted: 2 years ago.
Category: HVAC
Expert:  Tim H. replied 2 years ago.
Thanks for giving me the opportunity to help! You didn't give me the model number (which would be helpful), but it sounds like this is a 90% furnace. If so, you may have an issue with condensate not draining properly from your system. In this case, you need to remove drainage hose, condensate 'trap" and rinse out with hot water. These natural gravity drainage condensate systems have a tendency to build up "gunk" in them over time and this can cause a pressure switch error.
Customer: replied 2 years ago.
Tim -- There doesn't look to be an easy way to service this codensate trap. I assume that if air can move through it (by blowing) then vacuum pressure can move through it. Is that correct? Is there a way to test this theory? If I disconnect the lower vacuum tube from the pressure switch it should work accurately for me to test this theory -- correct?

Also -- you are correct. This is a 90% efficiency furnace. Model #PG9M. Hoses are routed correctly as according to the manual and it has been working flawlessly for 8 years.
Expert:  Tim H. replied 2 years ago.
The fact that you hear water gurgling is not a good sign. There should be no water gurgling if all drainage has occured as it should. I would start here as this is a common issue with 90% furnaces. If you provide model, I can see about "rounding up" a manual which may help you visualize the system and how it works.
Customer: replied 2 years ago.
It's a PG9M. I have the service manual. Isn't the trap just that -- a trap? I thought it was like a regular plumbing trap in that it should always have a bit of water in it to prevent air from coming back up? So, again -- Can I test your theory by checking whether the switch operates correctly with the lower vacuum line removed?
Expert:  Tim H. replied 2 years ago.
No, the vacuum lines need to be attached for it to work correctly. The board will look for an open pressure switch prior to starting and then as inducer comes up to speed will look for the switch to be closed. However, if there is water backing up in the collector box, and exhaust, this will affect operation of switch closing properly.
Expert:  Tim H. replied 2 years ago.
Page 3 of this manual shows the condensate trap and hoses located in the blower compartment. These need to be taken apart and rinsed out with hot water.
Customer: replied 2 years ago.
I'm skeptical because all of the lines are dry. There's no possible way that fluid has backed up as far as the pressure switch. I can funnel water into the condensate drain and it drains right out the side of the furnace.

I see similar answers where people indicate that when they remove the lines they see water come out -- this is not the case in my instance.

I'm going to further inspect the trap. If this is indeed the problem, I will accept your answers.
Expert:  Tim H. replied 2 years ago.
It's not the one that goes out the side of the furnace I'm worried about. It's the collector box and condensate trap I previously mentioned. Of course it's always possible you have a bad pressure switch. These are tested using a manometer, a device very few homeowners would have or understand how to test. The other possibility is just to replace the switch thereby eliminating that possibility.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3142
Experience: Experienced in all aspects of HVAC, both residential and commercial.
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Customer: replied 2 years ago.
I actually have a manometer for working on carbs. I didn't even think of using this.

I feel pretty dumb now. I thought that the condensator drained through that pressure switch. I didn't realize that it just measured vacuum that was generated by the condensator. Once I removed all of the lines from the trap, water started pouring out. What threw me off is that water was able to easily pass through the vacuum line (and the other small drain line.) These top ports must be seperated by something inside of the trap (if there's a reason for that, I certainly don't understand it.) Once I blew it out with an air compressor and soaked it in hot water for a couple of hours, water was passing through without issue. Thanks and sorry for being so difficult!!
Expert:  Tim H. replied 2 years ago.
No problem, glad you found it! This should be done annually.

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