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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22001
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I have a Bryant Plus 90 furnace, model number 350MAV036080,

Resolved Question:

I have a Bryant Plus 90 furnace, model number 350MAV036080, getting an error code 31 - pressure switch did not close or reopened. Prior to installing a new one, I verified the old one was not closing, but blowing on the burner enclosure hose did make it cycle but it seemed like an excessive amount of pressure to actuate it. I also checked the supply and exhaust lines outside the house, both were open and free flowing. After installing the new pressure switch, I have the same problem. What is the driving force for the pressure or vacuum? Any other areas to investigate?


Submitted: 5 years ago.
Category: HVAC
Expert:  BillyHvac replied 5 years ago.




first remove the rubber tube from the barbed fitting at furnace end...use a paperclip and poke the fitting clear. Then make sure the drain from the furnace condensate water is not blocked.


Check those 2 items first and let me know.



Customer: replied 5 years ago.

I remove the hose from the furnace side where the other end goes to the collector box side of the pressure swich and stuck a paperclip though it and worked it around, replaced the hose and restarted the furnace. Switch still not closing. I removed the drain hose off the condensate collector box on the side of the furnace and do feel air blowing out of that while the fan is running. Hose from that box to the pump-out box is free from obstructions.


Next step....?



Expert:  BillyHvac replied 5 years ago.

Remove the intake pipe from the furnace and leave just the exhaust pipe hooked up.


Then try unit with intake pipe off.



Customer: replied 5 years ago.
Intake pipe off, good draw, similar to what I felt outside on the intake line. Switch still not shutting.
Expert:  BillyHvac replied 5 years ago.


Thank you.


This is a sign of the following:


Plugged secondary heat exchanger - this requires attaching a shop vac to the drian applying suction.


Or pressurizing system from the vent side and blowing and then reverse and apply suction.


Make sure the rubber tubes do not have cracks in them.


The switch is to prove air flow through the furnace before ignition can happen...if anything in the exchanger, tubing, venting etc cause too little air to draw on the switch, then 31 appears.



Customer: replied 5 years ago.
I haven't tried the shop vac yet. I can take the vent hose off the combustion air intake, blow on it and the switch closes. I can not feel any air coming out of that vent. The paperclip goes in ~ 1 inch. Is there some sort of baffling in there? Possible problem?
Expert:  BillyHvac replied 5 years ago.


You have 2 tubes on the switch right?


One from burner box and one from exhaust?



Customer: replied 5 years ago.
Correct. One, which if I blow on closes the switch comes form the combustion air intake box and is also teed to the gas valve. The one on the vacuun side doesn't go directly to the exhaust but to the left side of the fan housing directly over the condensate drain.
Expert:  BillyHvac replied 5 years ago.

did you clean that fitting out ?...the one over the condensate?



Customer: replied 5 years ago.
Yes, that is the first one I did.
Expert:  BillyHvac replied 5 years ago.

Thanks. Yours is a bit more stubborn than will need to see if the secondary is plugged. Or have a tech check for you.


This happened often eneough to where they had to switch 20 year to lifetime warranty on exchanger replacements.


If plugged and not able to clear, the exchanger (parts) are under warranty.


They asl issue 400 dollars to the contractor towards the changeout. Customer final cost is around 4-5 hundred.




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Customer: replied 5 years ago.
Getting back to the air off the combustion air intake - should I feel pressure off that. What does "seeing if the secondary is plugged" entail?
Expert:  BillyHvac replied 5 years ago.

you have a primary heat exchanger...and then a second one...this is how we get 90% instead of 80% efficient. The second exchanger is where the water condensation is made. It can get plugged with "thickening water" and if unit was installed with an one inch slope to front the unit can get plugged.


You should not feel pressure coming from burner box..but you should feel suction coming from other tube port above condenste drain.




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