American Standard pressure switch

[dvschillin]

Okay, so I have this 90% furnace that is giving me a heck of a time. On the first call I observed that the pressure switch would only close every three or four tries I checked for any obstructions in the exhaust, intake, condensate, no water in the inducer or switch, so I had a new switch shipped overnight. I replaced it and the thing took off running like a champ. Switch needs 1.59 iwc and I am getting 1.9 all is good. I get a call two weeks later they are having the same problem. After I recheck all other possibilities I then check the pressure I am only getting 1.4 iwc while connected to the switch. If I pinch off the line at the switch I get 1.82 iwc. I think I must have gotten a bad switch so I put on a universal and once again the thing takes off running just fine. This time it is only a week and I get called back. I am still having the same problem which is with the switch connected I am getting 1.2 iwc but when I pinch off the hose I am getting 1.9 iwc. I have asked everyone I know and they are all as stumped as I am. I will be calling my distributor tomorrow but since this is Sunday I was hoping someone here could give me some ideas.

Thanks

[Keith73]

How old is the furnace?
Even a small restriction in either the intake or exhaust can cause a pressure problem ( such as a wasp nest).
Did you take the differential pressure across the switch (assuming a dual port switch)?
Enclosed combustion chamber? Typically those have dual port pressure switches and have a bleed port. (pressure change when pinching off line)
A failed heat exchanger can also do this. Have you preformed a combustion analysis?

[Glennhvac]

I can only relate my biggest PITA 90% pressure switch problem and it may not have a thing to do with your issue. Was a RUUD 90 I installed and ran fine the first couple years. The unit would drain fine until it got fairly cold out and longer run cycles. What I was seeing was the tube that went to the pressure switch would load up with water the minute the furnace cycled off. Often on the next call for heat those few drops stayed in the tube so the switch would not make.
Turned out one of those plastic squeeze clamps on the drain lines at the trap was loose. The thing must have been sucking in just enough air that waters was being trapped in the cover. It probably would not even happen if they put that 1/4 inch port up another 1/2 in higher.
May be some restrictions to combustion air flow internal to the furnace too.

[lions_lair]

Found this in a 90 percenter this fall.

Sent from my big,bad,beatiful...note 3

[dvschillin]

Keith, the furnace is 5 years old, it is an enclosed combustion chamber, I did not do a combustion analysis, and I did not take the differential pressure. I am not exactly sure what I would be looking for by testing the pressure differential.

[50hdmc]

Are you certain the hose is not leaking/cracked near the pressure switch....I like to install a "T" with my manometer so I can "see" exactly what the pressure switch is getting while it is running.

[dvschillin]

I did install a "T" that is how I got the numbers I posted. The reason I pinched the hose off at the pressure switch was to see if it would pressure in the hose which it did at the required pressure to make the switch, but when I let it loose the pressure is to low to make the switch.

[Keith73]

I did install a "T" that is how I got the numbers I posted. The reason I pinched the hose off at the pressure switch was to see if it would pressure in the hose which it did at the required pressure to make the switch, but when I let it loose the pressure is to low to make the switch.

That should happen with a switch that has a bleed port.
If the pressure switch has two hoses connected to it, you have to use a dual manometer to test that actual operating pressure the switch is working with.
While testing the switch with both sides being tested, if you open the burner compartment and see a major change in pressure, there lies your problem.
I've found systems that have had leaves sucked into the intake a trapped in a 90 so you could not see them.
I think more testing is needed.
Good Luck.

[dvschillin]

Keith, I disconnected the intake and had no change. Also the problem is that I never get enough pressure to make the switch to begin with which makes me think that it is a bad pressure switch, but the question is what is causing them to go bad? I will be investing into a new d manometer tomorrow with its own pump so I can check the pressure switches. Yes I see more testing on this one in my future.

Thanks

[gravity]

Could be weak inducer, leak in the back of inducer, blocked pipes, blocked h/x, blocked drain line....


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[dvschillin]

gravity,

checked and resealed the inducer, checked pipes, checked drain, and I am leaning toward the h/x but not sure how to check the secondary for leaks.

[Glennhvac]

Never done this and am wondering for future. Let say you have a gage on that pressure switch while the inducer is running and you are getting a reading of X. Would that reading change if the indoor blower is running if either heat exchanger has an opening? So as not to throw another variable into the mix, suppose we ran that test with the burner off.
Off the top of my head I would think that reading has to change once that blower starts considering we are talking 1-2" wc

[dvschillin]

Glenn,
My thought is yes and that will be my next test, did not think of that until after I had left and called some friends of mine. On the other hand I have been you tubing this and it looks like that might not always work. As I recall I was watching the meter while the blower kicked on and did not notice a change but wasn't really looking for that specifically.

[dvschillin]

I was watching a video where there were cracks in the transition piece at the collector. it seems to be the exact problem I am having.

https://www.youtube.com/watch?v=GYOS...5052A16308A4F5

[Glennhvac]

I'd be very interested to know if this method might determine an unsound heat exchanger.

Glenn,
My thought is yes and that will be my next test, did not think of that until after I had left and called some friends of mine. On the other hand I have been you tubing this and it looks like that might not always work. As I recall I was watching the meter while the blower kicked on and did not notice a change but wasn't really looking for that specifically.