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Heil furnace Pressure Switch issues

8K views 6 replies 3 participants last post by  yuri 
#1 ·
Hoping this forum can once again save my bacon. Jagans, Marty S., are you out there?

I have a 10 year old 90+ Heil furnace in my attic that died last night. I'll try to give as much information as possible so apologies in advance for the long post.

The furnace itself is of the N9Mp1 variety and it is installed "horizontal left through top" similar to Figure 16 in this PDF.

http://icpindexing.toddsit.com/documents/086477/44001102402.pdf

It was running fine last night. When I woke up this morning the upstairs level was 57F and the breaker to the furnace had tripped in the night. Outside temps were about 19F here in NJ last night.

It's been recently maintained, all hoses seem in good condition and as recent as last November I put a new exhaust motor in it which is turning freely still.

When I turned the breaker fuse back on the furnace tried to startup but it's not firing the gas, the igniter does not come on and the exhaust motor never spins up. Instead the honeywell smart valve flashes 3 steady lights which the manual says is a pressure switch problem "open when they should be closed".

System goes into 5 minute delay period, another cycle begins at the end of the 5 minutes. It does this repeatedly.

So I took the small 1/4" hose off the transition assembly (the box behind the exhaust) and sucked on it while cycling the unit. The small black hose running to the pressure switch in the attached photo. Everything came on, the burner lit, gas lit, and of course I had to immediately shut it down but seems like everything's working. Meaning I don't think it's a faulty pressure switch.

I checked the hoses but cannot find an air leak. What should my next step be?
 

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#2 · (Edited)
Marty and Jagans are gone or have not been here for a long time.

The inducer needs to run to create vacuum to close the switch. Check to see if it is getting power. There is a ST**** fan timer board in the fan area that starts the inducer as well. It may be shorting and tripped the breaker. It may be shot and not starting it. It may be intermittently starting the inducer as obviously it did it while sucking on the pressure switch which you should never do as it could get damaged.
 
#3 ·
The inducer motor is not getting power - I believe this is as designed. My manual says the sequence of operation is

1) Thermostat calls for heat
2) Pressure Switches proved open? No -> wait
3) Combustion blower energized (this never happens. Is this the inducer you're referring to? )
4) Pressure switches closed within 30 seconds -> No ( this seems to be where it's bombing out with 3 flashes).

I checked the fan board, there's a single fuse on there which looks ok. Will get out the multimeter I guess.
 
#4 ·
Yuri you called it. The ST board is effed. I didn't notice initially but there's a huge black blowout behind it. Something popped. I've shut off the breaker to the furnace and need to order a new one.

Any advice on where I can order/overnight one from? (I am not a contractor, just a guy with a cold upstairs).

And again thank you so much for your help. This forum saved my mental health.
 
#5 · (Edited)
americanhvacparts.com or supplyhouse.com

the inducer MUST start B4 anything else as it has to purge any unwanted gases out of the chamber plus supply combustion air and a draft thru the heat exchanger. The pressure switch must be open at that time and the board checks for that first, then it checks for it to be closed proving the inducer is running before energizing the gas and igniter.
 
#7 ·
Boards fail from age and use. The fan relay has contacts and eventually from arcing and switching inside they get hot and can short out or cause the board to blow on the back side from solder joints failing etc. Happens all the time with Whisperheat BCC boards and others.

They can make a board last 30 yrs but it will cause 3X as much and the consumer wants 10 yr quality nowadays and won't pay more.
 
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