Gas and Oil Home Heating Furnaces - Ignition Problems?

I have a warm air Heil furnace, 75,000 btu, model NHGK075AF01, serial number N912923789, Mfg # 928616229, built in 1989.

Here is my problem:

1. The T-stat called for heat.
2. The inducer/combustion motor was hot and not running, apparently it was frozen up. The heating cycle ended at this point.
3. I replaced the inducer motor
4. I replaced the electronic fan timer.

The T-stat now calls for heat and the inducer motor now runs,
however, the heat cycle does not complete!

The single tubed pressure switch, P-1, opens.

The double tubed pressure switch, P-2, does not open, therefore I do not get ignition, pilot and flame to run the circulation fan.

Now...I manually depress the double tubed, P-2, pressure switch and place a paper match stick between the contacts and diaphram and I then get ignition, flame and completed heat cycle, including shut down. Later on when the t-stat again calls for heat the unit properly fires up and runs through continous cycles!

Since I have 2 identical furnaces in my house, I have switched electric fan timers, pressure switches, and ignition modules and the problem unit performs the same.

The unit that I took the parts from runs perfectly with the parts from the problem furnace.

I have cleaned out the drain line from the floor drain to the furnace but not inside the furnace. However I have blown air back into the drain line and I get air flowing out of the line, but not condensation.

My questions:

1. Is it safe to manually by-pass the pressure switch to get igntion?

2. If so, can I keep it in this position until I fix this problem. Presently I warm this section of the house and shut it off after about 3 cycles.


3. What other parts, etc. must I check to determine if they are defective?

4. How about the Smart valve? what is it and where is it located?

5. What is an air proving switch. where is it and what does it do?

6. How about the panel behind the electronic fan timer and inducer motor, does it contain the above mentioned items and where do the plastic air lines go to inside this panel? I am thinking the one that connects to the P-2 pressure switch is not providing enough air pressure to activate the switch to cause ignition, because when I blow gently onto the pressure switch I get ignition.


Finally, when the T-stat calls for heat, in the problem furnace, the inducer motor does not turn on for at least 90 seconds. This never occured before and does not occur with the other unit. The respective inducer motors usually turned on instantly when you call for heat.

Thanks in advance and please keep the technical language to a mimimum, I am quite novice at this!!

Ron

Model: NHGK075AF01,
Brand: HEIL
Desc: GAS FIRED HEATING UNITS

SW PRSS 1150630 DRN SLND
SW PRSS 1150642 IN OUT DRIER
SW REV AIR 1150538 FLOW FILT 1150709
CTR TMR 1150489 FAN COMP
CTR LMT 1150492 THRM COUP
MOD IGN 1150558 CTR BND MTR 1150267 BLW ASY
MTR BLW 1150519 BND MTR 1150041 BLW
MTR IN ARM MTR 1150605 BLW
MTR COND KNB DOOR 99376
FAN BLD FILT KIT 1150524
WHL BLW VLV GAS 1150597 NAT CTR
PLY MTR PLT
PLY BLW MANI 1150587
CAP 7.5MFD 1150060 TRANSF 1150557 40VA
WHL BLW 1150518
BOX BRN 1150594 HOUS BLW 1150728
BOX CTR 1150617 CUT-OFF 1150604
BLW ASY 1150553 IND
WIRE IGN 1150510 BLW ASY 1150725
CNVR KIT 1148223 NAT-LP BLW ASY 1150731
CNVR KIT 1148262 LP-NAT COIL COND 1150576
BRN PLT 638009 DRN COND 1150585
ORIF 638196 LP KIT VNT 1150717 + DRN
ORIF 638174 NAT BAG VNT 1150719 + DRN
BRN 1150599 INSHOT
HEAT EXC 1150505 PRI

The cut and paste didn't work so great, but that up there is your parts list.

I'd replace P2. To answer your guestions:

1. Is it safe to manually by-pass the pressure switch to get igntion? NO,NO,NO, NEVER, NEVER, bypass a safety!

2. If so, can I keep it in this position until I fix this problem. Presently I warm this section of the house and shut it off after about 3 cycles. See above.


3. What other parts, etc. must I check to determine if they are defective?
Sounds like you found it. See, the inducer has to run to "prove" negative ventalation in the heat exchanger, All the safeties have to operate before the gas valve will open and ignition can begin.

4. How about the Smart valve? what is it and where is it located?
That's the trade name for the gas valve. It's supposed to be smart. Pretty reliable too. If this were bad, you'd have no ignition.

5. What is an air proving switch. where is it and what does it do?
The air proving switch is a safety. It "proves that the inducer motor is operating and doing it's job, thus allowing the rest of the ignition sequence to happen.

6. How about the panel behind the electronic fan timer and inducer motor, does it contain the above mentioned items and where do the plastic air lines go to inside this panel? I am thinking the one that connects to the P-2 pressure switch is not providing enough air pressure to activate the switch to cause ignition, because when I blow gently onto the pressure switch I get ignition.

Can't help you here with out a visual. Try going here: http://www.achrnews.com/ and search for articles about your type of heater. They may have written an article about the basic workings of a unit like yours.

Good luck, and stop jumping out safety switches, especially if your a pizza man and not an ac man

Thanks for the advice. I will disconnect the bypass on the P-2 switch.

Now..I am confused even more! Remember, I took off the P-2 switch from the problem furnace and replaced it with a known working switch from an identical furnace, then I put the questionable P-2 switch on the working furnace and they both worked!

Something is causing the P-2 switch on the problem furnace from doing it's job(s).

Any thoughts?

Also, how about the air proving switch? you stated this has to work to permit ignition. I cannot locate or identify this particular switch. Is it also known as the P-1 switch, since this is also a safety switch or is it another switch located behind the panel, inside the furnace?

The only items located on the exterior of the panel are the electric fan timer, ignition module, 2 pressure switches, transformer, and inducer.

I have an underestanding of what the sequence is for proper ignition, however therr seems to be something interrupting the performance of the P-2 switch.

check inside the inducer assembly, if the hose is sound, no leaks, and you know the switch works properly on the other unit, there may be something blocking the tap where the tube ties in to the assembly. have seen the hole not completely drilled out, trash blocking it, and even paint chips, baked off of the inside of the draft assembly.

Re-reading your initial post, after hvac4u's I think I know what the problem is (corect me if I'm wrong hvac).

P1 with on hose should be checking for negative pressure in the combustion chamber.

P2 with two hoses, should be checking to ensure airflow accross the heat exchanger. Meaning it is making sure that the indoor fan is running before allowing ignition. On the switch near where the hoses connect look for "high" or "low" or both. "High" would connect (via a hose) to the blower outlet and "Low" would connect to the inlet.

Like hvac4u said there maybe something blockig one of those hoses. Or there maybe something restricing airflow, such as dirty filters, a failing blower motor, or some such thing.

Verify that you have good airflow, and (if you can) that the hoses are connected to the correct port. Try dissconnecting the hoses from the switch and blow through them to ensure they are clear.

By switching out the P2 switchs you verified they both work. No you need to find out what is causing the switch on the problem furnace not to make (close).

Thanks for removing the bypass. Those switches are there to protect you and your loved ones. Don't put them at risk just to save the cost of a repairman's visit.