Check that heat exchanger. Cracks can trick a pressure switch. Also check the wiring and connections. You can use a mirror to look behind circuit boards, sometimes a solder point will go poof and drop the electrical connection in and out.

 

Had one that worked for awhile, until condensation would build up and get in the di and shut it off. Then it would drain slowly.

 

My apologies for the length of the post, wanted to make sure I gave you all of my efforts to date. Went out on a no heat call Sunday… Walked up on the furnace ran it through the start-up sequence for heat. Inducer Fan starts but no hot surface igniter.

Troubleshooting Steps:

1. Jump out the pressure switch, furnace starts on every try for the next 10 cycles
2. Check voltage (122v) to the inducer fan/amps (~1 amp)/resistance (0 ohm) – no issues other than motor is noticeable loud intermittently.
3. Check vacuum port from inducer motor using manometer (.8 - .75 w.c.) running for 1-2 min.
4. Check Pressure switch (.50 w.c.) – connect meter and check resistance when inducer starts. Switch opens but there is some resistance reading
5. Pull the flu pipe off of the inducer to check for anything in the flue and restart inducer without flue connected to bypass any possible blockage. No improvement
6. At this point, I have a system that starts every time when the pressure switch is bypassed, so I install a new pressure switch – new one ohms 0 when it opens and system starts a couple of times in 5-6 cycles, but issue is still intermittent.
7. Next, I pull the vacuum hose off the inducer, with the inducer running the system starts every time that I manually pull the hose and replace it on the inducer port, so I replace the inducer motor assembly – thinking with intermittent noise and play in the motor shaft may be causing some operational issues.
8. After replacing inducer motor and pressure switch, system starts every time for 10 cycles.
9. Next Day, customer calls saying everything worked great for the remainder of the day, but house was cold next morning… system doing the same thing.

At this point, I’m thinking system board, except the “#$#5” thing starts every time I bypass the pressure switch.

Any thoughts, suggestions, verbal beatings - 

Did you mean closes instead of opens?

 

I did mean to say the pressure switch closes, but had some resistance reading. Thanks for the catch.

 

Its gotta be the ignitor

Sent from my iPhone using Forum Runner

 

What about hose to pressure switch ? I worked on one that ran fine until hose heated up, and drooped and kinked.

 

Check drain. Remove the trap and flush it every which way. Make sure its full of water. Make sure the drain after the trap is sloping evenly away from the furnace (no dips or double traps).

I had one similar to this. The furnace would run perfect while I was there and then quit during the night. Here was the problem:

There were two traps, one at the bottom of the flue pipe, and one under the secondary HX. After each trap, the two drains were Tee'd together and then sloped perfectly to the drain. What caused the problem was the trap below the flue pipe never filled with water, and the combustion air from the flue pushed back into the secondary hx through it's drain. Since it created negative pressure the water would back up into the heat exchanger until tripping the pressure switch. Of course this never happened unless the outside temp dropped below 20 and forced the furnace to run long cycles. Every time the furnace stopped, the water drained out of the furnace.

So anyway... make sure the drain is good and there are no outside influences.

Also, make sure there is a good seal between the inducer and the transition/collector box.

Tee the monometer back into the pressure port. Grab a five gallon bucket. Turn the thermostat up to 80. Sit on the bucket. Turn on the power to the furnace and watch the pressure on the monometer. Let it run for 20 minutes. Patience is everything when it comes to drain issues.

 

Wow, inducer assembly and pressure switch - that's one pricey experiment.

Are you saying this pressure switch closes at 0.5 IWC? If so, since you were reading 0.75-0.8 IWC, that proves there's acceptable draft and the inducer assembly didn't need replacing. If the pressure switch wasn't closed under that pressure, the pressure switch is faulty. If the pressure switch was closed and the IFC didn't go the next step and energize the HSI, the IFC is faulty. Did you check the HSI? Did you have 120V? What was the resistance on the HSI?

Of course, this is assuming all your connections are tight from the board to the switch and back to the board.

 

It was the board, but here is why I didn't go directly to the board... Every time I bypassed (Jumped out) the pressure switch the system started and ran to temp. I checked voltage, resistance and pressures to everything before replacing parts. So HSI, flame sensor, gas value all worked as long as the inducer and p/s were bypassed. I agree that the fan and the p/s checked out, but when the system starts like clockwork with the p/s bypassed, it led me to believe the issue was at the inducer & p/s. Doesn't make sense that the furnace works as expected when the inducer fan and p/s are bypassed, nothing about that says control board to me. Although, it will in the future. At any rate, the customer got a great deal -

 

Great job!

 

So not a SLP98?