New pressure switch and inducer and still incorrect drafting

kevintabb

Comfortmaker GUH075A012CIN (about 15 years old, previous owner had service to replace circuit board and to reseal the inducer maybe 10 years ago)

Timeline:
3 blinks = pressure switch open. I replace the tubing on both ends of the pressure switch. I made sure the condensate line was clean. I removed the exhaust line at the inducer to rule out exhaust line blockage. No dice. Using a mityvac with a pressure gauge I can tell the inducer isn't pulling enough to trip the switch (just by comparison.) I find a thick mud wasp blocking a good portion of the fresh air line, but after knocking it out I don't see any difference. Since there's record lows in Atlanta, I open and close the switch by removing the leads at the pressure switch to get a single heat cycle at a time. The exhaust outside is slightly warm, I can hear the sump pump flush at regular intervals, and the furnace is producing adequate heat.

I order and replace the pressure switch to no effect. Inducer motor is drawing ~125V/.6A. It's rated 120V/.7A, which looks fine to me. I couldn't detect any shorts in the windings or case and the resistance was low. I removed the inducer and cleaned off old silicone with alcohol and resealed with some high temp RTV silicone. After the silicone set, turned it back on and noticed no appreciable difference.

I had already ordered a replacement inducer assembly so I went ahead and replaced the existing setup even though the above tests looked decent. Now I notice the pressure switch moves more, but still not enough to close. Also, curiously the switch moves more when I temporarily remove the exhaust line from the inducer, but still not enough to close the pressure switch.

I've noticed that I can blow on the other line to the pressure switch (the line from the combustion chamber), and that combines with the negative pressure from the inducer to close the switch. Should I have positive pressure coming from the combustion chamber at startup? Unfortunately I physically cannot remove the intake to check for blockage and there is no cleanout opening.

I'm afraid that the only thing left is the secondary heat exchanger is leaky. Anything else to try before giving up and calling a tech? Or, given the unit's age (compressor and furnace both 15+ years) should I just go ahead and get estimates for a replacement?

beenthere

Could be a cracked primary heat exchanger. or restricted secondary. Or a restriction in the flue.

Are you sure you have the hoses on the correct ports of the new switch.

kevintabb

I appreciate the reply. I'm pretty sure on the hoses and I think I swapped them temporarily as a test to rule that out, but I will try again tonight. If there is a restriction in the flue, wouldn't disconnecting the pipe at the inducer and allowing it to exhaust into the furnace room eliminate that as the root cause? I have noticed that there is a drip hole where the pipe connects for condensate to drain. Since the flue pipe is no longer cemented to the inducer, maybe that's a source of pressure loss?

I have read about using a shopvac to clear out gunk, but I assumed that meant the line to the exterior wall. Can that help clear a restricted secondary? I'm unsure where I would actually suck out any potential gunk.

The unit needs to be serviced anyway, but I wanted to know as much as I could before proceeding. Thanks for your time!

beenthere

I thought you had said you couldn't remove the exhaust pipe.

A shop vac won't clear out a secondary.

kevintabb

No, exhaust comes off easily. It has old dried out pvc cement on it that probably never bonded to the old inducer assembly (which is definitely not made of pvc The intake, however, is very well cemented to the combustion chamber.

beenthere

The restriction could be in the intake then.

A hack saw, and a fernco coupling can take care of that.