Trane XV90 inducer error / fresh air intake or pressure switch problem

hommealone

I'm having trouble with our Trane XV90 gas furnace.

Description of the problem:
- Call for heat;
- Inducer motor starts, and steps up;
- Inducer motor steps up: faster, faster, faster... "Inducer Error" shuts down the sequence before the igniter starts to glow.

Background:

Last fall, we had the same problem. Our propane supplier came for a service call, and with shockingly little troubleshooting, said we needed a new motherboard. I wasn't so sure, so I did my own troubleshooting: testing the motor and pressure switches for continuity; checking pressure switch hoses and pressure switch hose connectors for blockages. So far all checked out.

Then I tried another troubleshooting step. I removed the cover from the ignition chamber and tried to start it, and the furnace lighted and ran. Our fresh air intake and exhaust pipes are not long - 6 foot max. I snaked, and then vacuumed out the outside air intake pipe with a shop vac, replaced the ignition chamber cover, and tried again. The furnace then worked fine, and continued to operate normally for the rest of the winter. It was a blocked intake air pipe causing the problem.

Recently I tried to turn on the furnace for the first time this fall. I ran into the same exact problem as last year. I went right to the "remove combustion chamber cover" troubleshooting step, figuring it was blocked again. Same result: it fires with the cover off, but not when I put it back on. I again tried cleaning out the pipe; I'm sure it is completely clear now, but that hasn't fixed the problem this time.

Even if I pull the combustion chamber cover off just enough to let some air sneak around the edges, it lights. If, after it lights, I push the combustion chamber cover back tightly into place, the furnace continues to run. But if I again try to start it with the cover on? Inducer motor spins up faster, faster, faster... "Inducer Error".

I've double-checked all of the pressure switch hose connectors, and none are blocked. I've checked for continuity again, all OK.

If I pull the hose off the combustion chamber, it lights. That hose tees and attaches to both the low and high pressure switches. If I re-attach the hose to the chamber, disconnect the high pressure switch hose at the tee and cover the open tee connector opening (leaving an open hose to the high pressure switch) it lights. If I re-attach all of the the hoses, disconnect the low pressure switch hose at the tee and cover the open tee connector opening (leaving an open hose to the low pressure switch), it fails. (If I gently puff in and out on the hose to either pressure switch, I can hear the switch opening and closing.)

It is getting outside air - but maybe not enough to satisfy the high pressure switch, which perhaps is bad? I'm thinking that, perhaps, the high pressure switch is bad, but I do not own a manometer to check the switches. I've read online that while the pressure switch can go bad, that's only rarely the problem in Trane furnaces of this type.

Other than a blocked outside air intake pipe - again, I'm almost certain that it is now clear - what could cause it to light with the ignition chamber cover off (and continue to run once I push it back into place) but not light with the chamber cover in place?

Are there any other troubleshooting tests I can run, short of using a manometer on the switch? ANY ideas what's going on here?

EBEBRATT-Ed

Most of the time with pressure switch problems the tubes get plugged. Do't forget you working with very low pressure so it doesn't take much

A spider or some dust/dirt can do it

If you don't have a manometer shut the power off to the furnace and set the stat to call for heat.

Turn the power on and jump 1 pressure switch as soon as the inducer starts and see if it runs. If not try it with the other pressure switch.

Your going to have to look at the diagram and see what position the switches are supposed to be in when the inducer starts and makes air flow.

Re check the flue and intake pipe sizes with the manual to see why your not getting enough air

ratio

I want through this on a Trane job a few years ago. Pressure switch wouldn't reliably make on startup. I checked all the usual suspects, including the calibration of the pressure switch, everything was good. Trane tech support condemned the HX with the logic that the switch was doing what it was supposed to do, if it's not making & everything else is good it can only be the HX. Unfortunate, it was under warranty from another contractor so I can't say if they replaced the HX & that fixed it, but I believe they did.

EBEBRATT-Ed

To test the hx put Saran wrap over the flue and intake pipes with duct tape or ty raps. Have someone start and stop the blower while you watch the Saran wrap.

If it moves the hx is suspect

hommealone

Sorry, "hx" is the... heat exchanger? And if so, how does that effect the startup sequence up to and until the igniter starts to glow?

@EBEBRATT-Ed
You wrote:
Most of the time with pressure switch problems the tubes get plugged. Do't forget you working with very low pressure so it doesn't take much ... A spider or some dust/dirt can do it

Thanks, I'll check those more thoroughly.

You wrote:
Re check the flue and intake pipe sizes with the manual to see why your not getting enough air

Is it safe to assume that since it has been working fine for years, the pipe configuration is good enough? They are all standard PVC 2" pipe, less than 6' length, not too many bends.