Carrier Weathermaker 8000 help

moose404

Hi Everyone,

I have been dealing with an intermittently operating Carrier Weathermaker 8000 for the past several months. From time to time it will stop working, and then usually after several hours of reseting it, it will continue to work fine for several weeks at a time. I have already had a repair company come and look at it, and they told me it was a faulty inducer motor, which I had replaced but the problem persisted. Since then they have been back on 2 other occasions on warranty calls but never actually did anything.

I have given up dealing with them, and have started troubleshooting it myself. When I power it up, the blower motor starts up for 90 seconds, and then the inducer motor should start. However, the inducer motor only briefly turns (I'm gonna guess there is power applied for less than a second) before everything shuts down. I get error message 31 which lists:

PRESSURE, DRAFT SAFEGUARD, OR AUXILIARY LIMIT (when used) SWITCH DID NOT CLOSE OR REOPENED - If open longer than
five minutes, inducer shuts off for 15 minutes before retry. Check for:
- Proper vent sizing and condensate pitch
- Vent Restriction or High winds
- Defective inducer motor or start capacitor
- Low inducer voltage
- Defective pressure switch or connections
- Inadequate combustion air supply
- Disconnected or obstructed pressure tubing

I have checked the pressure switch, and it is working properly. I have also put the furnace into test mode, and the inducer motor starts and runs fine in this mode, along with the blower and the hot surface ignitor. There is no Auxiliary limit switch installed, and the draft safeguard is closed as its supposed to be.

Any tips on what I should try next? I'm reluctant to sink too much money into try to repair it, as its about 16 years old, but I would like to get it working again if possible.

Thanks in Advance,
Mark

how

That pressure switch is supposed to be measured as open before the inducer motor starts and then close when the inducer is up to speed. Almost all of those listed faults for error 31 require the ID to get up to speed to get that fault code. Since the inducer shuts down after a second of power , I wonder if the electrical switch in your pressure switch has been occaisionally sticking shut.
Just how did you test out that the PS was OK?

Do you have a capaciter for the ID motor?

The other potential fault is the motherboard.

moose404

I checked the pressure switch by removing it from the unit and measuring across the contacts as I sucked into the pressure tubing gently. It was normally open (~990 ohms) and operated normally from what I could tell without any sort of sticking (0.2 ohms in closed state).

I don't think there is a start capacitor, as usually these are attached directly to the motor terminals, which is not the case here. As I mentioned before, in test mode the inducer motor starts and runs fine for the 10-15 seconds that's specified in the manual. The only difference is that the blower motor is not also running at the time it starts in test mode, which makes me wonder if its some kind of low voltage error to the board itself.

The motherboard does seem like the next likely thing to try replacing, but I am contemplating just replacing the whole unit rather than the motherboard, as I don't want to sink too much money into replacing parts only to still end up with an furnace that is still flaky overall.