[jazzsax101]

Hi everybody,


My Carrier 58MCB080 furnace is flashing a 31 - pressure switch. The board recognizes a call for heat with a click, there's a slight buzz in the inducer motor, and after 20 seconds, the 31 error flashes.

The thermostat connection seems ok - I get 24-27 volts at the board, and I can audibly hear a click as the board gets a call for heat. But the voltage going out to the inducer (black and white wires in the cradle) is only 2V during a call for heat (and ~1.7v when not calling). I've checked the high limit switch and it has continuity (did not show OL/infinite resistance).

Is my only option to replace my control board, or are there more possibilities that I should test for? I'm in Northern California, so it is still cold and will be for awhile - and with the whole COVID-19 quarantine, I think its best I do this myself if possible, so I appreciate any help you guys can provide.

 

[beenthere]

If the board isn't sending 120 volts to inducer after you hear the relay click. Board needs to be replaced.

 

[roughneck]

X2, if it’s calling for heat and no power is being sent to the inducer before the code trips, the board is likely junk.

 

[TarheelTerp]

Is my only option to replace my control board, or...

Depending on age/model (~2014?) the board might be covered by warranty.
Either way it's not crazy high $$ @+/- $120

 

[user_12345a]

that unit is affected by a class action lawsuit - failing/plugging secondaries.

normally i would say have it tested before buying parts but in your case, you can't.

it can be inspected visually but can still look okay on the outside but be bad. You can also remove the trap, rinse it out and see if it's full of debris/rust flakes - can be a sign of failure.

search this board "carrier secondary" for more info.

 

[roughneck]

Here is the link

http://hipspro.com/pubs/Furnace-clasaction-detailed_notice.pdf

 

[jazzsax101]

Appreciate all the quick responses.



CAPACITOR: One thing I checked on a whim was the capacitor. It's a 3uf capacitor (+/- 5%), but only shows up as 1uf on my multimeter. I don't see how that could be causing the board to not send enough voltage to the inducer, but I'm wondering if maybe the capacitor breaking caused some feedback issue to the board that might have damaged it.


SECONDARY EXCHANGER: Yes, this is a great point and thanks for flagging that. I saw the lawsuit information, but I'm not seeing any of the tell-tale signs: my trap was completely clean, as were all the tubes around it; I don't see any rust or flakes anywhere in the unit; and when the unit did work, I didn't hear any odd sounds from the inducer. The blower motor works fine without the heat on and also sounds normal, if that matters.

that unit is affected by a class action lawsuit - failing/plugging secondaries.

normally i would say have it tested before buying parts but in your case, you can't.

it can be inspected visually but can still look okay on the outside but be bad. You can also remove the trap, rinse it out and see if it's full of debris/rust flakes - can be a sign of failure.

search this board "carrier secondary" for more info.

 

[user_12345a]

The secondary heat exchanger is checked by seeing if the burner box is getting very hot and checking carbon monoxide at a minimum in the exhaust. full combustion analysis is better but many won't have the tool.

To spot any rust the blower needs to be pulled - as it rusts from the inside out, it can still be bad but look fine on the outside.

You'll need a new board and inducer capacitor.

A bad inducer capacitor won't damage the board but you should replace it so the motor doesn't get wrecked, as it's very costly to replace.

 

[jazzsax101]

The secondary heat exchanger is checked by seeing if the burner box is getting very hot and checking carbon monoxide at a minimum in the exhaust. full combustion analysis is better but many won't have the tool.

To spot any rust the blower needs to be pulled - as it rusts from the inside out, it can still be bad but look fine on the outside.

You'll need a new board and inducer capacitor.

A bad inducer capacitor won't damage the board but you should replace it so the motor doesn't get wrecked, as it's very costly to replace.

Got it - very helpful. Seems like the next step is a no brainer from your perspective - I'll replace the board and capacitor. I've ordered those parts and will have them in a few days.



Once things are up and running, I'll check the burner box to see how hot the burner box is getting, and research what I need to do to check CO at the exhaust - thanks for the tips.

 

[user_12345a]

CO meters/analyzers are very expensive.

The front of the burner box should be cool to luke-warm, the sides can be very hot closer to the heat exchanger outlets.

The sides near the front where rollout switch is located shouldn't be hot.

Run the furnace for 10 to 15 minutes before checking.

Sniff the exhaust too - bad combustion can produce unusual smells in the exhaust.

 

[jazzsax101]

Really appreciate the guidance from user_12345a, tarheelterp, roughneck, and beenthere on the control board - you guys were absolutely right. The new control board came today and I was able to swap it in pretty quickly. After basically learning HVAC repair over the last two weeks (mostly from you guys in your other posts), I was ecstatic to hear the burners kick on - and the wife is certainly excited to feel heat again, she'd been sleeping with a ski hat on.

Regarding the secondary exchanger - I checked the front and front-sides of the burner box: felt cool to luke-warm after about 15-20 minutes of running, could leave my hand there no problem. Closer to the flame they were hotter. No smells.

Once this is all over and I don't need to worry about somebody sneezing COVID-19 into my air vents, I think I'll bring an HVAC tech in to double check my work and do an investigation of the secondary.

Once again, I really really appreciate it - you guys were fantastic and straight forward, both in this post and all the others I read before I finally learned enough to share details.



(Of course, following a pattern I've seen, I'll probably be back in two days with a different error flashing because I unintentionally broke something in repairing the first problem... )

 

[user_12345a]

Sounds good.

 

[jazzsax101]

Well, as predicted - I'm back. The furnace worked for a solid 4 days there, provided heat every time when asked. Woke up this morning a little cold, went downstairs and heard clicking from the furnace - isolated it to clicking on the new control board itself. No power going to inducer motor. Symptoms all seem the same as before, except for the clicking now.


Is it possible I have two bad CBs? Or is there something in my system that is frying my CBs?


Here's a video of what is happening: https://drive.google.com/open?id=19bVLECgP_TRWJaehJMPX2EocEfEkoOxU

 

[user_12345a]

it's asking me for permission - i can't view the video.

Did you replace the inducer's capacitor?

Having the motor fail to start and draw locked rotor amps can be hard on the relay.

Do make sure there's a stable 24V +/- AC between W and C with a call for heat and the door switch depressed.

Do you by any chance have a nest thermostat connected with no common and or wire saver device?

 

[jazzsax101]

Yes, I did replace the capacitor.



I'll check the 24V +/- VAC between W and C - but wanted to respond quickly before checking because yes, I do have a Nest thermostat. I installed it about a year ago. I'm not sure what common/wire saver means, but there is no wire coming into the "common" port on my control board. Is there a known issue here?

 

[user_12345a]

A wire saver device allows two signals to be sent over one wire to free up a wire to use as common; some people on here say they can cause problems.

The nest is a power-stealer thermostat and tries to charge itself up by drawing power through the control circuit- modern circuit boards with electronic processors don't play nice with power-stealers.

The technology works okay on mechanical relays.

When there's no a/c, the stat can only steal power from the heating circuit which is much more problematic than cooling as board's cpu controls heating operation, it's not strictly relays and switches.

It must have the common connected so it doesn't run in power stealing mode. Even when wire up with the common, nest has a poor reputation, causes problems.

**

Disconnect and tape off the W wire going to the stat and jumper R and W on the board - if the furnace starts working properly, try running a new wire and hooking up the common.

 

[roughneck]

It would not be the first time I’ve seen a nest cause damage to the equipment.
A common is mandatory with them despite what the instructions show.
The nest is among the biggest pieces of junk on the market today. I’d not have one if you paid me to use it.

 

[jazzsax101]

I think I fixed the video permissions - https://drive.google.com/open?id=19bVLECgP_TRWJaehJMPX2EocEfEkoOxU. Not sure how informative it will be.

Taped off white coming from the thermostat, jumped R+W; furnace did its 90 second blower startup then tried to respond to the call but inducer did not power on. Flashed 31 after a little while of trying.

I'm getting a steady 26.7v across W+C with no call for heat; 27V steady with a call for heat.

I read up on the lack of common wire. Definitely a problem - this is gonna be a pain to wire in though. I've still got my old battery-powered thermostat; I can try wiring that back in but I'm guessing if the jumping isn't working, then going to the old stat now won't do anything either.

 

[roughneck]

No, the old thermostat would not be any better.
Although not for certain, the Nest likely killed the board. This is not really out of the ordinary, and to live with the Nest means you have to be prepared to deal with all sorts of problems and irregularities.

 

[user_12345a]

You can try warranting the part if you bought it from a generous seller.

If you bought directly from amazon (not a separate seller using the amazon platform) and they shipped it, you may be in luck; they don't really check returns, and the minimum wage slave workers who handle returns definitely wouldn't know how to check a furnace control board.