Furnace not Working - Need HVAC Help!

[Super Sport]

UPDATE: Replaced the board, problem solved! Thanks for all your help!

Hey guys, I'm having trouble getting my nat gas house furnace working. It's an older Bryant unit and I'm getting a rapid flash on the LED - which is listed as line voltage (115v) polarity reversed. Currently the fan and A/C are working when turned on, and the fan comes on for the heat, but no gas or ignition. I did get into this unit this summer to fix an issue with the A/C - but only touched the 24v wiring. I have bypassed the thermostat and manually called for heat, but same result - no gas or ignition.

We have not run the furnace since I was working on the A/C previously, but it worked fine up to that point. I'm thinking that I either crossed some of the 24v wires or otherwise touched something causing this, but cannot find anything. I did not touch the incoming power and don't believe I could have caused reverse polarity there.

I'm pretty stumped and would prefer not to wait until Monday to get a tech out here to service. The wife is cold! I'm not great with electrical but have a multimeter. Please give me some guidance!

 

[Done That]

Was the transformer changed out during your previous work on the unit?

 

[PCustoms]

Where do all the wires in your first pic go?

 

[Super Sport]

Where do all the wires in your first pic go?

Those are the 24v wires that run to the t stat and A/C condenser.

 

[Super Sport]

Was the transformer changed out during your previous work on the unit?

No, only things tampered with were the 24v wires, thermostat, and some parts outside on the A/C condenser. At that time the A/C was not working and the furnace was producing a different error code (I don't recall which one) and the cause seemed to be that the 24v wires outside got crossed/shorted out.

 

[cstmg8]

Worked in HVAC before elevators. The things I'd check in order are:
1. Grounds on the equipment/feed.
2. 24 volt wiring you changed, specifically anything going to the 24v transformer.
3. Bad circuit board. Price a replacement online.

Hope this helps.

Sent from my Pixel 2 XL using Tapatalk

 

[Done That]

No, only things tampered with were the 24v wires, thermostat, and some parts outside on the A/C condenser. At that time the A/C was not working and the furnace was producing a different error code (I don't recall which one) and the cause seemed to be that the 24v wires outside got crossed/shorted out.

Ok, just checking. A unit that has worked for years doesn't just "lose" polarity all of the sudden unless supply wires get swapped. A transformer also needs to be in phase with line voltage so a change out can screw that up, but you are clear there. Other culprit can be a poor ground. Hate to say it but my gut says the control board may be going south. I don't think your previous changes would induce this fault, but anything is possible. Wish I could offer more help, but not much coming to mind.

Not to be a jerk, but are you 100% sure on the flash code? They can be tough to read sometimes...

 

[Caddybill]

Check line voltage into circuit board. Then ck for 24 volts out. Most of the time if you have line voltage in but not 24v out you have a bad board.

 

[Super Sport]

Ok, just checking. A unit that has worked for years doesn't just "lose" polarity all of the sudden unless supply wires get swapped. A transformer also needs to be in phase with line voltage so a change out can screw that up, but you are clear there. Other culprit can be a poor ground. Hate to say it but my gut says the control board may be going south. I don't think your previous changes would induce this fault, but anything is possible. Wish I could offer more help, but not much coming to mind.

Not to be a jerk, but are you 100% sure on the flash code? They can be tough to read sometimes...

No offense taken. It's a rapid, constant flash. I figured out how to read them the last time when the A/C wasn't working, so I'm confidant it's not another error.

 

[Done That]

My own system, i'd prolly swap the Red and blue wires right under the fuse, just to see. Those are the transformer secondaries, and swapping them might make the pcb happier re polarity.

Of course I have a free stash of replacement boards stacked like firewood at work so it would be an acceptable try it for me.

 

[mrobins297aaa]

you said you fooled around with the 24v wires, make sure that the W terminal wire the white is going to the thermostat, that white wire is not a common wire it is the heating wire. if that's ok.
turn off the 120v power for a few minutes and then start checking
make sure you have 24v between R terminal and com on the circuit board.
if that's ok jumper a wire between R and W and see if the heat cycle starts.

 

[mike88]

if you jump between the r terminal & w terminal remove the white wire first. if you have a older thermostat you could short out the heat anticipator. with the therm set for heat the white wire will become the hot wire so you should have 24 v between the w term & the com term. hope this helps

 

[mrobins297aaa]

after looking at the last picture it looks like the thermostat wiring is correct.
If you don't have 24v at the R and com terminals you've probably cooked the transformer.
how old is your thermostat if you have a old dial type and you shorted out the low voltage circuit even just for a second you will cook the heat anticipator in the thermostat.
make sure you haven't some how lost the the 120v ground at the furnace, maybe it came lose those furnaces need it to work properly

 

[mike88]

needs lower panel on also to get power thru door sw

 

[mrobins297aaa]

good point

 

[PCustoms]

Those are the 24v wires that run to the t stat and A/C condenser.

All 7?

The 2 terminals doubled up seem odd to me. If this is what you messed with, start there. As another post said be 100% sure of the error code.

 

[Done That]

All 7?

The 2 terminals doubled up seem odd to me. If this is what you messed with, start there. As another post said be 100% sure of the error code.

That's pretty much standard. For example, Y is compressor demand from Tstat, and then the second Y wire is it being sent to the OD unit contactor. Same with the common, to the tstat and to the OD unit.

 

[yeldogt]

I'm wondering what is happening when you say the fan is coming on for heat?

The unit should start the small fan for the flue -- after it checks the pressure. The ignition process should start ... the fans should only come on after the gas/flame is up and running.

What type of igniter ? The spark type typically run forever -- the ceramic require occasional replacement -- easy (if you get the correct one). Line polarity reversed on 110 would not just happen -- strange code?

 

[welder4956]

May not be related, but just about every year I have to pull that short piece of hose off and clean out the furnace pressure port with a piece of wire. Seem to get clogged over the summer with the AC running so much. It's worth a try if everything else looks good.

 

[Done That]

With a true polarity fault none of the tstat input stuff matters, it's the 120v input power, transformer power/phasing, and ground. The micro is looking at those signals only. That's why if they are confirmed good it's most likely a failed pcb.

 

[brewchief]

There is a good chance that it's a bad board, before throwing money at it the heat exchanger needs to be checked, I've replaced a bunch of those furnaces and they have a cracked heat exchanger more often then not.

 

[mike88]

the extra 2 wires on the y terminal & com terminal are the low voltage power wires that go to the contactor coil at the outdoor unit for a/c to run. the low voltage wiring looks correct. there a series of safety switches you may want to check some are auto reset some are manual reset. the switch towards the top with the orange wires looks like a manual reset safety, also you said fan comes on when calling for heat are you talking the blower fan down below or the small draft fan above the gas valve. there is a switch that has to close to prove that the draft fan came on and that a draft was created either by vacuum or pressure. yours looks to be by vacuum, its mounted to the right of the draft fan, make sure the rubber hose is attached at both ends and not cracked or brittle. this all can be checked with a volt/ohm meter.

 

[crazy wheel]

Hi temp limit or roll out switch open?

 

[Super Sport]

Mike88 offered to help walk me through some tests and he believed the board was the culprit (as many others here also suggested). I ordered a replacement board and after swapping them the furnace seems to be working just fine. Thanks everyone for all of the help and a huge thanks to Mike!

 

[mike88]

Glad you got going