A couple of furnace questions

[HighUintas]

I'm with Elknut on this one. Sounds like the flame sensor isn't providing the resistance needed to keep the valve open.
Only other issue is the "ignitor glows for a few seconds and then locks out". If you have an electrical problem has anyone checked grounds? Loose grounds cause electrical nightmares and these units are extremely finicky when dirty power is involved.

I'll order another flame sensor just in case that's it, but it never actually lights the gas when it has this problem. It doesn't get to the required 17s of igniter warm up for the gas to open. So I don't think it's the sensor.

I'll go through all the grounds on it today, but I'm pretty sure they're all good

 

[DanMan]

Doesn't sound like a flame sensor problem. Flame sensor only shows problem when a flame is present.
Really sounds like weak HSI. After 15 years it has to be weak.

 

[ElkNut1]

In the OP's original posts he mentions he gets ignition & flame for 4 seconds & then the furnace shuts down, this is why the flame sensor was considered.

ElkNut

 

[HighUintas]

In the OP's original posts he mentions he gets ignition & flame for 4 seconds & then the furnace shuts down, this is why the flame sensor was considered.

ElkNut

Sorry, my first post was tons of information.

What it had been doing this winter was:

- after a long sit with no voltage going to the furnace, it would get 4-5s into the igniter coming on but then solid red error.
- when it would retry, it would only go through inducer pre purge for a couple seconds then solid red error.
- retries and does it endlessly without lockout

 

[StuckoYota99]

Sorry, my first post was tons of information.

What it had been doing this winter was:

- after a long sit with no voltage going to the furnace, it would get 4-5s into the igniter coming on but then solid red error.
- when it would retry, it would only go through inducer pre purge for a couple seconds then solid red error.
- retries and does it endlessly without lockout

any updates?

 

[HighUintas]

any updates?

It's been working perfectly!!

No blockages in the flue at all

 

[Jpsmith1]

Hi. Hvac guy here.

Flakey electronic issues very often trace back to a grounding issue.

 

[HighUintas]

Hi. Hvac guy here.

Flakey electronic issues very often trace back to a grounding issue.

I forgot to check all the grounds. I'll do that

 

[HighUintas]

I checked all the grounds and they looked good.

The furnace quit working again today. It did the same exact thing. I tried unplugging the ignitor again and it didn't work this time, so I started from square one.

It quits during the inducer motor cycle, most of the time very shortly after it starts... Maybe a couple seconds. I know the gas valve and ignitor and flame sensor don't come into play until the inducer has been on for 15 seconds. And the solid red code I get is most often quoted as being a bad board. I know the board isn't bad because I swapped it with a new one and it did the same thing. So, it must be electrical issue coming from a part connected to the board.

I checked the transformer and inductor coil. Both had continuity and low resistance. I checked voltage and it was 120ish and 24ish. Those are likely not the issue.

I checked the high limit, auxiliary, and rollout switches... They had continuity and low resistance.

I checked the flue and there's no blockages. I checked the inducer pressure switches. Hmmm. The first stage pressure switch seems to act funky. I sometimes get continuity when sucking on the back tube and sometimes not. I hooked the thing back together and turned the furnace on to check voltage on the pressure switches.

The first stage pressure switch measures 27v prior to the inducer motor starting and 27v after. The 2nd stage pressure switch measures 7ish volts (I think?) before the motor starts and then 0v after. I'm unsure of the correct voltage for the second stage switch but I think it should be 27v also.

Well..... I happened to have a spare new pressure switch set from a couple years ago when I thought that was the issue and it started working again. I never installed it. So, I swapped it out and it worked just fine. 27v prior to inducer motor starting and 0 v after.

I can't believe it has been the pressure switches the entire time the last 3 years. I really don't know why it would throw the solid red error instead of the error that's actually related to the pressure switches. Perhaps because the 2nd stage switch was also being activated and sending a signal?

I don't know. I'm also not sure how the couple tech visits missed that or how I missed it when I checked previously. It could have just been coincidence and timing of when they were checked, since it was an intermittent issue.

Thank you all for the replies and help. It's great to have some folks to bounce ideas off of when everyone I know would rather just put a new furnace in.

 

[Jstumbaugh]

Good job figuring it out. I was about to respond to check the pressure switch based on what you saying it is doing going through the sequences now but went back to finish reading the post. Pressure switches can be finicky.