A couple of furnace questions

[TaperPin]

I'll get another flame sensor to try in case it quits working again.

Flame sensors are solid metal - with a little film of crud on the surface it has a harder time passing electricity through the flame. Steel wool or extra fine sandpaper will take the crud off and it will be good as new. It’s one of the few parts that will never actually wear out. Some techs will clean them and some will follow their sales managers instructions and change them out to suck a little money out of you. lol

 

[HighUintas]

Flame sensors are solid metal - with a little film of crud on the surface it has a harder time passing electricity through the flame. Steel wool or extra fine sandpaper will take the crud off and it will be good as new. It’s one of the few parts that will never actually wear out. Some techs will clean them and some will follow their sales managers instructions and change them out to suck a little money out of you. lol

Urgh. I hate that!

I have kept a small scrap of emery paper by my furnace the last few years for that haha. However, I'm not sure why it'd get covered with junk so quickly. The flame is nice and blue... No yellow

 

[StuckoYota99]

Without being in front of it diagnostics are tough.
I've seen intermittent issues caused by voltage drop through by bad contacts in a limit switch or pressure switch.

Back to basics though. 120vac on L1? 24vac from transformer?
jumper R to W.
Inducer motor starts and you can confirm 24vac through the pressure switch.
Confirm 24vac through the limit switches.
120vac to igniter and igniter glows bright orange?
24vac to gas valve?
Low fire may vary but usually it's around 1.5" wc on the manifold side.
3.5" wc hi fire with natural gas on the manifold side of the gas valve?
Does the flame go out because it loses flame rectification or because a limit or pressure switch opens?
Could be a shorted coil in the gas valve
Could be bad contacts.

If the control board has been replaced, limits and pressure switches are good to go. Then would suspect a shorted gas valve which could cause the control board to fault out or restart the ignition sequence.
A weak ignitor can also fail to light the gas during ignition trial.

 

[StuckoYota99]

Flame sensors are solid metal - with a little film of crud on the surface it has a harder time passing electricity through the flame. Steel wool or extra fine sandpaper will take the crud off and it will be good as new. It’s one of the few parts that will never actually wear out. Some techs will clean them and some will follow their sales managers instructions and change them out to suck a little money out of you. lol

unless you have an 80% trane on propane. We change those flame sensors out often since they rapidly degrade. flame sensors are a wear part just like anything else. Also flame sensors will fail due to the ceramic forming micro cracks.

 

[StuckoYota99]

Also what's your draft on the pressure ports with the inducer running? And does it change when the furnace ignites?

 

[WV Mountaineer]

You shouldn't have to clean your flame sensor once a year. You may need to adjust your flames at the gas valve. Your flames should be blue with little yellow showing. If you have dominant yellow flames then that will soot up your flame sensor prematurely. Seriously, I'd put in a new one for the 10.00 - 15.00 - Yours may be acting up but not out.

With proper flame adjustment you can go years without cleaning. Mine is going on 9 years without a single cleaning on natural gas.

No, not a licensed Hvac guy but I do a lot of repairs on them. (grin)

If it's only a partial obstruction then yes your ignition & flames will light but then it can shut down.

What's throwing me here is you have ignition & flames for 4 seconds then shuts down. that's generally a flame sensor issue. Maybe the control panel isn't reading the sensor?

ElkNut

Check the connection at the control panel. My buddy went through this for a couple years until it finally quit a couple months ago. The connection at the panel was bad. Causing it to malfunction.

 

[TaperPin]

unless you have an 80% trane on propane. We change those flame sensors out often since they rapidly degrade. flame sensors are a wear part just like anything else. Also flame sensors will fail due to the ceramic forming micro cracks.

Wear part - that’s funny.
A piece of stainless steel with a ceramic insulator is as durable as a spark plug.
You change them out because people believe you and don’t know any better.

 

[DanMan]

Trying to get straight all the symptoms that you are seeing.
Are the problems always in trying to get ignition ? Or does it sometimes get ignition but then shuts back down pretty quick ?

 

[HighUintas]

Without being in front of it diagnostics are tough.
I've seen intermittent issues caused by voltage drop through by bad contacts in a limit switch or pressure switch.

Back to basics though. 120vac on L1? 24vac from transformer?
jumper R to W.
Inducer motor starts and you can confirm 24vac through the pressure switch.
Confirm 24vac through the limit switches.
120vac to igniter and igniter glows bright orange?
24vac to gas valve?
Low fire may vary but usually it's around 1.5" wc on the manifold side.
3.5" wc hi fire with natural gas on the manifold side of the gas valve?
Does the flame go out because it loses flame rectification or because a limit or pressure switch opens?
Could be a shorted coil in the gas valve
Could be bad contacts.

If the control board has been replaced, limits and pressure switches are good to go. Then would suspect a shorted gas valve which could cause the control board to fault out or restart the ignition sequence.
A weak ignitor can also fail to light the gas during ignition trial.

The tech who said he thought it was the board confirmed the correct voltages coming from each of those. But, didn't measure gas pressure or draft on pressure ports.

 

[HighUintas]

Trying to get straight all the symptoms that you are seeing.
Are the problems always in trying to get ignition ? Or does it sometimes get ignition but then shuts back down pretty quick ?

Problem is always trying to get ignition.

After removing main power for awhile (10+ min,?) on the first try the igniter will usually glow for a couple seconds then it gives solid red error (supposed to glow for 17 and then gas opens)

When it retries, it will only do a couple seconds of the inducer motor before giving solid red error. Inducer needs to hit 15s before igniter comes on. It will do this endlessly without ever going into lockout.

 

[Rotnguns]

I had the same problem, changed board nothing. Hvac guy came out changed the inducer pressor switch and cleared the line. Worked for about a week. back to the same. Now it just takes 2-3 times to fire up. Had the gas company (propane) check pressure. 35 degrees seems ok, but at night it seems to either wick back or the tank freezes up? . Propane guy thinks at high pressure regulator at the tank to the house then low pressure at the house will solve the problem. Anyone heard of this. We're at 6000 ft. I will clean the flame sensor, never looked at that.

I suspect that the HVAC guy would know this, but just in case, here's a thread on an HVAC forum regarding derating gas furnaces due to altitude:

Question about derating gas furnace for altitude.

The #1 HVACR Community

hvac-talk.com

 

[MallardSX2]

Here all along I thought that my camper heater was a pain in the a$$. Lol

 

[DanMan]

Did the tech ohm the ignitor?
I'm wondering if it's weak but not totally broken.

 

[Jstumbaugh]

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.

 

[Weldor]

I suspect that the HVAC guy would know this, but just in case, here's a thread on an HVAC forum regarding derating gas furnaces due to altitude:

Question about derating gas furnace for altitude.

The #1 HVACR Community

hvac-talk.com

We did the high altitude kit when we installed it. New orifice kit etc. They checked the propane pressure with a manometer. Don't know if the pressure changes when it is really cold but it sure seems like it. I started putting a halogen lamp under the tank and covered the mess with a heavy tarp. It seems to make a difference?

 

[Jstumbaugh]

What size tank do you have? Depending on temperature and how full it is, the vaporization rate of the propane changes.

 

[Weldor]

It's a 150 and we have a keep full on it. Part time use. Of course they only fill 80% so it does not take long to run it down if it is very cold out.

 

[Jstumbaugh]

Yeah. They are definitely affected by temp. Not as much surface area as a horizontal 300-500gal.

 

[Weldor]

It's horizontal, not a vertical tank. but smaller for sure.

 

[HighUintas]

Did the tech ohm the ignitor?
I'm wondering if it's weak but not totally broken.

Not sure on that, but I don't think so. I think he just checked voltages.