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tophersig · Jan 23, 2010, 03:21 PM

Hello.

I have a similar problem to this gentleman with a Carrier unit:

https://www.askmehelpdesk.com/heatin...re-309949.html

Same furnace model #.

My unit intermittently does not produce a flame. What I observe:

1. The inducer motor spins.
2. The pressure switch outputs 25V after the inducer motor gets up to speed
3. The glow/ignitor gets red hot.
4. The gas value clicks open after 25V is applied to it.
5. No flame at all. The unit then turns off the gas and glow plug de-energizes. It then reports an ignition proving failure.

This occurs about 40% during calls for heat. Sometimes it recovers with subsequent attemps, other times it results in a lockout.

Sometimes I've seen the flame just start to ignite, but it started so late that the flame detection timed out, and shutoff the gas valve.

Questions:

1. Since the glow plug got hot, does that mean my pressure switch is okay? Is there another reason the pressure switch may be at fault? Does the pressure switch just ensure that the inducer motor is running? Why is it also connected to the gas valve?

2. Local gas company checked the presssure from the outside and its good. Since I intermittently have this issue, I assume there is no blockage in my house gas piping. Is this reasonable to assume?

3. Gas valves -- local technician doubts this could be the problem, since "when they die, they totally die". Is this true? Could the gas valve be intermittently blocking enough gas from entering the burners?

I know there is a lawsuit for my unit concerning the secondary heat exchanger. Could the sec heat exchanger cause an issue like this?

I really do appreciate any time you could spend on this. Wife and kids are getting restless. :)

Thanks,

Chris

hvac1000 · Jan 23, 2010, 03:38 PM

Clean the flame sensor with sand paper or steel wool if so equipped.

tophersig · Jan 23, 2010, 03:48 PM

Thanks HVAC1000... but since I don't get any flame ignition at all, I wouldn't think the flame sensor would come into play.

(BTW -- the tech that came did clean it, and no joy.)

Thanks -- any other thoughts?

hvac1000 · Jan 23, 2010, 05:09 PM

Did the tech do this test?

I always look for the simple stuff first and then go on to spend the big $$$.

tophersig · Jan 24, 2010, 07:04 AM

Hi HVAC1000.. thanks for your time,

I believe he did do that test and passed it, but I have a DVM and I can run that test today and report back to you.

But, I guess I don't follow your logic... since I don't get flame ignition at all (but the glow plug lights), how would the flame detector come into play? Wouldn't that be an item of interest only if I get a flame, then since the flame detector does not detect the gas valve shuts down?

But in this case, I get no flame at all. I would think the issue is earlier in the ignition process, than the flame detector.

Am I incorrect in thinking this way, or do you think faulty wiring on the flame dectector may prevent any flame ignition attempt at all?

-thanks, Chris.

tophersig · Jan 24, 2010, 03:49 PM

HVAC,

I tested the flame sensor, and when a flame is present, I get amperage.

However, when I don't get a flame at all, no current on the flame sensor.

I noticed (from another post) that a user with similar problem tried "venting" the combustion box by loosening the door to the combustion chamber. If I do this, I get flame every time. If I lock the door back down, I return to low percentage of flame.

Could this be a clogged intake?

Would I also look to my pressure switch? It is connected via hose to my gas valve, and the intake portion of the combustion box.

Feel like I'm getting closer...

hvac1000 · Jan 24, 2010, 04:16 PM

Could this be a clogged intake?

Check both intake and discharge to the outside to be sure no clog is present.

Could the sec heat exchanger cause an issue like this?

Yes

tophersig · Jan 24, 2010, 04:50 PM

Thank you. I will check both intake and discharge.

Concerning the sec heat exchanger (which my unit is known to have issues with), can you explain how it would cause my no-flame issue? Just trying to learn what the sec heat exchanger does, and how it can prevent the flame from producing.

Also - how would a faulty sec heat exchanger be proved?

Again, I really appreciate your time.

hvac1000 · Jan 24, 2010, 05:16 PM

Secondary HX is used to extract the last amount of heat possible from the unit.

Leave testing of the HX to a professional.

tophersig · Jan 24, 2010, 05:53 PM

Thanks, HVAC, I wouldn't try to mess with the sec HX, just want to learn what it does.

Should I hire my local tech to look at it, to see if it's the source for my no-flame issue? How would the sec HX cause that?

Also what about the pressure switch? My local tech thinks it could be the problem, but since I am getting the ignition plug energizing, I don't think it's the problem.

Thoughts on that?