Intermittent problem with (furnace) firing burners

[Puddin' Man]

Little bungalow in US midwest. Heil gas furnace on it's 2nd season. Standard forced air, std install. Gen'lly works well.

Heat cycle begins:

1.) t-stat calls for heat 2.) Inducer fan spins and pressure switch closes (15+ sec) 3.) Igniter is powered (17 sec) 4.) Gas valve opens, burners fire. Flame senser keeps gas valve open when heated. 5.) Igniter is powers down 6.) Blower runs after 30 sec

Works OK about 4 of 5 times.

The fifth time, it works OK thru 3.). The igniter is like a glow- plug: I can see it red-hot. The gas valve opens, and the burners fail to fire. It then flashes the code for burner-fire-failure, and goes back to 3.). It usually (but not always) works on the first re- try.

There is a lockout after 3 failed tries. This hasn't happened yet. It's a good thing: it's damned cold here.

Inside of the unit looks clean as a whistle. I blew compressed air thru the lead burner anyway. I've inspected the igniter and flame senser. They look fine. The contractor's roving trouble-shooter suggested goosing the (gas) manifold pressure, which I've done. No apparent help.

Not certain where to go from here. Any/all info/suggestions etc much appreciated.

Cheers, Puddin'

"Take Yo' Hand Out My Pocket (I Ain't Got Nothing What Belongs To You)!" - Rice Miller, who probably never even _heard_ of GW Bush, Paulson, etc

[Limp Arbor]

Check your ignitor's amperage draw. They can glow but still not draw the correct amps this will cause the gas valve to operate incorrectly or not at all. I think the flat ones are supposed to draw around 3.5 amps. Googling ignitor amperage and your furnace name should get you the specs.

[Nate Certified Heating and Coo]

You are either getting intermitent voltage to the gas valve or the gas valve is opening intermitently. NEVER "goose" the gas pressure! It's at a rated water column for a reason. Use a volt meter and check for voltage at the valve(24-27v). If you have voltage and it doesn't fire then it's most likely the circuit board. If you do have voltage and it doesn't fire then it's the gas valve.

[Puddin' Man]

Might blow my po' self up, eh? Well, anything is possible, I s'pose. :-)

I wish it were that simple.

I've stood over the unit and observed the cycle many times. The gas valve *always* clicks (*always* actuates). 80% of times the burners fire. 20% of the time they don't.

I suppose the igniter could be borderline hot enough to exhibit the intermittent behavior, but the tech didn't seem to think that to be likely.

I'll ohm-test the igniter when I can ...

P

"Take Yo' Hand Out My Pocket (I Ain't Got Nothing What Belongs To You)!" - Rice Miller, who probably never even _heard_ of GW Bush, Paulson, etc

[ds549]

make sure the smoke stack is clear and open .

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[Mark1]

So when you see the igniter glow and you hear the gas valve click and it doesn't fire, did you also hear/smell the gas flow?

Maybe the valve clicks but doesn't really open?

Maybe the ignitor is not located correctly to ignite the gas?

Mark

[Puddin' Man]

Didn't notice. Didn't stick my po' face close enough to the point of possible ignition. :-)

I guarantee it opens about 80% of the time. Not certain about the rest of the time.

Nah. Inspected carefully. It looks fine. Ohm-tests OK, too.

P

"Take Yo' Hand Out My Pocket (I Ain't Got Nothing What Belongs To You)!" - Rice Miller, who probably never even _heard_ of GW Bush, Paulson, etc

[Nate Certified Heating and Coo]

It is that simple, you are over complicating it. Again, do a voltage test as I described. Staring at it will not solve the problem. When that "20%" happens there will either be voltage or there won't, it's that simple.

[Limp Arbor]

:

Ohming it is a good start but it has to have the correct amperage draw to open the valve. The ignitor on my oven appeared to be working (glowed every time) but the oven would not always light. Amperage draw was low, I changed the ignitor and it has worked fine since.

The suggestion to check the flue is also a good one. My brother's friend had two different techs tell him he needed a new circuit board. Turned out there were leaves in the flue which caused it to keep shutting down/not firing.