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QUESTION:I have a Lennox natural gas furnace (Manufacturer's #4134768849 with a
Honeywell VR8204A 2027 gas valve) which will occasionally fail to
light. I discover this when I hear the fan running for a long time and
realize the house is cooling down. The furnace is in the attic, so I
can't just walk over and look at it whenever it is trying to start.
When the thermostat calls for heat, I can hear the ignitor clicking
and (looking through the louvers at the bottom of the furnace) will
see a weak blue flame which disappears after a few seconds, then
reappears for a few seconds, disappears, etc. Each time I have
watched, the gas valve will eventually open and the burner lights and
everything is fine - the furnace will run until the thermostat is
satisfied. But, it sometimes takes a minute or more (haven't really
timed it) before it lights.
This initial flame appears to be a pilot flame, but from what I have
read I think this furnace might have a MFS (Mercury Flame Sensor) and
doesn't really use a pilot flame. If that is the case, why do I see
this rather weak flame which appears to go out, come back on again, go
out, etc. before the burner eventually lights?
When I called a service company and explained what I saw, the owner
said that what I described sounded rather odd. He said that although
they are willing to come out and look at it, they weren't sure if they
could fix it unless the furnace actually malfunctioned so that the
burner wouldn't come on at all while the technician was there. I hate
to spend the money on a service call ($55) only to have them tell me
they can't do anything because it was working when they came out. So
far, the furnace has failed to light only a few times, but I doubt
that the furnace will fix itself.
If I understood a little better how this works I might be able to
watch it closer and give them better information.
ANSWER: Try to figure out if there's a pattern to when it malfunctions. It may be
trying to start each time, and finally starts up, or it may only do it under
specific conditions. When you do observe it, write down exactly what it does
and things like weather conditions, wind, etc.
The company was being honest with you. Sometimes it's almost impossible to
find a problem when the heater isn't malfunctioning. Luckily, (most of the
time) a problem will keep getting worse, and the likelihood of a tech
finding the problem will increase. Just don't wait until the middle of the
night to call someone, or it will cost a lot more than $55.
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