The yearly oil furnace inspection my wife ordered included a switch to
a smaller fuel nozzle. Performance improved (it was short-cycling)
but the next day the furnace cut out and would't come on again. She
called the people back and this time a crew of 3, after four hours of
effort, get it working and chalk it up to "a bad transformer."
But yesterday it wasn't working again, so I looked at it. Nothing
nada dead - not even a click from the control relay. I check the
thermostat and there's 24 volts AC that switches to 0V when it calls
It is a 5 year old in-floor furnace that appears to be in good shape.
I pulled the air circulation blower and beneath it found what I
suspect they replaced and called a transformer - a Honeywell control
module that has an input for the flame sensor and seems to control the
whole shebang - and appears to be brand new. All connections are good
but the red button on the top is up and resetting it returns the
furnace to operation only to have it stop again after a few hours.
There's a small spring loaded trap door in the cast iron firebox/heat
exchanger that allows me to see the flame.
I suspect an air/fuel mixture problem either causing a flame out
(can't run it for hours with the inspection port open) or failure to
reignite. flame is blue/yellow
My questions are how is the flame mixture adjusted - by flame color?
There appears to be a sleeve around the burner assembly with a series
of rectangular holes so that moving the sleeve would open or close the
air intake. How does the flame sensor work? Is this something that
requires a probe to be placed where it senses heat like a thermopile
or light like a photocell? I couldn't see the actual sensor just the
wires to it.
What should I be checking and can I tinker with the fuel mixture in
safety? (I'm a retired electrical engineer - and good at repairing
And do these guys seem competent based on what I've written?