We have an (older) Coleman 7970-656D furnace in a mobile home. For a few years
now we have the same issue(s) -- it attempts to start okay....ignitor lights,
gas burns for a few seconds, then it clicks a couple times, and flame goes out.
Wait a couple minutes and it tries again with the same outcome. We've changed
thermostats, thermostat wiring, ignitor, ignition modules. After changing a
part and get it going, it may work for a while and then trouble starts all over.
Any ideas??? Would appreciate ANY advice to get this fixed once and for all.
If I had a picture of the burner and control section, I can point out a
possible cause of the problem. Many furnaces have an "air prove switch"
and what it does is to detect that air is flowing through the combustion
chamber and it is in series with the other safeties or it may have a
direct connection to a control board or smart gas valve. The combustion
air blower may have a centrifugal safety switch too. The air prove
switch, often called a pressure switch, detects the pressure difference
caused by airflow through the combustion chamber if the "draft inducer"
blower motor is running. There is a time delay in the control circuit to
give the combustion blower time to spool up and get air flowing through
the combustion chamber. The pressure switch is a round diaphragm
type that detects the low pressure difference when the combustion blower
is running. There can be one or two barbed fittings on the switch with
at least one connected to a metal tube which goes into the combustion
chamber. If you can pull the tube from the metal tube, blow into or suck
on it, you will hear the switch click. If there's nothing wrong with the
contacts, the probe tube may be plugged up. A piece of wire can
be run through the tube to unblock it. ^_^
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