A few points.
The OP gravity draft furnace doesn't have inducer fan..
sip your martinis, don't gulp them.
Blower all the time might be a bad fan limit switch.
Ten commandments, not twelve.
The "glow bars" I've seen have been 110 VAC.
We refer to her as Virgin Mary, not Mary with the Cherry.
You don't sound like you know furnace troubleshooting.
We refer to Father Son and Holy Ghost, not Dad, JC, and the spook.
Christopher A. Young
Learn more about Jesus
wrote in message
Probably a bad idea to just start replacing parts.
It's strange that the main blower starts up at all, because normally it
doesn't start until after the flame is detected by the flame sensor and
the burners start (so it doesn't blow cold air), unless you have the fan
always on at the thermostat.
The sequence should be:
1. Draft inducer fan starts up and closes switch (via suction) so the
controller knows that the draft inducer is operating and it's safe to
light the burner. You can check this with an ohm meter, or jumper across
it to test it (however it must be open when the furnace starts, then
closed after the draft inducer fan starts, you can't jumper it closed
before the draft inducer fan starts).
2. Glow plug heats up (or spark ignitor starts sparking). You can check
that a glow plug is getting power with a voltmeter (should be 24VAC) and
you can see it glow red. A spark ignitor should make a noise.
3. Gas valve opens and burners ignite. Audible thunk as the solenoid is
activated. Controller knows that the burners are lit by looking at the
flame sensor and will close the gas valve after a short time if there is
no flame. You can test the gas valve by turning off the gas to the
furnace, unplugging the wires to the valve, and connecting 24VAC or so
to the valve from a small transformer. If you hear a thunk then the
solenoid is activating and the gas valve is probably okay.
4. Main blower starts up.
If one of the flame-out sensors is bad (open) the gas valve won't turn
on. There are usually three or four flame-out sensors in series. If
there's a crack in the heat exchanger then flames can shoot out the
front of the furnace and the flame-out sensors will shut off the gas.
When my furnace broke a couple of months ago it was an intermittent gas
When my friend's furnace broke last month it was an intermittent draft
inducer fan switch (sometimes it's just a clogged suction hose, but this
time the switch was bad).
If it's an older furnace with relays on the controller board you can
hear the relays click as various things are activated. The draft inducer
fan relay will click, the ignitor relay will click, and the gas valve
relay will click.