Bob, that is a possibility, but I'm sitting here looking at a bag of 50
thermal fuses that I purchased just for this problem. My last job was a
Trane unit with a bad thermal fuse. I cleaned around the burner
entrance and replaced the fuse and problem solved.

J


Bob wrote:

My money is on a sticking relay, but I agree that he should hire a
professional.

"Joey" wrote in message
...

I would think you are on track. First, verify the gas is making it to
the furnace. Make sure you hear the 'click' that opens the gas valve
for the ignitors to fire. It's an electrical activated solonoid that
you can hear or check with an ohm meter. Next, find that thermal
protection fuse which should be located near the burners. Look on a
site that sells them so you'll know what you're looking for. They have
two leads and should read zero ohms if it's good. I would certainally
hire a professional if the problem is beyond this.

J



wrote:

Hello.

I have an Amana Air Command Hi Efficiency 90 Gas Furnace that just
stopped working this afternoon. I noticed that cold air was blowing
out the heat registers and the temperature in the house was 8 degrees
below the thermostat setting.


This furnace is a pilotless unit with a hair-dryer-shaped exhaust fan
to vent the exhaust gases.


Here is how the furnace normally operates (to the best of my memory):

- the thermostat "cool-off-heat" switch is on "heat"

- the thermostat fan "on-auto" switch is on "auto"

- I turn up the thermostat setting, requesting heat

- there is a short delay (for the igniter to heat up?)

- the gas kicks in and ignites and the exhaust fan turns on

- there is another short delay (for the heat exchanger to come up to
temperature?)

- the blower fan kicks in and voila, there's heat




Here is the present abnormal operation:

- the thermostat "cool-off-heat" switch is on "heat"

- the thermostat fan "on-auto" switch is on "auto"

- I turn up the thermostat setting

- the blower fan *immediately* comes on and starts blowing unheated air
throughout the house

- the exhaust fan never comes on

- there is no gas smell, and the burners never fire up

- I cannot see any indication of an igniter heating up (not sure where
to look, but I think if there was something glowing in there I would
see it)


Here is my working hypothesis: There is a thermal safety sensor/switch
somewhere in there that thinks the heat exchanger is overheated. So
even though the heat exchanger is dead cold, it wants to keep the
blower motor on to cool it off. And it inhibits the ignition and the
release of gas to the burners. So when I turn the thermostat up,
requesting heat, all I get is the blower motor coming on.

Does my reasoning make sense?

And, is it pretty straightforward to replace the thermal safety switch?

And, where is said switch/sensor?

And finally, is there a quick and simple way to momentarily bypass the
thermal safety switch to test my hypothesis... so I don't waste time
and money buying the wrong part?

Thanks.

EJ