Gas and Oil Home Heating Furnaces - Plus 90 furnace won't start

I have a Day & Night PLus 90 upflow gas-fired condensing furnace model # 398AAZ installed in '91 which won't start when I turn the stat to heat. The fan comes on when I set it to "Fan" position, but nothing happens when I turn it to heat. A while ago, the small fan would come on, but the pilot wouldn't light. Now, nothing happens. Any suggestions?

Well we know you have 120 power at furnace and also transformer for 24 volt works.

But this does nto guarantee, the 24 volts is going through the W terminal connection through the stat and back to the furnace W connection where the stat wires all hook up indide. You would need to test for that.

Also, try to reset furnace because unless you turn it off, then on again in about 30 seconds, you may be "locked out". If it were only "locked out", the small motor will start again and we then will only have one issue on our hands really, that being the pilot situation.

Let us know also if you have a power damper, connected to vent pipe inside the furnace, or not.

Let us know, when you remove the big furnace door, to check things out, if you also hear anything hum when you turn it to auto-heat, and identify source of hum if you hear one.


But we have to see to it we get that small motor going first before going further, as that has nothing to do with anything wrong with the prior pilot problem.

There's no hum when I remove the big door. How do I check for a power damper? (What does it look like?)

You have a high efficiency furnace, it won't have a damper. Put the blower door back on and see what happens. See if it tries to run again. Make sure the power is on.