gas forced air furnace not starting in AM

[nesss01]

Hi,

we have an ~15 year old luxair gas furnace. I've found that around 30% of the time it does not start in the AM.

When it works, the wall thermostat triggers, the furnace becomes very hot, the blower starts and everything is fine.

When it doesn't work, the thermostat triggers, the furnace becomes warm

and stays warm, the blower does not start. After an hour or more, the furnace gets very hot, the blower starts etc.

I belive this only happens with the first AM trigger. If the furnace goes off and comes on again later in the day everything is fine.

We'll have the HVAC guys in next week, but can anyone diagnose this problem in the meantime?

thanks,

seth

[Ken]

I am not a HVAC person, but I would ask if blower start using ONLY the blower/fan switch on the thermostat at this "Early in the morning time?" Most thermostats have a Heat, Cool, and Fan control. Put it in the Fan position and see if the blower starts.

If it does, then it would mean the blower is probably OK, and that your failure to start the blower after the heat starts, is due to a switch that senses the temperature rise after the burners ignite. My furnace starts the blower a short time after the ignition and keeps the blower running some time after the gas has turned off. You might try this test.

Of course there are other measurements that could be taken to determine where your failure is, but you would need to have the proper tools and knowledge. Calling the HVAC man is probably the correct thing to do unless you have them.

[m Ransley]

It needs a cup of coffee

[barry]

Almost certainly, you have a "fan switch" in the blower-motor circuit. When the heat-exchanger warms to its set-point (shortly after burner lights), it closes and blower should run. Separate switch cuts out burner if temps soar (like if blower fails.)

Sound like fan switch is flaky- cheap fix.

J

[Stormin Mormon]

Pour it into the white plastic tube that comes out the side of the house.

[loch]

But in this case the burner isn't warming up as it should. When its working normally the flue is too hot to touch. When it doesn't work the flue just gets warm.

I think the fan switch/thermostat is just reacting appropriately to the burner tempearature.

[CJT]

It sounds to me like the either the gas valve is sticking or the burner needs cleaning (or perhaps your gas pressure is low because everybody in the neighborhood is trying to get their house warm in the a.m.). The fan is controlled by the temperature inside the furnace, so if the temperature stays low, the fan won't start.