Furnace (no heat) Diagnosis?

Old 02-08-08, 10:38 AM
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Question Furnace (no heat) Diagnosis?

I seem to have lost my heat yesterday so I was wondering if anyone might be able to steer me in the right direction so that I may be able to accurately assess the situation..

Anyway, it goes like this:

I have a forced air furnace (with cooling unit) and (obviously) the heat doesn't come on (I should also say that the furnace part uses gas).

I've done some preliminary leg work to see what might be wrong and I have noticed that:

1. when the switch on the indoor thermostat is on "Heat", then I can turn the thermostat dial all the way backward and all the way forward and the heat never comes on.

2. if I leave the indoor thermostat on "Heat" and switch the fan from "Auto" to "ON", then I can feel air moving out of the duct/vents into the house...however, the air is not warm at all.

3. I can switch the unit from "Heat" to "A/C" and I can turn the indoor thermostat dial so that the air conditioning comes on.

4. When I got the air conditioning to come on, then I went outside to observe the unit in action and noticed that the blower fan (on top of the unit) does indeed work and seems to spin just like it normally would when the air conditioning is on.

5. When I switch the indoor thermostat to "Heat" and turn the fan switch from "Auto" to "ON", then when I go outside to observe the unit in action, the only thing I notice is that I can hear the slight rumble of air moving inside the unit.

I also took the liberty of taking a few digital photos of my unit (and the indoor thermostat):

(Tag & unit specifications on outside of unit):

(gas hookup for furnace):

(Indoor thermostat):

Anyone got any ideas?...

Could it just be a bad thermostat?...or could it be that my (gas) pilot light has gone out?..or is it something more sinister than that?

A big thanks to all those who reply!
Old 02-08-08, 12:26 PM
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Sorry about posting this to the incorrect forum...I should have posted this to the Gas & Oil Heating forum instead.
Old 02-08-08, 03:03 PM
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Your unit is a packaged system with gas heat and electric cooling (compressor). It has an inducer blower and Hot Surface Ignition (HSI).

The company that manufactured it, if I'm correct, now calls their newer versions of this same unit "Comfortmaker".

Your unit is quite old, I have not being able to retrieve the schematic, but from knowing the type of ignition (HSI), and having an inducer blower, this is what could have gone bad:
(1) the pressure switch
(2) the ignitor
(3) the inducer blower
(4) the electronic module

If you think you can get the schematic (by the unit or if you have the manual), scan it and post it via PhotoBucket.

In the meantime, check as follows:
* see if the inducer blower comes ON at all (not the house blower, but the one outside the house, inside your unit, the one that moves the combustion gases)
* If the combustion blower runs, see if the ignitor gets cherry red (there must be a glass visor there somewhere)
* the module is likely a Honeywell or a Robertshaw. If you can take note of its M/N, I may be able to tell you what to check for.

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