Oki doke a couple of things first. If you are turning on the heat at the T-stat and then walking to the furnace with the access panel already off and press the door safety/interlock switch, your machine may go into a delay and then the indoor blower motor comes on because the board (assuming it has one) registers that sequence of events as a fault. Some units receiving a heat call from the T-stat upon a power loss and start up go through a different sequence of events than receiving a heat call while power is applied to the unit.
Faults on heat mode always default to the indoor blower motor energizing as a way to remove heat from the exchanger. The machine might not understand exactly what went wrong or why something weird happened; however, it sensed and "Awe &#*t" moment and went into "Save me" mode by turning on the fan.
If the same sequence of events happens with the furnace all put back together, door on, everything as it should be then you have to suspect a few things:
1. A limit switch is open or tripped. The switch could be defective or may require a manual reset on your part to put it back into service.
2. Your control board (again assuming there is one) is defective and not sending 24VAC to the coils of your gas valve.
3. The coils of your gas valve solenoid are open (no continuty)
4. The valve is good but the valve seals are stuck together and the solenoid energized is not enough to pull them open.
5. There is a defective, loose, or missing connection in the circuit. Maybe a loose ground, poor crimp splice, shove the wire too far into a screw on connection and hit the insulation instead of the actual bare copper connector.
Sounds like you are handy with a meter so with all power to the unit off, find each and every safety (you may have to look a little harder for some) and with the wires removed check for continuity through each and every one. If you have more than a few ohms (ideally you will have 0 if its good and you are not holding onto the connections with your fingers) then you should consider changing it.
Put all of that back together and then pull the wires off of the gas valve and check for an ohm reading through it. You dont know what the reading should be; but, you should have some reading and it will prove continuity through the circuit.
Put that back together and make yourself a fused jumper wire (cut and alligator clipped wire in half, splice a 3 amp car fuse into the cut ends of the wire with a crimp on connection or electrical tape if you have to). Clamp off one alligator clipped side to the R or any known 24VAC source, press the door switch to energize the furnace and controls, and then touch the other end of the now engized jumper to the 24 VAC inlet side of the gas valve. You should hear the valve snap open or click as you touch the coil side.
If so then the valve is good and the burners should try and light. If the burners dont light you may have a gas pressure problem, dirty orafice, or stuck valves.
Try those things and see what happens and then we can go from there.
Last edited on Tue Oct 21st, 2008 11:21 pm by dkpd1581
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