Furnace not Wanting to start properly

More likely a sail switch issue.
When the T-Stat calls for heat the furnace starts the blower
If the blower is not running fast enough the sail switch won't sail
if it's "gunked up" it may not sail either
(you can check at the multi pin connector on the furnace control board using a voltmeter or test light, and of course the manual so you know which pin.

Once the sail switch closes the control board starts the spark generator and sends power to the gas solenoid Possible bad solenoid, out of adjustment spark gap. bad control board dirty points.

Gas lights and heats the rod. On many this rod is also the spark rod.. Dirty rod may not heat fast enough

Rod generates about 0.480 volts and thus some current. Sensor chip on control board sees that and heat continues.. Or Well on many the flame sensor and the spark rod are the same. and there is only one wire so there is a solid state "Switch" that changes from Make Sparks, about 1,000 VAC to sense about 1/2 volt DC.. imagine what happens if the switch flips with 1,414 peak volts on the lead Pop goes the sensor.

Now.. I do not know if that last item is what killed my furnace but when I ordered a replacement control board from Dinosaur Boards (Very well made board and I have the training to say that with authority) I noticed a neat little protective device (Gas discharge tube) right about where I suspect that sensor chip is.. There to protect the sensor by clamping the voltage to .. I'm not sure what (most of those style are 90 volts but you can make them less)

Old-Biscuit wrote:
When Thermostat Closes......does the furnace fan come ON?

If NO...then you need to check that DC is going to furnace...
*On/Off Circuit Breaker ON furnace tripped --- mauanl reset
*DC Source Fuse Blown

IF YES....does the Spark Electrode Fire (CLICK CLICK CLICK)
Does the flame light off?

If No...then need to check that airflow from fan is not obstructed
*Mud Dabbers/Nest/Debris in Intake/Exhaust Ports
*Return air path obstructed
*Sail Switch stuck/paddle not moving...under the motor cover

If Spark Electrode fires/flame lights then goes out
*Electrode dirty (soot/carbon)
*Bad Electrode



2008 RV-----Time to open up the exterior access and clean/test things out

Thanks Old Biscuit, very profound advice, much appreciated.

When Thermostat Closes......does the furnace fan come ON?

If NO...then you need to check that DC is going to furnace...
*On/Off Circuit Breaker ON furnace tripped --- mauanl reset
*DC Source Fuse Blown

IF YES....does the Spark Electrode Fire (CLICK CLICK CLICK)
Does the flame light off?

If No...then need to check that airflow from fan is not obstructed
*Mud Dabbers/Nest/Debris in Intake/Exhaust Ports
*Return air path obstructed
*Sail Switch stuck/paddle not moving...under the motor cover

If Spark Electrode fires/flame lights then goes out
*Electrode dirty (soot/carbon)
*Bad Electrode



2008 RV-----Time to open up the exterior access and clean/test things out

Hi,

More commonly called a sail switch.

But it sounds is if there may be other problems, too.

Flapper switch may have mud dubbers, hornets nest inside of heater. Needs cleaning.