GMPN 100-4 furnace. Blower motor runs when not calling for heat.
GMPN 100-4 furnace. Main Blower motor (not draft inducer)runs when stat is off and auto/on fan switch is auto. Disconnected stat - and blower motor still runs. Unit is NOT trying to heat. Fault code on board is 5 blinks (False Flame Sensed)but again the unit is not trying to go through the sequence of operations to heat (or cool).
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Induced fan motors on older units are wired through the high limit safety on the stack - in series. When the furnace calls for heat, the induced fan motor comes on and after a time the furnace will light.
Older units have a temp switch in the stack that rotates to bring on the burners and blower motor. . When there is a call for heat, the induced draft fan motor comes on and then the burners will light.
After the heat exchanger is warmed up the limit switch will bring the blower motor on. That's basically how it works - there are two circuits - one for the induced draft motor and the burners and the other for the blower and high limit.
New units have a plug on the board you just plug in.
The first thing I would do is to verify that you have power to furnace. Check breakers and there may be a swith with a fuse in it mounted to side of your furnace or close by check fuse. You can use volt meter or test light to check for power. After you're sure you have power and thermostat is calling for heat if draft motor does not start you need to replace it. Draft motor is the first step in furnace start up.It should run for 30-45 seconds.then spark igniter or hot surface igniter should light pilot. It should stay on for about 20 seconds then burner should light. After burner lights the main blower should come on in about 2 mins. If you can get draft motor to run then procede to next step. Hope this helps you. Thanks
I would suggest that you have someone turn thermostat up to call for heat while you are watching. Being that old it may or may not have a induced draft motor on it. It it does the draft blower should come on first, then spark igniter,then pilot ,next the main burner,and after a short period the main blower. If you do not have induced draft motor the spark igniter should come on as soon as thermostat calls for heat. If it doesn't I would check all electrical connections making sure they are all clean and tight fitting. Just follow it through step by step . You can use volt meter to check across red and white wires from thermostat to where they hook into furnace. When you are calling for heat you should read about 24 volts. I would be inclined to replace spark igniter but if it does what it's is suppose to then I would go to gas valve. It might be worth your time to check connections on t-stat. I have occasionally seen wires that were broke under screw and some time make connection and other times not. You can also remove red and white wires from t-stat and temporairly touch them together. this should turn heater on. Hope this helps. Thanks.
Sounds like some one needs a new induce draft fan relay, if relay is located on the main control board you have to replace control board,follow wires from inducer motor and you will find the relay. I could help you more if i knew model # and serial # of the furnace, Good luck.
Your doing well just right. When the call for heat is on the draft motor gets a call from the board to come on. When this happens if no voltage is present at the motor then you ahve abad baord. Replace it.
If it is calling for heat the draft inducer it is the first device energized, the pressure switch will want to be proved and then the ignitor will glow and the gas valve will energize and the sensor will prove the flame and the heat is on.
If one of the above is not in the right time and place the rest will not work. About the fan it is either a shorted control wire for the fan or the limit switch (the high temperature limit) is made or bad.
If that is not the case the the thermostat has a switch in the back between the gas fired or electric. That will determine if the fan should be on every call for heat.
be sure the fan setting on the stat is set for auto, not on. in this mode the fan should only run when there is a call for cool or heat.if the fan continues to run check at the control board to see if you are getting 24 volts low voltage between green and common. if you are then the stat is bad. if youaren't then your electric control board is possibly bad. a lot of gas furnaces fall into a failsafe mode when a sensor or function fails and the blowers stay on 24 7. you may well have several problems from what you are saying. if you turn everthing off and start a new cycle for heat the first thing that should fire up is the induced draft motor , then the ignotor, then the gas valve. you can check each item with a meter , generally 120 volts to the induced draft motor, then 120 volts to the ignitior, the 24 volts to the gas valve. then ignition and 120 volts to the motor. most furnaces have a heat terminal and a cool terminal on the board. common to heat should get 120 volts on a call for heat after the sequence of operationb has taken place. 120 volts should be sent to cool terminal on a call for cool. a continueous blower is a sign of a failed safety. perhaps the motor failed and the safety opened and the motor was replaced but the safety never reset?. look at all the little micro temp sensors around the burner. they are like little breakers and need to be pushed in to reset . usually like a little stem that pops out from a metal or plastic dic. some are size of a quarter, some the size of a dime.
most forced draft fans are powered by 120 volts . close to the fan is usually a plug that unsnaps. create a call for heat after turning off to clear safetys and see if your getting 120 to fan on initial call for heat. if you are then fan draft motor is bad, usually sold as an entire assembly replace exact as it came out. sequence of operation is..... stat calls for heat via common and white wire to furnace , 24 volts. ......board sences call and sends 120 volts to forced draft ........ motor runs and creates pressure on proofing switch,....... switch closes 24 to board and tells it draft fan is on......... board then energizes ignitor,....... after ignitior glows gas valve is opened and burner lights ........ either ignitor or seperate sensor rod tells board the burner lit........ and a few minutes later the fan comes on and hot air is blown out. if a step is missed or doesn't proof the furnace either trys up to three times to light or locks out. turning off power usually resets safetys and it will resume attempts to lite normally.