Gas and Oil Home Heating Furnaces - gas furnace blower
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09-20-00, 02:03 PM
The blower and the power vent fan on my gas furnace cycles on for three minetes and off for two, constantly. The only way to stop it is turn off the control power. Any suggestions Thanks
09-20-00, 02:13 PM
Sounds like you have a fan limit switch prob. That's the little switch that turns your fan on and off and is controlled by the temerature in your heat exchanger. It's either malfunctioning (was it working fine before?) or has been somehow reset. It's been a few years but if memory serves, I used to set the fan to come on at about 125 and off at 100. The suggested settings may be on the door of your furnace or inside the service panel. The only trick here is not to let your fan cut off too quickly before the heat exchanger cools off after the t-stat is satisfied -- and, not to let the heat exchanger got too hot before the fan clicks on before the t-stat is satisfied.
Good luck!
Good luck!
09-23-00, 09:48 PM
Both the Blower fan and the Inducer fan are coming on.
The fan and limit switch controls the blower fan on older models, BUT you have a draft inducer so I beleive the cuircit board is controling the two differt motors.
Test the termostat for cycling to fast.Put a small wired between "R" and "W" at the thermostat. The furnace should run continually. If it is shutting off you are tripping an automatic swich that resets itself. This could be a LIMIT SWITCH.Some of these klixon switchs are anoly designed to be used a few times, they are not always acurate. This switch trips because the blower did not come on before the limit tripped. Try turning the FAN SWITCH at the thermostat to fan on. See if the cycling stops. If it stops the limit is a problem along with air flow could be a problem. Is the filter clean. On large furnaces you need a return into both left and right sides of the furnace OR the bottom of the furnace.Try running the furnace momentaraly(it is not safe to leave this way)for testing purposes the Blower door open . You will have to tape or push the door switch. Test the furnace. If it ran you have inadquite return duct work. and will need to add more return(s).
It is possible that a coil could be dirty. Try checking the temp delta across the furnace. On the name plate it will show you the design temp delta across the furnace. Take the temp of the air entering the furnace, subtract that number from the air leaving the furnace(not right at the furnace but a couple of feet down the ductwork is fine. Example 60 deg entering air, 140 deg leaving air (140-60=80) If 80 degrees is on the name plate Great otherwise increase the blower speed to get the right differce.
GOOD LUCK
The fan and limit switch controls the blower fan on older models, BUT you have a draft inducer so I beleive the cuircit board is controling the two differt motors.
Test the termostat for cycling to fast.Put a small wired between "R" and "W" at the thermostat. The furnace should run continually. If it is shutting off you are tripping an automatic swich that resets itself. This could be a LIMIT SWITCH.Some of these klixon switchs are anoly designed to be used a few times, they are not always acurate. This switch trips because the blower did not come on before the limit tripped. Try turning the FAN SWITCH at the thermostat to fan on. See if the cycling stops. If it stops the limit is a problem along with air flow could be a problem. Is the filter clean. On large furnaces you need a return into both left and right sides of the furnace OR the bottom of the furnace.Try running the furnace momentaraly(it is not safe to leave this way)for testing purposes the Blower door open . You will have to tape or push the door switch. Test the furnace. If it ran you have inadquite return duct work. and will need to add more return(s).
It is possible that a coil could be dirty. Try checking the temp delta across the furnace. On the name plate it will show you the design temp delta across the furnace. Take the temp of the air entering the furnace, subtract that number from the air leaving the furnace(not right at the furnace but a couple of feet down the ductwork is fine. Example 60 deg entering air, 140 deg leaving air (140-60=80) If 80 degrees is on the name plate Great otherwise increase the blower speed to get the right differce.
GOOD LUCK