January 7th, 2009 by webmaster
When my gas furnace comes on, the blower motor turns on and off 3 to 4 time before it will stay on. Other than this start up problem, everything works normally.Do you have a newer furnace with forced fan exhaust on the heat exchanger, that exits the side of the house like a dryer vent instead of through the roof like older furnaces? If so, check that exhaust piping, and make sure there is no downward slope in it anywhere. A downward slope can trap condensation from the exhaust, and cause the unit to cycle on and off.In most furnaces the blower motor is attached by a thermocouple to the heat exchanger. When the heat exchanger gets hot enough it triggers the thermocouple which acts like a switch to turn on the blower motor. I suspect your thermocouple is going bad. Recommend professional help.call a repair man.. or buy a new one.. dont f around a blow ur freakin house up.. there is a thermal switch not to be confused with the thermocouple. this switch actuates the fan when your furnace reaches operating temp. I will use a fixed switch for an example. It has a number F110-30. This means that the switch comes on when the temp is above 110 degrees but does not go off until te temp falls below (110-30=80) 80 degrees. if you are trying to keep your thermostat at a cool temp to conserve energy this is what happens.: lets say you keep your thermostat set at 68 degrees. When the temp falls below 68 your thermostat closes and tells the furnace to put out heat. the gas valve opens and the pilot light or the ignitor lights the gas and you have a flame in the furnace. The furnace itself may only be at a slightly higher temp because of the pilot. because the heat was taken away by the fan until it got below 80 on the last operation. anyway. so things start to heat up. finally the temp of the furnace gets to 110 degrees and the fan switch closes turning on the fan. The air coming through the furnace is less than 68 degrees and the furnace is just starting to warm up. when the cool air gets pulled through the the furnace it replaces the air the furnace has been heating up and the switch falls below 80 and shuts the fan off. the thermostat is still closed so the furnace stays on and the cycle begins over. now the furnace has had a chance to get a little more heat built up and all the metal parts that get hot in the furnace are almost to top operating temp but the fan switch closes again and the previous cycle repeats. finally when the furnace can radiate enough heat into the incoming air at a fast enough rate to keep the fan switch warm enough so it doesn't fall below 80 the fan stays on. when the thermostat is satisfied the flame goes out and the fan switch stays on to scavenge the resident heat out of the furnace until the temp of the furnace again falls below the 80 mark.
Some fan switches can be set for on temp and range.it is also possible to replace the existing fan switch with one that is more appropriate to your needs. F110-40 possibly but the air coming out of the vents will feel cold at the end of the cycle.#If you have any other info about this subject , Please add it free.# |
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