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hwuaiyuc · Oct 30, 2005, 03:53 PM

I have a problem with my furnace blower not starting automatically. It works good when I swiched the fan option to "ON", the furnace was burning and fan was running. However if I pick "AUTO", only the furnace burns periodicaly, and the fan totall gets stuck. I am wondering if it is a wiring problem, but not so sure. I will appreciate any help from anyone.

labman · Oct 30, 2005, 04:54 PM

You have 2 temperature limit switches in the furnace. One turns on the blower once it is hot enough when the furnace comes on. The second one turns off the gas if the blower fails to come on, or it gets too hot for some other reason. If the blower fails to start, the gas will keep coming on until the upper limit switch shuts it off.

Some furnaces use a different relay and coil for a higher speed for fan only and A/C is you have it. This means the problem could be the relay too. Most of the controls on newer furnaces except the limit switches, are on a board making diagnose and repair difficult. If the limit switch seems to be working, you may be best off buying and installing a new board, hoping that fixes the problem. You could call a repairman, and pay another $100 in labor for him to replace the board. $$$$$$$

hwuaiyuc · Oct 30, 2005, 06:31 PM

[QUOTE=labman]You have 2 temperature limit switches in the furnace. One turns on the blower once it is hot enough when the furnace comes on. The second one turns off the gas if the blower fails to come on, or it gets too hot for some other reason. If the blower fails to start, the gas will keep coming on until the upper limit switch shuts it off.

Some furnaces use a different relay and coil for a higher speed for fan only and A/C is you have it. This means the problem could be the relay too. Most of the controls on newer furnaces except the limit switches, are on a board making diagnose and repair difficult. If the limit switch seems to be working, you may be best off buying and installing a new board, hoping that fixes the problem. You could call a repairman, and pay another $100 in labor for him to replace the board. $$$$$$$[ Thank you very much. The interesting thing is fan worked perfectly in the summer time when I uesed A/C. At your point, there are 2 control systems to the fan for heater and cooler separately? ]

hwuaiyuc · Oct 30, 2005, 06:47 PM

Thank you very much, Labman. The interesting thing is fan worked perfectly in the summer time when I uesed A/C. At your point, there are 2 control systems to the fan for heater and cooler separately? By the way, where is the board, I checked manu and there is not a "board".

labman · Oct 30, 2005, 07:04 PM

The older furnaces were much simpler and cheaper to fix. Start at the blower motor. Follow its wires back to the relay or relays. The relay should have 4 wires, one connecting to the power supply from the house, one to the motor, one to a transformer, and one to the limit switch. If you have 24 volts at the wires from the limit switch and transformer, power from the house, and the blower isn't running, the relay is bad. You may be able to confirm it is the problem by giving it a knock. If the blower starts, the relay is definitely bad.

hwuaiyuc · Oct 30, 2005, 07:19 PM

Thank you, I know where it is, and will work on it.

hwuaiyuc · Oct 30, 2005, 08:15 PM

Yes, you are right. My furnace was older, around year 1985. But I found 5 wires connecting to the relay instead of 4, of which 2 were connected to the limit switch. Do you know the 5 wires are right? :)

labman · Oct 30, 2005, 08:28 PM

The fifth wire could be from the A/C. That makes it look like the limit switch could be bad. On the other hand, if it set for a while between the cooling season and when you needed heat, the relay could have stuck.

hwuaiyuc · Oct 30, 2005, 08:49 PM

Do you think it might be a problem of wiring on the limit switch? Someone repaired wires to A/C before summer came this year.

labman · Oct 31, 2005, 08:53 AM

Yes. The switches close when the air in the furnace heats up. If somebody
Disturbed the wires to the limit switch. It would look like it was open, the air
Not hot enough to start the blower. If you have a multimeter or a test light,
Check the wires to the limit switch. With the power off, it should show
Conductivity through the switch. Sometimes just bending a wire a little can leave it broken off inside the insulation.