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.

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. $$$$$$$

[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? ]

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".

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.

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

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? :)

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.

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.

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.

I tested relay by using multi-meter, and it sounds good. Now I am concerning about the wiring. Right out of the blower motor, there is a small box that contains a transfromer and a relay. On the box is a switch board, all wires connecting to the thermostat are fixed on this small board. How to check if those wires are rightly connected? By the way, how I re-ignit the little light in the furnace, it disappeared when I checked the limit switch? :confused:

The guy could have messed up the heating wires. The basics:

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

As for the little flame, it is the pilot light. If there aren't instructions on lighting it posted on the furnace, look for a knob with off, pilot, and on positions on the gas valve. Turn it to pilot, hold it in, and light the pilot light. Hold it on a minute or 2 to heat up the thermocouple. Then release it and turn the knob to on.

Thanks Labman, I figured out the flame, and I am working on the wiring.

Hi "hwuaijuc",

I have similar problem as you had last month with your furnace. It runs perfect on "Fan On" mode through thermostat. When it's on "Auto" mode, the blower doesn't kick on.

Did you resolve your problem. I hope you did. Would you please share with me your findings and provide me with some tips?

I am thanking you in advancing for your assistance.

Thanks, Ravi :confused:

The blower operates completely differently in heating mode than On or A/C. Usually there is a lower speed coil in the blower motor controlled by a different relay which in turn is control by a limit switch. Check the limit switch first. It should have power either from the white wire from thermostat or directly from the red wire on the transformer. Power should flow to it and back once the furnace is up to temperature. It then activates the blower relay, powering the low speed coil. Often as not, the problem is the relay. Unfortunately, the newer ones are on a circuit board, challenging to diagnose and replace. The board is expensive to replace. You want to be very sure the low speed blower coil and the limit switch are working before replacing the board. If the board is bad, you can save $50-100 by not having a technician to come out and check it and replace it.

OK, I still had this on my clipboard from my last answer.

Hi "hwuaijuc",

Were you able to fix your furnace? As I mentioned earlier, I have the similar problem as you had.

Labman is helping me out with his comments. I am trying to follow him. In the mean time, I would like to know whether you were able to identify and fix the problem.

Please let me know.

Thanks, Ravi :confused:

Hello boys, my first post and I learn a lot reading this forum, especially from Labman, a true Rennasance man. I never could spell.
I think probably the fan is running on high speed when you put the thermostat on "ON", here the thermo is sending out a G signal, and is probably going to high speed by powering the cooling relay. When the thermo calls for heat on "AUTO", it sends out a "W" signal and should work the low speed, or heat fan relay. I sounds like your high speed circuit is working but not your low speed. To check your low speed circuit, do what Labman has suggested by following the voltage from the thermostat, limit switches, etc. To be safe, you can check the continuity of the components without the power on using the ohm meter. You can remove the limit switche, for example, and bench check them with a hair drier or flame. You should have a schematic, maybe you can find it and maybe post a picture or scan of it. This would help immensly. :D

Hi Michella, :confused:

Thanks for the response.

How do I find out whether the limit switch is a part of the control board or outside of the board?

Please help me out.

Thanks, Ravi

Quote:

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Originally Posted by ravithas

Hi Michella, :confused:

Thanks for the response.

How do I find out whether the limit switch is a part of the control board or outside of the board?

Please help me out.

Thanks, Ravi

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My omission. The limit switch must be inside the furnace around the heat exchanger. I also failed to mention the upper limit switch. It is set higher. And shuts the gas off if the blower fails to start. The limit switches may be connected directly to the blower relay and gas valve, or to logic on the board. If the limit switches connect to the logic, it is harder to sort them out. You may need a schematic. Might be easier to yank them both out and hit them with the hairdryer as suggested. I think the lower one should close when warm, and the upper one open.

I believe I see only one limit switch, which is located inside the furnace and above the heat exchanger. There are two thermocouples attached with a one cent size round tab connected and placed inside the furnace above the heat exchangers. Is this the limit switch. How do I make sure the limit switch is working?

Please advise me. As I mentioned earlier, I am running the furnace with "Fan On" mode on the thermostat. The blower doesn't come on with "Auto" mode.

Your help is highly appreciated.

Thanks, Ravi :confused: