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

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

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:

Aruuuuuuuuuuuugh! As furnaces become more complex, they become a bigger challenge to the DIY, even the HVAC technician. The old limit switches had a bimetalic spring or snap disk that closed or opened contacts. If those are really thermocouples, you would need to connect a sensitive volt meter across the right contacts on the circuit board to measure the voltage. You would then need to know the typical voltage at the temperature. Best you may be able to do is to go to where they connect to the board, and check for conductivity with one lead disconnected. If the leads don't seem to disconnect, they could be thermisters, no easier to check. Thermisters and thermocouples are less likely to go bad than limit switches. May be time to gamble on a new board or a visit from a technician.

Hello All,

I am having the same issue... but rather go on with a number of repeat questions, I will enclose a picture of my furnace. I have been reading the replies, but it's easier for me to have a picture to refer to:

http://tinypic.com/hvx6vm.jpg

As I mentioned before, I had a similar issue as the blower doesn't kick on in "Auto" and I had to run the blower at "Fan On" mode. I had a burnt out relay on the control board which powers up the low speed blower during "Heat" mode. I replaced the control board and works like new. Please check for voltage reading. You shall have 120 V AC between the heat tab and the neutral.

Good Luck. :)

Hwuaijuc,

You mentioned that your furnace is 20 years old. As it has a standing pilot (the little flame), would make your furnace very inefficient. Probably less than 80%.

Contact your local HVAC guy and have them bid a new furnace for you. There are some out there that are over 96% efficient.

In our first house, we had one of the old large frame furnaces (30" by 60"!! ) and was at most 60% efficient. We went to a 92% efficient furnace and wow, what a difference. Not as drafty and it really helped the old pocketbook.

ModerDaze, looking at your furnace pic, it seems a little newer than hwuaijuc's but not by much. There is rust on it and that is never a good sign. You would probably want to get a bid on a new system as well.

The only drawback is that as Mr. Labman (typed with proper deference) stated:
Aruuuuuuuuuuuugh! As furnaces become more complex, they become a bigger challenge to the DIY, even the HVAC technician.

Like most people here, I love to fix things rather than replace them as I am too cheap. Plus, I would rather do it myself than hire out. But, when things are that old and they are starting to break down, I would rather pay the big $ and get a new, extremely efficient unit.

There are a lot of good manufacturers out there. I have had good luck with Carrier and Bryant. My current system is a Bryant and has variable stage blowers AND burners so it only uses as much gas as needed to keep the temp where we want it.

Good Luck, Dan

My furnace does not work on auto fan but will work when fan is on.

Usually the older furnaces have what is called a fan and limit control that only controls the blower in the heat mode. Some of them look like this.

My furnace the t,stat calls for heat the burner lights the fan won't run is this the high limit?

Post a separate thread for your new question.

My furnace blower fan will not start in auto, and eventually the flame is shut off by a high limit on the squirrel cage. The safety is working so that's good.

Now I know that it should start, as I can see the fan control temp dial turning on the L4064B2954 Honeywell combination fan blower limit switch, and it definitely getting well past that setpoint. When I see that, I simply start the fan in manual, and then pull it out into auto, and it stays running. It shuts down fine.

I had to change out a bad blower motor a few days ago, and this is when the auto began to fail. Is it possible the bad motor damaged this combination fan limit switch, and is there a way for me to repair the switch some how?

I can buy one, but they are hard to find. 3-4 days to deliver from Thunder Bay.

Just looking for any kind of advice that's out there. My experience tells me it's pooched, but I just want a quick fix before Christmas.

Thanks, Albert

At least you can still get heat while waiting to receive a new fan limit switch. Buy a new fan limit switch and keep the old one just in case the new one malfunctions in a different way.

For example no blower at all when gas comes on and then blower comes on after gas turns off upon hitting upper limit. Which is what happened with my Rheem Furnace.

Best,