Fan limit switch help - DIY Chatroom

stubborn1 · 10-28-2009, 09:10 PM

Hi,

The blower fan on my furnace runs non-stop. After doing some internet troubleshooting, I'm guessing it's running non-stop due to the fan limit switch. I'm missing the outer cover as well as the little push/pull knob thing that takes it from manual to auto operation. Does anyone know if I can purchase just the knob piece or do I need to buy the whole thing together? The only parts I see online are the entire assembly.

Is there any way to "rig" the manual/auto switch? I did some poking around (with power off) and couldn't find any way to manually set the switch.

Houston204 · 10-28-2009, 09:30 PM

That is your low speed fan control, can you hear the fan speed up when you set the thermostat to FAN On ?

I can only see one pin, the other two are behind that violet wire.
The first pin is fan off, the second pin is fan on, the third (highest) pin turns off the gas (usually due to low airflow)

What are your 3 setpoints?

Plumber101 · 10-28-2009, 10:17 PM

Low speed fan control ????? How so wrong

That is a fan/limit with heater

Since the fan runs all the time a couple things to check.

Thermostat - make sure fan setting is on auto and not on
If the stat is on auto take something nonconductive and lightly tap on the fan/limit. Sometimes the dial will get stuck because it binds from lent, dust and the control that turns the dial becomes too weak.

They are simple to replace but you will need to remove and measure the length of the shaft that stick into the heat chamber area.

Here's a troubleshooting link

http://www.inspect-ny.com/heat/Fan_L..._Procedure.htm

Houston204 · 10-28-2009, 10:52 PM

The fan limit typically energizes a lower speed than your G terminal on these older furnaces. A simple test would be to turn the stat to the Fan On position. I stand by my statement.

The dpdt fan relay could also cause this, or shorted G wire. I recommend checking this before replacing that fan limit.

Plumber101 · 10-28-2009, 11:05 PM

That would be a true statement if we knew what blower speed tap was used for heating. Yes I will agree that sometimes the speed of the fan controlled by the stat is different that the speed of the fan that is used for heating. So if the two speeds were the same what does turning the stat blower switch to on do?

I will also add that this device is not called a low speed fan control.

And this person is trying to get the fan to stop running.

stubborn1 · 10-28-2009, 11:10 PM

Regarding the fan on/auto setting on the stat, it's set to auto. My stat is only a two wire stat, so I don't have a wire to the G terminal - only the R and W terminals.

I watched the operation of the fan limit and it still rotates as it heats up and rotates when it cools down, so I don't think it's stuck. The movement does "jump" a bit, so it may need cleaning and/or replacement.

I have fan off set at 80, fan on at 120, and the high limit is fixed around 200 I think.

Plumber101 · 10-28-2009, 11:13 PM

Try what I stated before about tapping the limit.

Shut the furnace down and wait for it to cool and as the dial reaches the 80 mark tap on it

Seen this many times

Also move the 80 to 90 or 95

Houston204 · 10-28-2009, 11:14 PM

Since fan limit was in the initial title, I didn't see the need for naming it, only identifing that it controls a lower speed.

Good point about the possibilty that cooling and heating are operating the same speed. It is a very simple procedure to check this though.

Plumber101 · 10-28-2009, 11:20 PM

so true