Fan keeps on cycling even when the system is off

agentp · 07-03-2009, 04:29 AM

I am having a problem with my HVAC system as described below:

The thermostat (White Rogers) has two settings for the fan (auto and on). Right now, the fan it is set to auto. It has another switch with three settings (i.e. cool, off, heat) and it is set to off.

The fan still comes on for a few minutes every now and then. Since, the system is set to “off” and the fan is set to “auto”, the fan should not come on at all.

The furnace works perfectly and when I turn on the heat it turns on the fan too. So there is nothing wrong with the system other than this fan running every so often.

Here is what I have tried so far:

Changed the batteries in the thermostat.

Changed the air filter.

Reset the thermostat to factory setting so that it is not programmed.

Please help me fix this problem.

**beenthere** · 07-03-2009, 12:46 PM

Is the furnace in the attic.

agentp · 07-03-2009, 08:52 PM

Yes, it is in the attic.

**beenthere** · 07-03-2009, 08:54 PM

Its probably the high limit bringing the fan on. It may be getting weak and opening. Its not uncommon

agentp · 07-03-2009, 09:14 PM

Thanks for the quick response. The system is off and it is not running. The problem is that the fan comes on even when the furance is not running.

Below are the setting on the thermostat:

Fan swtich:
Options: "on" or "auto"
Current Setting: "auto"

The other switch on the thermostat:
Options: "cool", "off", "heat"
Current Setting: "off"

As you can see, the furnance is set to "off" and the fan is on "auto."

At this setting the fan should not run at all. But the fan still cycles and it DOES NOT blow hot air. That tells me the furnance is not on.

I am a novice, so please let me know if I am missing something. Thanks again.

**beenthere** · 07-03-2009, 09:18 PM

The switch on the thermostat, doesn't turn the fan off.
When the furnace's high limit opens, it will run no matter what settings you have the thermostat at.

The air may not feel warm or hot to you.

Yoyizit · 07-03-2009, 09:18 PM

Quote:

Originally Posted by agentp

Thanks for the quick response. The system is off and it is not running. The problem is that the fan comes on even when the furance is not running.

Below are the setting on the thermostat:

Fan swtich:
Options: "on" or "auto"
Current Setting: "auto"

The other switch on the thermostat:
Options: "cool", "off", "heat"
Current Setting: "off"

As you can see, the furnance is set to "off" and the fan is on "auto."

At this setting the fan should not run at all. But the fan still cycles and it DOES NOT blow hot air. That tells me the furnance is not on.

I am a novice, so please let me know if I am missing something. Thanks again.

He told you.
The high limit switch turns on the fan when the furnace temperature gets too high. If the furnace is off the temp. can't be too high, so the switch must be erroneously turning on the fan, because the spring inside the switch that keeps it off is weak.
When this switch is on the 'stat is overridden.

agentp · 07-03-2009, 09:25 PM

So basically I will need to replace the "high limit" swich on the furnance. Is it an easy fix? Can I do it myself or does it need to be done by a professional? Thanks.

**beenthere** · 07-03-2009, 09:53 PM

Its not a hard repair.

Just need to get the right one.

You'll need teh model number of your furnace.

30yrhvactech · 07-03-2009, 09:53 PM

Either that or the fan control, if not built into a combination fan limit control

Yoyizit · 07-03-2009, 10:05 PM

http://www.hogslat.biz/images%2Fprod...2FG20-5046.jpg

Maybe it's $20 if you're not in the trade.

bluefitness · 07-03-2009, 10:27 PM

Did you just move into the house? My old company used to install fan managers in systems. It would basically just come on to circulate the air for a set period of time, so the air wouldn't become stagnant. I know it is a shot in dark.