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Furnace fan won't stop running after the heating cycle is done

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My furnace fan won't stop running after the heating cycle is done. The only way to shut it down is to turn off the main power switch located near the furnace. I replaced the limit switch a while back (1-2 years ago) and things seemed to get better. Now it won't shut off at all. I've had it diagnosed twice. The most recent repair guy thought I have a short in the thermostat. When the fan is working properly it seems to have two speeds. If I turn the fan on manually at the thermostat, the fan runs on high. When it is running after a heating cycle (to cool the combustion chamber) it runs at a lower speed. I’m guessing the thermostat is not at fault because the fan is running at the low speed. I’m hoping to get a better idea if the thermostat is truly at fault before I replace it.

Furnace: RUUD, UGLD-08NE-GR

Thermostat: Carrier TSATCCPAC01-A

I also have an air conditioner located underneath the furnace.

Optional Information:
Make : RUUD
Model : UGLD-08NE-GR

Already Tried:
replaced limit switch replaced filters

The heat and the air conditioning fan works differently on furnaces. The fan for air conditioning (and when you turn the fan switch to on) work through a fan relay. The heating fan either works through a fan-limit switch on the older units or through the circuit board on the newer units. Is the unit staying on in high speed all the time? Or is it just running for a while after the heat shuts off in low speed?

Customer: replied 6 years ago.
The fan that runs when the heat is on, and now continues to run unless the power is turned off, runs at low speed. When working properly, this fan continues to run for about a minute after the furnace turns off to clear the heat chamber. Like I said, it now runs on low speed all the time regardless of what I do. If I turn the fan to "on" using the thermostat, the fan kicks into a higher speed. I am guessing that the thermostat doesn't control the lower speed and therefor can't be the problem. I think all signs point to the limit switch - but it is fairly new.
It most likely is receiving a signal from a defective fan-limit to run. I'm assuming the furnace is older and does have a fan-limit control such as this? If it does have the Honeywell fan-limit control, then replace.
Customer: replied 6 years ago.
It does have a limit control swith like that, and that is what I replaced recently when this first started acting up.

Did you put the correct one in? should have an 8 inch fan-limit with the heater in it. Should have a Ruud part #472054592.

Customer: replied 6 years ago.
I'll have to check later tonight. I bought the limit switch from a local appliance part store; I do remember there was a slight difference between the new and the old. The guy at the counter that it would work. It certainly did work better than what I replaced (the fan was running all of the time). I couldn't get the limit setting to work perfectly. When I first installed it, the heater would come on and after about a minute the fan would kick in normally. The only trouble I had was that the fan would cycle on and off a couple of times when the heater first turned on in the morning. At the end of the heat cycle, the fan would run properly for a minute or so until the chamber cooled off. Would a slight difference in length cause a problem?
it should be an 8 inch Honeywell fan limit (L4064) with the heater.
Customer: replied 6 years ago.

Looks like the parts man sold me the right part. I have a L4064B2236 and it is 8" long. L4064B2236

May be your settings then. Your fan off setting may have to be pushed up further.
Customer: replied 6 years ago.

Here's a picture of the dial. The fan off setting is well above our indoor temperature.Here's a picture of the dial. The fan off setting is well above our indoor temperature.

I hate to say it, but if your settings are correct and wired correctly and fan still runs, then fan limit is bad if it doesn't shut off.
Customer: replied 6 years ago.
Is there any way to test it?
Take the dial and turn it slightly with you finger. If fan stops, you have a sticking fan limit.
Customer: replied 6 years ago.

I turned it in both directions and the fan did not stop.

Pull one of the blower leads out (turn the power off first) and then turn it back on and see if fan still runs.
Customer: replied 6 years ago.

I pulled the red wire, fan load, off and sure enough the fan continued to run.

Then your fan is probably running off the high speed lead through your fan relay. This can either be on a circuit board (newer furnaces) or a separate fan relay (older furnaces).
Customer: replied 6 years ago.

Is the fan relay easy to find? or is this not really a DIY project. I am happy to hire someone, I just haven't had much luck finding an experienced trouble shooter. Does it happen to show in any of these photos?




Fan relay is the black thing below the transformer in your first picture.
Customer: replied 6 years ago.

To be sure, it's this thing right? Does it need replacing? Is there anything else I can try? Thanks for all of your help so far.


That is your fan relay, correct.
Pull the relay out on the picture (above). See if the fan stops. The relay is the device just below the transformer from your last posted picture. If the fan stops, replace either just the relay or the whole fan control. See here.
Tim H. and 2 other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
The fan definitely stops when I pulled the relay out. I'll order the part. Thanks, ***** ***** that works.
it is quite common for fan relays to fail, especially if they're older.
Customer: replied 6 years ago.

I replaced the whole fan control and the problem is now slightly different. I probably have some wires mixed up. The original fan control was a Rheem 47-19807-01, see attached pictures. When rewiring, I substituted the brown wire (new control - the Honeywell from your link above) for the yellow wire (old control). Otherwise, everything is in the same place. Now when the thermostat is set to auto (for fan control) the fan runs continuously. If I switch the thermostat to on, the fan turns off completely (won't come on even when the limit switch hits the on temperature).original control back

original control front

You'll have to refer to your wiring diagram to see how it's set up. I believe the fan relay normally closed contacts go through to the fan-limit control for heat and the normally open contacts on the fan relay would pull closed for high speed or A/C.
Customer: replied 6 years ago.
OK, I reviewed the wiring diagram and sure enough the colors were different between the old fan control and the new. Unfortunately, after I fixed all of the wires, the fan still runs all the time just like it used to.
There's only 2 places your blower motor can get voltage--the fan limit (which you replaced) and the fan relay. You said when you pulled the old relay that the fan stopped, so that narrowed it down to the fan relay. Unless something got messed up when you installed, you shouldn't have the blower running unless your thermostat is sending a signal to the G terminal at the furnace.
Customer: replied 6 years ago.

Right now I can:

  1. pull all of the wires out of the fan limit - fan continues to run
  2. pull the green wire from the thermostat - the fan continues to run
  3. pull out the fan control relay - the fan stops

The fan is running on low speed, when I turn the fan "on" using the thermostat, it runs on a higher speed. Could there be something wrong on the cooling side? Here is the wiring diagram for the furnace and fan control.furnace

fan control

Is this a downflow model? I see there is a TDC (time delay control) on these units that is probably not on the upflow units, apparently. In this case, if the contacts on this TDC were closed due to a defective control, it could send power directly to the low speed blower. If this doesn't apply, then you either have a wire shorted somewhere and sending 120 volts to the blower OR the new relay--contacts may be welded shut. In which case you need to begin to look for reasons for excessive amp draw such as a bad blower motor, or a poor wiring connection somewhere. Pull the relay out and if you have an ohmmeter, ohm across the normally open contacts. Does it show continuity?
Customer: replied 6 years ago.

This is a downdraft furnace. I pulled the fan control relay and ohmed accross the red/black normally open contacts - it does show continuity. Does that mean the TDC contacts are stuck shut? Here's a picture of what I'm guessing is the TDC:


Here's the inside. I checked the continuity inside and there is continuity from red to red and from red to black. TDC interior

Customer: replied 6 years ago.
OK, I took out the relay and hit it with my screwdriver. The red to red was then open - no continuity. When I reassembled the furnace, it all worked normally. I'm guessing I should replace this relay. Here's a picture:tdc-relay
replace that piece. Thanks for the wiring diagram as this one had me somewhat stumped. If it weren't for the fact that you had a downflow model with this TDC, nothing else seemed to add up. We had covered all the bases. Just to confirm this is the TDC, both reds should go to your junction box and the the blacks should go to C and M on your gas valve.
Customer: replied 6 years ago.

Thanks for all your help - this was a really great experience. It is very satisfying to have the heater back up and running without the fan going all the time. Ruud wasn't very helpful in finding this part and neither were my local part stores. Hopefully I'll be able to track it down on the web.

Thanks again,

If you want, you can just eliminate it. I don't believe it's absolutely necessary as it was optional per the diagram. You'd have to try it and see how it works. Cap off the ends of each red and each black and see how furnace operates. If ok, then take the red and black wiring out completely or just wire nut and tape the ends of each wire.