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I've replaced both limit switches on my Comfortmaker furnace plus the control board. Blower continues to run and the unit will still heat my home. Component changeouts did not make a difference in it's operation. I was thinking that the problem might be in the transformer, but since I am getting heat when called for from the main thermostat, I'm thinking that isn't the source of the problem. Thermostat fan control is set on Auto. All of a sudden, the blower motor would not stop running though it continues to heat the home. Furnace motor capacitor looks good. I disconnecting wiring at thermostat but blower continued to run. Wiring at transformer looks good... no fraying or burnt or discolored wiring. You still think it could be the thermostat?

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I have learned to not dispute things Tester has said, it is bad for one's reputation in this part of the world. I will not say he/she is wrong in this answer.

I will not be insulted if you just pass by this answer.

--- WARNING ---

I write too many words and use too much detail in my writing, always have and always will. Even my warning was too long.

Having said that, I offer this simply as data to show how Tester could have reached his/her conclusion and provide some background information. Hopefully it will help others looking for detail.

If anything I write seems useful, but confusing, let me know, and I will do my best to explain my thoughts. I am not an ass, it just seems like that sometimes.

--

Home HVAC control is more complicated than it needs to be, this is because as features were added - Air Conditioning, Heat Pumps, 2 stage heat, more control wires were added to the system.

The advantage is that everything is backwards compatible, clear back to the day that thermostats were invented.

A couple of disadvantages are that some wires are now redundant and there is not a standard for wire colors.

The color code became an issue, because when what was then 'standard' thermostat cable ran out of wires, the solution was often to pull another standard cable. This results in 2 of every wire color. This is where things stop being simple.

Terminal Designations and -- 'conventional' wire colors, don't place bets on them being right --

R - Power - 24 volts, to the thermostat - like the Hot wire in a light circuit - Red

RC - Cooling power - Air conditioning power into the thermostat - Red

RH - Heating power - heat power into the thermostat - can be the same wire as RC - Red

Y - Air conditioning compressor power - this is the piece outside of the house - Yellow

Y2 - Second stage A/C compressor power - not all compressors have this feature - Light Blue

W - Heating terminal - Heat power out of the thermostat - White

W2 - Second stage heat - Not in all furnaces, for Heat pumps this is Emergency Heat - Brown

G - Fan Power - Green

C - Power to thermostat - this is similar to the Neutral wire in a light circuit - Black

O or B - Power to heat pump Reversing Valve in outside unit - Orange or Dark Blue

E - Emergency heating on heat pump, a different way to get emergency heat - None

X or Aux - Back-up power/auxiliary, another different way to get back-up heat - None

S1 & S2 or Outdoor1 and Outdoor2 - Temperature Sensors, not connected to thermostat wires


Now, to Charlie's question:

I read '... continues to heat the house ...' as the furnace stops heating when expected, and not that is always continues to provide heat. This is confirmed with the statement ... heat when called for ... .'

First, remove the wire on the 'G' terminal of the controller in the furnace.

    1. If the fan stops, the problem is between the wire that was on the 'G' terminal out to the thermostat and the and back to low voltage (24 volts) transformer located inside the furnace.
    1. If the fan keeps running, the problem is from the 'G' terminal on the controller to the transformer. See step 5.

-- 3, and the fan stopped, attach the G wire back at the controller in the furnace. Expected result is that the fan will start again. At this point, go to the thermostat, and remove the wire from the G terminal. Move to step 4

-- 4, If the fan stops the problem is a short on the wire that was attached to the G terminal on the thermostat and the wire that was on the G terminal on the controller in the furnace. Most likely a pinch from something. It will most likely to have happened where the wire is tight. I have found problems in my system just fractions of an inch from the screw terminal in the thermostat.

This will be a short, and not a broken wire, a broken wire would cause something to stop, and not continue running. Since the short seems to only be between the G wire, and a power wire, expect it to be somewhere that the wires are not inside the jacket of the cable.

This is because it would be unlikely for the short to have happened on the jacketed segment, and not seem to have effected any other wires.

Crushing the entire jacket would likely, but not certainly, short more than the 2 wires. If you find the short, either separate the wires, and some tape on them, or use another wire, if you have a spare in the cable.

Hopefully you can find the problem in this 1 one run of cable, and you will live happily ever after.

-- 5. Remove the wires, after marking them, one at a time, starting with red ones, then black, from the controller terminals. If you remove a wire, and the fan stops. The problem is from that terminal on the controller to the transformer.

If all the low voltage wires are removed, the problem is either the controller, or somewhere between the fan harness and the fan. If you disconnect the fan harness from the controller and it continues to run, the problem is a short to ground from the harness to the motor. Or, and this is unlikely, demonic possession. Unless there are many wires, more than than three, then you have a state of the art, electronically controlled fan motor. The problem could be either the motor, or the 'module'. Replace the both parts, you will thank me later.

If the fan stopped when it was removed from the controller, the problem is in the controller. Replace, reattach all the wires, carefully, and live happily ever after.

--- If it is still not working. Then it is time to start drinking.

But seriously, let me know and we will get serious about fixing this thing.

Best of luck to you.

  • Very Helpful info. Before going through all this verify the hi-lo limit switch. – user46471 Dec 10 '15 at 0:29
  • Very precise answer someguy, I can tell by your coment Tester 101 doesn't much care for anyone's answers but his own that he searches the internet for. It's too bad one guy can try to drive knowledge out of a forum just so he can be the only one answering. Lol Have you checked the thermostat to see if there's a fan switch and it's turn off since you want to run it in the least economical fashion? – Richard Feb 13 '16 at 16:23
  • I'm not sure how I got dragged into this thread? I'm here to learn just like most others. If I've said something that's wrong, feel free to correct me. – Tester101 Feb 16 '16 at 19:54
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I had a similar issue I have two yellow wires on a screw named twin, the 2 wires are for heat and a/c, when I removed the a/c wire the fan stopped!!! I then realized something outside with my a/c was telling the furnace fan to continue to run! When I traced the low voltage wire to my outside compressor I found a very rusted part which is goodman part B1360324 this was very rusted and I suspect shorting out! This part is called a " condenser double poll contactor relay", basically a fancy word for an electrically operated switch!! When the thermostat calls for A/C this turns on the contactor via low voltage then switches on the High voltage compressor and the furnace fan! Warning turn off your circuit breaker if attempting this fix!! Very high voltage!

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I had a similar problem with my furnace/ac. Fan would always run even if the thermostat was set to auto or disconnected. Heat worked and ac worked fine. The only way to get the blower to turn off was to shut off the breaker. After much troubleshooting I found the heat strip element was broken and shorted to ground, this was allowing the heater to still work fine, and the shorted current found its way to the blower relay causing the fan to run continually. I fixed the heat strip and problem was solved.

  • What's the heat strip element? Would you please snap a photo of your furnace and the part that did the trick. Would be very helpful for us. Thanks. – jxramos Dec 12 '17 at 2:47
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  1. Get a short piece of thin wire that is bare at each end.

  2. Locate the fan limit control switch on the wall of the furnace near the burners. It should be app. 1.5" by 3" and brown or tan in color, and will have two wires going to it. If it is still readable it will say "fan / limit control" along with numeric values such as "180 -20 degrees" or something similar.

  3. When the problem reoccurs, pull the two wires off of the switch and connect them together with the small wire. If the blower stops, or stops after 90 seconds (depending on which board you installed) then your issue is excessive heat/lack of air flow, or a faulty limit switch.

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Found someone had set outdoor unit relay to NC. When it was supposed to be on NO. That was causing the fan and heat strips to stay running even when the tstat was off or blower set to auto. I switched it and it started working properly

  • Welcome to Home Improvement! This is interesting, but doesn't answer the original question. Please take our tour so you'll know how better to contribute here. – Daniel Griscom Dec 18 '18 at 2:25

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