HVAC problem: blower keeps running

[garrett1812]

Hoping someone here might have some insight before I have to call a service company to look.

Bryant gas furnace, circa 1988. Has worked well until yesterday, when the blower started to run continuously. The heat cycle will otherwise work like normal, except the blower does not stop after its normal few minutes after the end of running the burners. If I power down the furnace and start back up the blower starts immediately.

The thermostat has the fan set to auto, not continuous. The blower will even still run with the thermostat disconnected, so I do not think the thermostat is the issue.

I checked the limit switches in the furnace. Both had very low voltage drop across their contacts, and measured ~20ohm. When I jumpered the limit switches on the circuit board, same behavior, blower kept running. Interestingly, when I disconnected the limit switches from the circuit board also the same behavior, blower kept running.

My thinking now is the relay that runs the fan is stuck. I gave it a few light taps but nothing changed. If I power down the furnace and start it back up, I do not see any movement in the relay (it has a transparent housing).

Am I on the right track, or is there anything else to check?

Thanks!

 

[mbatarga]

Not an expert -but seems that the relay looks like your culprit.

 

[Bronson]

Replace the fan relay switch.

 

[garrett1812]

Thanks. I found a replacement circuit board, including relays, new for $35. That will be my first try before calling for service.

 

[LS6 Tommy]

Check the fan/limit switch. The relay only switches between low speed (heat) and high speed (cooling).

Tommy

 

[Falcon67]

Check the fan/limit switch. The relay only switches between low speed (heat) and high speed (cooling).

Tommy

Only time I've had a blower "stick" on has been the limit switch hanging up at the low end. The furnace at out ex-house sometimes required a Fonzi-style calibrated whack on the air handler to get the switch off high center.

 

[oldgoaly]

save the old board, find a new relay, solder or plug a new one in. then that replacement board will never fail!

 

[joel63]

Check the fan/limit switch. The relay only switches between low speed (heat) and high speed (cooling).

Tommy

^^^ More than likely.

Let us know what you find.

 

[garrett1812]

Check the fan/limit switch. The relay only switches between low speed (heat) and high speed (cooling).

Tommy

The blower still kept running when I jumpered over the limit switch.

 

[Streetbu]

Newer high effieciency furnaces have condensate pumps, you'll often find that "scum/bacteria" build up inside the internal drain in the furnace that feeds the condensate pump and causes the blower to keep running. Listen closely to the blower, if it sounds like there might be water sloshing, find the two rubbers hoses connected to the blower motor and disconnect them, they are probably full of goo and then a bunch of water will drain out. Not sure if yours is new enough to have or need a drain on it but it's free to check.

 

[moog]

If the limit was open then the blower would run continously but the heat would not cycle. Sounds like the blower relay on the board stuck closed.

 

[LS6 Tommy]

If the limit was open then the blower would run continously but the heat would not cycle. Sounds like the blower relay on the board stuck closed.

That's not correct. The limit & the fan stat in a fan/limit switch are separate circuits. The blower not shutting off happens quite frequently.


Tommy

 

[acmikee]

does the heat work? if not your control fuse or transformer is blown. check for 24v at the stat or unit. if it does work disconnect the "G" wire at the control board that will eliminate the t-stat wire being shorted.

 

[garrett1812]

Yes, heat is working.

 

[acmikee]

relay then in the board

 

[Dustball]

I had a Nest thermostat fail on me and kept the blower fan running non-stop along with no A/C.

 

[Ohmthis]

Tommy's correct the fan relay/ limit switch has two circuits. One is the fan relay that gets energized at a set point temp, the other is a safety that gets broken if/when the temp gets too high in the heat exchanger. Check this switch before replacing the board. You may not be able to return the part (board) if you install it and that's not your problem.

 

[garrett1812]

Tommy's correct the fan relay/ limit switch has two circuits. One is the fan relay that gets energized at a set point temp, the other is a safety that gets broken if/when the temp gets too high in the heat exchanger. Check this switch before replacing the board. You may not be able to return the part (board) if you install it and that's not your problem.

Where is the other switch typically located?

 

[LS6 Tommy]

Most fan/limit switches are prominent in on the face of the heat exchanger baffle, above the gas train & burner assembly. They sometimes have a rectangular metal cover. Under the cover there is a circular, silver dial about 2" in diameter, set in a rectangular black or tan plastic housing. The dial has three "tabs" on it. The one farthest to the right is the "high limit" setting for the burner. It should be set around 200*F. The next one to the left is the "fan on" setting. It should be set around 130*F. The other is the "fan off" setting. It should be set around 80*F to 100*F. Sometimes there's some sort of a locking screw above the limit tab. With the furnace cold, if you carefully rotate the dial counterclockwise a little by hand, you should hear it "click" & the fan should go off. If the "fan off" setting is well above the furnace temp and that's what happens, or if it doesn't click/turn the fan off regardless of the "fan off" set point, it's bad.

Here's a typical example:



Better look at the control itself:



Tommy

 

[garrett1812]

I can not find anything that looks like that in my furnace. In that area there is a limit switch (not adjustable). Looks like this:

 

[oldgoaly]

Google the number of the part see if it is a fan or limit. should give you specs. if it is a fan switch is that stuck closed?

 

[LS6 Tommy]

OK, that snap disc may control your fan if you don't have a traditional fan/limit. There's probably a pc board that has your fan relay(s) on it. It may have a built in time delay that keeps the fan from coming on before the heat exchanger is hot enough, then keeps the fan on until the heat exchanger cools down. Can you post a photo of your wiring diagram? It should be inside the access cover. That would allow us to eliminate a LOT of guesswork. IDK why I didn't ask for that sooner...

Tommy

 

[Charles (in GA)]

Only time I've had a blower "stick" on has been the limit switch hanging up at the low end. The furnace at out ex-house sometimes required a Fonzi-style calibrated whack on the air handler to get the switch off high center.

My Parents house was like this for years, from sometime after we moved in in 1961 till the unit was replaced in the mid '80's. Open the door in the hallway where the unit was mounted and give it a bang or two with your hand.

Thermal switch that controls the fan on a gas furnace.

Charles

 

[garrett1812]

Today I replaced the fan limit switch, same problem.

Replaced the circuit board, and all is back to normal! Must have been the relay.

Thanks all for the help.