Furnace blower stays on, basic HVAC question I think

[jmartin5009]

Good morning. I was reading an older thread about furnace blowers being stuck on and wanted to double check the answer - because I am a novice in this area. The old thread suggested disconnecting the green wire that runs to the thermostat and if that stopped the blower, it is a bad thermostat. I did this and it stopped the blower. Looks like time for a new thermostat.

The older thread also suggested checking the blower relay in the furnace. I did take a look there, tapped around the relay, but nothing changed.

If it matters I am working on a Frigidaire furnace with a heat pump.

I appreciate any response, even if this is a simple question. Thanks. John

 

[Carsonb]

I'm really bad at trouble shooting electrical without looking at it, and I don't do anything with heat pumps. The thermostat is switched to heat but not calling for heat and the manual fan switch is off? If so and pulling the wire does stop the fan then I'd think the thermostat has shorted out. Any power fluctuations lately?

 

[jmartin5009]

Heat is working fine, but when the set temp meets the actual temp, the fan just keeps running. I have to turn down the heat and then the fan will stop. The set/actual temp right now are the same, but the fan has been running constantly for over 60 minutes. No power fluctuations lately. This has been an ongoing issue - I had thought it was my imagination, but clearly the fan is staying on. I can change the relay or the thermostat, I am just trying to avoid wasting money.

To better answer your question, fan is set on auto with the other switch to heat. When I flip the fan switch from auto to on, nothing happens. When I switch the other heat/cool switch from heat to cool, the fan stops. Right now the temps are equal and the fan is off. As soon as the next heat cycle comes on, it will heat fine, the fan/blower is just stuck on until I manually switch.

Maybe I should just replace relay and thermostat? Still cheaper than a service call. Thanks

 

[Carsonb]

Heat pumps do have a longer fan on cycle to extract all the heat off the coils than an oil furnace or gas furnace. Not sure how they set that on a heat pump but it should be adjustable. Old mercury thermostats have an anticipator to shut off the burner before the actual temp is reached to keep from over heating the house. As I said I don't work on heat pumps but I would think there's a setting on the heat pump its self or a fan timer on the furnace for how long the fan runs after the supply of heat is shut off. Maybe its just a setting that you want backed off a bit.

 

[rickairmedic]

J one other thing to look at especially if its an older furnace is the fan limit control which could also stick and cause the fan to keep running . Most furnaces the fan will run for a short time after the burners shut off in order to cool the heat exchanger back down .


Rick

 

[lametec]

Since you can turn the blower off by playing with the thermostat, there's no stuck relay or anything like that.

What kind of thermostat do you have? There might be a programmable setting that's keeping the blower on.

 

[jmartin5009]

I have to start by saying my initial info was wrong - sorry - not trying to waste anyones time. It is not just the blower staying on, it is the heat pump as well. They both continue running well after the set/actual temp reaches the same set point - I mean hours after the temps meet and the air coming out of the vents is pretty cold. The thermostat is a White - Rodgers.
After talking with a local HVAC guy yesterday, it sounds like my thermostat is the likely culprit. My all electric system has two modes for the fan/blower on or auto. It is set to auto. The heat/ac function has 4 settings. off/cool/heat/emergency. Emergency is electric elements only. Heat uses the heat pump and I am told the elements if the thermostat calls for it - but there is no indication of this mode on the screen - the HVAC guy called it auxilary. The house is about 7 years old so the system is as well. Again, sorry to provide you less than accurate info - I do appreciate the insight. My direction today is to get a good quality replacement thermostat and hope that there is just a glitch with the electronics on this old one. Thanks, John

 

[Carsonb]

No problem here and thanks for some follow up! We all learn a bit that way!

 

[Dkramer]

It is not just the blower staying on, it is the heat pump as well.
After talking with a local HVAC guy yesterday, it sounds like my thermostat is the likely culprit.

It may or may not be the thermostat. Most electronic stats will display the set point temp if the actual room temp is within one degree off it. So if set point is 70 degrees and actual room is 69, the stat will say 70. But the heat will continue to run until the room reaches 70 degrees. The fact that the heat pump is running doesn't mean it is working properly. Take the temperature of the air coming out of the nearest vent to the furnace you can get to. It should be between 15 and 20 warmer than the return air temp. (Which could feel cool because we are 98 degrees.) 70 degree return should be about 90 degree supply. If it is not then I would look at the heat pump first.
Thermostats rarely cause a problem, but they also can go bad.
Hope this helps.

 

[jmartin5009]

Dkramer, you are right on the money. I am open to demonstrating my ignorance in this arena of knowledge - I am more of a car guy. After a long drawn out process with an HVAC tech who seems basically competent, I think there is no issue. The average temp around here lately is 32 to 28 - I think the heat pump is just operating as best as it can and 70 degrees is the break even point - so the thermostat is not calling for auxillary heat - it just leaves the heat pump running almost constantly. Cheaper to run it than straight electrical elements - tough to know how much life is being taken off the heat pump. So replacing the thermostat did nothing but provide a fancier new thermostat. Now, the lesson here is "if it ain't broke, don't fix it." I believe the culprit here was my ignorance added to some paranoia, which in small doses can be healthy. It now appears that the "testing" done by the tech has shorted out the defroster circuit board in the unit. I am waiting to see of it attempts to defrost tonight when it gets cold. Yep, the heat coming out of the vent at 77 or 78 degrees does feel cool relatively, but it is less than half that outside.
Anyway, I appreciate the attention and advice. Together we all make a pretty smart individual - sometimes. Thanks