Furnace blower on all the time, good or bad?

[nate larson]

Just bought a house and noticed that the previous owner had the furnance set so the fan is always blowing. The furnace is 12 years old and seems to be running perfectly. Is there a reason to have the fan on all the time? It seems like our electricity bills would be quite high if I kept it like this. The system is connected to an air exchange system and also to an april air humidifier. I am assuming I need to have the fan on to run these systems? Should I just have it on all the time in the Winter and turn it to auto the rest of the year?

[BobT]

I'm not an HVAC specialist but it is my understanding that having the blower run continuously in a heating system is not a bad thing. I would also suspect that if the same system is used to control humidity it might also be a good thing.

As far as electricity use, I don't know what the horsepower rating of the fan motor is but assuming it is a 1/4hp, FLA @ 120vac single phase would be roughly 5A. I would be willing to bet that you are probably drawing less than half that while running continuous so let's go with 2.5A draw.

2.5A * 120vac = 300W continuous use.

1kwh / 300w = 3.3hours to use 1kwh.

24h / 3.3h = 7.2kw per day.

My electrical rate is about $.08/kwh so

7.2 * $.08 = $.58 per day

$.58 * 30 = $17.45 per month.

Something to remember is that a motor will draw up to 10 times its normal current during the starting phase. Although it takes a very short time to get started, if it is cycling on and off fairly regular, it can add up to more cost than just running continuously but it all depends on the cycle rate.

[BoxMN]

First find out if it is a dual or variable speed motor. If it is, the slower speed(s) are most liklely DC and use little to no electricity. Ours is (we also have April air filter, 2400, and it works great) is a variable speed Trane, and we leave the fan on all the time. Keeps from having hot or cold spots in the house, and keeps the air filtering all the time. The higher speed hardly ever kicks in, only when very cold or right when you start it up, or when the programmable thermostat kicks in to raise temp.

good luck.

[youngie22]

a variable speed blower would be best, blower on all the time has many advantages. Mostly leaving your home more comfortable and cleaner!

[nate larson]

I will check it out to see what kind of motor it has and let you guys know. Thanks for the info everyone!

[BobT]

First find out if it is a dual or variable speed motor. If it is, the slower speed(s) are most liklely DC and use little to no electricity. Ours is (we also have April air filter, 2400, and it works great) is a variable speed Trane, and we leave the fan on all the time. Keeps from having hot or cold spots in the house, and keeps the air filtering all the time. The higher speed hardly ever kicks in, only when very cold or right when you start it up, or when the programmable thermostat kicks in to raise temp.

good luck.

If the blower motor is an ac motor it is still using ac power regardless. But, you are correct that it may use less power at slower speeds, depending on how the slower speeds are accomplished and the load. AC motor efficiency can go down as you decrease rpm under some cercumstances. I'm sure the designers have taken this into consideration.

[BoxMN]

FWIW -

[Note From Admin: Please read forum policy before posting again. Thank you.]

Ha! okay, ooops...

[WallEYES]

Always run my furnace fan continuously...helps circulate air and knock down dust. The furnace is 6 years old and has run that long 24/7. I figured that it only takes anywhere between 9-12 dollars a month to run.

[RussDaBuss]

We run our fan 24/7 as well unless we are gone for the weekend or longer. My wife has allergies. Since we have started running this all the time and with better filters, she can tell a big difference.

[spearchucker]

I think most variable speed fans use DC motors, but probably not in a 12 year old model. Our new Lennox uses much less electricity than our old single speed AC fan. About 60-75W compared to 600W with the old AC motor. I can't hear or notice the fan running on our electric bill. An old AC motor running continuously will be noticeable on your electric bill.

[Smoker]

The DC motors have been around for around 15 years or so. The Pulse21V had a DC motor but not to many people bought it because the cost to replace the motor at that time was almost half the cost of the furnace. I agree with spearchucker of the noise and cost to run the fan continuously. We had people call us back because they don't think the furnace is running.We will ask if it is warm in the house and we will tell them it is working fine. As said before run the fan all the time. More even heat in the house and with a good filter it helps the allergies.

[youngie22]

I have also had calls that furnace isn't working, its blowing cold air! that was just the fan circulating air in the house w/ flames not kicked in. didnt feel warm to them so something must have been wrong right? Had to explain that was exactly how we wanted it to work!

[BoxMN]

Thank you Smoker and Youngie, that is what my post said, but was edited...

I was not trying to be a smart-acher or anything, just giving the same info you posted.

[BobT]

So what you're saying is that most furnace Mfr's replace the 120vac motor with a dc motor when installing a variable speed kit or are the newer systems using dc motors instead of the 120vac motors out of the box even without the speed kit? That would explain a lot. Pretty simple explanation too.

Looking at the schematic for my 5-year old Lennox it would appear that motor is a 120vac motor and I know you won't run it on a dc voltage so it would require replacing the motor to do so.

[spearchucker]

So what you're saying is that most furnace Mfr's replace the 120vac motor with a dc motor when installing a variable speed kit or are the newer systems using dc motors instead of the 120vac motors out of the box even without the speed kit? That would explain a lot. Pretty simple explanation too.

Looking at the schematic for my 5-year old Lennox it would appear that motor is a 120vac motor and I know you won't run it on a dc voltage so it would require replacing the motor to do so.

I don't think there is a retrofit kit. Either your model has a variable speed DC motor or it doesn't. What model is your Lennox. I believe the G61 and G71 have the variable speed motors. I have the G71 and love it.

[Acemac]

go to the stat and push the fan button to auto. that will turn it off.

[spearchucker]

go to the stat and push the fan button to auto. that will turn it off.

That wasn't the question.

[Smoker]

I found out today that there is a retrofit to put a DC motor into some furnaces. The motor is almost double the price of a reguler motor plus the time to put it in. I'm not in the service part so what it all invalves I have no idea.

[spearchucker]

I found out today that there is a retrofit to put a DC motor into some furnaces. The motor is almost double the price of a reguler motor plus the time to put it in. I'm not in the service part so what it all invalves I have no idea.

It would need to have a power supply and motor controller, so my guess is that it wouldn't be cost effective. Would be better off waiting until the furnace needs to be replaced.