Furnace fan on all the time? What's the cost?

I usually just leave the blower on all year round on our med/high efficiency gas furnace to circulate air through the filter. Our house is tall and narrow so it's always freezing in the basement and hot upstairs. Was hoping to even things out a bit. This morning the blower motor seized and needed replacing.

It got me wondering how much it is costing me in electricity to keep that motor running, even if the furnace isn't. The furnace is a 1993 model and the blower motor is the older AC type. The furnace guy said the newer DC blowers are much more energy efficient.

What are your thoughts on leaving the thing running 24/7 for circulation versus on Auto and running only when the fire/cooling is required?

And for bonus points, how do I calulate energy consumption on my blower motor. Furnace guy figured it to be about a 5 amp motor give or take but couldn't/wouldn't give me any more info than that. Is that enough to calculate electricity consumption?

5 amps times 120 volts=600watts times 24 hours=14.4 kilowatt hours times at least dime a kwh= $l.44 per day is that right? Sounds cheap? Somebody else go figure. Dave

I agree with letting the fan run. The principle of constant filtration and even temperatures through out the house is correct.

Once the motor is started, the energy consumption will average out, regardless if AC or DC. The majority of the electric bill will be based on the starting and stopping of a motor. During starting, it takes a lot more electricity to get her going, as opposed to just keeping the motor running.

So, the average motor on the residential heating/cooling system will start sometimes 25 or more times in a given day. The whole house will need to recapture that 3 deg. or so temp spread, heating or cooling.

Let the motor run.

John,PA Industrial blower guy. Refrigeration engineer, FRESH FOOD Products.

What is the HP, current,voltage and price per KW?

If your ducts are not insulated well and run through cold/unheated areas you may be cooling the house off at the same time.

It does help to eliminate hot and cold spots in the house though.

Typical blower uses 12 KWHs per day. Check your power bill and see what a KWH actually costs you (total bill with the added charges). Here in NY, if I was using grid power, it costs and extra $50 per month to run a blower constantly.

My furnace blower runs constantly, all winter, as long as the fire is going. It's on a big wood furnace. With the original settings, it cycled on an off too much. And with a wood furnace, there is constant heat, unlike a oil or gas fired furnace.

I checked into many types of AC and DC motors. End result? No gain with any of them, unless I bought a very pricey high-end AC motor that costs three times as much and runs about 1% more efficient.

Next time someone tells you about a magic DC motor, ask for specifics and you'll find out it's nonsense.

For rough and ready calcs u can use $10 per month per Amp for things operating 24-7. Realize that is rough and ready but it works out close enough at current rates here. For exact figuring the name plate amps doesnt mean ding because it is not drawing 100% nameplate amps,, unless by fluke. It is likely quite a bit less, depends on how much air you are moving, more air moving equals more amps. To get exact figures you need to know your rate per KW hour and put an amp probe on it.

Against the grain.
"If your ducts are not insulated well and run through cold/unheated areas you may be cooling the house off at the same time. "
Duct insulation isn't perfect. To convince you as to how much "cooling" you are inadvertently doing by running blower when heat (flame/etc.) is off, just insert a thermometer into the return air duct and take reading, then do same on longest run air duct & look at difference. This is a lot of heat loss.
Further more, your skin will always feel colder in moving air than still air.
Even-ness by constant blower operation cost you in more ways than one.
You measure, you decide...it it your house & your money.

Without putting a clamp on ammeter on a motor lead,its just a guess. It may have a 5 amp motor, but depending on the blower load, it may be drawing only 1/2 that.

Measured actual motor amps X volts = watts. Calculate that X the cost per kwh to get your cost per 24 hours.

(quoted from post at 11:23:14 02/19/10) Without putting a clamp on ammeter on a motor lead,its just a guess. It may have a 5 amp motor, but depending on the blower load, it may be drawing only 1/2 that.

Measured actual motor amps X volts = watts. Calculate that X the cost per kwh to get your cost per 24 hours.

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Yes, better than a guess, but still not the real answer, since it fails to take power factor into consideration, without which you still have a 10-15 % error.

Thanks for the correction, I was not thinking about it being a motor load and the need to consider power factor.

Some of the new digital thermostats will let you program the blower to run som many minutes per hour, that way it does not have to run all the time.

A furnace blower is a centrifugal fan. The power draw varies along a "fan curve", which is a function of the furnace, ductwork, filter, etc.

Blower motors are described as either "PSC" (permanent split capacitor) or "ECM" (electronically commutated motor). The former is an ordinary capacitor start, capacitor run induction motor, the latter uses electronics integrated with the motor to produce a motor capable of continuously variable speed.

Your 5 amp blower motor draws 600 watts under full load, most likey around 500 watts in typical application. Running 24/7 that's 12 KWH per day or 360 KWH per month, or around $50 per month at typical electric rates. I disagree with John T about the starting and stopping of the blower drawing more energy than leaving it on. The blower is an "easy starting" load. Even if it were not, let's suppose starting draws 5x the normal run current for 5 seconds and there are 30 starts per day. The motor would run in start mode just 4% of the time, at 5x power draw durring start the total starting energy use would be 20% of the energy of leaving it on all the time.

The PCM motor (sometimes called a DC motor) offers a huge advantage in that it can operate at a lower speed in circulation mode. The power draw of a centrifugal blower is proportional to the cube of the fan speed. So when the blower is running and the furnace is not, the blower slows down and the blower power drops to about 1/5. So that new tech blower would cost $10 per month to run continuously.

If you have ductwork run in an unconditioned basement or attic you do have duct losses to consider. The heat loss from moving air through such ductwork may be significant depending on how well sealed and insulated the ductwork is.

One inexpensive option you might consider is a new-tech electronic thermostat. These offer a "circ" mode, in which blower only mode is started a few times per hour if the blower is not otherwise running to circulate heat. These provides enough air circulation to even out differences between floors and filter the air, without leaving the blower on all the time
One discussion of furnace blower efficiency

Your furnace guy knows exactly how many amps the new motor is pulling. The furnace label will show the blower RLA & actual amp's should be about half that. He should have been honest when asked.

Leave it off for a month and compare that bill to one where it was running all the time to get a general idea of the cost. Comfort versus cost?

BTW - a dirty filter or one of the expensive 3m filters will increase the amp draw and cost you more money.

On 24/7 about $40. a month. Start, Run, Stop, about $15. per month.
Now factor in a new blower fan every 5 years or so on constant run vs 20+ years for Start, Run, Stop. My experiences anyway.

Leave it off for a month and compare that bill to one where it was running all the time to get a general idea of the cost. Comfort versus cost?

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Our fan has run 24/7 for many years now, all year, just more comfortable in our fifty year old house. My guess is about $30 per mo. with a 1/2 HP motor. If you have one of those new digital (TOU) meters, read it everyday at about the same time, with and without the fan motor running, that will give you an idea of where you stand. Generally, our daily use (two people) is 25 KWH per 24 HR period until the A/C is needed. Power runs about 10 cents/KWH here with all the extra BS charges included.

would'nt be easier to run a ceiling fan--smaller motor and easier to install