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Old 2007-02-22, 11:08 AM   #1
jayoldschool
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Default Furnace fan - "ON" or "AUTO"?

I have searched the forum without finding a definite answer for what seems a simple question:

Is it more efficient to use my furnace fan "ON" all the time, or place it to the "AUTO" setting? The HVAC supplier told me ON, but we all know that one persons opinion may not be the best one...
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Old 2007-02-22, 11:21 AM   #2
hugh
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My new furnace has a variable Speed Blower & Draft Inducer Motor which uses Less Electricity. The fan is always running.

For older furnaces, I think all the following would factor into the equation:

Do you have cold-air returns in your house?
What type of furnace do you have?
Is your house a bungalow or two story?

If you have cold-air returns, a more efficient furnace and a two-story house, I would consider leaving it ON. Comfort factor goes way up and likely the furnace will come on less often

If you don't have cold-air returns and you live in a bungalow, I'd say leave it off.
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Old 2007-02-22, 11:25 AM   #3
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Depending on the furnace, fan type, ducting, as mentioned by Hugh, the answer is "it depends". I found that my furnace fan (which is part of a high efficiency (97%) furnace) draws a lot of power, if left on all the time. I therefore have mine set to auto. I didn't notice any difference in comfort level.

I have been in some homes where the fan is so powerful that it causes a "breeze" in the home near the registers and actually makes you less comfortable with the "wind chill".
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Old 2007-02-22, 12:02 PM   #4
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My fairly new thermostat has a Vent function which I really like. If the heat/cooling system has not run in the past hour it switches the fan on for 5 minutes. It balances out the hot/cold spots without running the fan 24/7.
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Old 2007-02-22, 12:28 PM   #5
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Very interesting. Could we have more information on this thermostat?
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Old 2007-02-22, 01:47 PM   #6
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Sounds like Hugh and I have the same or similar HVAC system.
I had our 18 year old system (heat/AC/humidifier) replaced last May with a mid efficiency system by "Dave". The old programmable thermostat was replaced with same brand updated programmable. Three days ago the Recovery program went to the dark side Should have been up to 75F by 7am which is daytime setting but was only at 71F and slowly climbing. Dealer replaced thermostat same day but it took 2 days for computer chip or module to learn preset needs. This morning all is well again. I tried the AUTO setting during A/C season and early this heating season and opted for the ON setting which means fan runs at lower rpm 24/7 until heating is called for then fan jumps to high speed until plenum air temp has cooled sufficiently to drop fan back to lower rpm. The AUTO setting means fan runs about 35% of time, random starts and stops according to manual. Difference in electrical cost for ON or AUTO setting could be determined with some higher math -- determine what portion of your electrical bill is for running fan full time (good luck with that) then taking 35% of that value and subtract the two. Difference is your saving or not over a year. In our case the fan setting stays the same, summer for AC and winter for heating. In heating mode the humidifier only gets water during the high speed fan setting so there is no rotating drum/tray to get gooped up with slime and caked-on lime buildup over a heating season. No condensation on windows any more but now I draw a bazillion static amps from finger to door knob. At the moment I am opting for slimy media over this new fangled system. sailmaker
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Old 2007-02-22, 01:50 PM   #7
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It isn't pretty, but it works. I use it to control my humidifier as well. If you're not in a hurry, it goes on sale every now and then. I paid around $30 after combining a sale with energy conservation coupons. In case the link doesn't work the Canadian Tire product number is 52-2570-6.
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Last edited by stampeder; 2007-02-22 at 03:44 PM. Reason: Broken retail link removed
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Old 2007-02-22, 04:17 PM   #8
Bent
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I run my fan steady on, I have an HRV that uses the HVAC fan to assist in moving air - however, I'd leave it on anyway. By running it 24/7, I don't have to worry about wearing out the centrifugel start winding switch and not having any air handling at all.
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Old 2007-02-22, 04:58 PM   #9
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I was running my Furnace Fan 24/7, especially when I had the AC on during the 'hot' months of June - Sept.

We're in a 3500+ 2 story house built in the mid-late 80's. with 2 story ceilings in the main foyer. I believe the furnace is original.

That was until I was telling my neighbor / wife's cousin about our Hydro Bill for July/August. It was around $380 IIRC for 2 months.

He (who also is a electrician contractor) asked a couple questions, re: fridge in the basement and about the furnace fan. Our fridge was 15+ yrs old, and could barely keep bread frozen. Our fan was on 24/7. He said those were the culprits.

We turned our Furnace Fan to auto, Our fridge died shortly after that conversation, and was replaced by a new one. Our hydro bill probably went down $100 for the next 2 months...

I will know more next year when I have the same months to compare too, but I believe the fan does draw a lot of power.

P
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Old 2007-02-22, 05:15 PM   #10
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It should be possible to look at the "plate" on the fan motor to see (roughly) what it draws when running, assuming no "variable speed".

For example, 1/3 HP is about 250 Watts. On 24/7 that would cost about $250/year running full time (11 Cents/kWh). In auto, it'd probably draw about $50 (on 20% of the time average, if that). Interesting that currently (pun intended) Watts on full time roughly equals $/year.

It's not only the money, but that's over 2000 kWh.


Unless it's a really old refrigerator, it probably uses about 100 kWh/month, or about $130/year.

http://www.digitalhome.ca/forum/showthread.php?t=35430 (My watt-meter checks)
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Old 2007-02-22, 05:16 PM   #11
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While the furnace fan may indeed draw a lot of power, I'd be willing to bet that the fridge was the true culprit. Old, energy-inefficient appliances waste a lot of power...
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Old 2007-02-22, 05:23 PM   #12
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the nameplate on a motor indicates maximum values, there is no garrantee that it that it needs to dissipate 1/3 hp all the time - only a clip-on ammeter and/or power measurement equipent would confirm it's total amps draw, and consequently it's consumption.
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Old 2007-02-22, 07:57 PM   #13
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Thanks for the replies so far!

A little more info: The house is a new townhome. Finished basement, two stories. Three bedroom. The furnace is a "mid" effeciency, I believe (I will do more research when I get down there) - it has no chimney, just an ABS vent out the side of the house. Ducting to all rooms, returns as well. I taped all the seams I could when I first moved in, and I close/open vents on the different floors in season.

It is a little complicated because the main floor (where the thermostat is) also has a gas fireplace with a fan kit. It can easily heat the entire floor during the evenings when we have it on (almost every night between Nov and March). Just one more thing to consider.

I am aware that the motor draws power. I guess the real question is what costs more: having the fan on all the time, or making it push when the burner kicks in?

Great debate. Keep the info and opinions coming.

Jason.
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Old 2007-02-22, 10:30 PM   #14
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I have my fan on all the time right now to get rid of the condensation on my windows.
I've had pretty bad condensation in the master bedroom windows for years & I don't know if it's 'coz I turn the furnace down to 20░C at night or if the windows need to be re caulked.
Having the fan on all the time seems to help with the condensation.
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Old 2007-02-22, 11:18 PM   #15
jayoldschool
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"I turn the furnace down to 20░C at night "

Wow! What do you have it at during the day? Mine is tops 20 daytime, around 17 at night...
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