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Old 04-06-11, 03:50 PM   #1
MN Renovator
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Default 800CFM of air, how much power?

I was always curious why my electric bill in December and January was so much higher than my other months. I don't use much energy, mostly the refrigerator, electric clothes dryer, laptop, and climate control.

The refrigerator wasn't running more in a colder house, my electric clothes dryer is used about the same as any other month because I wash everything about the same rate and the number of loads per month are about the same. Laptop is never off because it doesn't use much power running and I'm almost always on it when I'm at home and its usually doing something for me anyway such as file serving or something else. ...but I'll get the point.

My extra power usage, which I always thought was from the furnace, really is using a heavy amount of power. Today I was curious about how much a blower uses on a furnace so I pulled out my clamp meter pulled the cover over the blower and did a little test.

I attached the picture of the ammeter. 6.26 amps after its been running for two minutes. The updates about twice a second, its initial startup draw that the meter read was 9.48 but since it doesn't update often enough I know that it was only within about the first second but have no real knowledge of the real peak. Usually motors have a peak draw of 3x what they run at. 3 seconds after its running its at a little above 6 amps and then goes up to 6.26 after about 15 seconds, what I assume is some pressure building in the ductwork.

Either way, I don't really get it because this motor is a 1/6 horsepower shaded pole motor, so for it to consume 1/6 horsepower electrically it should be 125 watts or so, add a little bit more for inefficiency and then we call it a day. ...or so I thought.

Turns out it is using a full horsepower. I'm pretty confident this is the source of my high electric bill in the winter but I didn't think its use was that much. The motor is lubricated and it takes a good deal of time to spin down when its shut off.

What do you guys think, is it expected for a furnace blower to use this much electricity? Not quite $10 on the bill but this thing runs much longer hours when its pushing air when cooling the house with the air conditioner. I'll be clamping my meter on the outdoor condenser too when summer comes around.

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Old 04-06-11, 04:18 PM   #2
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My furnace uses roughly the same amount of power when it kicks on according to my TED.
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Old 04-06-11, 04:49 PM   #3
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The blower on a furnace does tend to be the single largest consumer of electricity in a home, that is why sealing and insulating not only save fuel but they save electricity.
Mine tends to add $10-15 per month as well in the heating season.
You can clean your duct work so there is less resistance, changing your filter more often and using a high quality filter is often worth the cost of the filter, you might also look in to a dual speed fan, not sure on retrofitting one, but if it can be done it might be worth it.
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Old 04-06-11, 08:21 PM   #4
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I think the refrigerator is actually using the most in my house right now. In the summer its the air conditioner. The furnace didn't run that much.

I used 76 therms in January $67 total, which includes the $10.50 for fixed service fees.
I used 206 kWh for 34 days from the middle of December to the middle of Jan $34.
I used 131 kWh for 29 days from the middle of February to the middle of March $27.

About 75 kWh where most of it was the furnace. If it was pulling 6 amps for the roughly 110 hours it ran from the middle of December to middle of January, it comes to about 80kWh, so I think that is about right. I still think the refrigerator might have done more of the dirty deed than that but 80kWh is still a significant portion of the bill.

I'm not sure I have much to complain about with almost $10 in electricity in the winter, its the summer chunk that it pulls since the blower runs at high speed, which oddly enough doesn't seem to draw all that much more power. I pulled the tap from tap 1(highest) to tap 4(lowest) and the difference wasn't even an amp less. Furnace is tap 3 and AC(2 ton, needs 800CFM) is tap 1. I did some research on permanent split capacitor and shaded pole motors, it seems it doesn't matter which speed they run, they draw roughly the same power.

Good to know that I'm pulling similar power as other people are, I thought this was nuts when I first saw it but I'm still a little confused why a 1/6HP motor is pulling so much amperage. I figured a 1/6 HP motor might be pulling a little less than its rating and would burn up pulling 6 times the electric power than what the HP rating would lead me to think.

The filter is a pleated filter and isn't too restrictive, not hard to see light through it. When I did the test the filter wouldn't affect it since the whole blower door is off so its getting an easier time since its not sucking air from the return ductwork since it has an easy path through the front and the ducts are going to be clean on the supply end since the filter keeps that side clean.

I think the ECM or X10 or other BLDC type variable speed blowers are the key to energy efficiency and cut the cost of operating a furnace tremendously. Whenever a major expensive component of my current HVAC setup fails, my next furnace and air conditioning setup will have one of these efficient blowers for sure. With my insulation efforts that will be going on soon(upcoming thread) and the air sealing I did at the beginning and throughout this winter, my heating and cooling needs will be reduced, hopefully with some significance as I'm thinking going from 7-9" of cellulose in the attic to R60 and filling in various insulation voids I've tracked down and some other issues I've found preventing insulation from doing its job in a few places I think I can cut my energy use even more.

The problem is that buying more efficient equipment suddenly becomes financially stupid because I reduce my costs so much to the point where I don't see how I could possibly recuperate the money spent. I've got a 2200 sq ft house that I spent a under $70 in gas to heat for December and January each and under $70 electric bill in July when air conditioning. I just don't see how spending so much on equipment is going to save all that much money once I've insulated and cut these costs down more. I'm not ranting, I hope its not coming across this way, I'm just trying to figure it out because if my equipment fails, going from a 76% efficient to a 95% efficient furnace and a variable speed blower would be great to know that I'm getting 15% more heat from the natural gas over an 80% and a 16 SEER versus a 13 SEER to replace my current 10-12 or so. Do they make 80% efficient furnaces that pull in outside air like the 90%+ furnaces do? I'd prefer one that pulls in outside air and it would pull me towards a 90+ really quick if forced infiltration isn't required for combustion air. It all seems great but the savings seem to be there for the inefficient house. Has anyone else had this thought process?
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Old 04-06-11, 08:51 PM   #5
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Quote:
Originally Posted by MN Renovator View Post
800CFM of air, how much power?
Power = flow rate * pressure
So how many PSI?

Btw, does your fridge still use much power, with as cold as you keep the kitchen?
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Old 04-06-11, 09:25 PM   #6
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I don't have any way to measure the pressure that I know of. I figure it would take at least a little bit of time to develop pressure in the ductwork, at least more than 3 seconds because that is all it takes before the blower settles to 6.0x amps and stays there, I tested it again and it seems to hang at 6.0x amps or so on the highest speed tap(fan on/air conditioning wiring is connected to this tap). Any easy way to measure the pressure? I don't have one of those water column meters and my tire gauges wouldn't work.

The fridge seems to run just as long and just as often, at least it doesn't seem like its reduced all that much, even when it was 40 degrees in the house for a few weeks, just a little less after I open it since its not dumping as much cold air out. FWIW, the fridge had produces a few gunshot like noises when shutting off a few times when it was at its coldest. I might be able to find a video I took of it when it did it, easily reproduced by unplugging it while it was on. I wouldn't think that it would reduce efficiency by being cold, but maybe it introduced some sort of operating issues. The fluorescent tubes in the kitchen didn't like to strike sometimes, the dishwasher filled with cold dishes had an issue with the 'water heating' function turned off, I found out that once it filled and ran for 5 minutes and I added a dish the water would barely be lukewarm so I turned the function back on to avoid getting sick by having the dishes washed in cold water. The rest of the fluorescent lights had their issues too. Ever see a CFL glow purple for a minute, stay dim for the next 2 minutes, and finally be fully bright after 3-4 mins? 40 degrees does that sort of thing. Alkaline batteries in my digital camera don't work at 40 degrees either, I figured it had to be colder but take 3 pictures with brand new batteries and the camera thinks they are dead already until I heat them up in either a glass of hot water or in my pocket.

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