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How much electricity does a woodstove blower use??

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Verynycegirl, Nov 30, 2005.

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  1. Verynycegirl

    Verynycegirl New Member

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    I have a blower on my woodstove which I love since it really helps warm the house up, but I'm wondering if I'm just going to pump up my electric bill? Would it be more cost effective to buy one of the stove top fans that were be discussed on the forum? Any idea how much those fans cost??

    Tracy

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  2. elkimmeg

    elkimmeg Guest

  3. begreen

    begreen Mooderator Staff Member

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    The stove top fans that we were talking about are self-powered and not plugged in, so no power consumption. The use the stove's heat to power them. That mean one needs a surface temp of at least 300 degrees to get them running. Many woodstoves qualify here. The price is about $110-140 depending on model. The cheaper electric fans are under $50 and draw very little power. If you are having an issue with heat collecting above in the cathedral ceiling, sometimes a ceiling fan can really help distribute the heat more evenly.
  4. Verynycegirl

    Verynycegirl New Member

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    I actually have two ceiling fans. One in my livingroom and one upstais at the top of the stairs (which we call the landing)
    Woul;d i run one or both? Would I run it at a low or high speed?
    Tracy
  5. saichele

    saichele Minister of Fire

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    1 amp blower? The 4" circulator fans (like in my insert) are all stamped either 20W or 25W.

    0.480 KWH/day, about 15 KWH/month, or somewhere in the neighborhood of $1.20/fan per month.

    I've got 4 of these little guys going (2 in insert, 2 on bookshelves blowing air around) more or less 24 hrs a day, with basically no change in our electric bill. The fans themselves are $20-30, and last maybe 3-5 yrs. No serviceable parts, so eventually the bearings dry up and you're done.

    Steve
  6. Rhone

    Rhone Minister of Fire

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    If you go here:

    http://www.electricmotorwarehouse.com/fireplace.htm you can go through the blowers on inserts, some of them I believe you need 2 blowers not one.

    Things like the Majestic uses a 0.50 Amp blower (I think you need two = 1 amp)
    A Regency Wood insert uses a 1.50 Amp blower
    Heritage a 1.1 Amp Blower
    Osborn 0.86 Amp
    Pacific Energy Esteem a 0.92 Amp
    Pacific Energy Summit a 1 Amp blower (according to the manual)

    I'm not a pro on electricity but that sure sounds like something is up. A computer fan ranges between 14W - 66 Watts and they are nothing compared to an insert. I don't understand, maybe the parts are labelled that way but the system as a whole uses more? I tried to find how much power the WWL uses, no luck. I can't find how much mine uses either.
  7. Sundeep Arole

    Sundeep Arole New Member

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    The amp rating is misleading. It tells the current (amps), but does not tell you what voltage things are running. 1 amp at 110 V is not the same as 1 Amp at 24 V, for example. The only meaningful thing is the wattage, which is voltage * current. That will compare different blowers with a apple to apple comparison.

    To compare, if only amps are listed, then you must find out the fan voltage somehow. It can be measured with a voltmeter provided you have access to the fan terminals and you know what you are doing, or otherwise you can directly measure the power consumption of plug in household appliances via a device called the kill-a-watt. Search for it online.
  8. saichele

    saichele Minister of Fire

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    I checked the manual for the WWL the last time that thread with the calculation ran, and all it said was 120V, 15 amp. If it even sucked 1amp all the time I'm pretty sure I'd have noticed it on our electric bill.

    Steve
  9. Rhone

    Rhone Minister of Fire

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    Interesting topic. A 15 amp, 120V would have to be on its own breaker as breakers usually only supply a total of 15 amps. 20 amps in bathrooms or if you paid for better wiring in your house. I understood that when I got a 12 amp vacuum cleaner and had to shut lights off when I turned it on else I blew a breaker. Once started though, the vacuum doesn't draw near the poewr and I can turn lights back on.

    Hmm... makes me think of something. Hotflame is it correct to assume a blower is like my 12 amp Kenmore vacuum where it does draw 12 amps but only during startup. My vacuum has in big bold letters 12 amps and dims what few lights I have on in the house when I turn it on but once it's going doesn't draw near the power. Do you think these 1 amp blowers, only draw 1 amp during startup and from then on draw less but have to spec 1 amp?

    I don't think there's a power converter in these blowers do you? You think they'd drop 120V household current to 24V DC? Their electronics seem too small to include a converter or voltage drop device. With Eco-fans Craig showed a movie of a different type and it doesn't start until you give it a spin. Once you've started the process it takes over from there. Likewise I'm thinking maybe the 1 amp is only what's required during startup, after that along Steve's lines once started it uses very little power to maintain speed.
  10. saichele

    saichele Minister of Fire

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    I wasn't suggesting that it takes 15 amp, just that manual was rather unhelpful in determining specifics.

    I suppose it's not inconceiveable that a 25w motor might need a full amp (120w?) to get started.

    Steve
  11. Rhone

    Rhone Minister of Fire

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    Yeah, and I think chances are that's exactly the case. Glad you popped in Steve, I've been wanting a battery backup for my blowers but using 1.2 amps in my formula's it wasn't worth it. This completely changes things. I also hope Frank Ivy pops in, as I used the 1 amp all through the formula's so he thinks it costs $8/month to run. If someone pops in and confirms the electricity requirements are only at startup, it may be like you said $1 - $2/month.
  12. Eric Johnson

    Eric Johnson Mod Emeritus

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    Just as a side note, a fan running 24/7 will last a lot longer than one subject to periodic start-ups and shut-offs. It's the stop and go and kills motors. Of course, running continuously does consume more electricity, so it's kind of a trade-off. The 15 amp mention in the manual is probably a reference to the minimum breaker size it can be connected to. And yes, even that's a basically meaningless guideline.
  13. Sundeep Arole

    Sundeep Arole New Member

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    Rhone,

    Well, it always takes more energy to start a motor than to keep it going, because of inertia and also something called back emf. 15 Amps seems quite large, though. My feeling is that is the minimum electrical service the fan should be hooked up to. I doubt they convert down to 24V DC, but I woudn't be surprised to see it stepped down to 24V AC or something such. You may also have multiple blowers making up your unit, all wired in a series configuration. That will effectively split the voltage across them.

    Well, if the blower contacts are easily accesible, you could just put a voltmeter across them and measure the voltage. It isn't complicated, but you do have to know what you are doing, so if you decide to do this, read up on how to do it.
  14. begreen

    begreen Mooderator Staff Member

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    A little clarification here, the running load on a motor will be significantly less than the start current. When a motor starts it does require much more current to get it going, but this is for a very brief time (<1sec.). I'm willing to bet all of the motors on most pellet stoves are running at 120vac and none will ever be wired in series. The 15 amp rating is for the circuit, not running load. That is the minimum house 120v circuit size. My pellet stove has a start current of 4.1 amps and a running current of 1.1 amps. That's because, the biggest starting current on my stove is not the motors, it's the ignitor. It's on for a minute or two and for a battery backup system that load would need to be calculated in.
  15. Sundeep Arole

    Sundeep Arole New Member

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    Allright, so I decided to sit down and work this thing out. The job of a fan is to move air, and since air has a mass, one can compute a absolute minimum amount of energy a fan of a given air intake size would need in order to produce a desired CFM. It is simply the amount of energy needed to move that much air. Of course, there are energy losses, due tio friction, some enrggy is lost to sound, etc. but we are talking about a minimum number here. Assuming a fan is 100 percent efficient, has a intake area (either blade area or total area of intake vents in housing) of A (in square inches), desired air displacement D (in CFM), the minimum energy needed for the fan to move the air is..

    P (in Watts) = 0.0001565 * (D^3) / (A^2)..

    Note, if you keep a fan the same size, but decide to spin it faster so you gan get double the CFM, you will need 8 times the power! Similarly, if you double the size of the fan blades, you only need a quarter as much power to get the same CFM.

    I'll post a full derivation later in the ash can if there is interest.
  16. Rhone

    Rhone Minister of Fire

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    Thanks BeGreen and Frank Ivy. I know gobs more about plumbing than electricals. Boy, that's twice my dream of battery backing the blowers on my insert got foiled ;) I got my hopes up.

    So VeryNyceGirl figure it to cost about $8.27/month to run your blower 12 hours a day give or take. One of those fans won't be a replacement for your blower. Your stove is special, it's a convection wood stove. It has channels in it that convert more of your heat into convection heat (hot air). Your blower pushes air "through" these channels of your stove to maximize it's ability to heat air and distribute it through the house. One of those fans resting on top moves air that's "risen" off your stove. Anyway I think it's probably worth it, it won't be as effective as the blowers but they work when there's no power, they're silent, a conversation piece, and save the environment. Difficult to put a price on that. It will work well with your stoves ability to heat extra amounts of air but here's a recommendation. Your blowers (if they're like mine) really help heat the house and move the heat around. Without them on I've found myself wanting to put more wood in to try to get the heat out as it doesn't heat my house near as well as when the blowers are on (which has been discussed a lot). It's a personal preference. So, this is what I recommend. I recommend you get it and keep the blowers off and use the fan except for the coldest days/months. Your blowers will maximize the heating of air and distributing it so you can really take advantage of your blowers on the coldest days. On the rather cold days, the fan will probably be sufficient enough to spread the heat around without using power, being silent, being a conversation piece, and save wear & tear on your blowers. Best of both worlds. Make sure your unit can handle the blower being turned off if you decide to get one. There's a few units where the blowers have to run while a fire is going else the blowers/parts get too hot and melt.
  17. saichele

    saichele Minister of Fire

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    Nice work, HotFlame. I took a simpler approach, and bought a new fan at Radio Shack. I actually needed one more for my house air circulation.

    The result - a 4.69" x 4.69" fan delivering 65 CFM is tagged 20W @ 120V, and 220mA @ 120V.

    I ran it through your equation, and ended up with 0.088W?

    Any thoughts onthat one?

    Steve
  18. Sundeep Arole

    Sundeep Arole New Member

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    Oops! I must have screwed it up. I can't expect it to be off by that much.
    I'll look over the calculations and get back to you.
  19. Sundeep Arole

    Sundeep Arole New Member

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    Well, the dimensions are right. Got to check the unit conversion factors now.
  20. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    VNG I have looked at all of these scientifc calculations everybody is having so much fun with and, being math challenged, I have determined that you could run those blower fans for about three or four years for what that ritzy fan is gonna cost ya.
  21. Verynycegirl

    Verynycegirl New Member

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    Well, since the whole idea of burning wood was to save money....I think I'll just stick with my nice little blower....It actually does a great job and I'm thrilled to know it's not going to make my electric bill sky rocket!!

    Tracy
  22. Sundeep Arole

    Sundeep Arole New Member

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    Steve,

    So I have learned a lot in this exercise, which is really why I enjoy doing this anyway. BB, I can appreciate your sentiment, but I am not doing this for the final number; rather, I just cant resist a fun problem. For me, figuring things out just gives me enjoyment; it's probably wierd, but that's just me. Of course, in practice, if I was really interested in a fan's power consumption, I'd just put a wattmeter on it and measure it. But, doing the physics is just plain entertaining!

    After re-checking the calculations, it looks like this is indeed the correct equation for the minimum power you need to move a given quantity of air. The density of air is a variable, but it doesn't change things much. But I have learned a lot about fan efficiency in general.

    So, we can run the numbers. First point, 20 W is inconsistent with 220mA at 120V, since the latter would give 26.4 W.

    As far as area, the intake is really not square, it is round. The fan blade diameter is 4 inches, since I think your numbers are for the housing size.

    For a 2 inch radius, we have a area of (3.14)*(2*2) = 12.6 square inches. Now, we must subtract the hub, since no air moves from there. I am guessing the hub radius is 1 inch. That gives a hub area of 3.14 square inches, for a net intake area of 9.46 square inches.

    How much power would we need to move air at the rate of 65 CFM through a opening of 9.46 square inches if we could move it with 100% efficiency? That is the question the equation will answer. So we plug in the numbers -

    P = (0.0001565)*(65*65*65)/(9.46)*(9.46) = 0.48 W.

    Now, this is a long way from the 20W rating the fan claims. And I agree. There are two factors here.

    One, the fan is not 100% efficient. This is really where I learned something. Fan motors, with their current designs, aren't very efficient. Most of the energy goes to heat, and 12% to 15% actually is transferred to the shaft! So if we assume a 100 percent blade efficiency, the overall fan efficiency is still 15 percent. So to move 65CFM using this fan, we would need a power of 0.48W / 0.15 = 3.2 W.

    Still a long way from the 20W rating - well yes. When the fan starts, the blade has to get moving, which needs energy. You can figure this out too! But it is a bit complicated, as you need to compute the moment of inertia of the blade, and know the fan's steady state rpm. We'll leave that calculation alone, but I'm willing to bet that is where the extra 17 W or so goes. The published rating has to handle the worst case, which includes the extra juice needed for fan startup.

    I'd say if you measured the current being drawn by your fan while it is running, it will be a whole lot less than 220mA. It will be closer to 22 mA. I'd try it out and see.

    What is the point in all of this - well, I'll spin it this way - we don't think too much about improving fan efficiency because the power consumption is low anyways. But given it is 15 around percent max, we are being quite wasteful. We can go a long way with better motor and blade designs.

    From a scientific standpoint, it is just fascinating to learn that, other things being equal, if you need to get twice the CFM without making the fan blades bigger, you will need a motor that sucks eight times the juice! Or that if you use a fan intake twice as large in area, you will only need a quarter of the energy to keep the same CFM. Someting I didn't know. Before this, I'd have thought nothing of reducing the area from which air can go in through a fan housing, but as you can tell, for a given fan motor, if you reduce your intake area by just 20 percent, the CFM will drop by 34 percent!

    I'll post how I came up with my numbers in the ash can when I get a chance.
  23. saichele

    saichele Minister of Fire

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    Still good stuff, Hot Flame.

    As far as I was concerned, it ws actually a question of how much juice I'm using. If the initial 1amp numbers were accurate, and I a
    have 5 of those guys going 24/7, I'd be using about $75/mo in electric. Kind of takes the edge off the savings from wood heat.

    But at a worst case of ~20W, all 5 are equivalent to a 100W bulb, 2.4 KWH/day, or about $0.20/day at my rate. Once they're up to speed, it's probably more like a nickel a day. That I can work with.

    But it's been an informative exercise.

    Steve
  24. Rhone

    Rhone Minister of Fire

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    I wonder if it's because you have a muffin fan in your insert and we have blowers.

    Your muffin fan is probably plastic blades? I'm trying to think of weight, as my blowers have metal blades, probably 6" long each, and 2" in diameter. Looks like this blower. Your fans look like Muffin Fan which size wise and weight the muffin fans sure look significantly smaller, compact, and lighter compared to a blower. My blower probably weighs several pounds and I can't imagine your muffin fans weighing as much.

    Hotflame, great job. I found your post very interesting, I love learning about things like that.
  25. saichele

    saichele Minister of Fire

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    Yup, twin muffin fans blowing into the base of the insert, pusing air out the top grate. Might weigh a pound each. The a couple in strategic locations pusing air around the house.

    Maybe not as husky, but 1) readily replaceable, and 2) cheap, both to replace and operate.

    Steve
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