Energy Efficient Window Fan?

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briancof
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Energy Efficient Window Fan?

Post by briancof » Wed Nov 16, 2016 2:22 pm

Has anyone had luck finding energy-efficient window fans?

I recognize that it's hard to define "energy-efficient" without knowing more about fan specifics. In a perfect world, fans would have a metric that quantified CFM/watt or something - a measure of how much air they move per unit of input power. I have not seen that for consumer-grade window fans, and so my next best idea was to find something that used a premium style of motor, which would have large bearing on fan efficiency. The best (and probably costliest) fan motors are (what's called) "ECMs" (electronically commutated motors), which rectify AC from the wall into DC before "inverting" it back into AC at whatever voltage and frequency is required by the fan. Without getting more technical, the presence of an ECM is probably a good indicator that the fan was built with low operating cost (i.e., high efficiency), rather than low purchase price, in mind.

We're in Chicago at a time of year where outdoor air is pretty cool. To run air-conditioning long-term when there's perfectly good and fresh outdoor air feels wasteful and silly to me. I'd hazard it does to some others of the Boglehead philosophy, too.

Here are the specifics of my situation that's led me here, in case any are curious.
  • -Rented condo, 2 bed 2 bath, top floor of 4 in a large building

    -Has been reaching mid/high 80s Fahrenheit during sunny days with mid 50s outdoor temp with windows open, curtains drawn and blinds closed.

    -Exterior wall is large, south-facing, and has floor-to-ceiling glass. It's a greenhouse, and the daytime sun just roasts us.

    -We have windows. However, they're all near the floor (where cool air sits) and hinged at the top (so as to shed rain), which means that they're serving, effectively, to dump out any cool air we have and trap hot air in the house. The windows hinge <30 degrees from vertical, and so there is not enough space for what gets described as a "box fan." We're probably limited, cross-sectionwise, to something around a foot tall.

    -We do have a door to the outdoor balcony which makes a huge difference if opened. However, there is no screen door and so we can't leave it open without locking our cat somewhere. I am going to write our unit owners about having a screen door installed (Is it greedy or unreasonable to suggest that they should contribute to that effort? It seems like an obvious property improvement and, frankly, I cannot fathom how no previous tenants had the idea).

    -Whole-house fan is probably not an option here.
Thank you in advance for any advice you can offer!

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tyrion
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Re: Energy Efficient Window Fan?

Post by tyrion » Wed Nov 16, 2016 5:02 pm

I'm not sure I would worry too much about getting the most energy efficient window fan. Any of them will be much cheaper than running AC.

Before I installed a whole house fan I had a 'window fan' in the room closest to the large shade tree. It had a thermostat on it which would automatically turn on at a certain temperature, so we used that to blow cooler air into the house during the afternoons. At night we would reverse it and have it exhaust air which would pull cool air in from any cracked windows throughout the rest of the house. It also had a 'circulate' setting which would have one fan going in each direction, to mix the air in the room.

I would measure your window and buy some sort of window fan that fits, preferably one that can do all 3 functions (intake, exhaust, circulate). Then you might want to buy another fan for inside so you can circulate the air in the house rather than having cool air down low and super hot air at the ceiling.

bloom2708
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Re: Energy Efficient Window Fan?

Post by bloom2708 » Wed Nov 16, 2016 5:16 pm

We have one of these in each of our upstairs bedrooms where the temp is always a few degrees higher than the other two levels. We use them as needed in summer or winter.

https://www.amazon.com/Lasko-4000-Stik- ... +tower+fan

I plugged it into my kill-o-watt and after a week it calculated about $5.00 to run the fan for the year. They aren't big, but they keep the air moving. Two speeds and oscillating on or off.
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BHUser27
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Re: Energy Efficient Window Fan?

Post by BHUser27 » Wed Nov 16, 2016 5:19 pm

How about one powered by a Solar panel? I see a few out there on Google.

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mrc
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Re: Energy Efficient Window Fan?

Post by mrc » Wed Nov 16, 2016 6:24 pm

bloom2708 wrote:We have one of these in each of our upstairs bedrooms where the temp is always a few degrees higher than the other two levels. We use them as needed in summer or winter.

https://www.amazon.com/Lasko-4000-Stik- ... +tower+fan

I plugged it into my kill-o-watt and after a week it calculated about $5.00 to run the fan for the year. They aren't big, but they keep the air moving. Two speeds and oscillating on or off.
Thanks for the tip on a small footprint fan. Don't you just love Amazon's pricing: $26 (with Prime) for 1 unit, $72 for two (no Prime). :confused
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DSInvestor
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Re: Energy Efficient Window Fan?

Post by DSInvestor » Wed Nov 16, 2016 7:07 pm

tyrion wrote:I'm not sure I would worry too much about getting the most energy efficient window fan. Any of them will be much cheaper than running AC.

Before I installed a whole house fan I had a 'window fan' in the room closest to the large shade tree. It had a thermostat on it which would automatically turn on at a certain temperature, so we used that to blow cooler air into the house during the afternoons. At night we would reverse it and have it exhaust air which would pull cool air in from any cracked windows throughout the rest of the house. It also had a 'circulate' setting which would have one fan going in each direction, to mix the air in the room.

I would measure your window and buy some sort of window fan that fits, preferably one that can do all 3 functions (intake, exhaust, circulate). Then you might want to buy another fan for inside so you can circulate the air in the house rather than having cool air down low and super hot air at the ceiling.
We have the Bionaire Twin Window Fan which was around $40 from Costco and can blow air in, out or exchange air.
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briancof
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Re: Energy Efficient Window Fan?

Post by briancof » Thu Nov 17, 2016 1:54 am

Thank you to all taking the time to respond to my question! I think I'm going to roll the dice on a "USB fan" plugged into one of those AC-to-USB power supplies one gets with (e.g.) smartphones, such as:

https://www.amazon.com/gp/product/B01DZ ... bw_c_x_1_w

BHUser27's suggestion of solar got me thinking about a fan designed to run off a DC source, which usually means batteries or solar, both of which usually mean they're designed to be parsimonious with power. Although solar would marry nicely with our sun-caused heatload, I'm not ready to forego the opportunity to operate without it quite yet. But it searching I did come across those USB fans that seem to be sippers. Anything running on five volts has to be lightweight. What's harder to figure out, with all of these, is what the resulting airflow is. But to bloom2708's point - not worry about the rest since they're all likely to come out ahead relative to AC. I can always order more.

DSInvestor's Bionaire looks nice but will not, unfortunately, fit the kind of top-hinged windows in my unit.

@mrc - what is Amazon, but one big arbitrage invitation?

leonard
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Re: Energy Efficient Window Fan?

Post by leonard » Thu Nov 17, 2016 11:16 pm

You might do a search and find my recent whole house fan thread.

Toward the end of that thread, someone posted about a window fan that sounded like a pretty good option, if your house is the right lay out for it.

BTW - in addition to temperature - you should consider humidity levels also - when considering moving outside air in. The temperature may be cooler - but will it cause humidity (or lack of humidity) problems?
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briancof
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Re: Energy Efficient Window Fan?

Post by briancof » Sat Nov 19, 2016 4:33 am

@leonard - I saw the thread and the fan in question. Thanks for suggesting, but it won't work with our oddball top-hinged windows.

re: humidity, I do not for a second doubt that it's central to comfort. I'm actually not sure that we have a way to independently control it, beyond running a dedicated (de)humidifier. And, if we're cycling outdoor air through the house, we'd just be fighting the ambient, right? Otherwise, with the house buttoned up, we wind up with whatever level of dehumidification the AC delivers. In any case, for the time being it is a secondary consideration to getting the house down from the mid 80s F daytime temperatures driven by our huge, south-facing glass walls. If we are complaining about humidity, I'll feel fortunate.

Aspirationally, I love the idea of building a near-"passive" house, aggressively insulated and smartly designed to maximize whatever the environment provides. Ideally, it would need only minimal corrections from an active HVAC system capable of independently controlling temperature and humidity. It's harder to do, from my very rudimentary building knowledge, without a greenfield design. But that's the dream. I have a lot of reading to do..

indexingfun
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Re: Energy Efficient Window Fan?

Post by indexingfun » Sat Nov 19, 2016 5:59 am

I don’t know much about energy efficiency in fans, but I would definitely look for ones with digital temperature controls that enable them to turn off when the temperature comes down to a selected point. Probably the biggest element of waste with fans is running them when they are no longer needed. I also find reversible direction and multiple speeds handy. You can have one fan pumping air in and the other out in the same or another window to foster circulation.

Something like this — at 9” high — might work. Given the poor circulation, a ceiling fan might also be useful.

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just frank
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Re: Energy Efficient Window Fan?

Post by just frank » Sat Nov 19, 2016 6:48 am

four things:

1. Like a whole house fan, the number of days a year that this can be of use, and the need for user intervention (opening the window) hurts the economic case. That is why there is no big high efficiency window fan market segment. Part of the issue is humidity....high indoor humidity contributes to a variety of allergy and health problems, and the careless use of these fans in the summer and fall can load the house with moisture (outside of very dry climates).

2. That said, I would aim for 'cross ventilation' powered by wind as much as possible, and aim for fans with the largest and slowest rotors possible. Those have the highest CFM/watt. CFM/W is rated for ceiling fans. Or look for an energy star bug. A big floor fan aligned with a window or open sliding door might move a lot more air for less W than a dinky plastic guy in the jamb. Remember than fans push much better than they pull. If the fan is not sealed around the edges it will push out a window (due to air momentum), but the same fan will have little ability to pull air into the room in the other direction.

3. Modern central ACs are very efficient when indoor and outdoor temps are close. A SEER 16 (current typical) unit will move >5 Watts of heat for every W used. If you keep the place closed up all day and want to cool it off after the sun goes down and it gets cooler outside, you are still saving by running your AC when it is cooler out, rather than during the hot part of the day. You can use a programmable stat to make this semi-automated. And the system will keep indoor humidity low.

4. I would try to engineer a secure screen door solution for the balcony, and put a big floor fan in front of it.

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czeckers
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Re: Energy Efficient Window Fan?

Post by czeckers » Sat Nov 19, 2016 7:53 am

As an apartment dweller, a window fan is certainly a more realistic option than a whole house fan.

Window fans are definitely more energy-efficient than window air conditioners.

A fan is a simple thing: an electric motor attached to a fan blade. There isn't much you can do to make one fan more efficient than another.

The amount of energy used will be directly proportional to the area swept by the fan blade(s) and to the speed you run the fan at -- the bigger the area and/or the higher the velocity, the more work is done by the motor(s) and thus more current draw. A bigger fan running slower will be a bit more efficient than a smaller fan running faster.



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leonard
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Re: Energy Efficient Window Fan?

Post by leonard » Sat Nov 19, 2016 9:41 am

briancof wrote:@leonard - I saw the thread and the fan in question. Thanks for suggesting, but it won't work with our oddball top-hinged windows.

re: humidity, I do not for a second doubt that it's central to comfort. I'm actually not sure that we have a way to independently control it, beyond running a dedicated (de)humidifier. And, if we're cycling outdoor air through the house, we'd just be fighting the ambient, right? Otherwise, with the house buttoned up, we wind up with whatever level of dehumidification the AC delivers. In any case, for the time being it is a secondary consideration to getting the house down from the mid 80s F daytime temperatures driven by our huge, south-facing glass walls. If we are complaining about humidity, I'll feel fortunate.

Aspirationally, I love the idea of building a near-"passive" house, aggressively insulated and smartly designed to maximize whatever the environment provides. Ideally, it would need only minimal corrections from an active HVAC system capable of independently controlling temperature and humidity. It's harder to do, from my very rudimentary building knowledge, without a greenfield design. But that's the dream. I have a lot of reading to do..
I'd suggest looking at average weather data for your location during the period you would be running the fan. If humidity in your area is high during that time, you might get moisture problems in the house that you may not have had otherwise. If humidity is very low, you might feel the effects of dryness - with dry skin (a problem I have in low humidity), and a few others. One can usually find a chart of average temps and humidity for their area - to it's at least worth verifying you won't be creating humidity problems with the fan.

Also, I really wouldn't call it a "secondary consideration". Moisture levels at extremes can cause problems.

One other thing - whatever fan you go with - I'd make sure to get a fan that moves more CFM than you think you need. I got what I thought was an oversized WHF and we have ended up running it on it's highest setting a few times this year. In hind sight, I probably should have gone with the size up, though larger sizes had other issues.
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Re: Energy Efficient Window Fan?

Post by Valuethinker » Sun Nov 20, 2016 8:25 am

briancof wrote:Thank you to all taking the time to respond to my question! I think I'm going to roll the dice on a "USB fan" plugged into one of those AC-to-USB power supplies one gets with (e.g.) smartphones, such as:

https://www.amazon.com/gp/product/B01DZ ... bw_c_x_1_w

BHUser27's suggestion of solar got me thinking about a fan designed to run off a DC source, which usually means batteries or solar, both of which usually mean they're designed to be parsimonious with power. Although solar would marry nicely with our sun-caused heatload, I'm not ready to forego the opportunity to operate without it quite yet. But it searching I did come across those USB fans that seem to be sippers. Anything running on five volts has to be lightweight. What's harder to figure out, with all of these, is what the resulting airflow is. But to bloom2708's point - not worry about the rest since they're all likely to come out ahead relative to AC. I can always order more.

DSInvestor's Bionaire looks nice but will not, unfortunately, fit the kind of top-hinged windows in my unit.

@mrc - what is Amazon, but one big arbitrage invitation?
Can a USB port really provide enough juice for a fan?

A good AC fan, I really wouldn't worry about it too much. If Energy Star has a rating system for fans, by all means go with one.

Overall we are talking tiny amounts of electricity. What is most important is that your fan fits the opening, and has enough "suck" ie flow through.

briancof
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Re: Energy Efficient Window Fan?

Post by briancof » Mon Nov 21, 2016 1:05 am

@leonard @just frank - I am guilty of having spoken glibly about humidty, and very much hear your warnings about its affect on indoor comfort and, at high enough levels, effects on health, mold, and the interior of a building. Having lived few years in Miami, Florida, I would be near the front of the line to testify. During the hot (by Chicago standards) months, we are absolutely operating air conditioning, which feels to be doing a plenty good enough job by the sniff test (I do not measure it). My question was really to the time of year (maybe starting early October) when the outdoor air feels much more comfortable, when we stop cringing that the though of al fresco dining. The dewpoint feels to have dropped enough to make humidity a second priority to exhausting some of the mid/high-80s air created by the daytime solar gain from the greenhouse windows in exchange for the 50s/60s outdoor ambient.

@just frank - Good advice on the pushing vs. pulling. I suppose if we're trying to "suck" in air from the outside , we wind up "churning" more of whatever air is inside the room rather than getting a clean throughput. Working against the outward-facing idea is that the windows are pretty obstructive to flow, so even a hard shove by the fan might not leave the space as effectively as it would through an open aperture. We will have to do some experimenting - maybe put inward-facing fans in our bedroom at night (where we most want the cool air) and some out-facing fans at the other end of the house to create some net movement.

@just frank - I hear you on AC efficiency, but suspect that even a perfect "Carnot" unit is going to have a hard time catching a well-placed fan that can "create" 60 F air from outside rather than having to create it from the 85 F air indoors.

@czeckers - I might dispute the idea that you can't make a fan better. Ignoring anything about the blades and placement, there are some *really* terrible motors (e.g., shaded-pole) that get used in small, cheap fans. Very good point about the large/slow vs small/fast tradeoff.

@Valuethinker - A USB fan, no doubt, will be quite limited. But since we are probably limited to small fans anyway, I'd though they may be among the more efficient of the bunch. We will have to see what the "feel test" tells us. I am ready to eat some humble pie. I do not believe Energy Star rates window fans, although (in my personal opinion) they ought to consider it. Unrelated, but if your post count is not a typo then my hat is tipped. >8 per day, average, for nearly a decade!

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just frank
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Re: Energy Efficient Window Fan?

Post by just frank » Mon Nov 21, 2016 6:25 am

briancof wrote: @just frank - I hear you on AC efficiency, but suspect that even a perfect "Carnot" unit is going to have a hard time catching a well-placed fan that can "create" 60 F air from outside rather than having to create it from the 85 F air indoors.
Having pushed the numbers years ago, I think that unless the air outside is more than 5-10 degree cooler than the inside, the fan can actually be less efficient.

Wind however, is free. :beer

EDIT: Looks like table fans are 20 cfm/W, figure 10 CFM/W blowing through a window/screen, perhaps a bit optimistic (blowers that can handle back-pressure are usually more like 5 CFM/W).

10 CFM/W and 1.08 for air specific heat, if it is 5°F cooler outside, I am getting that 1 W of fan power moves 15W of heat, so it does beat an AC, which moves closer to 5W of heat for every W of power put in.

That said, if your fan is poor, with a lot of 'churning' or blowing against the wind, or my 10 CFM/W is too optimistic, then it might be a wash for temps less than 5°F cooler outside, relative to running the AC

Wind is still free.

fan ratings: http://www.geek.com/gadgets/best-desk-f ... l-1589462/
Last edited by just frank on Mon Nov 21, 2016 10:12 am, edited 2 times in total.

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jimb_fromATL
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Re: Energy Efficient Window Fan?

Post by jimb_fromATL » Mon Nov 21, 2016 6:52 am

... But it searching I did come across those USB fans that seem to be sippers. Anything running on five volts has to be lightweight. What's harder to figure out, with all of these, is what the resulting airflow is. But to bloom2708's point - not worry about the rest since they're all likely to come out ahead relative to AC. I can always order more.
It does appear that there are no rating for efficiency per CFM or other measure for window fans, but IMO you're over-thinking the energy efficiency. No more power than they use compared to running the AC, I doubt it would make much difference how efficient the motor or fan blade design is.

What IS important is how much air you can move, and I doubt that the USB fan will accomplish what you say you want. That is a "desk-top" fan intended to cool you at your desk or chair by blowing a little air over you to improve your own body's cooling by evaporation. It does not move enough air to significantly affect the cool air pulled in or hot air pushed out of the room.

If you can't put one of those low-profile double or triple-blade window fans in the window opening, I suspect you'd accomplish a lot more than the USB fan just by putting a cheap box fan on the floor near the window. Or maybe make a wood frame to hold one of the low-profile double or triple blade window fans upright.

There are CFM (Cubic Ft per Minute) ratings for some box fans and window fans. For example a lot of the cheap $15-$20 box fans move about 1400 CFM. I found discussion about a couple of twin-blade window fans rated at 1400 CFM, too.

Here are some thoughts about how you might be able to compare that USB fan to the bigger ones:
  • I'd guess that the "up to 3.7 m/s" spec for the USB fan means that the air moves at 3.7 meters per second on its highest speed.

    If the blade itself is 6 inches in diameter, the area of air displace by the fan blade would be Pi R squared.
    3.14 x (3 ^2) = 28 .3 sq. inches.

    (Never mind that everybody knows that pie are round)

    Since a square foot is 12 x 12 = 144 inches, that is about 28.27 / 144 = 0.196 square feet. Making a wild guess that about 75% of the area of the fan is actual blade surface, that would mean about 0.147 square feet of air being moved.

    If the air is moving 3.7 meters per second that is about 12.13 feet per second x 60 = 727.8 feet per minute.
    Moving 0.147 sq ft of air x 727.8 feet is 107.2 CFM.
That suggest that a cheap box fan or a twin blade window fan moves about 1400/107.18 = 13.1 times more air than the USB fan.

To double-check the numbers, let's suppose that a 20" fan moved air at the same rate of 3.7 m/s on a medium speed.
  • With a 20 inch diameter, the area of air would be 3.14 x (10.0^2) = 314 .2 sq. inches.
    That is about 314.16 / 144 = 2.182 square feet.
    Making a wild guess that about 75% of the area of the fan is actual blade surface, that would mean about 1.636 square feet.
    If the air is moving 3.7 meters per second that is about 12.13 feet per second.
    Moving 1.636 sq ft of air 727.8 feet is 1191 CFM.
    That's not far off the ratings of 1400 CFM on a higher speed for a 20" fan.
jimb

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Re: Energy Efficient Window Fan?

Post by briancof » Mon Nov 21, 2016 11:43 pm

@ just frank - I am with you every step of the way here. We are in the probably-uncommon case of a 30 F rise above ambient (e.g., 55 F outside and 85 F inside) and so it is hard to imagine a fan of nearly any type, replacing inside air with outside, would beat an AC unit handily. But I do hear your point that the conditions matter. I'm reminded of learning how, above a certain vehicle speed, the aerodynamic penalty associated with opening windows exceeds the energy required to AC the cabin. The "crossover" depends on vehicle shape, of course, but my memory was that it's at a lower speed than folks imagine.

@ jimb_fromATL - I am definitely overthinking this relative to any benefit it could possibly deliver, and at this point has to be filed under "recreational nerding" in frugality's clothing. You (and others) were spot on about the desk fans. I bought two of them and they're feeble at best.

You are spot on with the (impressively detailed!) airflow calculations. Your estimate that 1 box fan equals 13.1 desk fans feels very reasonable. If there is anything good that can come from this thread, its that we have both theory and evidence that substituting desk for window fans is a fools errand.

I do have to quibble with your claim that it doesn't make a different how efficient a fan motor is. This topic is well studied. Here is, e.g., a report about supermarket display cases citing loss reduction of 70% by replacing shaded-pole motors (inefficient) with ECMs (efficient). https://goo.gl/fyJWt3 Further, my understanding is that the difference grows at lower speed settings.

It's one data point and I won't pass it off as any kind of scientific conclusion, but must take away that all fan motors are created equal. I suspect that window fan manufacturers do not use "good" motors because few consumers would understand (and be willing to pay for) the value proposition of reduced electrical consumption. It's all about low first cost.

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jimb_fromATL
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Re: Energy Efficient Window Fan?

Post by jimb_fromATL » Tue Nov 22, 2016 7:57 pm

briancof wrote: @ jimb_fromATL - I am definitely overthinking this relative to any benefit it could possibly deliver, and at this point has to be filed under "recreational nerding" in frugality's clothing. You (and others) were spot on about the desk fans. I bought two of them and they're feeble at best.

You are spot on with the (impressively detailed!) airflow calculations. Your estimate that 1 box fan equals 13.1 desk fans feels very reasonable. If there is anything good that can come from this thread, its that we have both theory and evidence that substituting desk for window fans is a fools errand.

I do have to quibble with your claim that it doesn't make a different how efficient a fan motor is. This topic is well studied. Here is, e.g., a report about supermarket display cases citing loss reduction of 70% by replacing shaded-pole motors (inefficient) with ECMs (efficient). https://goo.gl/fyJWt3 Further, my understanding is that the difference grows at lower speed settings.

It's one data point and I won't pass it off as any kind of scientific conclusion, but must take away that all fan motors are created equal. I suspect that window fan manufacturers do not use "good" motors because few consumers would understand (and be willing to pay for) the value proposition of reduced electrical consumption. It's all about low first cost.
I suspect it's not just the high first cost, but overall cost-effectiveness for their purpose. It appears that ECM motors can save money for commercial applications like 24-hour per day freezers and coolers, and even at home in furnace blowers for whole house HVAC and maybe for the condenser fans in the outside AC condensers (or outside unit of heat pumps).

But I doubt that you can even find a box fan for home use that has an ECM motor. You can buy the common and very popular 20" box fans for $15-$20. The cheapest ECM motor that looks like it might work that I could find was around $150 -- compared to about $6 for a replacement shaded pole motor for a box fan.

After adding a DC speed controller, I'd guess that an ECM box fan -- if there were such an animal-- would probably cost about $200, which is about $180 more than a conventional home box fan.

Just to keep the old brain from fossilizing any faster than it already is, I did a little more research on the subject.

I found a site showing the energy consumption of a popular 20" box fan to be in the range of 100 watts on high, 77 on medium, and 56 watts on low speed.

If you use it 8 hours per day, that's 0.80 kwh. At the national average of about $0.10 per kwh for electricity, that's $0.080 per day; $2.40 if you use it for 30 days in a month; and $12.00 to use it 8 hours per day for 5 months per year.

If a DCM fan saved 70% for electricity, that could easily save $8.40 per year. So the savings in electricity would recoup the extra $180 cost in about 180/8.40 = 21.4 years.

Heck, you could probably get your money back in less than a couple of decades if electric rates go up; and IF you live in the same place that long and IF the fan lasts that long. Maybe even less than 10 to 15 years if you live someplace where electricity is really expensive.

jimb
Last edited by jimb_fromATL on Wed Nov 23, 2016 4:17 am, edited 1 time in total.

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Epsilon Delta
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Re: Energy Efficient Window Fan?

Post by Epsilon Delta » Tue Nov 22, 2016 8:44 pm

For efficiency controlling the airflow is probably more important than the efficiency of the fan motor. The ideal is cross ventilation where cool air comes in one window and warm air out another on the other side of the room or house. The fan should fill one of the windows so air can't just cycle round it. This is easy with sliding double hung windows, they make fans with wings that will completely fill the opening. It's harder with hinged, casemate windows. You could DIY it with a sheet of plywood or plexiglass that fills the window opening and has a hole for the fan in it.

Disclaimer I have not modeled the airflow in a house, but I have done it with industrial equipment and we could use a much smaller fan if we ducted the airflow to where the cooling was required rather than just blowing it into the box and hoping it did the job.

briancof
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Joined: Tue Sep 13, 2016 11:20 pm

Re: Energy Efficient Window Fan?

Post by briancof » Mon Dec 05, 2016 3:24 pm

Jumping back in here after some holiday distractions!

@jimb_FromATL

Thank you again for the detailed and thoughtful reasoning on this. Without disputing your conclusion (which I don't: that more efficient fan motors are unlikely to payback in any reasonable time), I'd be surprised to see a $200 price differential for ECMs in the wild. I don't know what your source was, but would guess Grainger-type online retailer willing to sell singles. That price is probably a considerable overestimate (perhaps even an order of magnitude) of what a fan manufacturer would be paying an ECM motor maker for parts at production scale. I don't have hard evidence to back up this statement, but view it as suggestive that the technology is standing on its own legs (as you noted) in other applications, albeit high duty ones.

Those are probably higher-wattage applications. However, the "cheap" motors will get relatively more efficient as power grows (and OEMs start to substitute shaded pole motors for permanent split capacitor and, eventually, polyphase squirrel cage), and so it's not clear to me prima facie that lower wattage applications (of similar duty cycle) would not produce an even greater advantage for ECMs.

Somewhat relatedly, I have to think that the variable-speed capability would almost certainly be built into the motor at production scale rather than added, posthoc, as a purchased part.

The failure of such a product (i.e., efficient box-type fan) to appear on the market could be because: (1) that it would never pay back, even at scale, (2) that consumers would not recognize the value of such a product, or (3) some of both.


@Epsilon Delta

I suspect you're right on with this. Someone earlier in the chain started to get at it, too - "crossventilating" (I don't know if this is the proper term but it's commonly enough used) as much as possible and, even better, taking advantage of the pressure gradient created by the prevailing wind. If there were enough of that, one could even ditch fans entirely. In our case, all windows are on one side of the building, so we probably will not see much of this would jury-rigging some kind of exterior wind "scoops."

We do have the problem you mention - that to "duct" or "seal" a window fan with our hinged windows would take some amount of plywooding/plexiglassing that would not thrill my wife.

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