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Community and Q&A

Sizing a whole-house fan for a larger house?

whitenack | Posted in General Questions on

Hi All,

Considering a whole house fan for our soon-to-be-built “Pretty Good House” in Central KY, zone 4A.

The house is on the larger size, and the calculations for the type of WHF I need are off the charts in terms of what is available in the marketplace (I assume), what I have attic venting for, etc.

So, keeping in mind that the fans in the marketplace are about, say, one third or one half the size it needs to be sized correctly, is there still a benefit to including one in our build?

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  1. charlie_sullivan | | #1

    On the one hand, a pretty good house won't heat up as much as a standard house during the day, so you won't need as much air to cool it at night, so you are fine with a smaller fan than the guidelines say.

    On the other hand, once you have a well insulated house, the heat removal that the A/C has to do is reduced, but the humidity removal job is not reduced by the insulation. So the work that your A/C is doing is shifted to being primarily dehumidification. It looks like in your climate, the summer night-time moisture content of the air is pretty high, so bringing in that air overnight might actually increase the work the A/C does the next day, assuming you are running A/C. There are some shoulder season times, such as late May and early June, where you can benefit from the WHF without these problems, if that's your objective. Or maybe your tolerance for heat and humidity is higher than mine and you'll be fine using it as your primary cooling mechanism further into the summer.

  2. whitenack | | #2

    Thanks for the reply. Yes, humidity is a real problem around here. Thanks for the reminder about humidity removal...all the more reason to make sure the HVAC contractor doesn't grossly oversize the heat pump.

    I don't think my wife is going to let me operate the WHF during the humid summer months (and in a way I can't blame her), so it would be for the shoulder seasons when the night and early morning temps drop below the interior temps.

  3. Expert Member
    Dana Dorsett | | #3

    Since there is no standard for what comprises "...on the larger size..." it's hard to even take a WAG at how big a whole house fan might be appropriate.

    Active attic venting (rather than whole house venting) is almost never a good idea, since in almost all instances, at code-min attic R it would use more power than any cooling power it might offset.

    Any time the outdoor dew point is under 55F using a nighttime ventilation strategy will not impose a significant humidity load on the house. (A dew point of 55F corresponds to a relative humidity of 50% @ 75F indoors.) Whether that happens often enough in your location during the shoulder seasons for a whole house fan to be "worth it" is a judgment call. If the cooling load has been pretty much designed out of the house, during the lower loads of the shoulder seasons it might only take opening up some windows rather than a fan to gain the nighttime ventilation cooling benefit.

    If the morning outdoor temp is 65F and the outdoor dew point is 60F ventilating with outdoor air would begin to add a real latent load, resulting in 60% RH @ 75F, which is below the serious mold threshold, but on the high side for those with dust mite allergies.

    When outdoor dew points are much higher than 60F it runs pretty quickly into household mold/mildew and human skin-fungus territory at mid-70s room temps.

  4. whitenack | | #4

    Thanks for the reply Dana. I saw a sizing calculation that said you take the total cubic feet of the home, multiply by 30, then divide by 60, which gets you the recommended CFM. If that's the case, I need 15,000 CFM. If that is truly the needed CFM, will a typical 4600-6000 CFM fan offer any noticeable cooling, or would I just be wasting energy running the fan motor?

    Your humidity vs dewpoint info, while over my head from a details standpoint, makes more sense when I looked up the average humidity/temp/dewpoint for our area. Average dewpoint starts to creep above 55 about June, which coincides when it starts to feel uncomfortably humid around here. Then, about mid-Sept., the average dewpoint drops back down below 55 and stays that way for the rest of the year.

    So yeah, we wouldn't be using the fan to replace the use of AC during these hot/humid months. The windows would stay closed day and night. The fan would be used for the shoulder seasons when it gets warm during the day but the nights are still cool. However, I am used to poorly/normally insulated houses that heat up quickly in the spring sun and need the cool nights to flush the heat out. Would a "pretty good house" not be subjected to these swings? Or would they be even better suited for a fan because they wouldn't heat up as much during the day and you can bring in the cool air in the morning and then close things up and not gain much heat during the heat of the day?

  5. GBA Editor
    Martin Holladay | | #5

    You probably want to read this article on whole-house fans: Fans in the Attic.

    It sounds like your house measures 30,000 cubic feet. The most common sizing guideline for whole-house fans recommends that you choose a fan with a cfm rating that is 1/4 of your home's cubic feet.

    So you take your cubic feet and divide by 4:
    30,000 / 4 = 7,500 cfm.

    So you don't need at 15,000 cfm fan, even using conventional rules of thumb.

    You don't have to follow conventional rules of thumb. You might want to install two Tararack fans, each rated at 1,600 cfm, for a total of 3,200 cfm. That won't hit the rule-of thumb sizing, but it will work.

    Here's what I wrote in the article: "Tamarack fans have lower cfm ratings than most other whole-house fans, but the low power ratings confer certain advantages. The fans are quieter; they use less electricity; and they are smaller than other fans, and therefore easier to air-seal and insulate when not in use. Moreover, since a Tamarack fan blows a smaller volume of air than the typical whole-house fan, it usually doesn’t require any extra vents in your attic."

  6. Expert Member
    Dana Dorsett | | #6

    Q: I am used to poorly/normally insulated houses that heat up quickly in the spring sun and need the cool nights to flush the heat out. Would a "pretty good house" not be subjected to these swings?

    A: A "pretty good house" can still heat up quickly and see large temperature swings if there is a lot of unshaded window area. Using north facing windows for daylighting rather than east or west facing windows makes a real difference in how much it heats up, as well as the peak. South facing windows offer useful gains during the heating season, but designing roof overhangs to keep them out of direct sun during the days in summer can take the edge off it, as will exterior operable shades.

    Peak cooling loads can nearly double if you have large "sunset view" west facing windows, since the solar gains of west windows occur when the outdoor temps, roofing temps and siding temps are already elevated. Limiting the total area of the west facing glass to a minimum takes a big chunk out of the peak.

    Even north facing windows are net heat gainers during the daylight hours, but the temperature rise in the house is still much smaller than the gains attributable to other windows, even the south facing windows shaded with properly designed overhangs/awnings. The amount of window area it takes for reasonable daylighting isn't huge.

    Solar gains through the attic are pretty low in a pretty good house. Even at the IRC 2012 & later R49 code minimum it makes a BIG difference in peak ceiling temperatures if you have been accustomed to R19 or lower attic insulation. (And if you "shade" the south facing roof pitches with solar panels it can also make a measurable difference, though not so big that you would notice it much with an R49 attic. :-) )

    Is this an exising house, or currently under design? Site factors make a difference too:

    I live in zone 5A, and one of the additions has a lot of west facing clear-glass (= very high SHGC) double panes. Without the wooded hill shading the west side from about 2:30PM and later it would be an afternoon oven. The east side windows face a tree lined street with 2 story houses on the other side of the street, and the south facing windows are shaded by 2 foot overhangs. With the benefit of only limited filtered sun on those large west windows I can pretty much cool the whole house with a single half-ton window-shaker upstairs until outdoor temps hit the high 80s or low 90s, and my attic/roof insulation is nowhere near R49. (The 1% outside design temp in my area is 83F using the 25 year average, but some years the 1% bin would be high 80s.) Without the shade factors on the east and west sides my cooling loads would easily triple or even quadruple. When the dew points are low (as has happened more frequently this May/June than most years) I'll open an upstairs window and a couple of downstairs window when the outdoor temps are a few degrees lower than the interior, and it stays in the 70sF indoors even without air conditioning. When outdoor dew points are high I usually run the (ridiculously oversized 5 ton) central AC to bring the humidity down quickly if it gets too sticky indoors, or fire up the half-ton upstairs to keep the indoor humidity reasonably bounded rather than ventilating with humid air at night for sensible cooling. YMMV.

  7. charlie_sullivan | | #7

    I'm glad to hear you are realistic about when it can be useful. If you start with Martin's sizing rule (1/4), and then discount it for the house being pretty good, and then discount again for shoulder seasons, you might end up with a 1/10 sizing rule, or 3000 CFM. Which is the same recommendation Martin ends up with.

    But I'm not convinced that Tamarack is still the best available option. There are other companies, including AirScape, Invisco, and Quietcool, who offer fans with ECM motors. It might be that a few of the Tamarack fans use them too, but it looks like the HV-1600 does not. ECM motors start out more efficient than single-phase induction motors (used on most fans), but where they really shine is that their energy use drops rapidly when they are run at lower speed, whereas with typical induction motor setups, the energy use barely drops at all at low speed. If you aren't in a hurry to cool the house down in the evening (because your insulation prevented it from getting terribly hot during the day), and you can run the fan on a low speed that can result in substantial energy savings.

    I don't know enough about the specific companies and products to make a definite recommendation, but the energy consumption specs on some of them look great--for example, this one can do 1300 CFM consuming only 40 W, whereas the Tamarack HV-1600 consumes 207 W at its low speed at which it's rated 1150 CFM. There are bigger ones that do even better at similar airflow rates.

  8. GBA Editor
    Martin Holladay | | #8

    I like the Tamarack fans because they have motorized insulated shutters. It looks like the Airscape fans do too (top photo below), but the Quietcool fans appear to lack the motorized insulated shutters (lower photo below). I don't know about the Invisco fans.


  9. charlie_sullivan | | #9

    The 62 F temperature in my living room this morning reminds me that sometimes just opening windows is more than adequate for shoulder season night cooling, without needing a fan at all.

  10. whitenack | | #10

    Thanks all, for the replies.

    Martin, I had read that article before, but thanks for the link again. It's always good to read and re-read for things you missed the first few times. Yes, the house is roughly 30k cubic feet. Thanks for the more accurate CFM calculations. I couldn't find any discussions about CFM = 1/4 cubic feet, other than now when I re-read your article :). That is a more attainable result than what I had found.

    Dana, the house is currently under construction, so the position of the house is already set in concrete. It is sited mostly south/southeast, but a little more East than I would have wished for. I wanted to sight it directly due south, but my wife vetoed that idea since it would have been cockeyed from the road. And yes, it has a lot of north/northwest windows. When it was being framed this winter, I didn't think it was going to get too much late afternoon sun, but now that the days are longer and the sun travels further west, I see that it gets more than I had hoped. I invested in some low SHGC windows, so I'm hoping they pay off, plus I'm going to be planting some shade trees to help in the future.

    Charlie, sometimes the temperatures don't even have to be that cool! I was doing some work at the house last night. I was working down in the basement, and things were very humid. I took a break and came upstairs to check the angle of the sunset into the living room, and opened a sliding glass door to step out onto the soon-to-be back porch. All the windows were shut, but the front door hasn't been installed, and when I opened the sliding door, the cross breeze was amazing. The house sits up on a bit of a hill, and it seems to get regular breezes. The temps were probably still in the lower 80's, but it felt great. A lot of that had to do with me being soaking wet, though.

    I guess the big question is whether 1500-3000 CFM would be effective/worth the trouble. I'd have equipment and installation costs, a big hole to air seal/insulate, making sure the attic can exhaust the air, etc. And to do all that just for a fan that only manages 6 ach (if my math is right), I'm not sure if it is worth it. For example, I see that a simple box fan is rated for 2500 CFM. Maybe I'm wrong, but I wouldn't think setting a simple box fan to blow air out of an open window would make much difference. If I go to the trouble, I think I need to go with a fan that can get the job done.

  11. GBA Editor
    Martin Holladay | | #11

    New homes that include air conditioning rarely have a whole-house fan. In most cases, opening windows during the swing seasons works for nighttime cooling with your type of house.

    The reason that whole-house fans are fading away is because of the disadvantages associated with having a big hole in your insulated ceiling.

  12. user-6258531 | | #12

    One way to see if it is worth doing would be to purchase a more powerful window fan like the Air King 9155. The specs for it say it has a capacity of 2470 cfm on high. I use it our 60's ranch in Vancouver, WA. It does a decent job of cooling the house down at night and we rarely ever run our air conditioning unless outside temps hit 90. It is pretty loud on high so you would want to have a window somewhere other than your bedroom to mount it. I have used box fans for this purpose and the Air King moves a lot more air. I believe it was around $75. There is a larger model that pulls even more.
    If doesn't work, you saved cutting a hole in your ceiling.

  13. whitenack | | #13

    Thanks Steve. I'm still under construction, so if I'm going with a whole-house fan, now is the time to decide. I think I have decided to pass on the fan. I'm hoping that I have so much insulation that I won't need much cooling during the shoulder seasons, and our humidity levels during peak summer months are so high that we wouldn't get the benefit at night.

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