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Community and Q&A

Whole-house fans: Good, bad, safe?

W Ramsay | Posted in General Questions on

We’d planned to install a couple of Tamarack HV5400 whole house fans in our new build in St Paul MN. The purpose is for both improved IAQ and more energy efficient cooling when the weather is appropriate (<80°f / < 60%RH).

However, our local building inspector is giving us all kinds of grief. He was originally requiring a 1:1 make up air system which would not only be expensive but also defeat the purpose. He’d backed off that and said if we could insure that enough windows were open he’d approve it. We did that and now he’s saying that he wants an engineer to certify that our plan would provide enough passive make up air. This is getting to be a major pain.

Two questions:

1) Is a whole house fan beneficial? To me this is a very easy yes. On some days we can get enough air through our house with natural breeze but many days there is either not enough breeze or it is coming from the wrong direction and so a WHF creates the breeze and with windows open provides fresh air, reduces or eliminates VOC’s & CO2, and cools the house for a lot let energy and $’s than running the AC. Am I nuts? Is a whole house fan as beneficial as I believe? If so, why are they not more popular?

2) Are they safe? Can they be made safe. I’ve only been able to find 1 death attributed to a WHF and that was some high school kids who’d been smoking pot for a considerable time and turned it on, without opening windows, to get rid of the pot smell. Otherwise I’ve not been able to find any deaths or injuries. It seem the tens of thousands of people who have them are able to open windows just fine. Is anyone aware of any research paper, study or similar that talks about how to make them safe (without installing a 12000 CFM MUA)?

Thanks,

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Replies

  1. Zephyr7 | | #1

    I love whole house fans. They work great to quickly cool a house off an night, they work great to bring in fresh, clean air, and they’re cheap to operate. In northern areas, whole house fans can often replace air conditioning during the spring and fall.

    I see no dangers operating the fans as long as you are reasonable (don’t stick your hand into the fan, etc). I’m not aware of any safety concerns aside from the obvious physical ones from the spinning fan blades, but those are out of reach and shouldn’t be a problem.

    For your inspector, if you can show an open aperature or window that is at least as big as the area of the fans, I can’t see how you’d have any problems with negative pressure inside the house. Even at work where I design generator rooms for multiple multi-thousand horsepower megawatt size diesel generators, I just size the ventilation openings for 50% larger area than the radiator fans and I’ve never had a problem with air flow or restriction.

    It sounds like your inspector is going a bit overboard with your fan project. It wouldn’t be difficult to put alarm-type sensors on your windows and build a “lock out fan unless at least x number of windows are open” controller. You wouldn’t even need any fancy microprocessor-based system to do that. I can’t see why your inspector would be trying to require anything more complicated.

    Bill

  2. User avatar
    Dana Dorsett | | #2

    DO tell us the details of the fatalities attributed to a whole house fan! (Seems pretty unlikely to me.)

    The inspector doesn't seem to grasp the difference between exhaust ventilation fan for removing indoor air pollutants and a whole house fan designed for purging heat from the house.

    The efficiency of using nighttime ventilation with a whole house fan depends entirely on how it's used. During higher humidity days it can potentially bring in more latent cooling load than is "worth it", but most people can figure out how to manage that by feel. The lack of latent cooling and lower sensible loads in modern tight, insulated homes is probably why they are less popular now than they were during the 1920s through 1950s.

    Nightime ventilation schemes can be VERY efficient in drier climates like the west coast states, but just make it clammy indoors in much of the eastern states, delivering a higher indoor relative humidity than using a modest amount of air conditioning. With the higher latent loads of using whole house fans in higher humidity climates it's not clear whether a whole house fan would always use less overall power than 20+ SEER heat pump/ AC (such as a cold-climate mini-split), which would always have at least some latent cooling going on.

  3. Jon R | | #3

    Absolutely do not base needed intake area on the size of the fan. It's about CFM (in equals out) and how much depressurization you can tolerate. Some type of safety alarm would be a good idea.

    Consider installing the opening and wiring but leaving the actual fan install for later.

  4. Timothy Tucker | | #4

    We had one when we moved into our house here in SE Michigan, but removed it after being in the house for a year. House was build in the mid 90's and the fan was of the same vintage -- not at all insulated like the Tamarack models.

    Overall, we found it much more effective at creating an unwanted stack effect (losing warm air in the winter and conditioned air in the summer) than it was at pulling in outside air to effectively cool the house.

    I'd wonder if spending an equivalent amount on solar to what you're thinking about spending on fans would give you an overall lower electric bill.

  5. User avatar GBA Editor
    Peter Yost | | #5

    Your building inspector needs to focus on building performance issues that warrant an engineer, and this is not one of them.

    I have found the folks at Tamarack Technologies to be really helpful; I would give them a call and i would bet they can provide you with whatever guidance or "proof" needed as to conditions required to safely and effectively operate their whole house fans.

    Peter

  6. W Ramsay | | #6

    Thanks all.

    The danger he is concerned about is depressurizing the house. Hopefully he won't read this and decide that fan blades are a threat as well :-)

    The system we proposed provided enough open window for 6 CFM / sq inch (E.G., an 11" duct for each 600 CFM which seems a somewhat accepted standard for passive MUA). And he had even mentioned the 11"/600CFM once so we were surprised he didn't allow this one. He seems to want some kind of official document saying that whatever we're doing is sound from an engineering standpoint.

    Based on recommendations from another thread we proposed using a differential pressure switch which would actually be a much better solution but that was too complicated for him and getting any kind of engineering letter would likely be much more difficult than the open windows route.

    We also tried to explain to him that there are no engineering studies on this because they are not a problem - tens of thousands are installed every year and they aren't a problem and aren't viewed as a problem by other building inspectors.

    Agree & agree, Nelson and the rest of the folks at Tamarack have been excellent.

    1. Zephyr7 | | #12

      Maybe the tamarack people can send a letter that would satisfy your inspector? It wouldn’t be an engineers stamp, but it would be documentation from the manufacturer. Might be worth a shot.

      BTW, axial fans like are used in whole house fans aren’t really capable of generating significant negative pressure. It really takes a centrifugal blower or positive displacement pump to generate levels of negative pressure that can damage things. If your inspector is worried about negative pressure causing structural damage, it just isn’t possible here. If his concern is backdrafting in combustion appliances I suppose that is a valid concern, but also easy to address.

      Bill

      1. W Ramsay | | #13

        Thanks Bill, good point about axial fans vs blowers. In this case his concern isn't structural but backdrafting. Of course he's concerned about backdrafting sealed combustion appliances.

        1. Zephyr7 | | #20

          It shouldn’t be possible for your whole house fan to cause backdrafting for any sealed combustion appliance. “Sealed combustion” means the combustion air is separated from the air within the structure — such appliances have both exhaust AND supply air vent lines to the outdoors. They are immune to back drafting as a result (at least backdrafting related to negative indoor air pressures).

          Bill

          1. W Ramsay | | #21

            Yep, we've talked with him about that. This is a code issue, not a physics issue.

            And physics does whatever code and code inspectors say it will do d*** it.

        2. User avatar
          Dana Dorsett | | #22

          >"Of course he's concerned about backdrafting sealed combustion appliances."
          --------------
          >"We’d planned to install a couple of Tamarack HV5400 whole house fans in our new build in St Paul MN."

          On a NEW build you have the option of not installing ANY combustion appliances of any sort, (sealed combustion or naturally drafted.) There are heat pump solutions to heating that work just fine in a St.Paul, MN climate, without a huge premium in operating cost over natural gas, and usually cheaper than propane or #2 oil.

          Spending the foregone gas hookup charges on rooftop PV will often more than pay for any difference in operating cost. (I have no particular insights on your local market though- you'll have to do your own financial math on that.)

  7. W Ramsay | | #7

    I think one of the things I'm beginning to debate is if a WHF is important enough to keep fighting. I think it is but the fact that they are not more popular, at least among folk interested in energy efficiency and good IAQ, makes me wonder.

    Perhaps WHF's are not as beneficial as I've believed?

    1. Aedi | | #8

      I think the reason is because they have a rather narrow use case of a low-humidity temperate climate with moderate to large diurnal temperature changes in the shoulder seasons, making them useful in only some parts of the US and Canada. In addition, whole house fans require active involvement from the occupant in order to work correctly, so they are rarely installed in production homes. Finally, they only make sense in relatively large homes, as small homes can get sufficient air changes just by opening windows. These factor combine to make them relatively rare.

      In your case, a whole house fan makes perfect sense, and should be expected to save you a fair amount of energy.

    2. Jon R | | #9

      I find them OK if you really want to avoid AC use. Otherwise, it's far less hassle to just set the AC and not bother with frequently opening/closing windows as the weather (temp, humidity, rain) changes.

  8. Timothy Tucker | | #10

    Here are some more of my thoughts:

    IAQ
    - If you already have another system to circulate & filter air for either ventilation or heating / central AC, why introduce the complexity of adding another?
    - In colder climates, a WHF is only practical to run during warmer weather. If you're really concerned about IAQ, why wouldn't you focus on solutions that work year round?
    - If you're concerned about cooking smells, a vent hood over the stove is probably more effective.
    - Other temporary sources of indoor air pollution can also be addressed with lower power exhaust fans set to run when those areas are used (i.e.: bathrooms / attached garage).
    - You can cut down on VOCs through careful selection of building materials & furniture. MDF and other similar materials offgas VOCs at a sharply decreasing rate over time -- buying used can be a sustainable low-VOC alternative to all wood furniture & cabinets.
    - Replacing indoor air with outdoor air only improves IAQ if the outdoor air is cleaner than the indoor air. If you live in an area with industrial pollution or high pollen counts, the recirculated air from a central AC system may be cleaner than what you're pulling in through your windows.

    Comparison to ceiling fans:
    - In moderate temperatures, it feels just as cool (if not cooler) for us to run a ceiling fan in the room we're in vs running a whole house fan.
    - With the ceiling fan you can adjust to have the fans blowing on you or pulling air past you vs. whole house fans will only pull air past you.
    - Both a $1500 Tamarack unit and a $100 52" ceiling fan claim to move 2000 CFM, but the ceiling fan does it at 40W and the Tamarack does it at 300W.
    - We can turn the setpoint for the AC up a little and then run ceiling fans as needed without losing any of the conditioned air / pulling in more humid air.
    - Only 1 step is required to operate a ceiling fan (turn on) vs. 2 steps are always required with the whole house fan (open windows & turn on). We still have the option to open windows with a ceiling fan if we want.
    - I can install 10-15 decent quality ceiling fans for the price of 1 of the higher end Tamarack units.
    - Nothing special is needed to install them as long as you ensure that all your ceiling boxes for lighting are rated to support fans when building.

    Efficiency:
    - Central AC plus $1500 in solar will likely have an overall lower cost to operate than Central AC + WHF.
    - $1500 put towards additional insulation, air sealing, or better windows might give you more of a year-round payback in comfort.
    - If you can build near large shade trees (or wait for them to grow), they can reduce the indoor temperature for free.

    Other
    - Noise pollution is just as much of an issue to many people as IAQ -- while the Tamarack models are quieter than some of the old-school options, they're still much louder than most central AC systems, ceiling fans, or single-room exhaust fans.
    - If you want to pull smoke out quickly after cooking if the fire alarm trips, a vent hood to get the smoke out and an alarm with a photoelectric sensor to prevent false trips are more effective.

  9. Doug McEvers | | #11

    I see no reason to consider a whole house fan in MSP. Why bring in all of the night time humidity only to have to remove it via AC the next day? A really well planned, very airtight and highly insulated house will use very little AC kWh in Minneapolis. I doubt our 1978 vintage, somewhat thermally modified house uses $50.00 worth of electricity for AC during the summer here in the Twin Cities with a 78F setpoint.

    1. W Ramsay | | #16

      Ummm. Possibly. The house that was here before was a 2000 sq ft house built in 1972 (I think the absolute low point of construction quality in the U.S.). We used it for 8 years as our lake cabin. During that time we would very often cool it with simple open windows - breeze was enough.

      However there would also frequently be days when there was not enough breeze. Sometimes window fans would do the trick but not often. So, the thought to put WHF's in our new build on the same site.

      One issue that will be different this time around is humidity. We'd not worry about it much before other than when it was uncomfortably high we'd close up the house and turn on the AC. Now we'll watch it closer and likely only use the WHF when RH is below about 60-65%.

      The question then will be how often will humidity be a problem?

      1. User avatar
        Dana Dorsett | | #17

        Don't look at the outdoor RH (which is a RELATIVE number, relative to the unstated/unknown outdoor temperature), but rather the outdoor DEW POINT, which is a measure of the absolute humidity.

        A dew point under ~55F will be fairly healthy, comfortable air. At 75F a 60F dew point is about 60% RH, the threshold of where mold/fungus growth gets going (though it's not really raging yet...)

  10. Walter Ahlgrim | | #14

    It is clear that the inspector is being ridiculous. Move the conversation in to written form.

    Ask what he wants or need beyond the manufacturers instruction or chapter and verse from the code book. He has no authority to ask for anything beyond the code or the manufacturers instructions. If the instructions call for makeup air like the instructions for large range hoods do, then can he require it.

    My guess is once he starts writing anything beyond the instructions he will quickly begin looking for a face saving way out. If he claims a code citation get the manufacture to address his concerns.

    Walta

    1. W Ramsay | | #15

      MN Code requires mechanical MUA for any exhausting device over 400 CFM. There is no exception for WHF's. I'm not sure how it is written in other states.

      From what I understand most inspectors understand the function and purpose of WHF's and are OK with them. Ours is taking a strict interpretation of the code. On the plus side he does seem to be trying to find a way out - some way that he can feel like he did his job.

      1. Burninate | | #18

        Is there some reason it's impossible for a WHF to cause combustion appliance backdrafting with sustained negative pressure?

        I don't like the idea for a bunch of reasons, but I'm not sure why this isn't a legit concern.

        1. User avatar GBA Editor
          Martin Holladay | | #19

          Burninate,
          I addressed that issue in my article on whole-house fans ("Fans in the Attic").

          In that article, I wrote, "Because they depressurize a home, whole-house fans can cause atmospherically vented appliances located inside a home — for example, a gas-fired water heater — to backdraft. If the homeowner remembers to open plenty of windows before turning on the fan, backdrafting probably won’t occur. But the best way to avoid backdrafting problems in a house with a whole-house fan is to make sure that the house doesn’t have any atmospherically vented combustion appliances."

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