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Using a ‘whole house like’ fan for cooling

coram_deo | Posted in General Questions on

Have read your info on Whole House Fans, grew up with one and would like to use the concept. Would you comment and correct my thinking?

First I live in the Hill Country of Texas right on the line separating zones 2B and 3B. My home is of SIP construction with 7 1/2 inches of Styrofoam at the roof and 5 1/2 at walls. While the main level has heat and air, the attic does not. The attic is in the SIP envelope which helps. High temperatures creep above 80 in the summer and below 60 in the winter. I want to be able to use some of the area in the attic and to do so I need to bring the temperature down in the summer.

The thought is to cool the attic by using a shuttered exhaust fan on a timer in a whole house fan concept. The fan would be mounted in a window with the SIP envelope on one side and the outside on the other. There is a double hung window that would accommodate the fan. The fan would be turned on by the timer in the early morning when outside temperatures are close to 70 and run for 2 to 3 hours. Windows would be closed during the day and reopened at night. The goal is to lower the attic temperature when there is the greatest difference between inside and outside temperature with the expectation that it will not increase greatly during the day as it currently tends to stay about 80 without noticeable fluctuation and cooler air is not being introduced. The louvers on the fan will help to keep hot air, rain, bugs , etc. from entering and will be sealed in the winter. I do not know how many air changes will be needed and space and noise limitations will result in a fan with a lower cfm rating, thus it will be run for a longer period of time.

Well that is the basics. I would appreciate any questions, comments, concerns, etc.

Thank you for your thought provoking articles,
Ron

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ron,
    If I understand your question correctly, you want to use this fan to cool the attic, not to cool the house below. If that's true, then this type of fan is called a "powered attic ventilator," not a "whole-house fan."

    Powered attic ventilators are a very bad idea. The use of a powered attic ventilator raises your electricity bills two ways: (1) It will require electricity to run the fan, and (2) the fan will depressurize your attic, causing conditioned indoor air to be sucked through ceiling cracks into your attic. As this cooled air is pulled out of your house, it is replaced by hot outdoor air, and your air conditioner works harder than ever.

    If I misunderstood your question, and you just want to install a whole-house fan, then I have a different answer. Go ahead and install a whole-house fan if you want. But don't use your air conditioner on the days when you run the whole-house fan. You can't mix-and-match the use of a whole-house fan and air conditioning -- otherwise, all you are doing when you open your windows and run your fan is introducing humid air that has to be dehumidified by your cooling system. That wastes energy.

    All of these concepts are discussed in detail in this article: Fans in the Attic: Do They Help or Do They Hurt?

  2. coram_deo | | #2

    Mr. Holladay, thanks for your response.

    It shows a lack on my part of representing the situation. Since my home is of SIP constructions, I do not have a traditional attic. My attic is more like a second story that is not heated or air conditioned. The ceiling of this upper space is the roof of the house which is made of panels having 7.5 inches of Styrofoam for insulation. The entire house, both this upper space and the lower space (the air conditioned house) are inside this Sytrofoam envelope.

    If I view this space as a single story house without an attic it seems that the 'whole house fan concept' of blowing hot air out and pulling cool air in would be applicable. Cool air would be pulled in through opening in windows and hot air blown out with a fan, except my fan does not have an attic to blown into, instead it will need to blow the air outside of the space.

    Some thoughts about having an air conditioned space below this area: (1) the windows are open in the upper area for access to cool air (I do not know how many or how open) while the area below is 'sealed'. The expectation is that the air coming in will be from the windows. This being a SIP home there is not much insulation between the floor of the upper area and the ceiling of the lower, however, the upstairs is comprised of large areas over 4 by 8 T&G sheets and they are carpeted which should decrease infiltration. I want the air from the outside to be the source of air input as it is cooler than the conditioned air in the lower area. Although I am looking at about a max of 10 degrees difference, a cooler upper area should have a positive effect on cooling the lower area.

    The fan to cool the upper area would be used for 2 to 3 hours each night when the outside air is coolest to cool the upper level. A timer would turn the fan on and off. We would open the windows before the fan turns on and close them after the fan turns off. In regard to humidity. In general this area of Texas is not humid. The humidity seems to be mostly associated with rain, but if the weather is humid we would most likely not use the fan. If there is humidity I do not expect for it to move between the upper and lower areas, thus the air conditioner in the lower area should not be effected by any humidity in the upper.

    Your article: Fans in the Attic is the reason I sent my question. Since my 'attic' is not typical but the desire to cool an area by blowing hotter air out and pull cooler into a living area seems to be the same I was motivated to ask for your thoughts about the application of the concepts presented to my situation

    Thanks for your response. Your comments are appreciated and I hope that I have more adequately explained the situation. Please let me know your questions, concerns and comments.

    Thanks,
    Ron

  3. GBA Editor
    Martin Holladay | | #3

    Ron,
    It's rare for a home to have an airtight barrier between the first floor and the second floor. There are usually all kinds of penetrations -- wiring penetrations, plumbing vent pipe penetrations, plumbing supply and drain penetrations, even duct penetrations.

    Moreover, you haven't told us if you have an open stairway or a stairway equipped with a door. If there is a door, is there a door undercut -- or is the door carefully weatherstripped and equipped with a threshold that seals tightly against weatherstripping?

    I'm sure that you can understand what I am driving at. A whole-house fan installed in a second floor room will strongly depressurize your second floor volume. A lot of the makeup air will be coming from your first floor volume. The air that you have spent money to air condition will be drawn through cracks and leaks and sent outdoors.

    Finally, what happens in winter? Will you disassemble this whole-house fan and carefully seal up the window opening? Or will you depend on the leaky fan louvers to keep out the winter cold?

    If you are running an air conditioner to keep your first floor cold, that system can't operate successfully unless the cooled volume is surrounded by an air barrier. Right now, that air barrier consists of your foundation, your SIP walls, and your SIP roof. If you puncture that air barrier and insert a whole-house fan, you don't have an air barrier anymore. Now your air conditioner is struggling against large pressure differences, huge rates of air flow, and big holes in the envelope. That's not good.

  4. charlie_sullivan | | #4

    Looking over some plots of dew point vs. time of day at a nearby weather station for the past week or two, I see that the early-morning dew point is in the 65 to 75 F range. That means that if you get the temperature down to 75 F by your proposed method, you would end up with humidity between 70 and 100%, typically 85%. If your goal is condition the space for storage, I would think that 80 F and moderate humidity would be much better than 75 F and 85% humidity.

    It might be that the days I looked at are the outlier bad days when you would not use this approach. In that case it might work OK, but it still requires some thought about what conditions you really want for storage.

    Even with the open window(s), you will suck some air from the main part of the house. A possible solution is to have balanced powered intake and exhaust fans. If I were going to do that, I'd shop for some high-efficiency fans. In theory, fan energy consumption can be much less than A/C energy consumption, but there are a lot of pathetic fans on the market that use way more power than they should.

    If you want cooling "for free" in a moderately dry climate, without an indirect evaporative cooler might be another option to consider.

  5. coram_deo | | #5

    Wow - I am blessed with two men who do not want me to do a stupid thing, but will help me to see important considerations. Thanks to both of you Mr. Holladay and Mr. Sullivan.

    I am a believer in the 80-20 (90-10) rule - that 80% (90%) of the benefit can be obtained with 20% (10%) of the effort, time and expense. Further that the additional 20% (10%) will require an additional 80% (90%) of effort, time and expense. The key is in doing the right thing and you gentlemen are helping me in this.

    In light of this, is my upper area perfectly sealed from the lower area? -No. Any area for infiltration will be but a small percentage compared to the areas available by opening windows, but the stair area could be a problem even though there is no (almost no) place to replace any air pulled from this area. I do not believe that a lot of makeup air would come from the lower area but it is possible that some would. I do not know how to quantify this, so I am looking at doing an experiment with a fan that is not as powerful, does not have the options I would like but could provide some information.
    This is a three month a year situation where the temperatures are uncomfortably warm. During the other nine months the fan will be removed from the window and the window will operate as before.
    I have difficulty believing that there will be LARGE pressure differences, HUGE rates of air flow and BIG holes in the envelope. How can these items be quantified to include their relative influence?

    Mr. Sullivan, you hit on what had been a potential area of concern and have removed the potential. When you spoke of dew point you lost me. I have looked at hourly temperature and humidity values and the humidity has been a concern. Let me tell you my uneducated view and hope you will educate me. But first I need to tell you the intended use of the area. While initially I was interested in mostly storage, my wife saw too much area to be wasted on storage. So first walls were constructed to fence off anything under 6 foot (storage is behind those walls). We have two 'overflow' bedrooms for guests during periods of 'moderate' climate, there is a large living area surrounded by bookcases, my wife has a office (comfortable about half of the year), there is a bathroom so no one has to use the stairs and finally there is another area with office furniture and more bookcases. It is this area where activity could increase by making it a hobby / indoor workshop like area. There are many areas of storage and the temperature has never been a problem there. The notion is to make the other areas more usable when people do not want to go out into the heat.

    Now my current opinion on temperature and humidity for this indoor setting - If you were to plot temperature and humidity to have areas of comfort, marginal, and lacking: on one axis temperature would be up to low to mid 60s uncomfortable, from low to mid 60s up to high 60s marginal, high 60s to mid to high 70s comfortable, mid to high 70s to about 80 marginal and above that uncomfortable. Then to look at reasonably expected humidity in these ranges. for uncomfortable areas it does not matter you are already uncomfortable. For the comfortable range of high 60s to mid to high 70s it also does not make much difference. Then for the marginal range of comfort humidity, either too high or too low, can be a problem. The is the area where humidity concerns me. Based on this I would choose 75 F with 85% over 80F at ?% (65-75% or even the perfect 50-55%). If I could have 70 F in the morning heating to 75 F later in the day 85% would be ok. Data I look at has humidity for the hours of lower temperature from 50s to 90s, but about half the time numbers below 80 would be available. Please educate me on dew point and how it can be used to predict humidity.

    Mr. Sullivan, I had thought of the use of intake and exhaust fans as you mentioned. There seems to be obvious advantages - air availability, less time needed, less windows opened for longer periods, etc. My questions relate to matching of cfm ratings and periods of operation. Your thoughts on these as well as your thoughts on specific fans would be appreciated.

    Gentlemen, again I thank you. Please keep educating me. This summer is coming to an end but it would be nice to have something in place for next year.

    Ron

  6. GBA Editor
    Martin Holladay | | #6

    Ron,
    Q. "I have difficulty believing that there will be LARGE pressure differences, HUGE rates of air flow and BIG holes in the envelope. How can these items be quantified?"

    A. It's easy to quantify the rate of air flow; it's equal to the fan rating. For whole-house fans, that means 2,000 to 6,000 cfm. Yes, it's "huge."

    Is the hole in your envelope big? For a whole-house fan, it's in the range of between 4 and 9 square feet. Yes, that's "big."

    To quantify the pressure difference between the air in a second floor room and the air in a first-floor room, you will need to use a blower door. The difference in pressure when the whole-house fan is blowing between 2,000 cfm and 6,000 cfm will be significant -- I dare say "large."

    But you know what? It's your house. If you want to open your upstairs windows (and operate a 2,000 to 6,000 cfm exhaust fan through one of the windows) while your downstairs is being air conditioned, be my guest.

  7. prairieburner | | #7

    The main idea I'm reading from Martin's answers is that: since the "attic" is within the conditioned area of the home anything that you do within that area will effect the home as a whole (pulling unconditioned air into the building's envelope of conditioned air).

    Perhaps you should look into a small ductless minisplit air-to-air heat pump to make up for the lack of HVAC on that level of the home.

  8. iLikeDirt | | #8

    A whole-house fan makes sense for, well, a whole house. I have one in mine and I love it. But Martin is right: it makes zero sense to operate a whole house fan in only part of your house at the same time that you are conditioning the other part. These kinds of fans move a humongous amount of air. Your air-conditioned air is going to fly right out the window.

    If you want to lower the temperature of your conditioned attic, I think one of the best things you could do is to install light colored or bare metal roofing over purlins to create under-roof ventilation. This kind of thing is already hugely popular in Texas, and for good reason: it can drastically reduce the contribution of solar heat gain, giving you a smaller temperature difference through the insulation, which slows down the rate of heat transfer. This kind of thing is of course expensive and could be wasteful if you have a relatively new roof.

    Honestly, have you considered a window AC unit? As a believer in the 80/20 principle myself, that's what I would advocate for this situation. Just turn it on when you want to go up in the attic, and keep it off during all other times, removing it for the winter months.

  9. Expert Member
    MALCOLM TAYLOR | | #9

    Nate,
    You seem to have given a lot of thought to appropriate building techniques for hot dry climates. What do you think about completely decoupling the roof from the house?
    This link illustrates the general idea, but with many details that may be counter-productive.
    http://www.jetsongreen.com/2009/06/desert-modern-rimrock-ranch-house.html

    If you started from first principles, would it be fair to say that Texas houses would be best built as insulated masonry structures with concrete floors and de-coupled roofs, providing both shading for the house and the outdoor spaces the occupants could use?

  10. BobHr | | #10

    If your house is very well insulated and very tight then it wouldnt take much air flow from the AC. I would see if your could extend HVAC ducts to the attic. You have said you think you can keep the space cool just by purging the air and then closing the windows.

    I also believe it will be much hard to make the fan system air tight and well insulated. Air leakage will be a huge enemy.

    What you find is that during air conditioning season the air flow is from top to bottom in a house. Air leaks in at the top and leaks out at the bottom. As you chill the air in the house it becomes denser and and falls. This is what pulls air in at the top. You fan setup will make it easier.

  11. iLikeDirt | | #11

    Malcolm, that's an interesting idea. It's certainly true that shading and rejecting solar heat gain is priority #1 in a hot, dry, and sunny climate. If have masonry walls--even reasonably insulated walls--that are absorbing sun all day, they're going cook you at night. Outdoor spaces are critical, and in a dry climate, can be incredibly pleasurable. During the summer where I live it can be 100f outside, but under a tree with a breeze, you could be there all day.

    It's a little harder where I live, where it can get to be 110f in the summer and 10f in the winter with snow. I love puzzling through how to do this, but I am by no means an expert at all, just an interested layman with some self-taught building experience, the sense that a one-size-fits-all approach is routinely being used in places where it does not at all fit, and an interest in exploring ways to make a difference at some point.

  12. coram_deo | | #12

    Where to start? A few years ago when I was building my home I could keep up with a dozen or more things at a time. Now there is difficulty keeping up with one. I thank everyone for their thoughts and I have certainly picked up on several things.

    First of all I am NOT NOT NOT considering using both air conditioning and an exhaust fan for the same area. Further I believe that any potential for infiltration from the existing air conditioned area and the area I am considering cooling via a fan would be insignificant. Forget the phrase 'whole house fan' and just consider the concept of cooling an area by blowing hot air out and replacing it with cool air.

    To do this I am considering a fan with similar specs to the Tamarock HV1000 mentioned in the GBA article. The Tamarock requires 3 sqft for venting and runs at 1150 cfm. The 3 sqft is about .06% of the SIP envelope. The 1150 cfm provides a air changes rate less than that assumed for a typical house and that is not just open space but containing a fan that is discharging hot air.

    Nate, thanks for many positive comments. I have considered a window unit and it likely will be a fall back. Problems are the area is large and the possible positions do not address my wives office. Most windows are casement or sliders. There are only two double hung and their positions are not good.

    Mr. Hronek, thanks for your thoughts. Also thanks for reading what I wrote. But because of you I have been running from basement(home of a/c runs) to the upper level (where cooling is needed) looking at ways to add ducting. I had thought that this was not possible but you have caused me to rethink the situation. Yes, I believe that there may be nothing more needed than purging the air (occasionally). An occasional purging could be done by using both exhaust and intake fans as has been suggested. My A/C does not seem to be taxed at all and there is likely available capacity. I will check with the A/C professional that I value. With many comments made about the possibility of adding to the load on my current A/C I was thinking that if that were the case it would not be bad. But do not believe it is the case. An additional benefit of a duct is that it will work for heated air as well. Placement will be a problem but should be better than a window unit.

    Nate, on a separate item you mentioned your thoughts on masonry walls, their absorbing heat and discharging it at night. What you say is not theory. I have a basement with filled block walls and I did not insulate between the block and the rock veneer - big mistake. Fortunately half of the walls are below grade and the others are mostly on north and east sides. By insulating the above grade walls on the interior The heat discharge has been delayed. Walls cool down by the below grade walls and the slab over night and we start over the next day. A rec room in the basement did not break 75 F for the last two summers without any A/C. Before the discharge was delayed the temperature would increase by about 10 F daily and continue to increase until late at night.

    Gentlemen, thanks to your comments I believe that I have viable alternatives to solving my problem.

    Thanks,
    Ron

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Nate,
    I have a lot of sympathy for your approach of trying to avoid getting caught up in complex solutions and universal answers.

    We seem to spend a lot of time worrying about roof systems. Separating the functions so the roof only has to provide shelter against moisture and sunlight, and moving the insulation to another assembly just seems like a good idea to me.

    Although our climate here in the PNW is very different, a large roof, sheltering the walls and outdoor spaces, works very well here too, but the benefits of de-coupling the two functions don't seem to be as great in our temperate region.

  14. iLikeDirt | | #14

    It does not surprise me that insulating masonry walls on the interior has helped a lot. Because of this, most of the daily temperature change takes place within the massive block part of the wall, which is now effectively outside the thermal envelope. Insulation is important, but its location within a masonry wall sandwich important too in a hot and sunny climate. A wall that is going to be hit with the sun and get really hot should definitely be insulated on the interior side or in the middle (if it's thick enough and has a high enough heat capacity) so that the large temperature changes from the solar heat are not taking place within the thermal envelope. However, masonry that will not be exposed to the greatest sources of exterior heat (usually because it's shaded properly) can be positioned inside the insulated envelope, where its resistance to rising in temperature reduces the radiant heat coming off throughout the day, making the interior more comfortable than the air temperature alone may suggest. It's more comfortable in a room with 75f walls and 80f air than the reverse.

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