I live in a high desert climate, meaning low humidity and temps down into the low 70s F at night even during the height of summer. My passive cooling approach so far has been to cool the house down at night with a big fan, and tolerate a slow heating during the day. This works ok but not great. For a few weeks a year the house approaches 90F in the late afternoon, and ~ 85F peak for over a month. Not terrrible since humidity is low, but I know as the years pass we will get more spoiled. I could slow down the heat accumulation during the day if I closed all the windows but then the house humidity and air quality degrades too much to perceive an overall benefit. I'm wondering about incorporating an air/air efficient energy exchanger to cut down on heat entry while continuing to ventilate the house. I first read about them in zero energy homes in Germany, where they insulate like crazy for the winter, and then rely on an exchanger to bring in fresh air while not losing indoor heat. One example of a product sold in the US Intro to the topic. The advert blurb says "up to 70% of heat (or cooling) maintained, 100 watt power draw. The power draw sounds quite reasonable, but I cannot quite make sense of the efficiency because the home as a heat sink would seem to depend on heat capacity and heat transfer rate. Any ideas how to interpret the utility in terms of how many degrees the home is likely to heat up compared to without ? Perhaps I can just compare average ambient to average indoor air temperature with the exchanger ?
Using my 50 year out-of date science knowledge and the very old brain: It's true you have the home as a heat sink. But you might just discount this and treat them as constants like solar heat gain and the BTUs added by the humans in the home. So if the inside air is 80 F, outside 105 F I figure you get fresh air at something like 87.5 F when using the ventilation. CFM is between 30 and 160. It might take 3 to 4 hours to totally replace the air in a 2,000 house. Somewhere between 1 and 2 degrees EXTRA heat gain per HOUR when you use the ventilation. Figure a 6 hour total run time for the fan would would result in a 8 to 10 degree gain over not using the fan. But, you have the hot air at the ceiling and the cooler air at the floor if you are not using a fan. Mixing the air with a fan might make a human think it's even hotter. I would think you need a small(?) air-conditioning unit to make this work.
Thanks, xpcman. See the info link I added to my OP. My house usually heat up 10 degrees F through the day, with whatever air exchange it currently is at and an ambient average temperature of 80 - 85F. If I assume the device exchanges a similar amount of air that is 50 - 60% the heat content difference between ambient and home of straight ambient air, I think I can guess that the house will heat up half as much. That would be GREAT. I think even my wife would have little to criticize peak summer temps of the house of 80F. Now that I have thought about it a bit more, I think the fly in the ointment is getting the entire house ventilated without using large amounts of power. I know from my days of using whole house AC that air flow through the ducting requires a kW. That is just unacceptable, unless I can keep it to about an hour a day.
A 160 cfm or less fan like xpcman suggested doesn't use much power . It should be under 100 watts. You don't need much airflow to just accomplish air exchange. Something to consider is putting your air exchanger intake and exhaust as far apart as practical. You might be able to do something like that with a very few feet of ducting if you put the unit near an inside wall to an adjacent room We also use free night air to cool down our house although it's just to reduce AC use, not eliminate it. It helps if you get showering out of the way early in the morning and make generous use of bathroom vent fans before it gets hot outside. Any other humidity creating activity like cooking adds to the problem, so hot days are a good time to put off as much cooking as practical.
They do work very well, but use more energy than I care to waste. I also find that they encourage laziness, because it is too easy to push a button.
Nope. I don't have an attic, if you mean a crawl space. Just a couple of inches above the ceiling where the rafters for the roof are. I did change my roof two years ago from tar to foam. I'll have to check, I think this added R6 to the roof and has perceptibly improved the overall house insulation. My uber-fan is in an upper story window.
Sage- Courtesy Eaglesight333 this post was apparently intended for you. Don't miss the "Dads in Briefs" video. >>Beat The Heat: 10 Design Tips To Help You Live Without (Or Use Less) Air Conditioning : TreeHugger Read more: http://priuschat.com/threads/green-news-and-more-v3-29.112585/#ixzz21VMynMmL
First, if you house is uncomfortable with regards to air quality from closing the windows for part of a day, you should look for, and eliminate sources of indoor air pollution and moisture. It should not go bad that quickly. Before you get any benefit from ERV or HRV (air to air heat exchanger) you need to ensure that your house is well air sealed. And insulation should be maxed as well as that is likely to be a better economic option. Once you determine that you need a ERV, shop around a bit, 70% efficiency at 100 Watts seems pretty poor to me. How much heat it will save is a very complicated question, and depends a lot on your exact circumstances. If you really care, I would recommend a competent energy audit. Make sure, in advance, that the audit will provide all the information you require. In my climate, I generally recommend an ERV at some point in the process of making an energy efficient house, but it is rarely the number 1 priority.
