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There are three methods of heat transfer, conduction, convection, and radiation. The sun shining on the roof heats the roof by radiation, the hot roof transfers the thou the roof via conduction, the air in the attic is heated via convection. There would be some radiant heat transfer to the ceiling insulation. But the transfer of heat thou that insulation would be via conduction. If the attic space is well insulated they an attic fan will have very little if any effect on the room hemp below.
I have done some tests in my house. Outside hemp 90 beg. attic 120 beg with a van running, 124 without the fan. A four degree hemp difference is not enough to count.
The above discussion is about day time heating. The opposite happens at night. The attic hemp drops below the outside air temp. Again this is due to radiant heat transfer. I’m considering installing a reversed bathroom fan to blow the cold air from the attic into my shop at night.
Jerry
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Mike,
Please help me understand where the reference to hemp and the DEA came from.
The only plant on my 15 arces that looks anything like hemp ( Cannabis sativa L. ) is Sulfur cinquefoil ( Potentilla recta L.). It's a bit to short and missing a couple leaves to be hemp. I smoke Marlboro Lights and drink Bud, or any other free Beer.
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Jerry,
fg is transparent to infrared waves. So the underside of the roof heats the topside of the ceiling directly
-Rob
*All I can say is this....I built a 2500sf heated house in the humid state of louisiana...I put in an attic fan....it has a switch. I can turn it off and on....Im not talking about a powered attic vent Im talking about a "whole house fan". The lumber yards and manufacturers have a table as to the house size and fan size required. I followed that. I have the 13 SEER trane a/c units. My bill is much higher in summer when its too damn hot to run an attic fan at night....who wants 95 degree evening air when they can have 75 deg. a/c for a less than the cost of three tanks of gas in the truck each month. But....in the spring, early summer and fall...I can turn may fan on at night and let it run till a.m and close the windows....the a/c never kicks on until after lunch even though its 85 deg outside.....I know my pillows get damp and the humidity cost money to remove with a/c but nothing is better than feeling a breeze over you while you sleep. You dont hear the dogs bark or chickens crow cause the fan drowns out the noise.....My wife hates it cause it reminds her of camp meeting and hell fire preachers....I guess my conscience is not as guilty as hers cause I love mine. I just dont use it year around. It has a time and place. Its sort of like hunting or fishing. The meat costs more than at the store but it really does taste better fresh.......dreaming of sleeping in the hammock this fall...P.S......the fan wears the damn bearings out in my turbin vents ....I have seen them stop up gable vents when blown in cellulose insulation was used....Be sure to turn a/c off when you turn fan on....and I beleive there is a table to tell you how many square feet of doors/windows to leave open for the size fan you have......and if you live in the city get burgular bars or forget it lol
*P.S BearMy house is a two story also......be sure to locate at the highest level of ceiling.....Im lucky my stairs terminate in a hallway so the fan is located there...also I think you have to buy the shutter seperate from the fan
*the hemp came from your post ... i thot conditions were harming some of your plants......((If the attic space is well insulated they an attic fan will have very little if any effect on the room hemp below. )))make mine a 'gansett...
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Columbus, Ohio. 3500s.f. two story house. Can a whole house fan be effective in this size house? If so, what size fan? The objective is
to cut down on use of air conditioning.
Thanks.
*Bear,2700 sf house Toledo. Var. speed fan in well insulated house in the woods. Can get interior temp as cool as outside (with resultant humidity) at nite. Close it up during the day and only a cpl degree temp rise. It works. Locate centrally. Best of luck.
*BearDo a search of the archives on this subject. Not too far back, we had a long discussion about this and it brought up alot of other thing you need to consider, i.e. enough soffit vents, automatic shut-off in case of fires, etc.Vince
*Bear:Yes, they can be effective in getting warm air out and cooler air in and in getting the hot attic air out!The are other better ways to do this, and some us here will disagree that Whole House Fans are the way to go. Technically, you want to insulate the home, so the hot air does not permeate the inside, add an air conditioner, and the same insulation keeps the cold air in. But that involves a lot of insulation and carefull installation of vapor barriers and plugging of holes where moisture can get it. This is the best method, but it is expensive, time counsuming, and needs a high degree of care. A fan that can move the air all over the house will have a tremendous engery drain, and the energy savings will not be as significant as you think. Some studies (I say some, not all) suggest that you are money ahead if you concentrate on insulation and vapor barriers throughout the home, as opposed to Whole House Fans.OK, so you are cheap and sloppy and the home is a sieve (so you can't insulate it and plug the holes) and therefore want a fan. Fine. Most people strive for a complete air change in the home every 3-10 minutes. Take the square footage of the home (3,500) multiply it times 8', which is the height of the walls, and that should give you the cubic feet (28,000). Divide this figure by 1/2 for an air change every 2 minutes (you'll need a fan/fans with 14,000 cfm); by a third for an air change every 3 minutes (9,300); etc etc. Do the Math. I think there is a Table in the Grainger Catalog. You would need one big mother of a fan, or several large ones to really move the air in a 3.500 sqf home.My recommendation is to install 3-5 Certainteed Power Roof Vent Fans (I think they run about 900CFM) and hook them up on a thermostatic switch (some models have them built in) and put them in the attic. Adjust them so they go on at 98, then another at 100, then another at 105 etc., so that at 105, all five of them are blowing. They will blow out the attic and elevate the thermal line from the second floor into the attic. As you might know, one of the big problems in summer is the attic gets clogged with very, very hot air, and the the hot air on the second floor as no where to go. Remove the super hot air out of the attic, and the second floor becomes more manageable.Several ways to go on this, and I hope this helps.
*a whole house fan is a nice compliment to even the tightest, most well insulated house. I put them in houses that have real R-24 walls and R-50 ceilings. It is purely logical that it is a great way to get hot, stale air out of the house at night and get some free cooling to hold you through the day.The differnece is that a well insulkated house needs much less of a fan. Because the radiant surface temperatures are lower there is very little cooling required for "comfort."Attic fanbs are a complete waset of money and do NOTHING to keep the uppermost ceilings cooler. Everyone and their brother will tell you some anecdotal data, but when people test real houses with instruments and no "perceived improvement" based on the couple hundred bucks they just spent on fans, the data says attic fans are losers.Attic heat transfer is through RADIATION. All kinds of cool air can blow right through the attic, and the ceiling still gets heated by infra-red waves emitted by the bottom of the roof sheathing. This infra-red goes right through the fiberglass (regardless of thickness) and cooks the ceiling.-Rob
*Rob,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*There are three methods of heat transfer, conduction, convection, and radiation. The sun shining on the roof heats the roof by radiation, the hot roof transfers the thou the roof via conduction, the air in the attic is heated via convection. There would be some radiant heat transfer to the ceiling insulation. But the transfer of heat thou that insulation would be via conduction. If the attic space is well insulated they an attic fan will have very little if any effect on the room hemp below.I have done some tests in my house. Outside hemp 90 beg. attic 120 beg with a van running, 124 without the fan. A four degree hemp difference is not enough to count.The above discussion is about day time heating. The opposite happens at night. The attic hemp drops below the outside air temp. Again this is due to radiant heat transfer. I'm considering installing a reversed bathroom fan to blow the cold air from the attic into my shop at night.Jerry
*jerry , knock off the hemp before the DEA breaks down your door...good post .. nice points... i concur... insulate the hell out of it and add a small A/C....or move to RI and wait for the southwest breeze in the evening...man, i love summer...