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I read with interest Bob MacDonald’I question on page 17 of the September 2000 issue of FIB. His question said he heard of bad things from modifying ridge-vent systems with electric fans. The answers addressed Bob’I observed mold and moisture problems but not the “bad things” from electric fans.
I am about to install an attic exhaust fan (not whole house fan) to reduce our attic temperature. We have a 1 year old home with continuous ridge vents and continuous soffit vents. There is perfect vent in the eaves to assure good air movement and 16 inches of blown fiberglass in the attic.
Due to allergies, we run the HVAC with ESPs all the time. We have a Trol-A-Temp system for balancing temperatures in two zones of the home. Our home is on a hill so there is little tree canopy to shade the roof. The attic is hot and the AC seems to run all the time.
My thought was to increase the attic ventilation to reduce the temperature and thus reduce the cooling load.
Is this a good idea or are there “bad things” that will result from this?
Replies
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if you have an attic that is too hot , you will reduce your cooling load more effectively by blowing a cap of insulation on top of what you have...
the minimal cooling effect of the attic exhaust fan is probably not going to be noticeable..
in fact you may create a negative air pressure that will suck conditioned air out of your cooling spaces.. which will be replaced by hot outside air infiltration thru cracks around windows, doors and construction joints....
*What about an attic with a full ridge vent and a gable end vent. Why not install the fan in the gable end so it blows outside air into the attic which would force hot attic air out the ridge vent? Their are no soffit or other vents in the attic.
*Gable fan with soffit vents, yes, but not with ridge vent.
*If you're going to add more insul, consider a few inches of cellulose. It's not as transparent to radiant as FG.
*<<The answers addressed Bob’I observed mold and moisture problems but not the “bad things” from electric fans. >>What? I think of mold and moisture as "bad things."How hot is it in your attic? Have you checked that the vents are actually working? (E.g., are the soffit vents blocked?) Is your AC properly sized? Duct work properly balanced?Bob
*Mold is a "bad thing". I was not sure if the "bad things" were the mold or some other type of non-mold "bad thing".Thanks for your other points.
*Advanced Energy in North Carolina has proven that attic fans increase the cooling load in the conditioned space by sucking cool air out.The Florida Solar Reasearch Center has proven that heat gain through attics is 90% radiant, 9% convection and 1% conduction. You will be addressing the 9% part with your fan.Joe Lstiburek has proven that vented attics in Nevada are hotter than unvented attics in Florida. People effectively air condition houses in both locations.My point - address the radiant heat gain through your ceilings. Consider the fact that the house probably has other serious thermal defects which may increase cooling load. And lastly, an undersize cooling system running constantly is better than an oversize system that cycles - provided the occupants are comfortable. If the occupants are not comfortable, then you look for other thermal defects becasue comfort is found in the surface temperatures of the walls and ceilings and air conditioning can only help these indirectly.-Rob
*Not one of your examples "proved" anything.What they are, at best, are opinions on findings of specific situations in their studies.When will you learn, that a house is not a laboratory experiment under controlled conditions?Stop beleiving everything that you read, that's written by some obscure author, as being fact.Advanced Energy in North Carolina demonstrated that attic fans can increase the cooling load in a conditioned space that is not properly sealed by sucking cool air out. Nothing earth shattering here.The Florida Solar Research Center has indicated that heat gain through their test attic was 90% radiant, 9% convection and 1% conduction. And the relative point to the rest of the world would be.....And Joe L demonstrated that a vented attic in Nevada could be hotter than a unvented attic in Florida. And the average daily RH, temperature, wind current, climatic conditions would be exact duplicates of each other, as was the construction?The very thing that you accuse "the big manufacturers" of doing (slanting studies to sell their products) is being done by every wingnut group trying to get grant money or recognition on the speaking tour circuit.Close your books, step away and open your eyes and ears. Let the house speak to you and stop trying to redesign mother nature.Gabe
*Good point Gabe. It just seems like common sense to get rid of some attic heat. My dilemma involves more of a cost vs. benefit question. Factor in the price of the fan and the power to run it and are you really going to benefit enough, if at all, to make it worthwhile? I tend to think it might payoff in the long run, so I am thinking I may add one to my downwind gable vent. I just haven't found enough evidence to be convinced that it's worth the effort and expense.
*Gabe, since you didn't quibble with the 90% radiant heat gain statement, do I understand correctly that you accept that number, but just don't think it's significant?I believe radiant is one of the least addressed heat gain mechanisms because you can't see it or touch it. So we focus on insulation and venting (or not venting) and vapor barriers.I have developed a real interest in these topics and just out of curiosity plan to do some real world testing in my new house, which offers some real challenges. Foil-faced foam as a radiant heat barrier will be a prominent component. I assure you, my testing will be highly practical and well documented (I'm a skeptic by nature).
*Fred, sorry I went off on a tangent. I believe the attic fan, depending on it's location, is pretty much going to negate your ridge vent, due to the volume of air the fan is going to move. The suction pressure at the inlet of the fan is going to be much, much lower than at the inlet to your ridge vent, so as much of the air as can is going to go straight to the fan.But hey, if you can do it yourself, and attic vent fan won't cost much, especially if you can mount it at a gable. Try it and see if it helps.
*Crusty,Also use felt faced foam for comparison.I agree that venting removes "some heat" but when it adds cost, complexity or labor then what is the reason to add it?-Rob
*Fred, If you must add attic fans, consider two equal fans, one blowing in and one blowing out. This way you are less likely to unbalance pressures in the attic or house. Also if you have mold you may have crawlspace problems bring mold spores into the house. Look for can lites. They make a hole in your ceiling for air to go through. Many are now available with gasket kits to seal them up. This can reduce infiltration and improve performance of the entire house. Finally, seal any cracks or holes in walls, ceilings or floors. Check around switches or outlets, they can leak a lot of air. Maybe a good wind powered fan would help - it will not make as much pressure imbalance and does not use ANY electricity.Kevin
*Anecdptal data, but true. Had a house in Etlanner, GA. Attic fan croaked one day in early April a few yrs back. Upstairs got to 95 degrees. That was the temp of the conditioned air coming out of the vents. All the upstairs was supplied air through the attic spaces. Went up into attic; temp was130 degrees, actual, as measured w/ a thermometer. Fan was not moving - bearing frozen. Cursed a bit, went out and bought a replacement motor with permanently lubed ball bearings instead of oilite bushing. Regardless of what the political nuts in FLA (my ex-home state.) say about source of heat, the temp dropped to 95 degrees in attic spaces w/i an hour when fan came back in svc. My conclusion - conditioned air was getting roasted in ducts by 130 degree air. Sort of like our turkey got roasted today. Face it, an attic becomes an oven when it is hot enough. I am currently building a house w. continuous soffit and ridge vents. it's also goung to have a gable end fan. If it doesn't work, I've only shot about 200 bucks on the installation effort, and I'll let it sit idle. If it does work - the FLA folks can go pound sand. I sure could feel that heat! If the FLA folks are correct - just imagine the heat load from radiant heat on that day. Now, the failed fan house did not have a ridge vent, so I have no idea how well it would have cooled with one. BUT, based on my experience, I'm dropping over $100K to build this house. What percentage of that is an extra $200 if it is a failure? I've had some long conversations w/ my HVAC guys about just this problem. I'm gonna have some long runs of flex ductwork because we cannot get rectangular in. I have to provide a friendly environment for that duct work.I liked Gabe's comments - he's correct about the real world not being a controlled lab. That's why we build w/ such great safety factors.Don Reinhard
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I read with interest Bob MacDonald’I question on page 17 of the September 2000 issue of FIB. His question said he heard of bad things from modifying ridge-vent systems with electric fans. The answers addressed Bob’I observed mold and moisture problems but not the “bad things” from electric fans.
I am about to install an attic exhaust fan (not whole house fan) to reduce our attic temperature. We have a 1 year old home with continuous ridge vents and continuous soffit vents. There is perfect vent in the eaves to assure good air movement and 16 inches of blown fiberglass in the attic.
Due to allergies, we run the HVAC with ESPs all the time. We have a Trol-A-Temp system for balancing temperatures in two zones of the home. Our home is on a hill so there is little tree canopy to shade the roof. The attic is hot and the AC seems to run all the time.
My thought was to increase the attic ventilation to reduce the temperature and thus reduce the cooling load.
Is this a good idea or are there “bad things” that will result from this?