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I don't know if more recent studies have been done, but ones such as this one in Florida questioned whether forced ventilation systems result in a net energy saving.
Interesting. This was the key finding I took away from that article (saying basically that an attic fan draws more power than it saves):
Quote:
Increasing attic ventilation rates in existing residential buildings is often accomplished by adding forced ventilation using attic temperature activated attic fans. However, even those who are in favor of increased attic ventilation have often warned that the energy consumption associated with the attic fan motor is likely greater than any realized energy savings from its use (Wolfert and Hinrichs, 1974). Also, an early detailed study showed that while forced attic ventilation did reduce cooling energy use, the reduction was quite small and outweighed by the energy consumption of the fan itself (Dutt and Harrje, 1979). Another study in two instrumented side-by-side homes in Texas came to similar conclusions (Burch and Treado, 1979). Forced ventilation was found to reduce ceiling heat gain by 1.1 Btu/hr/ft2 (328 W) over soffit venting and gains to the attic duct system by 94 W. At a normal air conditioning COP of 3, the overall reduction in cooling energy use could be expected to be approximately 140 W against the measured consumption of 284 W by the ventilation fan. Measured reduction to the maximum cooling load was only 6% for R-ll ceiling insulation. Thus, the powered ventilation does not typically result in a net energy savings unless the attic is uninsulated.
Interesting. This was the key finding I took away from that article (saying basically that an attic fan draws more power than it saves):
Yep... I've seen many gable vents with powered fans without shrouding which doesn't really work at all ..... although "full" passive ventilation is important with continuous soffit and ridge vents being preferred.
This study from the University of Florida has interesting graphs comparing standard venting, full venting and a radiant barrier. Net net is a 30% reduction in heat flux going from standard venting to full venting with an additional 40% reduction with a radiant barrier.
That's a ridge vent. I have this particular one on my roof now. The roof ridge is cut out below it to allow heat to rise and pull cooler air in through the soffit vent(s).
RE: the economics of adding a wired power exhaust fan: I agree that the economics can be pretty close, but I sure would like to see an update of that FL study referenced above. All it's internal references were in the early and mid 70's, when energy was WAY cheaper.
But upon more surfing, I see a lot of studies and recommendations pro and con that lead me to conclude the economics are very tight. Tight enough that broad statements really shouldn't be made.
Always live in a single floor home (ranch) and you won't have a problem, seriously though, cold air is heavier so insulate all you want the 2nd floor will never be as cool as the first. A house ducted correctly for a/c will have returns near the ceiling and floor supplies, if your a/c is running efficiently you should have a 15-18 degree difference in the temps at those locations obviously the supply being 15-18 degrees colder.
In the past ten years larger houses here have 2 units, one in the attic and one in the basement and that one in the attic works its butt off fighting the heat, but it cools the upstairs. Also some houses I have been in have the unit in a closet upstairs and it cools the house much better than one in the basement.
During the a/c season make sure to change your filters monthly, I believe in using the cheap filters over the nice pleated ones since the pleated ones restrict more air. Try closing a few supply registers downstairs, maybe just two to start with as your a/c is a little sensitive and will freeze up if you restict too much airflow. Closing these hopefully will force more air upstairs. Also inline duct fans can be installed. My guess is more insulation for your attic will probably be a little more expensive than the duct fan. Personally I'd rather swim in a septic tank than insulate my attic.
The first year we were in the new house, the summer was ROUGH. The single unit barely moves any air out of the upstairs vents (2 story, single ac/heat unit).
We went ahead and re-did the plenum coming out of the air mover, as well as installed zone dampers, a zone damper controller, and an upstairs thermostat. We can now completely shutoff the downstairs and direct all air/pressure to the upstairs (and vice-versa). It made a WORLD of difference. Not even sure that it saves us much money, but the upstairs of the house actually does cool down.
Probably better to invest the $$ from an attic fan into a whole house fan. These take in the outside air and put it thru your vents. Normal lows for most of the spring/summer are actually in the 60's and low 70's, so pulling in outside air to cool your house is a very reasonable alternative to running the a/c all night. I know in my house, when I let it heat up during the day to 80*F+, I can't just turn the temp down and have the ambient air be 75*F and then switch my units off. The radiant heating of all the items in my house give off a lot of heat so I need to run the a/c even when the temps outside drop lower than my house temp.
Attics can hit about 150 degrees on a hot day. That heat radiates from ceilings & walls. An attic fan will help a lot. Set about 95 degrees. Also make sure insulation is up to date. That will help a lot.
Bottom line, I think next house I get, I'll put in a wind-turbine fan as a compromise.
Since those have no controls, you have to remember to cover/seal them in the winter so they don't continue to pull the heat from the attic.
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