Attic Exhaust Fan and cooling the attic

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Old 09-14-15, 11:08 AM
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Attic Exhaust Fan and cooling the attic

I have a powered attic exhaust fan on my roof that pushes air out of the attic. I've read a bunch of articles that say those fans tend to pull cool air out of the house into the attic, which raises my cooling bill (I live where we have about 40 days over 100 degrees per year). They say that when air is pulled out of the attic, something has to fill it back, and much of that translates to conditioner air being pulled out through cracks in your ceiling.

These articles suggest you use soffit vents in your attic or those spinning vents that just use the wind to vent your attic. But doesn't that present the same problem? When air is pushed in by the wind, the hot attic air has to go somewhere...wouldn't it just get pushed down into my house through those same ceiling cracks?

Also, would it help to replace my attic fan with one of those spinning vents? Can I do this swap without altering my roof?
 
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Old 09-14-15, 11:27 AM
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I would first stay with the powered exhaust. They work much better. 2nd yes you have to have a way for makeup air to get into the attic.
 
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Old 09-14-15, 11:35 AM
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Hi doorknob,
In an ideal home there would be no leaks between the house and the attic, but there are many. The tops of all of your walls, recessed lights and electrical boxes, plumbing penetrations and many holes drilled for the electrical. Some of these can be sealed from the basement (if you have one) or attic, but once the house is constructed, the opportunity to seal the drywall to the framing and all of the above becomes next to impossible.

Thus, the recommendation to not depressurize the attic with a powered fan as that will result in conditioned air flowing into the attic. In addition, as that conditioned air moves to the attic it also must be replaced by outside air, that 100° air you mention, flowing in through other cracks to add to your ac load.

The spinning vent you mention pulls air out of the attic and soffit vents only replace what your fan is taking away. Soffit vents can function along with ridge vents or other high vents for a bit of natural air flow, but it isn't a lot and certainly will not COOL the attic.

Even the powered fan is only adding already hot outside air. The real defense is to air seal everything you can and then add lots of insulation in the attic. That along with the soffit and ridge passive venting works best and uses no additional power.

Do you currently have any soffit vents or other intake vents to provide air flow for your powered fan?

Bud
 
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Old 09-14-15, 12:43 PM
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Bud9051: yes, there is at least one ridge vent and several soffit vents around my attic.
 
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Old 09-14-15, 01:26 PM
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Doesn't sound like enough venting. For static venting the recommend 1 ft² of (NFA) net free area of venting for every 150 ft² of attic floor. That venting is typically divided 1/2 high and the other 1/2 low. With a powered exhaust fan, the volume of air they can move needs all of the vent area available and more. I don't have any numbers, but a 2,500 CFM fan will need to pull that attic very negative to get 2,500 CFM of air to flow in through all of your vents. It is that negative pressure that also allows air from your home to leak into the attic.

Estimating your NFA is a bit of a pain since you don't have the mfgs test results. A ridge vent can be estimated at 16 in² per linear foot of ridge vent. They often advertise 18 in², but I feel they are optimistic as their bug screen has to reduce the flow somewhat.

For the soffit vents, calculate their length and width and use 50% of the total area. Total your number of sq inches and divide by 144 for sq ft. Divide your attic floor area by 150 and let us know how you are doing.

Bud
 
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Old 09-14-15, 01:53 PM
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Looks like there are plenty of soffit vents. And just a couple of ridge vents. I wasn't able to do that calculation, but in any case, adding more vents is not an option. Right now I can either keep things as-is (with power attic fan blowing), turn off the attic fan, or replace the attic fan with another spin vent or ridge vent.
 
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Old 09-14-15, 03:01 PM
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A spin type vent would not be close to the powered fan. You have the list, insulation and air sealing and then improve the ventilation if the calculations so indicate. Otherwise, keep it running while it is hot. Like a friend of mine said to me once, it's only money. Of course he had lots of it and I didn't.

stay cool,
Bud
 
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Old 09-16-15, 08:03 PM
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Late to the conversation here, but I'm thinking of powered attic ventilation myself. I'd like to question a couple statements made above. If one can successfully replace stagnant attic air with fresh ambient air in the the conditions described by the OP, wouldn't the attic end up cooler? Doesn't matter if you are drawing in 100 degree air if the existing attic temp is way higher. I live in Las Vegas. I measured 152 in my attic when the daily high was barely over 100. Slicing a third off the temperature in the airspace above your living area can only help.

The negative pressure concerns have a solution that come to mind. Rather than waste time calculating theoretical flow ability of all the various vents, if you can even see and measure them all, why not measure differential pressure? I'm just a DIY homeowner, but I assume a HVAC pro has the equipment to do so. But I'd take crack at doing it myself first. Figure out what it is. Add vents if required, and/or adjust your fan's speed (or size) to approximate balanced pressure ... pull in what fresh air you can without sucking into the core of your home. Even if an impressive air exchange rate is not achieved, continuous replacement of hot air with relatively cool air will have a benefit.
 
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Old 09-16-15, 08:30 PM
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Hi Zippin,
The mods may elect to move this to a new thread, but since the op looks to be done I'll just continue here for now.

The guidelines for attic ventilation are at best crude and they are applied to both hot and cold climates with little difference. So, yes, an innovative home owner who does a bit of research can most likely design something better for their specific location and climate. But offering dozens of options to the many home owners out there would probably be a disaster.

The best all around advice, both hot and cold climates, is to air seal that ceiling plane and add lots of insulation. Without those two improvements, any fan, intake or exhaust, will create a sufficient pressure difference to nullify the benefits from lower temps in an attic, that's assuming the house already has a reasonable amount of attic insulation.

Just picking some numbers here, but if an attic fan is pulling 50CFM of conditioned air out of the house, even if it reduces the attic temp to ambient, that energy penalty exceeds the benefits from the reduced heat gain. The actual leakage along with the insulation levels would all need to be known to get actual numbers.

I hope that helps some.

bud
 
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Old 09-17-15, 10:39 AM
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"Just picking some numbers here, but if an attic fan is pulling 50CFM of conditioned air out of the house, even if it reduces the attic temp to ambient, that energy penalty exceeds the benefits from the reduced heat gain. The actual leakage along with the insulation levels would all need to be known to get actual numbers.

I hope that helps some."


Yes it does. Probably difficult to quantify the actual "pull" out of the living space. And as others have said, sealing of plates and various holes in the walls and ceilings can only be practically accomplished during original construction. Not an option for me. But my gut tells me there still may be a net benefit to replacing the hot air in my attic, despite the impacts of negative pressure. I doubt it's a one-for-one tradeoff.

I'm a bit late for this cooling season, but I'll try to measure some numbers when I install a fan in my peak vent. I'm going to use an automotive radiator fan, a battery and a small solar panel. I'll try to size the battery so that it can realistically keep up with demand during the dark. Due to prevailing north winds, pulling air out my south peak makes sense (wind complimenting fan-induced airflow), with the handy benefit of south exposure for the solar panel. I'll put a rheostat on the fan so I can tailor CFM. I might also place a thermostat on it, and/or a remote switch, probably wireless. I have most of the components already. So whatever I have to buy is the total cost, and operation will be free. I hope to be able to measure differential pressure in order to optimize fan speed. Since we have many mirror-image days in LV with regard to sun, humidity, wind and ambient temperature, measuring performance should be easy. One day let nature run it's course, while plotting attic temp each hour from a wireless thermometer, and track both my cooling units' total energy consumption using an Efergy Elite. Next day, repeat measurements with the fan operating. The results would not reveal the specific impact the attic fan with regard to conditioned air loss. But it would prove whether or not benefits of running a fan exist; does the reduced thermal load in the attic reduce A/C burden to an extent greater than the increased workload caused by extracted conditioned air?

Somebody else must have gone down this path out of curiosity. Hopefully I'll be able to throw some number out there in a few months.
 
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Old 09-17-15, 12:09 PM
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I agree someone (many) have gone down this path before. One of the frustrating aspects of the energy profession is, it has been researched to death, yet most of that information remains buried in research papers I would struggle to get through. Most of what we read are just the conclusions, be they right or wrong, with no details about where those numbers/facts came from.

Your tradeoff is directly (inversely) related to the amount of insulation you have in your attic. If you are reasonably well insulated, then those high temperatures are not that expensive.

As for measuring the pressure differentials, passive pressures are in the couple of pascal range. I use a manometer specifically designed to compare two inputs (DG 700) and cannot reliably measure 2 or 3 pascals. HVAC contractors use inches of water. 1/4 inch of water is about 200 Pascals. Impossible to differentiate small numbers with their u-tube devices.

At least it is fun.

Bud
 
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