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Old 04-08-2009, 09:07 PM   #16
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A/C struggles in hot weather


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Originally Posted by hvac122 View Post
Yes the attic is cooler but studies have found that your pulling around 20% of that air from the house through the attic floor. This is air that you have already paid to condition. This also could put your house in a negitive pressure which could cause health problems and unsafe conditions.

You would be much better off sealing the attic floor (remove insulation, seal and then put insulation back. Also installing a radiant barrier at the roof line will stop the infrared infiltration through the roof.
I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter. How much air can these things move? It never occurred to me that these fans could suck THAT much air -- enough to pull 20% through the attic floor. At best, I figured they might gently exhaust the heated air, which would be replaced with air entering through the soffit vents.

Please tell me more about these health problems and unsafe conditions you alluded?

Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?

Thanks!

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Old 04-08-2009, 09:14 PM   #17
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A/C struggles in hot weather


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I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter. How much air can these things move? It never occurred to me that these fans could suck THAT much air -- enough to pull 20% through the attic floor. At best, I figured they might gently exhaust the heated air, which would be replaced with air entering through the soffit vents.

Please tell me more about these health problems and unsafe conditions you alluded?

Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?

Thanks!
depressurizing the attic by fan large enough to cause a temp diff is going to create a chimney effect with in the stud space and pull air from the up stairs are, sometimes even the lower floor. Just means you have to cool the new air thats being sucked up ward.
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Last edited by hvaclover; 04-08-2009 at 10:17 PM.
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Old 04-08-2009, 10:11 PM   #18
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A/C struggles in hot weather


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I wonder if we are talking about the same kind of attic fans? The two I have are mounted in the roof and appear to be like 10-12 inches in diameter.
Are you talking about a vent that lets the air out of the attic? Or are you talking about a motorized fan?
Quote:
Also, since my attic has blown-in insulation, I would assume that moving that back to seal the attic floor would be a hard task? What kind of money would I have to pay (say, per sq ft) to do that? And what benefit would I realize in terms of energy savings?
I sealed my attic myself this winter. If you're physically able to climb up there and move around, you can do this yourself.

Just sealing my attic and doing some other minor energy-saving things helped a lot. I compared my gas bills from March 2008 and March 2009. Because of the things I did, my usage went down 19%. That's with just R-19 insulation in the attic. Sealing yours will help a lot.
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Old 04-08-2009, 10:35 PM   #19
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A/C struggles in hot weather


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Are you talking about a vent that lets the air out of the attic? Or are you talking about a motorized fan?
These are motorized fans, thermostatically controlled to kick on at like 100 degrees or thereabouts.

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I sealed my attic myself this winter. If you're physically able to climb up there and move around, you can do this yourself.

Just sealing my attic and doing some other minor energy-saving things helped a lot. I compared my gas bills from March 2008 and March 2009. Because of the things I did, my usage went down 19%. That's with just R-19 insulation in the attic. Sealing yours will help a lot.
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Old 04-08-2009, 10:46 PM   #20
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A/C struggles in hot weather


You can get 100*f in your attic on a mild sunny day. Those fans are running more than they are off.

Your infiltration, because of them, is a major contributor to the excess cooling load.
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Old 04-08-2009, 11:03 PM   #21
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A/C struggles in hot weather


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These are motorized fans, thermostatically controlled to kick on at like 100 degrees or thereabouts.
And you got 2 of them.

They're moving ar least 2000 CFM between the 2 of them.
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Old 04-08-2009, 11:07 PM   #22
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A/C struggles in hot weather




i think Beenthere agrees with the excess infiltration.
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Old 04-10-2009, 10:19 AM   #23
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A/C struggles in hot weather


The health issues that I was referring to are possible backdrafting of your water heater. If it is a gas unit then it could backdraft and bring carbon monoxide into the living space.

Sealing the attic is labor intensive but would do a world of good compared to the attic fans. There is no way that the attic vents were designed to move as much air as you are trying to do with the fans so the air has to come from someplace. This will put your house into a negitive pressure and pull air up through the attic floor.

I would suggest that you have a blower door test run on your house before you do anything else to find out where the inflitration is comming from. You would be very suprised as to what you will find out and it would prove to you what everyone is trying to tell you.
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Old 04-10-2009, 10:29 AM   #24
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A/C struggles in hot weather


Would an infra red scan along with the blower test work in mild weather or be worthwhile? Also, putting the house in a negative pressure will suck in a LOT of dirt/dust, yuck!
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Old 04-10-2009, 09:33 PM   #25
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A/C struggles in hot weather


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the coil on that air handler can only do 18F difference on the entering and leaving air and you need to verify that the return air is true space air and noting from the attic is being be sucked in to jack that return up.do you just have a single return grill in the 2nd floor hallway,and does it go right into the back of the air handler.then the suply ducts are they insulated flex runs with minimal elbows and turns.the charge has to be up to operating levels if anything less your tonnage is less in doing that 18F split.if your condenser is being banged by the sun that adds to the operating conditions compared to a shaded area...start with the ducts then get the charge checked. 18F air splits on the air handler(drop) as mentioned then 10F on the condenser(rise)
Looks like I have 2 returns: both in the ceiling (attic floor), one near the a/c unit and another on the other side of the attic (this is a pretty large attic, say 1800+ sq. ft.) Does this make sense that I would have dual returns? I cannot say if the returns are "true space air".

The supply ducts are insulated and look reasonably straight.

Compressor is on the NW side of the house, so it gets afternoon and evening sun. No shade, tho.

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