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Old 07-19-12, 08:37 AM   #11
Xringer
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There are tar-like stains on the outside of my house, around the vents.
That's caused by out-gassing from the (1956) wood, and maybe roofing tar paper.
I have climb a ladder with pressure-washer sprayer to clean those stains.

The hot air in my attic accumulates in the peak, and had little effect on the ceilings of the living space.
But, as the day wears on and the peak hits 120f, the air down at the insulation becomes very warm.
I think the hot air just soaks into the insulation, warming the wood and the sheet rock under it.
Radian heat from the roof bombards the insulation directly.
The insulation and the ceiling it covers, warms to the room temperature, of the attic..

Before long, the living space starts to warm up. I can reach up and feel the warmth of the ceiling.
Radian heat is raining down.

When possible, we pull down the attic stairs and let cooler air updraft into the attic.

But, when we have the AC running and the house is sealed, there is very
little natural outflow of air.

Picture a can with two holes in the top, sitting in the sun. At first the air expands with the heat and some air flows out.
But once that's over, stagnate hot air will just sit in the can.

So, installing a fan in the east side vent (see pic above), sucks hot air out,
and pulls cooler outdoor air into the attic from the west side vent.
That air flow keeps the 120f air off the attic floor, and helps keep my living room ceiling cooler.
So, my Sanyo ASHPs don't have to work as hard..

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Old 07-19-12, 09:07 AM   #12
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Quote:
Originally Posted by bennelson View Post
The way I see it, it's all about "Delta-T", that is, the DIFFERENCE in temperature.

The bigger the difference on one side of the wall than the other, the more heat tries to push through to the other side to even out the temperature.

Insulation slows down this process, which is why we insulate our houses, to keep warmer longer in the winter, and cooler in the summer.

I recently put the remote reading sensor from my wireless indoor/outdoor thermometer in the attic of my house. It's been hot lately, and we've been running the air conditioning, set to 80 degrees F.

In the attic in the middle of the afternoon, it's been 120-125 degrees. So, that's a 40-45 degree difference than in the house. If it were winter, we might have the furnace set to 70 and the outside temperature might be 25 or 30. That's that SAME 40-45 degree difference!

Basically, the AC is working as hard in the summer, as the furnace is in the winter when it's below freezing! Imagine if you had a way to warm up the outside in the winter - your furnace would have to run that much less because of it.

That's kind of what you are doing if you can cool your attic. If you can drop the temperature there, it's that much less heat trying to beat its way into your house!

Anything you can do to reduce the Delta-T (including raising the thermostat in the summer) reduces how much energy is used to heat and cool the house. It just seems like a good attic fan is a simple, cheap, way of doing that.


(The summer/winter comparason isn't perfect. In the winter, heat loss is sort of everywhere - walls, attic, foundation, etc. In the summer, the attic is the hottest thing, but you also have to consider solar gain, and that the walls of the house are unevenly heated as well)
Hi,
This is probably all be good logic, but Soutface.org, which is generally a pretty good outfit has some reservations about powered attic vent fans: http://www.southface.org/factsheets/...s%2000-771.pdf

They mention two things
The power used by an AC powered vent fan might be more than the saving. Obviouosly this would not apply to a solar powered one.

If the living space ceiling is not sealed well, the powered vent fan might lower the pressure in the attic enough to pull conditioned air from the living space up into the attic. This seems like a valid concern to me as a lot of homes don't have well sealed ceilings.



Gary
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Old 07-19-12, 09:55 AM   #13
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Quote:
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If the living space ceiling is not sealed well, the powered vent fan might lower the pressure in the attic enough to pull conditioned air from the living space up into the attic. This seems like a valid concern to me as a lot of homes don't have well sealed ceilings.
That would only happen IF you have a very poorly sealed ceiling AND really insufficient attic venting.

Air flow takes the path of least resistance. Houses like mine will have vents on the underside of the roof overhang. That's how air gets into the attic. Air going out the ridge vent, end vent, or powered attic vent, comes IN through these vents on the bottom of the overhang.

Obviously, start with the "low-hanging fruit". As a general disclaimer on all home projects - start with the easy and cheap stuff - caulking, weatherstripping, and insulation.

If your powered attic vent is working so hard that it's sucking air out of conditioned space, you have worse problems!
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Old 07-19-12, 10:21 AM   #14
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Quote:
Originally Posted by Xringer View Post
So, installing a fan in the east side vent (see pic above), sucks hot air out,
and pulls cooler outdoor air into the attic from the west side vent.
That air flow keeps the 120f air off the attic floor, and helps keep my living room ceiling cooler.
So, my Sanyo ASHPs don't have to work as hard..
My single story ranch has vents just like that on the north and south side. I like the idea of your cross ventilation.

What type of fan are you using at your vent, how is it powered, and switched on/off?
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Old 07-19-12, 10:50 AM   #15
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Quote:
Originally Posted by bennelson View Post
That would only happen IF you have a very poorly sealed ceiling AND really insufficient attic venting.

Air flow takes the path of least resistance. Houses like mine will have vents on the underside of the roof overhang. That's how air gets into the attic. Air going out the ridge vent, end vent, or powered attic vent, comes IN through these vents on the bottom of the overhang.

Obviously, start with the "low-hanging fruit". As a general disclaimer on all home projects - start with the easy and cheap stuff - caulking, weatherstripping, and insulation.

If your powered attic vent is working so hard that it's sucking air out of conditioned space, you have worse problems!
Hi,
Not sure that's true.
I've read elsewhere that air movement from the living space to the attic space is the single largest source of infiltration/ex filtration in a lot of homes.
I'll try to find that reference.

I know when I added insulation to my attic I first pushed the old insulation out of the way and sealed every penetration for wires, plumbing, vents, can lights etc. -- there were lots of them All those little holes add up, and even if most of the flow out the fan is coming from the attic vents it seems like it would not take a lot of flow of conditioned living space air to offset the benefits of the attic fan?

Maybe someone can find some measurements on real attics?

Gary
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Old 07-19-12, 11:22 AM   #16
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Hi,
Not sure that's true.
I've read elsewhere that air movement from the living space to the attic space is the single largest source of infiltration/ex filtration in a lot of homes.
I'll try to find that reference.

I know when I added insulation to my attic I first pushed the old insulation out of the way and sealed every penetration for wires, plumbing, vents, can lights etc. -- there were lots of them All those little holes add up, and even if most of the flow out the fan is coming from the attic vents it seems like it would not take a lot of flow of conditioned living space air to offset the benefits of the attic fan?

Maybe someone can find some measurements on real attics?

Gary
A couple of careful studies on this:

FSEC-GP-171-00

Home Energy Magazine - Ventilation :: Drawbacks Of Powered Attic Ventilators

The FSEC study found a good reduction in attic temp (20F) and about a 6% saving in AC costs. But, the AC ducts in this house went through the attic and that may have been a big contributor to the AC saving. They found a payback period of 20 years, but they paid a lot for their PV powered attic fans.

The HomeEnergy on looked at 8 homes and found that powered attic vents did depressurize the attics and caused significant flow from the living space to the attic. Not sure what kind of ventilators they were using in this one, and they may have been more powerful than the typical solar ones. They also found some other problems associated with the power ventilators.

There are a bunch of refs out there saying that AC powered attic vents don't payoff.

Gary
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Old 07-19-12, 11:24 AM   #17
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Originally Posted by rav View Post
I am planning to have a solar attic fan installed this year. I was curious if any of you here have them and what's your feedback about them. My attic has the usual soffit and ridge vents, last fall i installed an attic tent and was very pleased with the heat savings compared to 2008.

Ravi

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ravi
Hi Ravi,
Just wondering what an "attic tent" is -- is that a radiant barrier?

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Old 07-19-12, 11:53 AM   #18
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Originally Posted by MarkM66 View Post
My single story ranch has vents just like that on the north and south side. I like the idea of your cross ventilation.

What type of fan are you using at your vent, how is it powered, and switched on/off?
It's a simple fan with it's own sensor that turns it on at around 110 deg F.
Looks a bit like this one.


When I first installed it, I was standing on the ground about 18 feet below it,
and I could feel the blast of very hot air coming down on my head!!

It was installed during the 1980s and I'm amazed that it's still working after all these years..

One thing I found out when my old roofing shingles were replaced, the tar paper underneath was so brittle,
it crumbled like saltine crackers. My Roofer told me that excess heat ages shingles a lot faster.
He won't install new shingles without installing a ridge vent, because of heat damage.
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Last edited by Xringer; 07-19-12 at 12:03 PM.. Reason: Roofing life
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Old 07-19-12, 04:50 PM   #19
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I just measure the temperature in my attic, midway between the roof and the rafters it's 138 degrees F. The outside temperature is 110 degrees, in the shade. Does that warrant additional attic ventilation?
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Old 07-19-12, 06:28 PM   #20
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I just measure the temperature in my attic, midway between the roof and the rafters it's 138 degrees F. The outside temperature is 110 degrees, in the shade. Does that warrant additional attic ventilation?
Wow, it's been cooking there since May 1st. Very Texas like...

Yeah, I would consider a ventilated attic and maybe a reflective barrier to bounce back the radiated heat from the roof..

Weather Station History | Weather Underground

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