Originally Posted by bennelson (Post 23138)

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)

Originally Posted by GaryGary (Post 23145)

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.

Originally Posted by Xringer (Post 23142)

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..

Originally Posted by bennelson (Post 23149)

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!

Originally Posted by GaryGary (Post 23152)

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

Originally Posted by rav (Post 6148)

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

Thanks,
ravi

Originally Posted by MarkM66 (Post 23150)

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?

Originally Posted by MarkM66 (Post 23161)

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?