HVAC question...


Active Member
I live in Seattle and in the summer we have a few weeks of warm weather but nothing extreme. Most people don't have AC, including me. People use open windows, celing fans, attic fans, skylight ventilation to cool things down during the 3 weeks of hot days.

Our house has one zone of that has a trunk that goes to the attic and blows down on rooms and the other trunk goes to the crawl space and blows up in rooms.

It has been suggested that I add an attic fan to clear out the hot air from the attic. Why couldn't I put a large motorized damper to route the air from the furnace (while in fan only mode!) right into the attic and out the ridge vent. Basically this would pull the air out of the house and clear out the hot attic, without needing a second fan and all of the associated systems.

An alternative idea would be to suck air from the (cool) crawl space and push the hot air out of the house. This would be a little tricky as it would have to control the return air from the house to the crawl.

I'm assuming that I could use the automation system to look at crawl, house, attic temps and do the right thing based on temps.

Where is the flaw in these ideas?

Large motorized dampers aren't cheap, even more so if you want a low-leak damper (which I would suggest you need, to keep the hot air out of the attic during heating season) . . a 20x12 low leak damper is gonna be about $170, plus about $300 for the motor (NY area pricing, commercial quaility) . .

. . and even w/ a low leak damper you'll be removing the insulation from a section of the duct in the attic, causing a good deal of heat loss. .

. . a separate fan w/ a gravity operated backdraft damper will probably be cheaper, and you would give your furnace fan some downtime, extending it's life . .

. . if you put a transfer (air-passage, use a filter grill and you can slip in a peice of foam board for the winter) from the crawl space to the house, a traditional whole house fan will draw air thru to cool the house and attic . .

Pete C
A couple of points you may want to consider...

- To have effective airflow from the furnace to the attic would require a large size duct. Air will take the path of least resistance which would be through your heating/cooling vents. If you used a smaller duct to the attic and closed off the heating and cooling vents, it would put a strain on your air handler motor.

- Using your furnace to "ventilate" your home is not a very energy efficient operation. You are using a 800-1500 watt device to get fresh air. You may want to consider using an air exchanger or an exhaust strategy with a barometric damper to bring in air from a controlled point.
Go to: http://www.broan.com/display/router.asp?do...&CategoryID=512

- I would be careful about taking air from a crawl space and venting it into your home. Because of the moisture that can be found in crawl spaces, you could be bringing mold and mildew issues into your home. Radon is also a serious issue. There are places in Washington state that have high radon counts.

Good Luck!!!
If it were mine and the access was good I would consider making it into a matrix.

Heat cycle.... Floor vent in rooms needing heat blow, ceiling vents in high temp rooms become returns.

AC cyle, Ceiling vents in rooms needing AC blow, floor vents in colder rooms become return.

It would be complex to ensure you had the correct amount of flow going at all times but I doubt it's impossible. That would also allow you to use the main fan alone to simply transfer air from one specific room to another. Sun room might become all returns in the winter.

Ditch the complex attic vent, just use a temp/humidity switch and a fan or two.
Our Boston climate was not much different than yours. We used a whole house fan in our last two houses back east. It can be used to pull in the cool morning air, and then again when it begins to cool down in the evening. It worked so well that I didn't bother to install the window A/C in our bedroom anymore.

One of the best home made whole house venting systems I ever saw in use was fabricated by an old buddy years ago for his home of about 1200 sq ft.

What he did basically was mount a squirrel cage type fan to a piece of plywood on one end of the opening to his attic on two hinges so he could swing it out of the way in the off season. It had a square opening the same size as the fan opening to allow air to be drawn in and foam all the way around kept it from passing vibrations.

He switched it on/off with a wall switch on the bedroom wall just outside the closet.

On the other end of that same attic opening was a hinged piece of plywood that was all gasketed and insulated for winter.

When he turned on the fan it drew air from the whole house and just vented it out through his existing attic vents (soffit and roof vents).

Twice a year he had to interchange the fan unit for the sealed unit (no crawling into the attic to check for open vents and stuff like that.) - just flip the fan out of the way and drop down the plywood sealed unit .

Of course nowadays you could operate the fan by temp/time etc depending on you needs.

He kept his place quite comfortable without a/c using this setup.

This is sort of what I'm after, something simple that pulls the air out of the house. If it is cheaper to add a fan that's fine. I don't think what you suggest will work for me because Ihave a ridge vent so it is not easy to seal off the attic. But I wonder if there might be a way to have the attic fan tie into the house system so I could pull air out of the rooms via the registers thus replacing hot attic air with warm house air, and (hopefully) the house with cool outside air...

I have no problem having the system be a winter/summer system with me going in the attic and physically moving things around.

A whole house fan is normally installed in the ceiling to vent into the attic. Basically, it pulls a slight vacuum in the house and pressurizes the attic. It forces air out all the attic vents. A good place to install one of these is in a central hallway or at the top of a stairwell leading to the upper level. Some install over the ceiling joists, and others require cutting one.

These move a LOT of air. Our nickname for it was the "hovercraft". Because of the reduced pressure in the house, air is drawn in any open window. So it is easy to adjust the flow depending on which rooms are currently in use. I don't think it would work nearly as well if it was forced to pull air through ductwork.


I think the ridge vent would be perfect for a wholehouse fan system!

I would think you could pick up a squirrel cage fan or two very reasonable from a heating contractor who does removals or a yard sale or even a junkyard.

Just wire it to an extension cord before you do anything permanent and try it out ( maybe things like variable speeds or even a two speed motor )

Then when you get it perfected we can franchise it ( and I will be the millionaire Canadian sole distributor of course ) !

I don't want to be a millionaire I just want my wife to be comfortable on hot days :). She's Candadian so you might have to arm wrestle her for the rights up north.

So I get it now. I need a whole house fan, independent of our existing heating system. These fans sound pretty loud... are there quiet ones or isolation mounts I should be looking at to keep things quiet. I am assuming that I'll use a few temp sensors and my HV pro to control this system. Variable speed would be great but I don't know much about this stuff...

Ours was rubber isolated and variable speed, which is hard to find now. It had a dial to set any speed from just spinning fast enough to open the louvers up to a gale force. At night we turned it down to wisper mode with just bedroom windows open. Pretty much the only noise at that speed was from the louvers rattling because they weren't fully open.

If you can't find a variable speed unit, I highly recommend one with at least 2 speeds. They do make a lot of noise at full speed. Ours was a 30 inch unit, and we only used max speed for a quick cooldown.

We're also in the Seattle area, without air conditioning. We had a couple of solar-powered attic fans installed. The cost was reasonable (under $200 a piece I think, installed), and the installation process took the guy less than an hour a piece.

These are the people we used:


The installers they use are employees, and this is all they do essentially (the attic fans and the solatubes -- which we also like). So the guys have seen and done it all, as opposed to using a handyman who doesn't really know what he's doing . . . .

The effect of the solar fan was pretty noticeable When we had the hot streak in 2006 where it didn't cool off at night it didn't do much. But before we had the attic fan installed, I felt that the home office was always hot, whereas now it doesn't get hot until late in the afternoon.
I've had several whole house fans in different houses and they all worked very well. They can be a little loud at high speed, but as suggested, make sure you get one with several speeds if not a variable speed unit.

Just keep in mind that you really need to seal up the louvers during the winter. I have a seprate power switch mounted at the fan (turn that switch off and the fan doesn't work - regardless of the position of the switch in the house). Then I have roll insulation that I put over the fan assemble or inside the fan cage on top of the louvers (depending on what works better for that particular fan). The added cut off switch prevents someone from accidentally turning on the fan inside the house and mincing up all that insulation.

If you don't seal up the louvers, it's like having a 2-3' square hole in your ceiling during the winter :eek: