fanpower needed to cool house overnight

[Lacustral]

I guess that is reasonable, as long as it isn't damp air.

Is that other people's experience? That taking cubic feet of airspace to been cooled, multiplying by the fan's CFM, times a fudge factor of 2-3, would give you about how long it takes to exchange out the air in the airspace?

I just need to exchange the air on about 4800 cubic feet. Not to have the fan strong enough to set up a breeze you can feel.

If I were exchanging the air 2-3 times/hour as you suggest, that would be a 240 CFM fan. If I were only exchanging air every 2-3 hours that would be a 40 CFM fan.

One person said, they need a 6000 cfm fan. But they have a masonry house which must have a giant heat capacity. I have a wood frame house. I have a total of about 1000 sq in of exhaust and intake ventilation in my attic.

Running a fan for just a short time wouldn't work, because on really hot days in the summer, the low of 70 F isn't reached until the early morning hours.

Ideally what would be great would be a variable speed control AND a thermostat, so the fan would shut off if the inside temp gets down to

65 F ...

Fantech's variable speed controls go from 0-100%. Does anyone know if a speed control will work fine at, say 10-20%?

Thanks for all the answers!

Laura

[Lacustral]

Do you know how many CFM your box fan is?

Thanks Laura

[jah213]

No, I don't know, sorry. It's a 20-inch fan, the cheapo kind that goes on sale at the hardware store for $12.97. Plastic housing, plastic blades. Nothing very special about it, but it works just great for exhausting the hot air and pulling in the cool air.

Jo Ann

Lacustral wrote:

[StLouisMike]

Like some of the other comments regarding exchanging air not affecting the overall temperature of the house, I would also add that a simple fresh air exchanger won't do the trick either. We have a very good system tied into our regular household ductwork (a $2,000 RUUD system not to control air temp as much as humidity and getting fresh air into our 'super tight' panelhome). Running the furnace fan 100% and the fresh air exchanger on high over night with the outside temp in the low

60's doesn't bring the air temp in the home down a degree. The heat build up in the structure itself does not dissipate easily.

Mike

[Lacustral]

I know leaving my windows open cools my house down much more than if the windows are left closed, so bringing in outside air helps. I wonder if you have a heat exchanger.

Laura

[Phil Scott]

[Joshua Putnam]

Just double-checking, do you have a very small home? 4800 cubic feet would be a 600 square foot house with 8-foot ceilings. A 240 cfm fan

*would* be enough to create a breeze you could feel in that small a space.

[jah213]

I don't know how many cubic feet I have, but my house is about 1600 sq. ft. and I have 10-foot ceilings downstairs, 8 feet upstairs. When I put my box fan in an upstairs window and open windows downstairs, I can literally feel the breeze coming up the stairs. The method that works best for me is to open windows at night and put the box fan in the window blowing out, which exhausts the hot, stale air in the house and brings in the cool night air. During the day, I keep the windows closed and curtains pulled where the sun is coming in. I don't open up again until it cools off in the evening. My house is well insulated and has good windows but, at 100+ years old, is far from airtight. This method works really well except during periods of hot, humid weather when it stays hot at night. At that point I resort to a couple small (5K) air conditioners and put my box fan on the floor to circulate air. It's not high tech, but it works great.

Jo Ann

Joshua Putnam wrote:

[Lacustral]

Yes. The upper story, the one that gets hot, is 600 sq feet.

Laura

[Lacustral]

Actually it's 640 sq feet (just measured it). x 8 feet high, = 5120 cubic feet.

Laura

[Hul Tytus]

For example: 2 story house, 20' x 30' with 8' ceilings. A 16 in. 3 spd fan from Graingers (~3000 cfm on hi speed) is mounted in the trap door to the attic with a venting window in the attic open.

This has proved effective. Having the fan in the attic is responsible for the quick cooling to some degree; hot air tends to rise and a fan pushing out a window moves it less directly.

Hul

[BobK207]

Stick a 3 speed 20" box fan in one of the upper room windows.......blowing outward. Place a carboard shield on the sides as needed to create a reasonable seal.

Open windows in the rooms you want to cool (open more windowns or open them further in the "most impotant" rooms to cool)

The rooms with the open windows will cool FIRST & the room with the fan will cool LAST.

Experiment with fan speed & window opening, adjust window openings & fan placement to get the desired result.

fwiw I have a 2300 sq ft home in OC SoCal. It's a 1.5 story design built in 1930 We typically get good nightime cooling (into the low

70's or less with low humidity) we can get temps in the mid / high 90's & even low 100's.

I used to have a Nat Gas AC unit but it bit the dust & I haven't replaced it.

A single 20" box fan funning on high speed will cool the house to outside temp by morning & will provide a comfortable (& adjustable) breeze in the bedrooms.

cheers Bob

[nospamtodd]

Some other things to consider:

1. The cooler it is outside, the farther the temp will drop overnight. Even if it doesn't drop much, at least you get fresh air in. In the fall, I was able to pull in warmer air during the day and raise the temp in the house by a few degrees.

1. Since you have to leave windows and doors open, consider the security issue.

2. You'll be pulling in dust from outside.

[Big Al]

Here is a tip. You can buy a cheap 14, 16 or 18" fan at Wal*Mart or Home Depot. The type of fan you set on the floor or a table that oscillates, NOT a box fan. They cost about $15 here. Take it apart and remove the base. Cut a piece of thin plywood that will fit in an existing window and cut a round hole in it about 3/4" larger than the blade on the fan. Remove the front of the blade guard and mount the fan by the outer edges of the guard, put the front of the guard on the other side of the hole. Nothing fancy, just drill some holes and wire it on. Instant window fan,. total cost about $20. This is important: Since all these fans have plastic blades, be sure to run the fan and be positive it does not vibrate before you start. If it does, return it and try another.

And they work well. If you know how to use a window fan. Try to find a fan with a three wing blade. I have found very good 18" fans on sale at Big Lots and Home Depot. As the fan blade gets larger the size of the window it will fit in naturally changes too. Here in Tucson that's a problem in newer houses.

If you're lucky enough to have an attic, you can mount it in the access hole and cool the attic at the same time for free. However it's easy to make a much better fan for that use.

Al

[Lacustral]

I don't want to use a window fan, I'll be using an inline fan in the attic to cut down on noise.

But, it would be helpful to know how many CFM are pulled by the window fans that people have been saying work so well. Probably more than 200 anyway.

Laura

[dkarnes]

i have used a built in whole house fan (up stairs hall) and window fans. the whole house fan does work quickly, however i am using a small cheap

2 motor fan in the window now for a small ranch. put it at one end of the house away from the bedroom and open the bedroom windows a little. i am very happy with the results and do not need the expense of putting a new whole house fan in. also consider where you live and the insulation needed to put on top of the fan in ceiling during the winter months.

[Una]

A key point that seems to need underlining here is that there are two elements. The air exhaust (via fan), and air *intake*. You can put in a humongous monster of a fan but if you don't open enough windows and interior doors throughout the house, the fan cannot get enough house air to exchange hence cannot cool the house. This is especially true in a newer, "buttoned up" heating/AC efficient house.

Also, a good reason to install a whole house fan in an attic gable end (if you have one) is so that you have the choice of running the fan to cool just the attic (indirectly cooling the house, but with less perceptible noise and no draft), or to cool the whole house directly, by opening the attic door, selected windows, and interior doors (especially the bedroom doors).

Catalogs (and websites) that specialize in farming equipment usually include a huge selection of ventilation fans.

Una

[Clark W. Griswold, Jr.]

And in this situation, you run the risk of back venting furnaces, water heaters and any other vented devices in the house, which is a very bad thing.

[Thomas Kendrick]

1. Any dust outside is gonna become inside dust.
2. You won't get any cooler than the outside air temperature. May work great at higher elevations.
3. Outside humidity becomes > [e-mail] (Una) wrote: