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Speedo 10-10-2007 08:34 AM

Ventilation Fan Installation
Hi- new to the forum here. I'd like to install a ventilation fan in my split level home in VA- mostly to expel the hotter air from the top floor into the non-climate controlled attic during the cooler hours. I've done some research on available models and understand the flow rates (cfm) and efficiency ratings (cf/watt), but have not found any info on what type of fan I should be looking for for my particular home. The fans I'm looking at range from 10cfm to 380cfm.

So my questions are:
1) Is there a rule of thumb for house volume vs flow rate?
2) Are there any strategies on where to place the fans, other than in the highest spot in the house?

Any info is greatly appreciated,


glennjanie 10-10-2007 09:39 AM

Welcome Pete:
Its good to have you, make yourself at home.
If you are putting the hot air into the attic, is it well ventilated? Is there at least 1 square inch of ventilation for each square foot of space? My opinion is that the best attic ventilation draws air from the soffits and expels it at the roof ridge. If those conditions do not exist in your attic, I would recommend that you vent to the outside air.
Most split level houses have 4 or more rooms on the upper floor and 100cfm per room is most satisfactory. So, the 380cfm model is the best for your situation.
The optimum placement would be as near the center as possible. If there is a bathroom on that level with a vent fan, the door should be kept closed allowing the bath to vent independently and having the windows open about an inch would assist in the ventilation.
In my house there is a 36" 'whole house vent fan' in the central hallway which can make the curtains stand out when the windows are opened. You feel like it will take the rug off your head when you pass under it.
Please let us know how it works out for you.

Speedo 10-10-2007 10:06 AM

Good info Glenn- thanks. I was also considering a whole house fan but I wasn't sure how that would work with respect to insulation in the wintertime. If I go that route I'll have to do some research on that as well as take a look into the attic ventilation as you mentioned. Oh, and I suppose I would need to look at the joists and see if I'd need to do some cutting. Man, it's a slippery slope :)

Speedo 10-10-2007 11:50 AM

I just did some measuring to address the ventilation question. I have 504ft2 of attic space and 576in2 of open vents to the outside (2 x 268in2), so it sounds like I have sufficient ventilation. There is also a ridge vent along the length of the apex.

However, the attic has a small volume (don't know if this matters) of approximately 756ft3 (max height is only 3ft). Would this be a problem with installing a whole-house vent and make a smaller fan venting to the outside more appropriate? The whole house is approximately 1400ft2.

travelover 10-10-2007 06:45 PM

Attic volume should not matter -the hot air is just passing through to the vents. They make some whole house fans that span the ceiling joists so you don't need to cut them. Also you can buy / make an insulated box to cover it in winter, from the attic side to cut heat loss.

In my house I just bought two $20 box fans and put them in downstairs windows at night, with the upstairs windows open. Works great - and cheap.

Speedo 10-10-2007 07:45 PM

Thanks. The box fan routine is what I'm doing now but I have 2 under 2 year olds and they are yankin on the cords. Thanks for the input I'll keep researching based on y'alls suggestions.


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