I'm installing a 4500 CFM whole house fan (Air Vent #54301) into my 1300 sq ft house. The recommended net free area is 864 sq inches.

My house is a tri-level, so there are basically three attic spaces. The spaces are all connected albeit by limited passageways. I'm sure air would flow in between them, but it would be constricted. I have a combination of ridge vents, gable vents, and soffit vents. I estimated the venting area for each attic space, then took 50% of that to estimate the "net free area" since the vents are all restricted with screens/etc:

Top attic (fan will exhaust into this attic): 372 sq in
Middle attic: 59 sq in
Lower attic: 262 sq in

Total for the entire attic space is 693 sq in net free area. I think this would be adequate if I considered the attics as one large space, especially since my 50% figure is probably pretty conservative, but I think I need to focus on the larger attic space that the fan will be venting into, which is only 372 sq in.

I figure I need to add some static vents to my roof (above large attic) which will get me 50 sq inches per vent.

Any advice would be appreciated!

 

Whole house fans are not my favorite. They have certain climates and temperature ranges where/when they will be beneficial. One of their major drawbacks is high humidity and Washington state can have some very humid areas. Filling the house and the building framing with high humidity as make-up air is drawn in can create issues.

As for the additional NFA, you are probably fine where you are. As you said, NFA isn't always 50%.

At times when the WHF is not running are you running AC?

Bud

 

Thanks for your response, Bud.

There is no AC in the house, so it has been pretty brutal the last month or so when the upstairs (where all the bedrooms are) is still >90F after the outside has dropped down around 70F. We usually just have box fans in the windows blowing the cool outside air into each room, which seems to work okay. I was surprised when I learned about WHF, since they seem like a perfect solution for homes in my area. I'm in western Washington, but inland a bit so humidity won't be an issue on hot days when I want to run the WHF. If I was near the coast then it might be something to be concerned about.

The static vents are cheap and easy to install, so I may just add four or six to my upper roof.

 

Give me a bit better location (I'm in Bothell) and I can give you a bit better information.

I use a 20 inch box fan in a window at one end of the house and open the windows on the other end of the house. Depending on what rooms I want to cool faster and whether or not the noise will be a problem I will either blow out or blow in. Last night I set the fan up to blow out and started with a RELATIVE humidity of 59% in the bedroom at a temperature of 72. This morning the RH had risen to 64% but the temperature had dropped to 66. In the kitchen (where the fan was) the RH was about 60% and the temperature at 68. The outside temperature was about 62 as I recall.

REMEMBER that "humidity" is relative to temperature. With the same moisture content a higher temperature will have a lower relative humidity than it will at a lower temperature. In my area the relative humidity of the outside air has been pretty high, up in the 80% range, at night. Dew point is an easier concept than relative humidity for many people to understand.

Do you have the Weather Underground site for your area? It will list all kinds of atmospheric conditions and generally have a station fairly close to your home. Here's the site for my area. Bothell, Washington (98021) Conditions & Forecast | Weather Underground

 

Hi Furd, I'm down south in Fairwood. I don't know where I was looking but the numbers I gave were incorrect. The RH numbers you gave are more accurate.

Looking at the hourly forecast for my area (for today), the RH hovers around 50-60% until early in the morning (3-7am) where it creeps up to 80% like you said. However, I would only run the WHF in the evenings once the temperature drops down to around 70F. From what I have read, most people can cool their houses down in less than a couple hours. I'll have the WHF on the timer, since I wouldn't want it running all night.

 

One other issue I did not mention is operating any naturally drafted appliances while the WHF is running. They are so powerful that a gas hot water heater or furnace will be backdrafting into your home. In my cooler climate we see people who turn the fans on and fall asleep. The thermostat detects the temp drop and kicks the furnace on and all dickens breaks lose. Most use oil over here on the east coast and you definitely know when the furnace is backdrafting. With gas you simple die quietly (potentially), you can't smell CO.

The argument I always run into is they will be sure to not run both, but in truth, that message will not reach everyone living there or will be forgotten and eventually it will happen.

Bud

 

The biggest problem with a whole house fan is that you need to open sufficient windows to minimize the negative pressure in the house. Otherwise you will be drawing air from the crawlspace or basement which might have odors. Also, you will draw in a lot of dirt from the outside air and leaving the windows open is sometimes not a good idea from a security standpoint.

Other than those caveats a whole house fan can work well in our climate IF you can make it through the day.

 

I've never had a WHF but a buddy of mine did while living in one of the LA suburbs. He said out there the purpose was to use the relatively cooler air of the evening when you got home from work to replace the hot accumulated air in the house. Hence, it ran for a few minutes and then was shut off.

Do many use them for long periods of time? Sorry, bit of a hijack here, I know....

 

Thank you both for bring up the issues with WHFs.

Regarding the backdrafting issue, both my furnace and hot water heater are located in the garage away from the living space, so the chances of backdrafting occurring is very slim. My gas clothes dryer, however, is in the living space. As long as all the windows are open I don't foresee any issues. Also, we have CO alarms in every room (as required by the seller when we bought the house).

Up until now we've had all the windows open throughout the night, so really nothing will change besides being able to cool the house off faster. Usually the temp drops down cool enough around 8pm, so we could run the WHF for a couple hours (if that) and then close all the downstairs windows if security is an issue.

Mitch, from what I've read, people can cool their houses off within 20 minutes or a couple hours. All depends on the size of your house, the temperature, and how much air your fan can push. But yeah, it usually isn't necessary to run them for long periods of time...at least that the conclusion I have drawn. I've read several recommendations, but you want to have a certain number of air changes per hour...I've seen it around 10-30 air change/hour. With my setup, I should have around 25 air changes per hour. That is a lot of air so I don't see it taking long to cool my house off.

 

A timer is a critical item to have on a WHF. Here in NJ the humidity levels run pretty high but when the nights cool off and the humidity is not high I'll run my 48" fan. It's a belt driven direct vent to outside. I'll set the timer to shut of by 2AM max. My house cools off in minutes.

I'll get up early and shut all the windows. It takes better than 1/2 a day before the temp rises enough inside to run the A/C. Usually the A/C comes on around 3PM.