20" Box Fan CFM

Determining the actual CFM is difficult even if you have the proper equipment, but if anyone out there does have an easy way to measure air flow I would be more than interested .

It sounds like your fan is doing a pretty much what you want it to do... whole house fans are more about causing a large pressure differential than the actual amount of air moved. The obvious goal of a whole house fan is to replace all the air inside the house with cool night air and purge all the hot air in your attic, which it usually achieves in a few minutes (this is assuming you're running your fan 20-30min). The not so obvious goal is to create a low(er) pressure inside the house and "suck" the air out of the walls.

I'd speculate that the 10% of the year you're unhappy with corresponds to the hottest 10% of the year and the problem isn't that you can't get your air exchanged, it's that the outside air simply isn't cool enough to do the job (or it's getting too hot, pick your poison).

Thanks for your response. You are correct that my least effective ventilation days occur when outside temps bounce off 100 degrees and overnight only goes down into the mid 60's. Those are the situations when my previous evening inside temps approach 82 degrees (again, no humidity) and I find it necessary to run the A/C for a while the following AM to bring the house down from a typical 72-74 (AM) to around 68. That happens in about 45min-1 hr. BTW, if I don't do the pre-cool exercise, evening temps become intolerable and the A/C needs to be used late in the evening. With a hot roof, and ductwork that's been baking all day in the attic, efficiency suffers.

The thermodynamics of the home are fascinating. Even after evening temps outside begin to moderate from daytime 100's, inside temps will continue to climb toward the aforementioned 82 as heat accumulated in the roof, attic and walls begins to make it's way through the insulation and the ceiling and walls become radiators. 9-10 PM is often the warmest time inside, even though the sun is long gone and outside temps are in the low 80's. By 11pm, it's cool enough to start the Wal Mart fan again, and the cycle repeats. Just wish I had a few more CFM available to pull the interior down closer to ambient...although a 7-10 degree difference is not bad.

Hey, what am I complaining about??

Regarding your comment on hot attic air, I thought the main coupling mechanism from that area is radiant, from the underside of the 150 degree roof to the top of the insulation and then to the ceiling. Hot attic air, not so much. Thoughts?

Thanks again

I would guess a 20" box fan moves around 350CFM. For an attic fan to do it's work, you have to open windows on the shade side and keep all other windows closed. When the shade side becomes the sunny side, reverse the windows open/close. Thermostaticly controlled attic fans don't cost much and can keep the attic cool.

That shaded side/sunny side routine worked well for me when I lived in Tampa. I had a reversible window fan and kept windows open on either side of the house and only had to switch the fan direction. Only needed to run the energy thirsty air conditioner/heater combination unit when doing heavy duty cooking. Opening windows shaded by a tree also helped. If no trees, installing Bermuda shutters provides the shade. Bare shutters are available at most hardware stores along with hinges, etc, You will be the envy of the neighborhood _{if your association allows them.}

Your walls will absorb a lot of heat and radiate it back into the interior for quite a long time. How is your insulation? Are your windows efficient? How is the sealing around doors and windows? An insulation contractor can do an IR survey of your house and tell you where the leaks are. You would be surprised to see how much leakage there is in the average house. Wall receptacles can let in a lot of heat.

Interesting to read up on what is done in the US to deal with the heat.

In India, in places where the humidity levels are low, evaporative air coolers are quite common. They work on the principle of using the heat to evaporate droplets of water and thereby reducing the temperature of the air.

If the humidity is low, this works no matter what time of the day.

Its cheap equipment, when compared with an AC, as all that is required is a fan and apparatus to create a thin film of water.

Are these evaporative coolers used in the US?

yes, evaporative coolers are used to some extent here. They also go by the name "swamp coolers". Other homemade, and on the market, cooling units use an ice cooler or trash can full of ice with fans and sometimes a small radiator, fan and pump to move the cool further from the cooler.

My quickie search on "20 box fan cfm" turned up numbers from about 2100 to 3200 cfm.

I am a member of an astronomy club in Northern New Jersey. We have a three story tall cement observatory with the typical hemispherical dome under which sits a telescope with a 26 inch Pyrex mirror that is about 4.5 inches thick.

During the summer months, the scope was virtually useless. The heat pumped into the 25 foot diameter dome would soak the mirror. Pyrex takes a long time to reach thermal equilibrium and it was not uncommon to have to wait until 2 or 3 AM before the scope could be used. For many this was not possible.

A dramatic change in heat build up was achieved simply by repainting the dome. It had been coated with aluminized paint. I argued to change it to white. The results while subjective, were still very apparent to all.

Our next step is to see how to insulate the interior of the dome.

I bring this up because the color of the roof is of paramount importance, although being in Arizona, it's likely you already know this.

Insulating the interior of the roof would seem to be the next step and since you live in an arid environment, the problems of condensation appear less likely. You can go to larger fans but that is still dealing with the symptoms.

Laughing Jaguar

Hello:

Some additional thoughts. As has been observed, air leakage is indeed a critical issue and I am fortunate to have a friend in the energy management business who owns a "blower door". We checked my home last year and, while I don't recall the numbers, we recorded some of the best residential pressure differentials he has seen. I've done BD tests in other (older) homes with him and with the system running, just the back of your hand can identify air leaks. Interestingly enough, spiders are excellent for locating those leaks. They find areas where there is air flow to be a productive location for webs, especially in crawl spaces where small flying critters abound :-).

Regarding the mention of roof color, DOE is promoting an initiative for "white/light colored" roofs as a mechanism for reducing solar gains. A search of their site will reveal astounding numbers. (BTW, look at aerial photos of cities in hot climates, apparently our ancestors did not need the DOE to tell them what works :-).

Reflective barriers were also discussed, but my attic is such a maze of trusses that installation on the underside of the roof would be a major chore. Placing material on top of the insulation is also an option, but accumulated dust can quickly degrade performance.

Finally, I occassionaly work for a construction company and have brought home enormous amounts of insulation from demolition jobs that was otherwise going to land fill. Can barely get into my attic (and yes, was careful to avoid the soffit vents!)

Thanks to all who contributed! Would appreciate a link to the 20" box fan CFM numbers...those figures sound quite optimistic (trust and verify :-).

Steve

Carson City, NV

Why insulate the inside? Seems insulating the outside would serve your purpose far better.

Something like Dow XPS rigid board covered with a stucco type of material.

This will give you the CFM for a 20" box fan.

http://www.mcmaster.com/#ventilation-fans/=824i23

Ron:

Thanks for the link! Poked about some more and found this page that seems to be close to what I have, although $12/Walmart vs $40/industrial leaves some room for flexibility:

http://www.mcmaster.com/#ventilation-fans/=825btm

Also, when I used the 3 X area = CFM numbers for whole house ventilation, I forgot to remember that capacity is for a fan to ventilate a home during daytime occupancy conditions. Clearly, those are not my circumstances so a considerably lower capacity unit gets the job done (to 90% satisfaction :-) overnight. And as someone else observed, the object is not so much CFM as pressure differential.

Some numbers to ponder...101 yesterday, 82 inside by late evening. Dropped to 61 ambient overnight and the fan pulled the house down to 70 by 6am. A/C ran for 1/2 hour to achieve 68 inside, and I'm ready to float through another day. If these 100 degree days subside, I'll be good to go!

BTW, I'm the son of a WWII vet (deceased) who was stationed near Anniston, Alabama and I arrived there when Mom took the train down to see Dad off. We returned to PA and I lived in the DC area till moving to NV in '02. Being able to say I was a Southerner by birth came in handy during a number of bar fights when I was in the Navy :-).

An update to the ongoing saga of the "20" box fan in the attic hatch", night time ventilation system. In a feeble effort to stay ahead of climate change effects here in Northern Nevada, I stacked a second box fan atop the first. Using the tried and true "how far does the closet door swing in" technique, I determined two fans are superior to one (duh) but have still not been able to move enough air overnight to bring the inside temp down to dawn ambient. Reluctant to stack a third fan :-), so currently in the hunt for a more appropriate air mover. Considering a gable vent fan mounted vertically on a piece of plywood in the attic a few feet from the hatch and connected to an insert in the hatch opening via flex duct. Should mitigate some of the fan noise we currently endure. Sure wish I knew enough about HVAC and fan laws to apply actual engineering to this effort. All Rube Goldberg, so far.