A bedroom AC won't do anything to cool down the entire house overnight,
which is what he's trying to do, by moving cooler night air into the
house. I agree with the advice to get a whole house fan. That's
exactly what they are designed to do, pull air through the whole house.
Of course, a lot depends on the climate.
That depends on where you live and the house thermal mass and insulation
and how cool you want it. Whole house fans can work almost as well as AC
for all but a few days per year near Philadelphia. NREL says June has
average daily min and max temps of 61.8 and 81.7 F. July is the warmest
month, with 67.2 and 86.1 and humidity ratio w = 0.0133 pounds of water
per pound of dry air. An indoor fan or a window AC for dehumidification
can help ensure comfort in July.
But you may need a large fan. Lasko's 2155A 16" window box fan has
a thermostat and sliding mounts to fit windows and it's efficient,
moving 2470 cfm with 90 watts on the highest of 3 speeds, and it's
fairly quiet. About $50 at ACE Hardware stores.
A house with an open window or two upstairs is a natural cool air trap.
At night, warm house air will rise up out of the window and cooler night
air will slide in and replace it. During the day, air stops moving. More
air and heat can flow with a fan that runs at night, eg an upstairs window
with a 1-way damper outside (eg plastic film, hinged at the top edge) and
an downstairs window with a damper inside and a differential thermostat or
an outdoor 70 F thermostat that runs the fan when outdoor air is cooler.
It can never reach the daily min, but a larger fan can keep it closer.
A Q cfm fan is like a 1/Q F-h/Btu thermal resistor or a Q Btu/h-F conductor
between outdoor air and the house mass. This may be the bottleneck, if the
exposed mass surface is large, with a 1.5 Btu/h-F-ft^2 airfilm conductance.
If a house has infinite thermal mass (most don't :-) and G Btu/h-F of
conductance and a Q cfm fan and a constant indoor temp Ti and 8-hour night
and 16-hour day temps Tn and Td and the fan moves 8Q(Ti-Tn) Btu at night
and the house gains 16(Td-Ti)G Btu/h during the day and these heatflows
are equal, Ti = (QTn+2GTd)/(Q+2G), approximately.
For example, G = 800 and Q = 100 and Ti = 80 and Tn = 70 makes
Ti = (100x70+2x800x80)/(100+1600) = 79.4. G = 400 and Q = 2470 makes
Ti = (2470x70+2x400x80)/(2470+800) = 72.4.
We might model a house with C Btu/h of thermal mass and G Btu/h-F of
conductance and a Q cfm fan and Tmin and Tmax indoor temps like this,
viewed in a fixed font like Courier:
1/Q (closed at night)
| | --------------
| 1/G | Tmax . . .
Tn/Td ---------www--------*--- Tmin/Tmax . . .
| Tmin . .
| ------- -------
--- C night day
RCn = C/(Q+G) and RCd = C/G and Tmin=Tn+(Tmax-Tn)e^(-8/RCn) and
Tmax = Td+(Tmin-Td)e^(-16/RCd), and Tmin = [Tn+(Td-Tn)e^(-8/RCn)
-Tde^(-16/RCd-8/RCn)]/(1-e^(-16/RCd-8/RCn), if I did that right.
For example, C = 6000 and G = 400 and Q = 2470 and Ti = 80 and
Tn = 70 makes RCn = 2.1 hours and RCd = 15 hours, so...
20 C`00'house thermal mass (Btu/F)
30 G@0'house conductance (Btu/h-F)
40 Q$70'fan cfm
50 RCN=C/(G+Q)'night time constant (hours)
60 RCD=C/G'day time constant (hours)
70 TNp'night temp (F)
80 TD'day temp (F)
110 PRINT TMIN,TMAX'min and max indoor temps (F)
Just for the record -- I have ~1600 sq. ft. in two stories. At night,
I put a box fan in one window, set on medium, blowing out. It matches
the window size almost exactly, so I don't bother to block the sides.
It is in a spare upstairs bedroom. By the time I walk back into my
bedroom, also upstairs, I can already feel the difference in temp. I
don't know how long it takes, since I just leave it on all night, but
when I get up in the morning, the entire house is cool.
Sometimes simple solutions will get you where you want to go.
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.
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
Catalogs (and websites) that specialize in farming equipment
usually include a huge selection of ventilation fans.
I've got one of these installed, in a small single floor house. A big
multistory house will require more. Very quiet.
If I had something to say, this is where I'd say it.
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.
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.
2. Since you have to leave windows and doors open, consider the
3. You'll be pulling in dust from outside.
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