kilowatt hour tech expert...

[Chuck(G)]

There are a lot of variables, Joe. What's the SEER of of your central unit, versus the EER of your window units? How many square feet is each expected to cool? Are your units installed in a shady place (can mean up to 10 percent savings)? Does the central unit exchange to outside air or is it a geothermal unit?

Generally, smaller is better, even with window units. So I'll venture that most things being equal or at least typical, your best bet is the windows units.
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You can save some more money by installing an attic fan and maybe a whole-house fan. See:

http://www.atticfans.com/

Makng sure that you have shade trees to the south west will also make a big difference.

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We have central air conditioning, but ti's a secondary part of a heat pump system, which is mostly used for keeping the house warm in the wintertime (much more efficient than a plain old electric forced-air furnace). Much of the house is in the shade, as is the compressor unit for the heat pump.

[Normal]

This doesn't exactly fit your situation but I found this information while trying to figure out what I want to do.

Economics of operation
Operating a properly sized 2-ton, 10 SEER air conditioner in Atlanta, Georgia costs over $250 per cooling season (1250 hours), based on 8.5¢/kwh, or roughly 20¢ per hour of runtime. A large 18,000 Btu/hr window unit air conditioner with a 6 EER costs more than 25¢ to operate for one hour.

By contrast, the whole house fan has a motor in the 1/4 to 1/2 hp range, uses between 120 to 600 watts, and costs around 1¢-5¢ per hour of use.

What Are The Drawbacks?
Temperature, humidity, and dust
A whole house fan has some drawbacks: the fan can only cool the inside of a house to the outside temperature; unlike an air conditioner, it does not dehumidify; and dust and pollen can be brought into the house.

http://www.eere.energy.gov/buildings/in ... sefan.html

I keep asking myself "why after over fifty years of no air conditioning do I think I need it now."

[Chuck(G)]

From yer gummint (http://www.eere.energy.gov/consumerinfo ... rcond.html):

Room Air Conditioners
Room air conditioners cool rooms rather than the entire home. If they provide cooling only where they're needed, room air conditioners are less expensive to operate than central units, even though their efficiency is generally lower than that of central air conditioners.

[ThomasDodd]

Assuming that we decide that we need air conditioning on a particular night, is it cheaper (as one would assume) to run two 110V 5000 btu window units in the two c. 18' X 18' X 9' bedrooms (with bedroom doors closed) than to crank up the 5-ton 220V job - which spews coolth throughout the 3000 sq.ft.? (These two l'il units are newish and clean.)

Where's the thermostat?

That's the biggest problem with conventional, central A/C stsyems. The temperature is regulated with a single thermostat in a, generaly, poor location.

If the thermostat is in the kitchen, and the bedrooms are upstairs, their is now whay to really control the tem,perature up there. It will be warm than the kitchen, so to be comfortable upstairs, you need down stairs quite cool. With the window units, each room now has a seperate thermostat, for better regulation of the temperature in each room.

So the 2 window units in the used rooms (with closed doors) should be less expensive.

I'm considering zoning my place to solve this. so the main day-use rooms are on a seperate thermostat from the night-use rooms. Best route seams to be servos on the registers (vents), controlled by the thermostats. But they ain't cheap. Maybe I can find some used R/C car/plane serves and make them work. The electronics are the easy part. I really surprized you cannot find the servo controlled registers for $15-$20, since standard registers are one $4-$10. Instead they are like $60 or more.

[ThomasDodd]

Even though the vents on one floor or the other are (80-90%) shut off, the intake vents in all rooms (of course) are still open. Using these strategies, the location of the thermostat (downstairs, near the front of the house) really doesn't matter that much.

Upstairs will still warm faster than down. So with the thermostat set so that upstairs is the correct temp, say(only and example) 75 upstairs means 80 downstairs. But it'll warm to 80 upstairs before it's warm enough downstairs, about 83, to turn the A/C back on.

This question (again) is this:

Is it likely at night that 2 newish/clean 5000 btu 110v window units individually cooling two closed-door 18X18X9 bedrooms is going to be cheaper to run than a 5-ton newish/clean central air unit (with optimum set-up on the vents) cooling off the entire 3000 sq ft?

I believe you've had 3 yeahs on that already

Chuck, the DOE (via Chuck), and me.

[Chuck(G)]

We're drifting off topic here, folks.

So how's the food at Pig Daddy's?



BTW: Some of the best looking net images of food that I've seen!

[Paul S]

The only thing I would add is that, depending on how good your house is insulated from the outside and from room to room, you need to consider that the inital heating or cooling of a room or whole house uses the most power and that once you get the walls, floors, furniture etc.. to your desired range, your heating and cooling costs are much less to maintain.

Some smaller units take a lot more energy surge to get running and quick cool than a big unit does to just idle without having to recool things in the room over and over again.

[Rick Denney]

Assuming that we decide that we need air conditioning on a particular night, is it cheaper (as one would assume) to run two 110V 5000 btu window units in the two c. 18' X 18' X 9' bedrooms (with bedroom doors closed) than to crank up the 5-ton 220V job - which spews coolth throughout the 3000 sq.ft.? (These two l'il units are newish and clean.)

Even without studying the energy efficiency ratings, I would suggest that the room units will be substantially cheaper. This is especially true if the room units provide a means of recirculating mostly indoor air back through the evaporator, as the central unit does.

It just takes less energy to cool 720 square feet than 3000 square feet. The difference is so great that it will overcome any difference in efficiency.

But it depends on the location of those two bedrooms in the house. 5000 BTU's might not be enough to cool a 360 square-foot room with south-facing windows and no shade, or on the second floor under a hot roof. I don't remember the head loads from the calculations I did last year, but I'm thinking 360 square feet is going to require more like 8000 btu's.

You could close off the vents in the rooms you don't want to cool and close their doors. That won't save as much as just cooling two rooms, of course, but it might maintain a slightly better comfort level.

Rick "who grew up on Houston but who now can't tolerate a hot house" Denney

[Lew]

One warning about closing vents in unused rooms with central air. The result of the cold air hitting the closed vents, and not circulating past them is usually condensation. This condensation tends to build up and result in black mold around the vents (bad) or rotted trim/soft wallboard (also bad). Shutting off the air to unused rooms this way is a bad idea.

This advice came directly from a relative in the air conditioning business in Florida (when we lived there), and first hand experience of neighbors in FL.

[MaryAnn]

Well, I am an engineer for a utility, and we would just love it if you would run that big ole whole-house air conditioner all night long instead of those little room air conditioners.

Nuf said?

MA

[Rick Denney]

Yeah...again, 100% shade - nighttime. 5000 do the trick...at least so far in one of the rooms.

As long as you don't use the room before the small unit has a chance to catch up with the afternoon heat, it will be fine.

I have a 13,500 BTU air conditioner on my (very small) 23' motorhome. In Texas last August, it would keep the inside of the motorhome down to about 88 degrees when it was 100 outside in middle afternoon. That's probably 60 degrees cooler than it would have been without the AC, though. At night, it was fine. It's about half the size of one of your bedrooms, but then it has six poorly insulated surfaces exposed to the outside air. In the shade, it's fine.

Rick "who runs the separate generator and powers the roof-top AC will driving down the highway--losing less than 1/2 mile per gallon to do so" Denney

[tubatooter1940]

Bought an older home (1916 masonry house with a wine cellar) that had a central air unit but the french windows leaked so much that when the A/C unit croaked we replaced it with just a heater and installed 13 ceiling fans downstairs and a huge attic fan upstairs.
The high ceilings made it practical for this system to cool our house with fans and only when the temperature passed 98 did the place become unbearable.We banked the money we saved by not air conditioning and spent it on traveling the world.
Dennis (who believes that sweating is good for you) Gray

[Rick Denney]

Someday, we're going to have to temporarily trade:

tubas-for-motorhome

for a week or two.

Heh, heh. I hope your mechanical skills are good. This motorhome is right up your alley, though. It's old, paid-for, well-made (like an aluminum airplane--no wood except for decoration), and needs a paint job in the worst way. It's the only coimplete motorhome made by one of the Big 3 automakers, but it was made in the 70's when quality was not Job #1. Thus, it requires a certain amount of re-engineering, and a good knowledge of Oldsmobile drivetrains. This would be the ultimate gig vehicle for out-of-town gigs: Enough room for a quartet and all the gear, including a place for everyone to sleep before driving home. (Just don't bring anything that won't fit through a 24" door.)

Rick "who never has to go through security to drive his motorhome" Denney