Your Place: Am I making efficient use of my home's cooling system?

Posted: July 23, 2010

Question: My house is three stories plus a basement. There is an open central stairwell that rises up through the center of the house from the first floor to the third floor. We have five bedrooms, three on the second floor and two on the third floor.

We have a two-zone, air-conditioning system with two compressors outside. Zone 1 serves the first floor (kitchen, living room, and dining room) and the basement. Zone 2 serves the second and third floors. We almost always keep the AC vents closed on the third floor and in the basement, as those spaces are not used very much.

The air filter for zone 1 is in the basement and the air filter for zone 2 is near the top of the central stairwell, just as the stairway reaches the third floor. A service tech who came out to do a "tune-up" recently told me that the whole system is about 12 years old.

We use zone 1 throughout the day to cool the first floor, with the basement vents closed, and with zone 2 turned off. When we go upstairs to sleep at night, we turn on zone 2, with the third floor vents closed, and turn off zone 1.

At night, with zone 2 cooling the bedrooms, and with the bedroom doors closed, I usually leave the windows on the first and third floors open. I assume this is circulating fresh air through the house, which is getting sucked up the central staircase and vented out the open third-floor windows.

During the day, when zone 1 is cooling the first floor with zone 2 turned off, I sometimes leave the second- and third-floor windows open.

Because heat rises, this whole scheme makes good sense to me and I assume makes efficient use of our AC system. On the other hand, it would not surprise me if, like my wife, your response is "What the hell are you thinking?" Any thoughts?

Answer: Despite your wife's incredulity, it seems as if you have the situation thought out well. I note that you are a lawyer, and your thinking reflects that.

The experts - I don't include myself among them, since all I do is ask the questions of them - probably are thrashing out the plausibility of what you have done here. My thought is whatever works for you is fine, even if it doesn't do the same for the next person.

All situations are just a bit different from one another. That's the reason why the source of a leak in one ceiling may be a hole in the roof, while another might be missing mortar from a chimney. The result is the same: a wet ceiling.

My first question is: Are you comfortable? The second is: Is your monthly utility bill reasonable? If the answer to both is "yes," go with it.

Solar-powered fans. From a mechanical engineer/reader in Cedar Rapids, Iowa:

Everyone doesn't realize that the biggest problem with solar-powered exhaust fans is the problems that they cause in the wintertime! These fans hardly operate in the winter with the sun so low in the sky or cloudy skies with not enough sun energy to get them to work, especially in the northern climate zones.

So if you don't have a ridge vent, you will not get rid of the moisture in your attic and you will get a nice collection of icicles in the attic doing costly damage to your roof sheathing, insulation, as well as ceiling gypsum board.

I had to hook my DC volt solar fan to a car battery to get rid of the icicles last December. Ridge vents with an electric fan(s) on a T-stat works the best for heat and moisture removal! I added a ridge vent this spring.

From a reader in St. Marys, Ga.:

Attic fans have been well researched and shown to be of no or negative value. Heat can transfer from one of three mechanisms: convection, conduction, or radiation. The primary source of heat transfer in an attic is by radiative transfer from the underside of the roof sheathing to the attic floor/insulation.

The replacement of attic air via attic fans will not affect the physics of the situation and can actually use more energy by creating a slight vacuum in the attic that draws conditioned air into the attic.

Successful kitchen remodel. A tip from Consumer Reports: "Don't get stuck on a size. In addition to being expensive, huge kitchens can be exhausting to work in and keep tidy."


Questions? E-mail Alan J. Heavens at aheavens@phillynews.com or write him at The Inquirer, Box 8263, Philadelphia 19101. Volume prohibits individual replies.

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