Re-Thinking my AC Strategy

For the first 12 years that I lived in this house I never had air conditioning. At the time I was working afternoons and evenings, so I was seldom home during the heat of the day, plus it was one of those luxuries I figured I could do without.

The house has an attic fan which pulls in outside air through open windows and pushes all of the hot air out of the attic. It does a reasonably good job of keeping the house cool once the outside temperature cools down. But during the heat of the summer, you pretty much have to run the thing all night long in order to get much cooling effect, because the outside air temperature never falls much below sixty degrees.

The main problem with the attic fan is that you have to have the windows open to run it, which, during allergy season made it sort of a "pick your poison" proposition. I could either suffocate from stifling heat or suffocate from the high pollen count.

Still, by employing the unheated waterbed tactic, and the occasional camping trip to the basement when it got really unbearable, I got by.

But then once I quit working and was home during the heat of the day, the weaknesses of my strategy started becoming quite apparent.

It was all made worse by the fact that my home office is on the west side of the house and has a sliding glass door, so that room just cooks in the afternoon sun, getting well over 85 degrees or so. During the worst of it, I'd actually sit at my computer with my feet in a bucket of ice water, wearing a wet t-shirt with a fan blowing on me just to make it bearable.

Soooo, when the government offered some nice tax rebates for installing high efficiency furnaces and air conditioners a few years back, I decided that the time had come.

I replaced the original 1954 furnace (which ran at about 40% efficiency) with a 95% efficient model and a central AC/air source heat pump. (A heat pump is basically just an air conditioner that can run either direction... so it can both heat and cool. They are much more efficient than any sort of a furnace when the outside temperature is above freezing. Once it gets really cold though, their efficiency plummets and you switch over to the gas furnace.)

Anyhow, my basic strategy had been to use the air conditioning only during the heat of the day to keep it below 80 inside, and then once it cooled off outside I'd switch back to my attic fan system. I'm used to the heat and I really don't mind it that much.

But recently a new problem has arisen. A family of skunks has taken up residence in our neighborhood. We're not quite sure where they're living, but they're out and about after dark most days, and trust me... those suckers are fragrant even when they're not spraying anything, and you do NOT want to pump a bunch of skunk scented air into your house at night!

So with great amounts of environmental guilt, sometime around the middle of last summer I started keeping the windows closed and running the AC at night. And to my utter amazement, my electricity bills actually went down!

I thought it was a total fluke, but then CatMan gave me a long lesson into the inner workings of an air conditioner, and it started to make sense. I won't pummel you with the details (and to be honest, I can't remember the details) but suffice it to say that an air conditioner works by moving heat from one place to another. This is accomplished by compressing and decompressing a gas trapped within the system. But it has certain physical limitations, namely that it can only heat/cool the air by about 30-40 degrees.

This is why the heat pump only works well when it's above freezing outside. Once it gets too cold, it just can't get the air warm enough. But it also means that if you run the air conditioner when it's 90 degrees outside, the temperature of the air coming out of your AC vents will be about 50-60 degrees. This will cool the house, but the AC has to work fairly hard to do it.

If, on the other hand, you run the AC at night when the outside temperature is 60 degrees, the air coming out of your vents will be closer to 20-30 degrees, meaning that it is able to cool the house down in no time flat.

Sooooo I now have a new strategy which is to run the AC at night to cool the house down to the low 70's. It really only takes about 20 minutes to get the house that cool. Then during the day I've got it set at 80 - but it has yet to come on during the day. Even with 95 degree heat outside, the inside temp is still holding steady at  a comfortable 76 degrees.

In the evenings I open the windows and run the attic fan for 15-20 minutes just to clear out any hot air that's still lingering up there, and leave them open for an hour or two, but close them before "the skunking hour" begins.

So far it's working great, and it's much nicer to sleep at night in a house that's 73 degrees as opposed to one that's 78 or so! Plus, it's really nice not to have to hear the noise of the attic fan running for hour upon hour.

So waddya think? Any thoughts on this new strategy? I'd love to hear how you utilize your AC to stay cool without breaking the bank.