Air Conditioning

June 2nd, 2020
Growing up, I thought of air conditioning as extravagant. And AC for a large house, especially if it's poorly insulated or you only use a small part of the house, can get costly. On the other hand, AC for a single room is just pennies per hour, and if it lets you sleep well on a hot night or makes you more productive during a hot day, it's probably well worth it.

To take an example, this small window unit is rated at 5,000 BTU with an Energy Efficiency Ratio of 11.0. This means we should expect it to draw 454W (5,000 / 11) on a hot day (95F, 50% humidity) when running on high. Living in MA we have some of the most expensive electricity in the country, and at our $0.21/kWh 454W is 9.5¢/hr. At temperatures below that 95F it will be more efficient because it will draw less (I measured 332W testing this weekend) and you won't need to run it constantly to keep a good temperature. This brings it down to about the power consumption of a PC, maybe between 1¢/h and 5¢/h. A workday's worth is then ~$0.09 to ~$0.45. It doesn't have to make you much more productive or comfortable to be worth it!

But maybe climate change means I should be avoiding AC for altruistic reasons? The dirtiest fuel still used is coal, at 2.21lb CO2e per kWh. At $10/T for carbon offsets that's 1.1¢/kWh. Not zero, but it's very low. If you're worried about longer-term sustainability when we run out of things to offset, at that point we're not going to be using coal (since if someone is still burning coal you could pay them to stop) and demand for AC lines up well with the production curve for solar.

You could even make an AC unit directly connected to a solar panel, without the efficiency losses of converting to alternating current or charging a battery. You should be able to make a ~4,000 BTU window unit that runs off a standard 65x29 panel and runs the compressor at whatever speed the incoming voltage will support. Probably not worth it, though, unless you need cooling year-round whenever the sun is shining, since otherwise the panel is wasted the rest of the time.

Fan pre-cooling is still a good fit for large houses, days that don't get that hot, and weather where it cools down a lot overnight. Working from home, with my 'office' being the corner of our third-floor bedroom with South-facing windows, however, this room is a much better fit for AC than pre-cooling. Anticipating hot days later this summer, I put in a window unit:

Since it's a narrow sliding window, it was a bit tricky. The opening measured exactly 16 inches, and standard window AC units typically list a minimum width of ~23 inches. They do make vertical AC units for narrow spaces, but they're generally large and expensive.

If you don't install the sliding accordion panels on a unit, though, you can generally use it in a substantially narrower window. The one I talked about above has body width of just under 16 inches, and without its accordion panels it fit perfectly.

That is, perfectly side-to-side. Vertically I did need to fill the space above and keep it firmly in the window. I cut a piece of plywood to exactly the vertical gap, and attached a bracket to keep it from moving. The panel slides in horizontally, and catches the top lip of the unit. For the bottom of the unit, since I wasn't using the included brackets I put on a narrow strip of wood to keep the unit solidly on this side of the frame.

If I wanted to make it nicer I could paint the wood, or cut out the middle section and put in something transparent like glass or plexiglass. For now, though, I'm planning to leave it as is.

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