From The Jersey Sierran, July - September 2022
Summer is approaching, and with it the need to cool our homes. A central house air conditioner is the largest summertime energy consumer in a home. It typically runs 30 percent of the time during the four-month warm season, for a total of 860 actual running hours. While running, it draws 3 to 5 kilowatts (kW) per hour. Assuming a conservative 3.5 kW, it would consume about 25 kilowatt hours (kWh) per day or 3000 kWh during a four month season. Based on the NJ average residential cost of 15.77 cents per kWh, that is roughly $473 per summer. For ethical reasons, such as reducing one’s carbon footprint, we should seek to minimize this energy consumption, and there are practical ways to do it without major sacrifices of comfort.
The strategy begins by controlling the unwanted flow of heat from the exterior to the interior. The best possible insulation, including double-glazed windows and tight weather stripping will retard unwanted heat flow between indoors and outdoors. Note that these measures contribute even more toward conserving heat in the winter when the temperature difference between indoors and outdoors is far greater. Equipping windows with shades, curtains, or blinds whose outside-facing surfaces are white will also prevent a significant influx of heat.
We should know that an efficient modern air conditioner will use less than half as much energy as an older (pre-1990) less efficient model.
Even more important, but less obvious, is a smart strategy for using the air conditioner. This plan invokes some basic physics, which in this context pretty much coincides with “common sense.” First, heat flows spontaneously “downhill,” that is, from high to low temperatures, but insulation can slow that process. Conversely, to force heat to flow “uphill” (from a cool to a warm place) requires a refrigerating machine (in this case the air conditioner) which must consume power to do that work. Just as in pumping water uphill, the greater the temperature difference (corresponding to height difference) and the more heat you pump, the harder the machine must work, and hence the more energy it consumes.
During summer nights, however, you want to create just the opposite effect by opening up the house. Specifically, when the outdoor temperature drops below the indoor, open the windows and allow the hot indoor air to exchange for cool air. Installing a large window fan or attic fan to draw air through the house will considerably enhance this effect. Such a fan consumes about 250 kWh, or less than a tenth of what an air conditioner uses. Ventilate until the interior temperature approximates cool outdoors. This usually takes 3 to 4 hours with a fan. In the morning as the outdoor temperature starts to rise, seal up the house to shut out the daytime heat. This scheme works almost every day, enabling you to “keep your cool” with the indoor temperature remaining below about 80 °F until you repeat the ventilating cycle the next night.
Unfortunately, a few nights will be so warm that one cannot achieve sufficient cooling by morning, and if the day is forecast to be so hot as to require air conditioning, then it is best to run it for only an hour or two in the morning. This has two advantages. First, the air conditioner has to pump heat across a relatively small temperature difference, so it consumes less power. Second, it avoids imposing a heavy load on the utility grid during the peak demand period in the afternoon.
And now the results: Last summer, an elapsed time meter connected to our air conditioning compressor recorded only about 25 hours of running time over the entire summer season, and hence less than 1/30th what an ordinary air conditioning routine would consume. And, as explained above, even when the A/C did run, it had to pump heat over the smallest possible temperature difference.
We will naturally repeat the plan this summer.