Urban Myths

Since starting the Cold House experiment, many people have expressed the belief (or at least, suggested the possibility) that it is more energy-efficient to keep a house at a consistent warm temperature than to warm it once or twice a day and let it cool in between. Most of these people have advanced academic degrees, so although the suggestion seemed absurd to me, I decided to think about it awhile.

But I still think it’s absurd, and impossible, except under one unlikely condition. Here is a thought experiment to convince the disbelievers:

Suppose that you like to eat hot croissants for breakfast, and as a bedtime snack (mmm, croissants…) So, you need to heat up the croissants in the oven twice a day, at your preferred temp of 325ºF. So what do you do? In the morning you turn on the oven, heat your croissant, then turn it off. By evening the oven has cooled down– maybe to room temp, maybe not quite– so you turn it on again for your bedtime snack, then off again while you sleep.

Is there anyone who will argue that it is actually more energy-efficient to leave the oven running at 325º ’round the clock, rather than heat it from cold twice a day? If you are going to argue this, what is your rationale? And why is your oven off, right now?

If you consider the house as the oven, and the outdoors as the colder kitchen, it is exactly the same.

The one possible exception I can think of: if you have some sort of furnace or heating system which can operate incrementally, and which for some reason is much more efficient when running at, say, 10% of its ability than at 100%, then it might make sense to keep throwing heat outside for the benefit of generating it more efficiently. But I’m not sure what kind of heating system that would me. My plain old oil furnace has no gradations: it’s either bellowing a great flame of oil, or doing nothing. If anything, it seems to be at its best when called upon to do a good bit of steady work, rather than provide short intermittent bursts of touch-up heat.

The whole notion that keeping a house hot all the time is more fuel-efficient than keeping it hot some of the time reminds me of the urban myth that cold water boils faster than hot water. Which I also hear stated more often than I can bear.


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