We’ve heard people say that it does no good to turn your heat down at night because it takes more energy to bring the house back up to temperature in the morning than you save by turning the heat down. One person even quoted her heating contractor making this assertion. It’s simply not true, and a couple of basic laws of physics can explain why.

The first law, usually called “The Law of Conservation of Energy,” states that the energy going into a system (in this case, your house) equals the energy coming out of that system.  The total amount of heat put out by your heating system equals the total heat lost through the envelope. You heat the house, and eventually that exact same amount of heat escapes!

The second law relates to how heat flows from warmer to colder regions. The heat flows at a rate that depends on the temperature difference between those regions. For example if you keep your indoor temperature at 70°F and it is 30°F outside, you lose heat at one rate, but if the heat is turned down to 60°F and it is 30°F out, the heat escapes at a rate about one quarter slower, so less heat escapes during the period it is turned down.

Of course it’s true that your heating system will work harder during the time it takes to bring it back up to 70 in the morning, but that extra heat will ALWAYS be less than what you saved by having the temperature turned down during the night because you were losing less heat during the night.

Another way of thinking of it is: Suppose you have a container of water that has some small leaks at the bottom.  The higher the water level, the greater the pressure, and the faster the water leaks.

To maintain a constant level you need to add water at the same rate that it’s leaking.  If you allow the water level to drop for a while the leak rate will decrease, because the pressure decreases.  To bring the level back up you need to increase the incoming water flow for some time.

However, by the time the water gets back up to its original level the total amount of water that has leaked out will be less than if the level had been kept high. Since the total water coming in has to equal the total water leaking out, you use less water overall.

So it is with the leaking heat. Less heat will leak at the lower temperature than at the higher, so less heat will be used!

You will definitely have some savings, but you may ask, “How much?” Well, here in central Maine turning down your heat by 10°F for 8 hours can account for between 8% and 10% of your heating bill, a noticeable difference.  If your oil bill is $1000 dollars in a given year this can save you $80 to $100 per year. In very leaky old Maine homes it will make an even bigger difference.

It should be noted, though, that if you have a super-insulated newer home you might not even notice the difference since you are using relatively little energy to begin with and it takes a longer time for the temperature inside of the house to go down.

Some people can have trouble remembering  to turn the thermostat down before going to bed.  A simple solution is to invest in a programmable thermostat. For under $50 you can get a “5-2” thermostat which allows you to change the setting four times per day with separate settings on weekdays and weekends.  This allows additional savings because you can turn the heat down during the day when people are out of the house. These thermostats are easily overridden if you want to turn the heat up or down at different times.

Given the savings mentioned above, the payback time for this investment in a programmable thermostat is less than one heating season.

So – Next time someone tries to tell you it’s a waste of heat to turn your thermostat down overnight or for long stretches of the day, you can patiently explain that, actually, not doing so is a wasted opportunity to save on heating bills.

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side.  He can be reached at [email protected]  Cynthia Stancioff, MA, Public Administration, is an amateur naturalist and wordsmith.

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