Electronic Digital Thermostat

DEAR TIM: I just brought a programmable thermostat for my home, and I started to program the unit to my own settings. I found out that the unit came with a factory setting such that when I went to sleep, and during the time I went to work the temperature dropped to just 60F.

I scaled this temperature back to 55 degrees, as I did not see any reason why the system would have to run at 60 when I'm not home. But here's my question: Is it better to leave the system at 60 or 55 degrees when I'm not home? Will it take more money for the unit to heat up from 55 degrees, than to heat up from 60 to 68F. Will the cost of the unit continually keeping the temp at 60 outweigh the unit having to pump out heat to go from 55 to 70F? Theresa H, Easton, MD

DEAR THERESA: That is a very common question and the answer might surprise you. As with many things there are some variables that come into play. We need to talk a little physics here so strap yourself in.

There is no doubt that you will save money the lower you set the thermostat. When you are gone from the home for extended amounts of time and/or are sleeping, it makes perfect sense to ratchet the thermostat back to 60 and even 55F. It makes even more sense as the outside temperature begins to plummet.

When the outside temperature is vastly different than the inside temperature, the heat pump or furnace needs to run longer to offset the increased heat loss created by the falling temperatures. But as the outdoor temperature rises and moderates to where it is maybe just 15 or 20 degrees F lower than the inside temperature, the heat pump has a far easier job keeping up.

This happens because the temperature drop of an object over time is not a linear function. As the temperature of an object starts to get closer to the temperature of its surroundings the drop in temperature slows considerably. You can demonstrate this easily with a standard thermometer that you run under hot water. Once the thermometer is removed from the hot water the mercury starts to drop quickly but then slows to a crawl as it tries to get back to room temperature.

When the thermostat calls for heat and you have to go from 55 to 70F, you will use more fuel or energy than if it only had to climb from 60 to 70F. But good heat pumps and furnaces can make up that 5 degree difference fairly quickly.

I can tell you for a fact, I would set back the temperature to 55F to save money. The wild card here is how long you will have the thermostat set to 55F. The longer the heat pump sits idle, the more money you save.

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