Programmable thermostat settings upstairs and downstairs

by Paul
(Boston, Mass.)

I live in the Northeast and have programmable thermostats upstairs and downstairs. In colder months, since my young kids are at school all day and really only use the upstairs to sleep and take the occasional bath, I leave that set at 60 degrees most of the time; it bumps to 65 for a couple of hours when they’re getting ready for bed.

Downstairs I set to 58 at night and 68 during the morning, when everyone is home and getting ready to head out. I work at home in the basement, which has its own heat, so set the downstairs (first floor) to 65 for the majority of the day, back to 68 around dinner time and down to 58 at bed time.

My question is, would it be more cost-effective to set the upstairs temp higher during the day? Is the heat from downstairs just flowing up and getting lost?

I appreciate your thoughts.

Answer from Green Energy Efficient Homes

It's true that warm air rises, so if you keep the upstairs thermostat set lower than the downstairs thermostat for part of the day, some of the heat from downstairs will rise to the upstairs. But turning the upstairs thermostat won't improve the efficiency of the house. At the very worst, you'll use the same amount heat but more of it will come from the upstairs heating more (and the downstairs heating less). But it's more likely that there are natural obstacles such as doorways, stairwells, and other constricted places that at list partially impede the convection of heat from the basement to the second floor, so I expect you will save at least some energy by leaving the upstairs thermostat set low.

An easy way to determine how much energy you'll save by turning the upstairs thermostat down is to compare the basement and second floor temperatures during the day when the kids are out. If the basement is warmer than the upstairs, you're already saving energy. I expect there will be at least a few degrees difference, depending on how open concept your house is.

Remember that the bigger the temperature difference between the indoors and outdoors (or between two floors of a house), the more energy flows between them, even through insulation. Insulation just slows heat transfer down. So the more heat you produce in any given space in your home, the more it tends to escape to other places. That's why you're better off keeping rooms cold when you don't need them heated.

I've heard people - even a neighbor who worked for a heating oil company - suggest that it takes more fuel to bring a home back up to comfort temperature than to keep it at a comfort temperature. This is misleading - it's true in one respect but most people draw the wrong conclusion from it. The easiest way to understand this is to imagine two identical rooms that start off at 70F on a winter day.

If you let the temperature in one room fall to 60F, and you keep the other room at 70F, and then you start measuring the amount of power consumed over a one hour period to get the room back to (or keep it at) 70F, the room that had dropped to 60F will require more heat energy to bring it back up to 70F. But that overlooks the first part of the equation, namely that during the period that the first room's temperature was dropping from 70F to 60F, more energy was used in the constant-temperature room to maintain the temperature at 70F, and the larger the temperature differential between the two rooms, the larger the relative heat loss in the warmer room versus the slower room.

The best way to save energy in your situation is to take a three-pronged approach: (1) lower the thermostats in areas of the house you're not using, as you've already thought of; (2) do what you can to reduce heat loss through windows, doors, walls etc. for example using energy saving window coverings or plastic window insulation; draftproofing doors, adding insulation etc.; and (3) adding barriers between parts of the house controlled by diferent thermostats, for example by closing doors or by hanging floor-length curtains in doorways that don't have a door. Just be prepared for the occasional complaint from your family when one of them comes home earlier than expected and has to walk into a cold room ... but with the money you save, you should at least be able to buy them an extra hot chocolate!

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