I'm tired of coming home to a cold house!

April 4, 2008 2:01 PM

Which approach really burns more heating oil - keeping the house at a constant medium temperature (say, 64 degrees) or keeping it very low most of the time (58-60) but bringing it up to a higher temperature (say, 66 degrees) once or twice a day?

We have a single thermostat (in the dining room, which is between the kitchen and the living room) which is computer controlled. Our home is a 1913 Queen Anne style house with an open floorplan (arches, not doors, between the four rooms on the first floor). In accordance with conventional wisdom, we let the house cool down to about 59 - 60 degrees when we're sleeping or at work, then heat it to 64-66 when we get up in the morning (about two hours) and in the evening when we're home (about four hours). (We have a 21-month old baby, so I don't like the house to be too cold when he's awake and home.) On weekend days when we're all home for the entire day, I will keep the house at about 64-66 degrees for the entire day.

What I've noticed over the course of the winter is this: it feels as though the furnace works longer and harder getting the house warmed up (from 60 - 64, say) than it does keeping the house at a constant 66. I haven't timed it with a stopwatch, but if I'm up early in the mornings with the baby I'll notice that the furnace kicks on more than an hour before the target time for the computer-set temperature, and this happens again in the afternoon before we get home from work (although the house cools more overnight than it does during the day). Then the furnace continues to work for 10-15 minutes every hour keeping the house warm for the 4-5 hours we want it warm in the evening, since the warm air is working its way up to the bedrooms and other rooms on the second floor that are in use.

By contrast, when we're home all weekend and are keeping the house at a constant 64-66, by Sunday morning the furnace is only kicking on about once an hour for perhaps five minutes or so. Also, the upstairs bedrooms and bathroom have a chance to get fully warm as the air circulates around the house. (They're always chilly during the week, since the single thermostat is downstairs.)

In short, from simple observation I'm always left with the impression that the furnace is working much less hard to keep the temperature constant than it is to warm the house twice per day.

I know what "conventional wisdom" says - it says let the house sit cold most of the day. But is this really true for all homes? Since I can't be home all week to run a stopwatch to see how long the furnace is running for, and I don't have any way to tell exactly how many gallons of oil we're burning per furnace run, is there any other way to check to see which approach is actually burning less oil?

posted by anastasiav to home & garden (11 comments total)

When I worked at an architectural consulting firm that specialized in energy efficiency, a lot of the projects they worked on for the state and major utilities involved promoting the use of programmable thermostats so that people would remember to reduce the heat at night and while away from the house. I suppose it's possible that your house is an exception somehow (the people who could calculate this would probably need information about the insulation, windows, climate, and solar exposure of your house to actually do so). But this "conventional wisdom" that for houses in the state of California on average, it's better to turn down thermostats at night and while at work, is coming out of a lot of studies that a lot of highly-educated specialists in this area (not that I'm one) got paid a lot of money to do. I'm not saying you're wrong about your house in particular, just that this idea is not some old wives' tale.
posted by salvia at 2:26 PM on April 4, 2008

I'm guessing that heat escapes your house at the same rate (dependent on time-of-day) regardless of internal temperature. So maintaining a higher temp isn't going to mean less energy needs to to be applied overall. The outflow of heat is the same.
posted by mrnutty at 2:40 PM on April 4, 2008

Keeping your house cooler saves more energy. Anything else would violate conservation of energy. The fallacy here is that temperature does not have "momentum" - keeping your house at a constant temperature is just a special case of the output heat of your furnace matching the insulation loss.

I'm guessing that heat escapes your house at the same rate (dependent on time-of-day) regardless of internal temperature.

This is exactly wrong. Fourier's law states that the rate of cooling is proportional to the temperature gradient.
posted by 0xFCAF at 2:44 PM on April 4, 2008

I think it would also depend on how long you are out of your house. Depending upon your insulation, keeping the house at a constant temperature for 8+ hours could possibly cost you more than keeping the house cold and heating it once.
posted by The Light Fantastic at 2:44 PM on April 4, 2008

Fourier's law states that the rate of cooling is proportional to the temperature gradient.

So, in general, will a hotter house lose heat faster or slower?
posted by mrnutty at 2:52 PM on April 4, 2008

So, in general, will a hotter house lose heat faster or slower?

It will lose it faster. The larger the gradient, the faster the transfer.

Dittoing what everyone else said. Keep it cool at night and while you're gone to save money on heating. (and, of course, the opposite is true for air conditioning).
posted by chrisamiller at 3:13 PM on April 4, 2008

Seconding salvia's suggestion of a programmable thermostat. I bought one from Amazon - dead easy to install (the wiring is explained in the instructions - took about 10-15 minutes). It has saved at least 10% on the heating bills for comparable monthly average temperatures this year -- I was constantly forgetting to turn the thermostat down as I leave the house, so this does it automatically at certain times! I can also adjust the temperature more precisely -- a change of -2 degrees still saves money but is difficult to make with an old-fashioned rotary thermostat.
You can get a wall-mounted fan heater with a pull-cord, to solve the bathroom temperature problem (safer than pushing electrical switches with wet hands). And OxFCAF is entirely right about Fourier's law. However, the temperature gradient (the difference between inside and outside temperature) is dependent on the amount of insulation in your house, particularly the roof. If upstairs is cold, you could probably save most money by putting another layer of insulation into the roof space, on top of the existing insulation.
posted by sgmax at 4:07 PM on April 4, 2008

Depending upon your insulation, keeping the house at a constant temperature for 8+ hours could possibly cost you more than keeping the house cold and heating it once.

Possibly? How could maintaining a certain temperature from t = 0 hours onward ever cost less than maintaining a certain temperature from t = 8 hours onward?
posted by Mapes at 4:11 PM on April 4, 2008

Funny, I hadn't even meant to be recommending a programmable thermostat. But sgmax is right -- if you got one, not only would you save money, it would also solve your "tired of coming home to a cold house" problem. You could set it to start warming up a half hour before you'd be back, or a half hour before you wake up. Ah, warm mornings...
posted by salvia at 4:16 PM on April 4, 2008

We have a single thermostat ... which is computer controlled.

He already has a programmable thermostat.
posted by sophist at 4:20 PM on April 4, 2008

It may seem like the furnace is running a lot longer to heat the house up for the 8 hours you want it warmer, but this is more than balanced by the 16 hours a day that the furnace runs little if at all.

This question comes up often. It would really be a nice feature, and trivial to implement, for those computerized thermostats to log the furnace running time per 24 hour day and allow you to scroll through the record. This would allow you to see the actual data showing that allowing the house to cool down really results in less total furnace running time.
posted by JackFlash at 10:07 PM on April 4, 2008