A recent conversation reminded me that many people believe it’s a bad idea to set your programmable thermostat too low, asserting that it will use more energy to bring your house back up to temperature than it would to leave the temperature closer to normal.
This is wrong. False. Myth. Not true. No way, no how.
(Update: 12/2010: More detailed scientific theory about why programmable thermostats will indeed save money, if you use them correctly in a new post.)
Every moment your house is warmer than the outside air, (heat) energy is leaking out. The greater the difference, the more energy leaks out.
Every moment your home heater is on, energy is being used. The longer it’s on, the more energy is used.
Say you set your thermostat to 68° in the afternoon after school or work, then down to 50° at night. Your neighbor, who lives in an identical house (I guess in some condo complex or something :-) also sets her thermostat to 68° in the morning then down to 60° at night. It gets cold at night, say 20
It will indeed take longer to raise the temperature of a house from 50° to 68° than it would to raise the temperature from 60° to &68deg; for the same house and outdoor temperature.
Over the course of a given day your neighbor’s heater will be on longer. And that’s all there is: a burner is either on or off, so length of time on is pretty much all that matters in energy consumption. (There are some ways to optimize, such as running a circulator pump after the burner is off, but that’s not relevant, here).
It is true that it will take longer for your house to get to 68°, and your floors may not heat up as fast as the air. I read a couple things about issues with condensation which might apply to summer, but not winter.
The link above is to a 5 + 2 day thermostat with four settings per day, from Honeywell — $28.90 right now. That means you can have different weekday and weekend settings. It installs with a screwdriver — no, really, it’s super easy — there are two low-voltage wires to disconnect from your current thermostat and attach to the new one. They are very light, so don’t need any fancy mounting to the wall, and use a 9V battery. If you’re home during a time it expects you to be away, there’s a simple override; it will switch back into automatic mode at the next change time.
We have ours set to:
- 4:30 am — 67° (morning warm-up)
- 8:30 am — 50° (burner usually off 7 hours)
- 3:30 pm — 67° (afternoon warm-up)
- 9:00 pm — 55° (burner usually off 7-1/2 hours)
It would be really nice if there were a thermostat that could adjust it’s start time to “reheat” the house based on outside temperature, since it takes longer the colder it is outside.