Five Percent: Conserve Energy

Climate Change Is Important: Energy Conservation is the First Step


January 20, 2009

Don’t Set Your Programmable Thermostat Too Low (Myth!)

Category: Conservation,Household,Save Fuel,Tips – Tom Harrison – 4:00 pm

programmable-thermostatA 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.

Period.

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.

68 Comments

  1. Mark M — thanks for the comment.

    I hope your explanation helps — it’s a good way to think about the problem.

    For many, the logic is far from a no-brainer, and that’s largely because the actual science is not quite as simple as all that (as I wrote about last year in another post.)

    Maybe even simpler is this:

    You turn your heat down during the day when you’re at work, and turn it back on before you get home so your house is comfy. One day you get the flu so come home at noon to your cold house: you crank up the heat to “comfy” for the afternoon. Compared to a normal day, does your boiler/burner run longer, the same, or less?

    If it runs longer (hint: it does), you use more energy.

    Thanks for your comment!!

    Comment by Tom Harrison — January 16, 2013 @ 9:01 pm

  2. One item not noted in the cool down/warm up situation is house damage. I’ve noticed more nail popping in my drywall when I cool down too much (or fast) to below 62 F. when -20 F. outside. Plus moisture could also be an issue with cooling down too fast/much. Anyone heard of any research on cooling rates vs home damage? I’ve used a 5 time period thermostat & it seems to have lessened nail pops.

    Comment by Ronald Christopherson — January 29, 2013 @ 6:36 pm

  3. Hi Ronald,

    If your home cools fast, you may need to evaluate the building envelope to find out why. My house was built in the 1840’s and I have had a programmable T-Stat in it for 20 years with no noticeable house damage.

    Comment by Charlie — January 30, 2013 @ 12:37 pm

  4. Tom,

    I actually ran my furnace at a steady setpoint for one night and the next night the temperature and wind conditions were the same. The 2nd night I set it back to 50F. Although the home did not drop to 50F, there was a good drop. My burner runtime was 40 minutes less on the programmed setback night. The time period was from 10PM until 730AM when another setback occurs.

    The Ecobee TStat has a downloadable csv file that gives you all of the parameters that you would ever want. The data on your web portal is updated at 5 minute intervals and furnace run time is reported in seconds.

    Comment by Charlie — January 30, 2013 @ 12:41 pm

  5. Hi Tom, thanks for replying to my statement, and I would like to “attempt” to answer your question about the furnace “staying on longer if I come home earlier in the day”, not trying to be argumentative, but if my thinking is wrong, I gotta figure this out. I believe that the furnace does not stay on longer…I just turn it up…sooner. so instead of it coming up to 70° at 4pm, I turn it up to 70° at noon, now once it’s reached 70°, the cycle on/off time will be the same as when it was 60°, or any other normal days. And btw, my thermostat differential isn’t 7 degrees like I thought, it’s more like 2 degrees. Ok so maybe I’m looking at this the wrong way, perhaps its the “therms” in the heat that the furnace is producing, that is drawing more “energy”? so obviously 70° heat will produce more therms than 60° heat, and isn’t that how the gas company calculates payments ?

    Comment by Mark M. — January 30, 2013 @ 1:31 pm

  6. With regards to nail pops, I have also been told that letting the temperature in the house drop to low can lead to cracks in the walls and ceiling, due to different degrees of shrinkage during cooling and expansion during reheating of the various structural elements in the house. Is this really a concern?

    Comment by David — January 30, 2013 @ 4:51 pm

  7. We installed a Honeywell wifi thermostat in our church sanctuary and its been great in many ways. The convenience of being able to modify the schedule remotely is fantastic. We typically keep the temp at 50 in the winter except for Sunday morning and evening. The unit learns when to start to ensure its exactly 68 when we walk in Sunday AM, regardless of outside temp. Meanwhile, the other 3 dumb programmables need to be adjusted to earlier start times the colder it gets. One thing we could use is a smart wifi stat that has separate fan schedule (this one has ON, OFF, or AUTO (when heat is on), as we have a large open space, and running the fan longer would help move the air around. The Honeywell also does not have usage tracking to see how often the furnace cycles. Will the Ecobee do these two things? How about any others? thanks!

    Comment by Bob — January 30, 2013 @ 7:54 pm

  8. Just to echo the comment half way down about programmables and heat pumps….not always a good idea, and usually not a good idea with a geothermal/ground source heat pump. With oil and gas, there is typically only one stage of heat. No matter how high you turn the thermostat,or how much of a set back it’s recovering from, the heating uses a constant amount of energy. Not so with heat pumps. They are often sized very closely to the space they are heating. Raising the temp by more than a few degrees, or trying to recover from a large set back, will often engage auxiliary heat strips, powered by electricity, negating any savings from the heat pump.

    From experience in my home with a geothermal system, having setbacks in winter caused second stage and many times aux heat to engage, costing $6 or $7 per day to heat the home. Now I’ve locked the programmables at 70f, and the usage is about $3 to $4 per day.

    Home is in bucks county, pa, 2000 sq foot, two story. Cinder block 1st story, new full 2nd story in 2010.

    Comment by Stuart — March 9, 2013 @ 9:37 am

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