Thermostat Strategies, Part II

February 11, 2011 at 3:40 pm 2 comments

Now for the exciting conclusion to our previous post on thermostat strategies

If you have an electric heat pump you need to take special care when adjusting your thermostat in the WINTER only—this does not apply to summer air conditioning use.

Most standard Energy Star rated programmable thermostats are set to have the heat pump’s back-up heat (a.k.a. ‘emergency heat’, ‘heat strips,’ ‘auxiliary heat,’ etc.) turn on when the thermostat temperature is more than 2°F than the current temperature in the house.  Simply put, this means is that if you turn your thermostat up 3°F or more the back-up heat will kick on—we will call this the “2°F rule.”

So what?  Isn’t having a back-up a good thing?

It is in terms of comfort it is a good thing because this 2°F rule was created because a limitation of heat pumps is that they usually take longer for a home to heat up than an equivalent gas furnace—so the back-up heat helps get your house warmer faster.  But, the back-up heat uses 2-3 times more energy than a heat pump alone—meaning that you will pay 2-3 times more to heat up your home on the back-up heat (you are paying a lot more for speed and comfort).  Also, heat pumps quickly lose their efficiency below 32°F, so the back-up heat is necessary to supplement the heat pump on very cold nights and days.

What this all means is that if you turn your thermostat down to 60°F when you go to work, but have it kick on one hour before you come home to 70°F, then your heat pump will use the back-up heat from 60° to 68°, and then at 69° it will use the heat pump alone.  This usually means that you spend more money re-heating your house than if you had left your house at 70°F all day!

So, how can I have a heat pump and not pay 2-3 times more for heating than my cousin who has a gas furnace?

  1. One way to deal with this is to set your house down 2°F below what you like it at when you are home.  To continue the example above, you would want to set the thermostat down to 68°F during the day and then back up to 70° for when you get home.
  2. Another strategy is to turn it down a few degrees more and then have it come back up in 2°F increments.  For example, you could have the thermostat go down to 66°F during the day, have it come up to 68° an hour before you come home, and then when you get home make the thermostat 2°F higher than the current temperature in the home.
  3. You can have a heating technician disable your back-up heat.  However, you may find that you have a hard time keeping your house at a comfortable temperature—especially on very cold nights.
  4. You can also buy a special thermostat that allows you to disable the heat pump’s default 2°F differential rule (one that has an “adjustable deadband” or “differential” for the back-up heat strips).  They usually cost 2-3 times more than a standard thermostat, but can make a huge difference in your heating bill.

There are different types of heat pump thermostats:

  • Most allow you to manually enable/disable the back-up heat at the thermostat.
  • Some allow you to control the 2°F rule to make it any temperature difference you choose (“differential” control can be set to 4°, 5°, 6°F, etc.).
  • Some allow you to control the 2°F rule by giving the house a chance to heat up on its own without the backup heat for a given amount of time (this is sometimes called a “stage timer” setting), and if the heat pump has not successfully heated the home within that time period then the back-up heat strips will kick on.
  • Others have a remote sensor outside that allows the back-up heat to come on only when it is below a certain temperature outside (usually less than 30-40°F)

The easiest thing you can do is to keep your thermostat at a relatively constant temperature.  Keep in mind that the lower this temperature is the more money you will save!

But to maximize energy savings, you might consider buying a special heat pump thermostat (Globe EnerSaver 59025 is the best example, see pg. 46-48—it can be bought here) and play around with the 2°F rule settings and  until you get a good balance of COMFORT and ENERGY-EFFICIENCY.  Also, check out this great blog entry about one man’s heat pump programmable thermostat adventure and the source of the recommended thermostat above.


Entry filed under: Energy Tips, Learn Do Teach workshops.

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2 Comments Add your own

  • 1. Daniel  |  February 11, 2011 at 7:29 pm

    As a proud owner of a Globe Enersaver, I’ve got to point out one caveat that actually hurts its performance with heat pumps. The thermostat’s logic asks two questions before it turns on the backup heat: how many degrees am I below the set point (differential) and how long has the heat pump been running (lag).
    You can adjust the differential by about 7 degrees, which is great, but you can only adjust the lag by an hour. So, if you expect the house to heat up from 66 degrees to 70 and it takes longer than one hour to do so, the back up heat will come on!
    You can manually disable the auxiliary heat by pushing the reset button on the front panel with a pin, then selecting “no furnace” instead of “electric furnace” but you’ll need to remember to set it back if it gets very cold outside!

    • 2. Clean Energy Durham  |  February 17, 2011 at 7:22 pm

      Thanks Daniel! I searched literally for hours to find one that had the lag and differential connection as you describe, but to no avail. Thanks!


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