155) A Myth: Turn down heat at night

by Gail on December 12, 2011

Heat pump

In a previous post I reported on a green building forum presented by the Sunshine Coast Regional District.

I didn’t tell you one of the most important things I learned from Murray Frank, the speaker from Constructive Home Solutions.

He told a story of his British father-in-law, whose lifelong habit was to turn down the heat every night, to save energy. After his home heating system was updated to an air-to-air heat exchanger (or heat pump), he continued to turn down the heat each night before going to bed. His son-in-law advised him not to turn it down, but he continued to do so, while steadfastly denying that he was. When s-i-l saw him turning it down, he devised a plan. He installed a fake thermostat in place of the thermostat f-i-l adjusted each night. Then, he installed the functioning thermostat in a hidden location, set it to f-i-l’s daytime temperature. F-i-l continued to turn down the fake, but reported to s-i-l that the system was finally fixed, because his heating bills were considerably lower!

What gives?

Well, the heat pump works on the differential between the air inside and the air outside. If the thermostat needs to jump 2 or 3 degrees suddenly in the morning the (necessary) back up heating system (a furnace) jumps in to get it to the higher temperature quickly.

The furnace - back up heating system

Last winter, I observed the “auxiliary heat on” notation on the thermostat every morning, and was frustrated because the outside temperature wasn’t very cold. Wasn’t our heat pump working as promised?

Now that we’re not turning down the thermostat every night, the heat pump doesn’t have to raise the inside temperature suddenly, and our heating bills have gone down. (An aside: Since last winter, we have installed a simple electric heater with a blower fan in the studio, so there are additional electrical demands, too. Our heating bills should be higher, not lower.)

So, I conclude that turning down the heat at night doesn’t save energy. Take that to the bank.

At least, if you have a heat pump!

{ 5 comments… read them below or add one }

kano December 15, 2011 at 12:22 pm

I love the debunking of incomplete or non-holistic perceptions of “energy efficiency”. It is so important to consider the context of our “sustainability conscious” actions, not just the actions themselves.

One of my favourites is the relative “efficiency” of incandescent vs compact fluorescent bulbs… but that could turn into a rant. Maybe I should do that on my blog instead of yours. :)

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Ina June 1, 2012 at 2:26 pm

Network Thermostat has settings where the thermostat learns to slowly increase the heat amount to the desired temperature setting, so that your back up heat doesn’t kick in automatically. Turning down the heat can save on your power bills, it just has to be turned up slowly so your heat pump can keep up with the demands.

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E3 Engineer December 17, 2012 at 1:35 pm

Point well taken – setbacks are ill-advised for most heat pump owners. Two mitigation strategies: (i) as per Ina’s comment – install a fuzzy logic T-stat which senses the temperature rise or ramp-rate. As long as ramp-up continues the auxilary heat is not activated. (ii) for geoexchange or ground-source heat pumps ONLY the electrical breaker for the aux heat can be switched-off thereby precluding the T-stat call for auxiliary heat. Caution: don’t use the 2nd strategy if you are away for an extended period, as the aux heat provide redundancy if the heat pump should fail in your absence.

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ChemE November 24, 2013 at 11:41 pm

For the all heating systems that do not have duel components setbacks are more efficient. The only reason you were showing a savings is that your heat pump either runs at a higher efficiency than your furnace or the fuel source used for the furnace is more expensive. Therefor the cost of your furnace running to reheat your house was greater than the cost of the heat pump maintaining the temp all night. By no means does this apply to anyone with a single source of heat, even a lone heat pump.

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Gail November 25, 2013 at 8:58 am

Exactly, ChemE.

Gail

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