Posted by: Li Ling Young | March 20, 2016

Your House is a Battery

(originally posted in the Vermont Zero Energy Home Pilot blog, where I write as a building science professional, rather than as a homeowner)

We used to get this question a lot, “Do I really save energy by letting my house cool off when I’m at work, since the heating system has to work so hard to warm the house back up later?”  Vermonters are frugal and it’s common to use a temperature “setback”: allowing the house to be cooler than is comfortable when no one is home, or when everyone is asleep. But, there is skepticism that it doesn’t actually save energy since warming the house back up (surely) uses so much energy.

My stock answer to this question is, Yes.  Courtesy of my colleague, Paul Scheckel, an analogy: When you want to make tea, do you keep the kettle hot all day, only to pour your orange pekoe at 4pm?  No, you turn the kettle on when you want the water hot.  The moral: there is no need to keep your house warm all the time if you’re only going to enjoy that warmth for a few hours in the morning and a few at night.  Caution, I’m going to contradict this later on: DON’T TAKE MY WORDS OUT OF CONTEXT.

Sometimes my questioner shoots back, “But my house doesn’t ever get down to 62 (or whatever the setback temperature is) degrees.”  The purpose of the setback isn’t to make the house colder, it’s to prevent the thermostat turning on the heating system (as frequently) during the setback period.

But why didn’t the house cool off?  Because it’s insulated.  Teakettles aren’t insulated.  The better insulated and air sealed a house is, the more slowly it cools off.  Very air tight and well-insulated homes cool off so slowly that they only call for heat infrequently, whether or not the house is in setback.  Efficient homes save less from thermostat setback than poorly-insulated and leaky homes.   Older, poorly-insulated homes can save quite a  bit of energy by using thermostat setbacks, and that’s probably why this strategy has taken hold in the Vermont imagination as a way to be frugal.

The reason heating the house back up after setback doesn’t incur an energy penalty is that most heating systems operate at the same efficiency no matter how much heating they have to do.  In fact some heating systems, particularly oil -burners, are a little more efficient when they run for a long time, as they would when warming the house back up.

Now comes a new type of heating system – the cold climate air source heat pump.  (Also occasionally known as: ductless heat pump, mini-split, heat pump, air source heat pump…)  The defining feature of a cold climate heat pump is that it’s variable speed; it doesn’t operate only in on-mode or off-mode, it can put out a little bit of heat or a lot of heat.  Cold climate air source heat pumps are most efficient when they are not working very hard: very much like a car rolling along at a constant speed.

Also unique to cold climate heat pumps is that they make less heat as the temperature outside drops.  So, if you combine the lower efficiency and lower output at low temperature you can see that you don’t want to ask your heat pump to do a lot of hard work when it’s very cold outside.

heat pump capacity temp

As the outdoor temperature (horizontal axis) drops, an air-source heat pump loses capacity to make heat.  This model only makes about 70% of it’s rated capacity (vertical axis) at -13 deg F.  As a side note, these heat pumps have been observed making plenty of heat at -24 deg F.  Cold climate heat pumps work great, but you have to size them properly for your climate and your house.

So how should one operate a house heated with a cold climate heat pump?  Do not set the temperature back, ever.  By keeping the house at the same temperature it’s like driving a car at a constant speed on a flat road: you only have to put a little energy in to keep going.  On the other hand, recovering from a setback period (changing the temperature of the house) is like accelerating a car uphill: you’re asking the car to work hard, it does so less efficiently AND your car has less power the steeper the hill is.  The car won’t be able to do what you’re asking , and it will use a lot of gas trying.  Or, to shed the analogy, your house won’t recover from the setback very quickly, and it will do so at the lowest efficiency.  So don’t accelerate and don’t drive uphill.

What about the teakettle?  Isn’t it wasteful to keep the house toasty warm when no one is home?   Remember, well-insulated and air-sealed homes don’t gain much from letting the house cool off.  In fact, given the unique nature of a cold climate heat pump it might actually make the most sense to heat the house up when no one is home.  If the warmest part of the day is at 1pm, when everyone is at school or the office, it might be best to ask your heat pump to get the house extra warm at that time, when it can do so at a higher efficiency, rather than expecting the heat pump to do a lot of heating work at 5:00 when everyone is home but the temperature has dropped 10 or 20 degrees.  But ONLY if the house is well-insulated and air-sealed so it can hold onto that heat energy long enough that you get to enjoy it later.

This is how a well-insulated house is like a battery.  You can fill it up with heat energy at the best time, and use the heat energy later when you want to be comfortable at home.  It only works if your house can hold the heat, and it’s only beneficial if you’re using a cold climate heat pump.

I have two purposes in explaining all this:

  1. Point out how important both building improvements and advanced heating systems are; and how they work together to make a whole greater than the sum of the parts
  2. Make a compelling case for why one operates a heat pump-heated home differently from a fuel-heated home.

Here’s my best advice on how to operate your heat pump for best efficiency and comfort

  • Set the temperature you want the house to be and leave the thermostat there all season (winter operation; in the summer just turn the AC on when you want it)
  • Leave all the interior doors open when no one is home (good idea all the time, but desire for privacy may trump this)
  • In very cold weather you can get more heat out of the heat pump by putting it in high fan speed.  It’s noisy, so I advise doing this when you’re asleep or when no one is home (again, using the battery-function of a well-insulated home)
  • Clean the filters regularly
  • Make improvements to the insulation and air tightness of your home!

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