Recently I’ve learned of two people running their heat pump in a way that costs them a lot of extra money. With heat pumps, there’s an extra setting on the thermostat.* In addition to heat, cool, and off, there’s a setting for emergency heat. But what is that setting for? What does it do?
A heat pump, as I wrote before, pulls heat from the outside air (unless you have a ground source heat pump, which pulls heat from the ground or a body of water). As it gets colder outside, your heat pump is able to pull less heat inside. Eventually it can’t meet the heating load of the house. That’s where supplemental heat—which is NOT the same things as emergency heat—kicks in.
What happens when your heat pump can’t keep up?
For most heat pumps, the supplemental heat source is electric resistance (strip) heat. When the heat pump can no longer pull enough heat from outside to meet the heating load of the house, the electric resistance heat comes on and supplements the heat pump. If you have an all-electric home, your supplemental heat source is almost certainly electric resistance heat.
If your home has natural gas, propane, or fuel oil, the supplemental heat may be supplied by a furnace. This is called a dual-fuel system. Most of these are connected in a way that when it gets too cold outside for the heat pump to supply all of the heat, the heat pump shuts off and the furnace supplies all of the heat.
If your supplemental heat is supplied by electric resistance, it’s 100% efficient. That may sound good, but it’s not. The heat pump, by that same measure, is 200 to 300 percent efficient, so when the heat pump by itself can’t supply all the heat your home needs, you at least want it to supply as much as it can. That gets you more of the 200-300% efficient heat and less of the 100% efficient heat.
Unfortunately, some people have a misunderstanding about how this works, and sometimes that misunderstanding comes from a surprising source.
What’s your emergency?
For some reason, a lot of people overlook that the thermostat says “emergency” (EM HEAT on the Honeywell thermostat shown* here) not “supplemental,” and think that when it gets cold outside they have to switch over to emergency heat. Now, here’s the kicker. Evidently they have good reason to think that, because in both of the cases I’ve heard about recently, their HVAC company told them to switch over when the outdoor temperature is “in the thirties” in one case and “below freezing” in the other.
How are people supposed to learn the correct way to use their thermostat for a heat pump when they’re getting such bad advice from the person who’s supposed to know!? If you switch to emergency heat, you’re going to pay a lot more, perhaps hundreds of dollars more, to heat your house.
Another interesting thing about one of these cases is that the owner had just had his electric furnace, which is all strip heat, replaced with a heat pump. He spent thousands of dollars to get a more efficient heating system, and the HVAC installer told him to bypass that extra efficiency and just run it the way his old electric furnace ran.
By the way, electric furnaces have now been banned here with the adoption of our new Georgia energy code. Hooray! Now we just have to teach owners – and installers – on the proper use of heat pump thermostats.*
Do it like this!
In short, if you have a heat pump with electric resistance heat as the supplemental heat source, keep the thermostat* in the “Heat” setting. Do NOT use the “Emergency Heat” setting unless it’s really an emergency; for example, when the heat pump doesn’t work. You probably won’t notice a difference in how well it heats your home, but you will notice a difference in your electricity bills.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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