Heat Pumps, Thermostats, and Bad Design

We have a gas furnace and a heat pump. When we put them in, a year or two ago, I calculated that at current prices for gas and electricity, it was less expensive to heat with the furnace than with the heat pump.

Lennox, the company that makes the thermostat controlling our heating/cooling system, has designed it on the opposite assumption. If I set it to “heat” it uses the heat pump by default, only switches to the furnace if the difference between the desired internal temperature and the actual internal temperature gets too large for the heat pump to bring the temperature back up in an acceptable length of time.¹ In order to get the furnace to operate instead of the heat pump I have to set the thermostat to “emergency heat,” which is not the obvious setting for routine use.

It isn’t just Lennox thermostats:

Heat pump thermostats are specially designed to control heating, cooling, and auxiliary heat systems, making them an essential component for homes and businesses with heat pump systems. Unlike standard thermostats, heat pump thermostats efficiently manage both heating and cooling functions by automatically adjusting to seasonal temperature needs. They also activate auxiliary heat when temperatures drop, providing additional warmth without overworking the heat pump. (From a different company’s web page)

My initial conjecture, after discovering that the inefficient default assumption programmed into my thermostat was not restricted to that model or company, was that it was due to federal or state regulation, possibly pushed by environmentalists imagining a future where efficient heat pumps powered by solar electricity replaced fossil fuel for home heating.

Poking around the web, I discovered that the conjecture was half right, half wrong:

Federal § 460.202 Thermostats and controls. (c) Heat pumps with supplementary electric-resistance heat must be provided with controls that, except during defrost, prevent supplemental heat operation when the heat pump compressor can meet the heating load.

…

California CEC Title 24 Residential Compliance Manual 4.2.1.2 Heating System Controls: The second control capability must prevent the supplementary electric resistance heater from operating if the heat pump alone can meet the heating load, except during defrost.

…

Both state and federal regulations specify that they are for systems where the alternative to the heat pump is electric resistance heating. A resistance heater simply converts electric energy to thermal energy, gets one joule of heat from one joule of electricity.² A heat pump uses electrical energy to pump heat from the colder outside to the warmer inside of the house. The electricity it uses is converted into heat, since energy is conserved, and added to the heat pumped into the house, so one joule of electricity provides more than one joule of heat, with how much more depending on the efficiency of the heat pump and the outside temperature.³ A heat pump is more efficient in terms of energy than a resistance heater, takes less electric power to heat your house. Whether it is also more efficient in dollars depends on whether the savings in the cost of electricity is more or less than the additional cost of the heat pump.

Since both electric resistance heating and the heat pump use the same input, electricity, and the heat pump uses less of it to accomplish the same thing, if you have both you are better off using the heat pump. So it makes sense to have a thermostat that does that — if your backup heating is electric resistance, which mine isn’t. It makes sense that the federal and state regulations require that kind of thermostat for a system where the alternative to the heat pump is resistance heating, as they do.

It does not make sense to control a heat pump/gas furnace system with such a thermostat.

Once you know how it works, there is a simple solution — set the thermostat to “emergency heat” and only switch to the heat setting if the furnace isn’t working or the price of gas gets high enough to make heating with gas more expensive. But there must be people who don’t realize that, who assume that “emergency heat” is for emergencies and scary, and waste a lot of money heating with the heat pump when they should be using gas.

There should be, perhaps are, thermostats designed for a heat pump/gas system, but I have not yet found any. I have not even found any installer web pages that make it clear that gas heating can be the less expensive option in a dual fuel system and recommend, if it is, the emergency heat setting on a thermostat that makes the heat pump the default.

I did find a web page on dual fuel technology, but its explanation of the role of the gas heating is:

if the heating demand exceeds the preset heating capacity of the electric heat pump, the heat pump pauses, and the gas furnace takes over until the indoor temperature reaches the desired temperature

It does mention that “The cost of electricity and natural gas in your location can affect the cost-effectiveness of a dual fuel system” but never takes the next step of recognizing that, where gas heating is less expensive than heating with a heat pump, you should only use the heat pump as an air conditioner unless the furnace isn’t working.

After reading that web page, it occurred to me that I might be letting my interpretation of the evidence be distorted by my political biases. I wanted to blame the inefficient design of my thermostat on government regulation, but in fact the regulation promoted an efficient choice, just one not applicable to my system. Perhaps I should blame the failure of firms that install dual fuel systems to make it clear to their customers that it might be cheaper to do their heating entirely with gas on a failure of the market. If they tell their customers that the heat pump they are buying is, under most circumstances, only a fancy air conditioner, they might not buy it.⁴

Diogenes went around Athens with a lantern searching for an honest man. I am now going around the Web looking for an honest heat pump installer. If any of you find one, let me know.

---

My web page

Past posts, sorted by topic

A search bar for past posts and much of my other writing

1. Or if the outside temperature is below 25°, the heat pump’s minimum, but I live in San Jose.

2. Electricity is usually measured in kilowatt hours, heat in joules or British Thermal Units, but they all measure the same thing: energy. 1 kilowatt hour equals 3.6 million Joules or 3412 BTU.

3. It takes more energy to move heat “uphill,” from colder to hotter, the bigger the difference in temperature. Also, if the heat pump is located outside of the house, some of the heat generated by running it will be lost to the outside, so in sufficiently cold weather the argument to show that the heat pump is more efficient than resistance heating might break down.

4. I bought mine, knowing that, for three reasons — as an air conditioner, as a backup heater, and in case electric prices went down or gas prices up by enough to make the heat pump the less expensive alternative.