When (And Why) Heat Pumps Suck

By Ron Barmby

When you need heat pumps the most, they pump heat the least.

This is according to an unbreakable law of physics that applies to the entire universe without exception, even black holes, and especially to heat pumps.

Heat pump promoters who claim otherwise must have defeated the second law of thermodynamics, which would theoretically also allow them to make time run backward.

In addition to denunciation by physics, the cost of operating heat pumps is often very high compared to natural gas-fired heating.

Heat pumps are often accurately described as an air conditioner in reverse. To understand how a heat pump works, it helps to know how an air conditioner works.

How An Air Conditioner Works

An air conditioner transports heat from inside a house to the outside in a continuous circuit using a chemical mixture called a refrigerant.

The refrigerant is piped into the house as a liquid under high pressure and at a temperature just above the outside air temperature. It expands into a gas across a valve that has a lower pressure on the other side and becomes very cold, about 4°C (39°F).

This part of the air-conditioning unit is called the evaporator because it causes the liquid refrigerant to evaporate into a gas. The cooling is a bit similar to when you let air out of a tire and the expanding escaping air feels cold. (If you want to know more details, look up the Ideal Gas Law.)

The pipe with cold gas-phase refrigerant is then passed in front of a fan inside the A/C unit (or furnace if it’s central A/C) to cool the room.

The second law of thermodynamics dictates that two bodies at different temperatures will equalize in temperature when brought into contact with each other.

How fast this equalization occurs is driven by the temperature difference between the two bodies: the bigger the temperature difference, the faster the heat exchange happens.

The pipe containing the refrigerant gets cold because it’s in contact with the cold refrigerant, and the air outside the pipe gets cold because it’s in contact with the cold pipe.

A fan speeds up the contact of the warm air in the room with the cold pipe and as the heat from the air is transferred to the refrigerant, the refrigerant gets warmer.

The low-pressure and now warm refrigerant is then piped outside the building to an electrically driven compressor, which reduces the gas in volume and increases its pressure.

The compressor puts energy into the gas causing it to heat up (the Ideal Gas Law again). The hot high-pressure gas is then cooled to near outside temperature by passing the pipe containing the gas for several loops in front of a fan that uses outside air to cool and condense the gas into a high-pressure liquid (which is why it’s called the condensing unit).

The high-pressure and near-outside-temperature liquid refrigerant is then piped inside the building and the whole circuit starts over again.

This circuit is called the Carnot Cycle after the French physicist Sadi Carnot who discovered it in 1824. It wasn’t until 1902 that American engineer Dave Carrier produced the first air conditioner.

How A Heat Pump Works

To convert an air conditioning unit into a heat pump, you place the evaporator (which attracts heat) outside of the house and the condenser (which expels heat) inside the house.

Furthermore, the heat pump refrigerant is chemically different so that it can be cooled in the evaporator to minus 25°C (minus 13°F). The refrigerant has to be colder than the outside temperature to absorb outside heat.

The two perceived advantages of a heat pump are:

• If the electrical energy to run the heat pump was produced without carbon dioxide (CO2) emissions, it’s considered a green heating source. But all electrical generation has some form of environmental impact, most likely out of mind at a remote site.
• Because it collects free heat from the outside and delivers it inside, the only cost to run a heat pump is the electrical energy for the compressor and fans. A heat pump is more energy efficient because it transfers existing heat, rather than creating new heat by the combustion of a fossil fuel or biomass.

But here is the catch: Energy efficiency is not the same as economic efficiency.

At an outside temperature of 10°C (50°F) for every unit of energy used to run the heat pump (purchased electricity), the heat pump can collect four units of energy from the outside (heat delivered into your home). At this outside temperature, the efficiency of the heat pump is 400%.

When the outside air temperature drops to minus 20°C (minus 4°F), the efficiency of the heat pump drops to 200%; one purchased unit of energy input delivers only two units of free heating energy.

This is a result of the ambient temperature (minus 20°C) approaching the refrigerant temperature (minus 25°C) and reducing the rate of heat exchange between the two. It’s the second law of thermodynamics at play. The smaller the temperature difference, the more slowly the heat exchange occurs.

A conventional natural gas furnace delivers only 0.9 units of heat energy for each unit of purchased energy, for an energy efficiency of 90%.

When Heat Pumps Suck

Where I am writing normally has temperatures colder than minus 20°C 20 days per year, and the retail cost of electricity is nine times more than natural gas on an energy equivalent basis.

With an outside temperature of 10°C, to heat a room to the same temperature a heat pump would use only 22.5% of the purchased energy of a natural gas furnace (22.5% = 90%/400%). But the total cost of that purchased energy would be twice as much (22.5% X 9 = 2).

Under mild weather conditions, the heat pump already has twice the operating expense as a natural gas furnace. Under common winter conditions of minus 20°C, the energy efficiency of the heat pump drops to 200%. It uses only 45% of the purchased energy, but that purchased electricity costs four times more than natural gas.

Your natural gas bill charges you by the gigajoule (GJ) and your power bill charges you by the kilowatt-hour (kWh). Currently where I live natural gas is $4.89/GJ, and electricity is $0.16/kWh. I have a 90% fuel-efficient natural gas furnace.

The following temperature scenarios provide a rough approximation of the difference in cost between running a furnace and running a heat pump where I live:

• The outside air temperature is 10°C or warmer—Divide the gas cost by 60 to get the kWh equivalent energy cost. If this number is smaller than the cost of electricity in kWh, then it is cheaper to operate a natural gas furnace than a heat pump. $4.89/GJ (gas) divided by 60 equals $0.08/kWh (electricity). In this scenario, heating my home with gas is half the cost of electricity needed to operate a heat pump ($0.08/$0.16).
• The outside air temperature is near minus 20°C—Divide the gas price in GJ ($4.89) by 120 to get the kWh equivalent cost ($0.04). My natural gas furnace cost is one-fourth of the cost of operating a heat pump.
• The outside air temperature is minus 25°C or colder—Most retail models of home-use heat pumps will probably not work at all because the refrigerant has to be colder than the outside temperature. Many heat pumps have a conventional electrical resistance heating element built in as a cold temperature backup. This is confirmation by the design engineers that they can not beat the second law of thermodynamics.

[Note to reader: One GJ of natural gas equals 278 kWh of electricity, and is roughly equivalent to one million British Thermal Units (BTUs).]

What Heat Pump Promoters Won’t Tell You

Granted, in moderate climates where winter home heating is more for comfort than survival, and especially where summer air conditioning is desirable, a heat pump that’s switchable to an air conditioner is probably worth looking into. They are more energy efficient, but there’s more to consider. For example:

1. Compared to natural gas furnaces, heat pumps can have a much higher operating cost, which governments attempt to overcome by taxing fossil fuels and using those taxes to subsidize “green” electricity.
2. Natural gas and other combustion furnaces provide instant heat and can warm up a cold house much faster than a heat pump can.
3. In very cold weather heat pumps suck.

If heat pump promoters deny the above statements, they must be Oppenheimer-smart and have found a way around the universal second law of thermodynamics. Ask them if they are working on a time machine next.

This commentary was first published at Climate Change Dispatch on February 19, 2024.

Ron Barmby (www.ronaldbarmby.ca) is a Professional Engineer with a Master’s degree, whose 40+ year career in the energy sector has taken him to over 40 countries on five continents. His book, Sunlight on Climate Change: A Heretic’s Guide to Global Climate Hysteria (Amazon, Barnes & Noble), explains in layman’s terms the science of how natural and human-caused global warming work.