Making Heat, vs. Moving Heat
Many highly educated people are confused when they are told that heat pumps for HVAC and water heating are more than 100% efficient. After all, it seems to be counter to the Laws of Thermodynamics that we learn in Physics class!
Don’t worry — heat pumps don’t break the laws of physics! The confusion stems from a misunderstanding of what is meant by “efficiency”. In this context, efficiency is a function of the following formula:
- energy added to (or subtracted from) the medium (the air or water), as measured in the duct
- divided by…
- energy put into the heating/cooling system (the water heater, furnace, air conditioner, or heat pump)
In other words, if you put 1 kWh (1000 watt-hours) (or kilojoules, or BTU, or kilocalories) of energy into the heating system, and you get 1 kWh (or kilojoules, or BTU, or kilocalories) of heat into the air or water, you are 100% efficient. If you spend 1 kWh of energy but get only 0.9 kWh of heating or cooling, you are 90% efficient. But how can you get more heat into the air or water than you put into the system (the heat pump)?
By the way, 1 kWh = 859.2 kcal = 3409.5 BTU = 3600 kJ
Traditional heaters work by directly generating heat. The best you can do is to transfer all of that heat into the medium. Heat pumps move heat between two mediums.
Traditional Electrical Heaters
A traditional electrical heater generates heat by passing an electrical current through an element — a wire or coil — that has high electrical resistance. The electrical energy (voltage) that is lost during its transit through the heating element turns into heat. In theory, the maximum efficiency of the element is 100%, but this assumes that:
- the voltage on the output of the element is zero volts, and
- that all of the resulting heat is transferred to the air (in the case of HVAC) or water (in the case of a water heater).
Neither of which is ever the case. The very best traditional electric water heaters have a Uniform Energy Factor (UEF) of 0.93-0.95, meaning that they are up to 95% efficient.
Traditional Gas Heaters
In a gas (or wood, or oil, or coal) heater, the fossil fuel is ignited, and the burning fuel releases heat — along with pollutants such as CO2, carbon monoxide, nitrogen oxides, hydrocarbons, and particulates that are secondary byproducts. That heat is then transferred through a membrane (usually metal) to the medium (air or water) that we want to heat. Think of it like heating water on the stove. That transfer is inefficient — some of the heat goes into the surrounding air or “up the chimney” (or flue, or vent pipe) and heats our atmosphere. The very best gas furnaces (used to heat air) use secondary heat exchangers, which capture some of the vent heat and claim UEF of 0.987, or 98.7% efficiency.
Tank vs. Tankless Water Heaters
There are two options when it comes to water heat — tank and tankless. Most water heaters heat the water and store it in a 50- to 80-gallon tank. The advantage of this kind of system is that it buffers the supply of water against demand, which means the heater itself does not have to create hot water at the same rate (measured in gallons per minute, or GPM) it is being demanded, which in turn means it does not have to instantly draw on as much electricity or gas to heat that water. The downside is that hot water loses heat through the walls of the tank as it sits, waiting to be used. This is an issue especially if the tank is poorly insulated, or if hot water goes unused for a long period, as in a vacation home, or in your primary residence when your family goes on vacation.
A tankless water heater generates hot water on-demand. If the system is sized appropriately, it means you have effectively unlimited hot water. But it also means that the capacity of the system has to be sized to generate as much instant hot water as will ever be demanded of the system at once, as from multiple showers, dishwashers, washing machines, etc.
And if that happens, it draws and extraordinary amount of energy. A tankless gas water heater burns a lot of fuel, very quickly. And a tankless electric water heater can pull a huge current. One way around this is to install separate tankless heaters at every point of use, but this is expensive and not always practical.
Tankless water heaters are more expensive to purchase, but they do save energy, and they last up to 2 or 3 times as long as tank heaters, as long as they are well maintained. And they take up much less space. They are therefore a better “green” investment than traditional water heaters.
But (and this is a big but) the efficiency of both gas and electrical tankless water heaters are still limited by the Laws of Thermodynamics. In other words, they are less than 100% efficient. They are therefore less “green” than heat pump water heaters.
How Heat Pumps Work
Heat pumps work not by generating heat, but by transferring heat between two media. Those media may be air, water, or even earth! It therefore does not have to create the heat itself… it basically steals the heat energy from one place and delivers (or “pumps”) it to another. In this way, a heat pump can be 300% or even 400% efficient, in terms of electricity input vs. heat delivered!
This is not a new concept at all. Air conditioners have done this for decades — they transfer heat from the air inside your house, to the air outside your house. This is the same way a heat pump works when in cooling mode. A heat pump in heating mode is quite a bit like an A/C that is working backwards… it transfers heat from the air outside your house to the air inside your house.
Factors that Impact Heat Pump Efficiency
The efficiency of a heat pump depends on the following factors:The temperature of the medium to be heated or cooled
- The temperature of the external medium to or from which heat must be pumped
- The refrigerant being used in the heat pump
- The efficiency of the mechanicals (the motors, coils, radiating fins, etc.)
Efficiency depends to a significant extent on the temperature difference between the two media. In other words, they are less efficient in heating when there is less heat to “steal” from the source (i.e. it is very cold outside) or in cooling mode, when it is harder to dump the heat (as when it is very hot outside).
For this reason, some heat pumps have heat exchangers that are buried underground, allowing them to transfer the heat of the ground to the house, or vice-versa. While more expensive to install, this is especially effective in extreme climates that get very hot and/or very cold, since the ground several feet down maintains a more consistent temperature than the outdoor air.
Heat Pump Water Heaters (HPWHs)
The same principles apply to a heat pump water heater (HPWH). The heat pump “pumps” heat from the air (or ground) into the water, which is then stored in a well-insulated water tank. A tank is generally required because, while several times more efficient, it is slower than an on-demand water heater, and requires the buffer against high instantaneous demand.
A HPWH heat exchanger can of course be outdoors, but unlike an HVAC heat pump, it can also be indoors or in the garage (mine is in my garage). The only caveat for an indoor HPWH is that there must be enough air volume around the heat pump to provide sufficient source heat. And bear in mind that an indoor HPWH will cool the air, in effect acting as an air conditioning unit. This is great in the summer, when you want to cool your house, but not so great in the winter, when it will add to your space heating burden.
Also unique to HPWHs vs. HVAC heat pumps is that the temperature of the incoming water also affects efficiency — if it is already relatively warm, it does not require as much energy to heat it to the target temperature.
Many if not most of the HPWHs made in the US are what is know as “hybrid” systems… they have both heat pumps and a backup electrical heating coil. The purpose of the coil is to provide supplemental heating when the air temperature is too cold to provide efficient water heating. Obviously, coils are far less efficient that heat pumps, and this affects the overall efficiency of the system.
Regardless, heat pumps are far more efficient than traditional gas or electric furnaces or water heaters.
About Working Fluids (Refrigerants)
Heat pump efficiency is also a function of the “working fluid” that is used as the refrigerant in the heat exchanger. This chemical is compressed such that it is near its “transition temperature”: when it is warm, it is a gas, and when it is cold, it turns into a liquid. The transition between those states either absorbs (as when going from liquid to gas) or releases (going from gas to liquid) a tremendous amount of heat.
There are several chemicals being used as refrigerants in heat pumps, these days. They vary based on their efficiency at different temperatures and pressures, and in their function as greenhouse gasses. There is a good explanation of this at this link.
To summarize, hydrofluorocarbons (HFCs) initially replaced chlorofluorocarbons (CFCs), which were banned by the Montreal Protocol because of the harm these chemicals were doing to the ozone layer in the upper atmosphere. These are still in wide use in the US. But they are powerful greenhouse gasses, with CO2 equivalence of 675 (for R-32) to 2088 (for R-410a)!
Using an HFC-based heat pump is still far better than a traditional gas or electric system, but there are better alternatives.
HC (hydrocarbon) based systems based on propane (R-290) are now available that have relatively low global warming potential and which operate well in the range required for heat pump systems. However, these hydrocarbons are highly flammable.
Even better is a CO2 based system. CO2 is future-proof as a refrigerant (CO2 equivalence of 1), and operates efficiently to below 0F (!!). They therefore require no supplemental electric coil to provide heating in cold weather, and are therefore incredibly efficient. I personally have a CO2 based HPWH from Sanden in my garage that has worked beautifully. The heat exchanger is a separate unit from the 80 gallon storage tank, so each can be replaced or serviced independently if necessary, but the quality of both is such that I anticipate a very long life from both.
However, CO2 based systems are less widely available, and require a high-compression system that must be more robust and is therefore either less reliable or more expensive. CO2 based systems are very popular in Japan from companies such as Mitsubishi and Sanden, but it may be harder to find qualified parts and service professionals, at least until they become more widespread.
but still in older systems, , the working fluid (refrigerant) used for most heat pumps nowadays is tetrafluoroethane, otherwise known as r-134a. It has replaced chlorofluorocarbons (CFCs) Unfortunately, r-134a has a CO2 equivalency factor of 1430, meaning that one kg of r-134a released into the atmosphere has the same impact as release 1430 kg of CO2.
A General Comment about Electrical Systems
Electricity is a secondary energy source — it was generated using a primary energy source. Most often (currently in the US) it is generated by a utility burning fossil fuels — which itself is inefficient and polluting — then transmitted and distributed to your home through wires and transformers that lose over 24% of that energy through resistive and inductive losses.
Ideally, the energy is generated locally, in order to minimize transmission losses, and it is generated using renewable energy such as wind or solar. So the best energy is rooftop solar.