Founder | BPI® Energy Auditor Certified Professional
Heat pump electric water heaters are usually hybrid water heaters that use ambient heat from the air to heat water. It is an interesting process and highly efficient.
Hybrid hot water heaters can reduce energy usage and save you significant money on your utility bills. In fact, hybrid water heaters can pay for themselves in under two years. Plus, they have a longer lifespan than standard water heaters.
Hybrid water heaters, also known as heat pump water heaters, utilize an electric pump or compressor to take warm air from the environment and use it to heat water. In case there’s not enough heat in the air at all times, they can also use electricity as a backup—that’s the “hybrid” element.
This article details exactly how a heat pump water heater works and why it is so efficient. It’s a combination of several processes that makes the water heater as efficient as possible to save you money.
How Does A Heat Pump Water Heater Actually Work?
If you are on this page, you probably have at least a vague idea that heat pump water heaters are an efficient way to heat water in your home. Let’s discuss how that actually happens. There are a few different stages between warm air entering the compressor and hot water leaving the other end. We’re going to break down the three main steps.
The First Step: Warm Air Into The Heat Pump
The first step in the process is bringing the ambient air into the unit. A fan is hooked up to a compressor within the water heater. The heat pump is the top area of the unit that houses the compressor, evaporator coils, and expansion valve.
You will see that heat pump water heaters tend to be taller than standard electric water heaters. The reason is that these extra pieces are housed in that area.
The ambient air from outside of the unit is brought into the unit. It’s important to note that the air doesn’t have to be “hot” as we think of it. Heat pumps are extremely efficient at absorbing heat energy from the air, even at temperatures as low as 45 degrees Fahrenheit.
Most manufacturers recommend choosing a location for your water heater where the ambient temperature doesn’t regularly go below 50 degrees. That said, the higher the air temperature, the more efficient the unit will be.
The air enters the unit and flows through the compressor and evaporator coils. The heat energy from the air is absorbed into evaporator coils. The now cooler and less humid air is expelled out the other side.
An added benefit of hybrid hot water heaters is they produce cool, dry air. This can help keep the area of the home where the water heater is installed cool and dry.
The Second Step: Refrigerant Increases Temperature
Refrigerant is pumped through the evaporator coils. Actually, this is going on while the air is being pulled into the unit. But now we’re focusing on the compressor itself.
Because refrigerant has a low boiling point (which means it will boil at cooler temperatures than water), the heat from the air being pushed through the system brings the refrigerant to a boil. This is the first heat transfer of the process: from the air to the refrigerant.
This boiling refrigerant then transforms into a gas. When gas is compressed, its overall temperature increases. The compressor then puts added pressure onto the gas to make this happen. As a result of this, the gas continues to increase in temperature.
The Third Step: Heating The Water
Now we will do another heat transfer. The first heat transfer was from the air to the refrigerant. Our second will be from the refrigerant to the water itself.
The heated refrigerant is housed in the condenser coils. Water comes in near the bottom of the unit and is then pumped to the top of the unit, where the heat is transferred from the refrigerant in the condenser coils to the water.
The water temperature is then checked and moderated. If the water is not hot enough, electricity will be used to heat it the rest of the way. For those who live in very hot areas, the heat from the air and the naturally warmer water means that very minimal electricity will need to be used.
How Does A Hybrid Water Heater Work?
Heat pump water heaters work so efficiently because the use of electricity is minimal. Some electricity is used in pumping refrigerant and water through the unit, but not much. If the water needs further heating after going through the heat transfer process, backup electricity will be used to bring the water up to the desired temperature.
A normal electric water heater uses electricity for the entire process. In fact, heat pump water heaters are 3.5 times more efficient than electric water heaters and 7 times more efficient than gas!
This can save hundreds of dollars on utilities each year. Hybrid water heaters have a return on investment of 2 to 3 years, despite being a larger up-front investment. And they can ultimately save a homeowner several thousand dollars in energy costs.
It is important to know that because hybrid hot water heaters tend to be taller than their electric or gas counterparts, they need to have a bit more space.
Additionally, because they use ambient air, they are recommended to be housed in a room or area that is a minimum of 100 square feet. This ensures that the unit is not just pumping its own cool exhaust air into the system, reducing its efficiency.
Final Thoughts on Hybrid and Heat Pump Water Heaters
Heat pump water heaters are incredibly efficient. While it is a fairly complex process that occurs within the unit, the basics come down to heat transfer.
Heat is transferred from the air to a refrigerant. The refrigerant is then compressed, heating it even more. Then a second heat transfer occurs from the refrigerant to the water.
The minimal amounts of electricity used to heat the water are the key to saving energy and money.
Hybrid water heaters use the heat energy in the air to warm up the water, reducing the electricity needed to heat the water the rest of the way.
It’s an interesting process all on its own. But you’ll be even more interested in how much you can reduce your utility bills and environmental impact!
I think it is worth considering where you are geographically and how your home is heated, before jumping on the hot water heat pump band wagon. Since most such units use ambient air from inside the dwelling, they are only transferring heat from the room to the water. This means you are really heating the water with whatever your home heat source is – furnace, boiler, woodstove, etc.
Such units might be fine where there is excess heat – a southerly location, or a commercial utility room, where removing heat is beneficial for reducing air conditioning load. Otherwise, for home-owners in moderate to cold climates, those promised savings might be elusive.
There are a very few manufacturers of air-to-water heat pumps for domestic water that have the condenser located separately outside rather than attached to the top of tank. Those would genuinely capture “free” heat from the atmosphere, but one has to be careful of the efficiency limitations as the outside temperatures drop.