Heat Pump Water Heater:
Frequently Asked Questions

What is a the difference between a heat pump water heater and a standard water heater?

What kind should I buy?

How larger of a water heater do I need?

How efficient is it?

How does it work?

What is the difference between a water heater and heat pump water heater?

A standard water heater heats water inside of a tank by burning fuel (like propane or natural gas) or using electric resistance heating.  

So then, what is a heat pump water heater?

Are you familiar with residential heat pumps?

The outdoor unit sits outside.......

Using a compressor, it takes the heat from the house and rejects it to the outdoors.

And in the winter.....just the opposite.

It takes heat from outdoor air, increases the temperature, and rejects it to the air inside the home.

So, then what about a heat pump water heater?

Very much the same!

Using a refrigerant compression/expansion cycle, it transfers heat from air surrounding the water heater.....to water inside the tank.

I know what you might be thinking.

If heat is taken from the air surrounding the water heater....

And it cools the air around it....

Is it, in reality, an air conditioner?

The answer is YES!

While heating the water inside your water heater tank, it is simultaneously cooling the air.

Good in the summer.....not so good in the winter!

Be sure to follow the manufacturer's requirements for maintaining the space temperature around the water heater.  Most manufacturers want the space to be maintained between 40 F and 90 F.

Note: If you have a geothermal heat pump system the attached water heater is also known as a desuperheater

What Kind Should I Buy?

Each manufacturer boasts different advantages:

  • Stiebel Eltron is a German manufacturer that does well in the American market.  It is energy star labeled and considered one of the highest quality products on the market. 
  • General Electric is one of the most popular appliance manufacturers in the world.  It is also sells heat pump water heaters and offers a very high quality product.  Click here to find out more about this GE heat pump water heater.
  • Rheem offers the Ecosense and Prestige water heaters.  They are slightly less efficient than the Stiebel Eltron Accelera 300 and slightly more efficient than the GE heater.

Click on the links above for more detailed information regarding each model.

How Large of a Water Heater Do I Need?

This question can be very subjective.  However, if we are talking about the standard "tank style" heat pump water heater then you may adhere to the following general criteria:

  • Mini tank (30 gallons) can generally supply hot water for 1 to 2 people living in a house.
  • Small tanks (50 or 60 gallons) can generally supply hot water for 2 to 4 people living in a house.
  • Medium tank (80 gallons) can generally supply hot water for 4 to 5 people living in a house.
  • Large tank (100 gallons) can generally supply hot water for 5 to 6 people living in a house.

How Efficient Is It?

When considering only the efficiency of the heat pump water heater, one will generally conclude that this is the most efficient centralized heating option available.

At times (depending on gas versus electric costs), a tankless or point of use heater may be more efficient.  It is usually best to contact a local contractor to determine what is the best option for your climate and house type.

There is one important thing to consider regarding these water heaters:

  • These water heaters act like an air conditioner by absorbing the heat in the air and rejecting that heat into the water.  In the summer....this is good because we all want the air to be cooler in the summer.  But in the winter, if the air is being used from a conditioned space, then the heat from your furnace will be utilized to heat the water.  If you have a very efficient furnace, this is not a bad thing.  However, if your furnace is 90% or 80% efficient (or worse), you could be losing the energy efficiency which you expected to gain, buying a heat pump water heater.

How Does It Work?


It uses the heat in the air around it and transfers the heat into the water.

See the picture below.

Heat Pump Water Heater Diagram

Just like a standard heat pump, the compressor pumps refrigerant.  The refrigerant absorbs heat from the air, moves through the compressor which causes it to be compressed to a higher pressure and temperature.  The heat is then rejected into the water heater, goes through an expansion device, and starts the process again.

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