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Future House | Understanding Heat Pumps

Ask This Old House home technology expert Ross Tretheway explains everything you need to know about heat pumps.

Home technology expert Ross Tretheway discusses the new age of home heat pumps in a Future House feature. Ross takes us through the four different types: water to water, water to air, air to air, and air to water. He details how those systems work and how they pull warmth from the ground. Ross talks to his father, plumbing and heating expert Richard Tretheway, about how heat pumps have evolved.

How Heat Pumps Work

Heat pumps work by finding and moving heat in and out of a building. A heat pump will take heat from within the home during the summer and let it dissipate outside, allowing the air conditioning system to work more efficiently.

In the winter, a heat pump can scavenge heat from the outdoors (even in temperatures as low as zero degrees) and pump it into the home for comfort. The only requirement? Enough electricity to run the compressors.

There are Two Sides to the Story

Heat pumps have two sides: the source side and the load side:

  • The source side is where the pump is gathering the heat from. In the summer, the inside of the home would be the source side. In the winter, the outdoor side of the system is the source side.
  • The load side is where the heat is being delivered to. The heat pump removes the heat in the summer and dumps it outside, making the outdoors the load side. In the winter, the heat pump sources heat from outside and supplies the home, making the house the load side.

Geothermal Heat

Many heat pump systems require geothermal heat. This is the heat trapped in the earth’s surface, where it stays at a consistent temperature year-round. These systems require drilling down into the earth and running a system of piping for water. The water then absorbs heat as it passes through the ground and back into the system for heating.

These systems are typically expensive, and they max out at around 120 degrees (boilers can go much higher). But, they can be more efficient than some other systems, making them an excellent option.

Different Types of Heat Pumps

Heat pumps are becoming more and more advanced, but there are four basic types: water to water, water to air, air to air, and air to water.

Water to Water

This type of heat pump requires geothermal drilling, which means drilling into the earth to tap into its heat. The water travels below the surface, comes back up, and travels through a heat exchanger. In the heat exchanger, the source water heats the load water. The load side water then travels through the heating system just as it would a force hot water system.

Water to Air

Water to air systems require geothermal drilling to source the heat in the earth’s surface. Then, the hot water runs through a coil in a duct. When air blows across that coil, it heats up, just as it would with a furnace.

Air to Air

In an air to air system, the heat pump sources heat from the air on the load side, which is then transferred to a refrigerant. This refrigerant then travels through a coil inside the ductwork, where load side air blows across it, similar to a furnace. These are the most common heat pumps.

Air to Water

An air to water system sources heat from the air on the source side and transfers it to a refrigerant. This refrigerant then cycles through a heat exchanger where it can heat the load side water to be pumped through the house.

Once in the house, this water can serve several purposes. It can be pumped to a zone manifold for hydronic heat, cycled through a heat exchanger for domestic hot water, or even pumped through a coil to supply a ducted system. In the summer, this system can remove heat from the home, cool the refrigerant, and supply the home with cooling.

There are a ton of advantages to this system over the others. First, no drilling is required. Second, you get the efficiency of hydronics. In some cases, homeowners relying on older heating systems can expect to save thousands of dollars each year on heating costs, as well.


Ross explored four different types of heat pump systems: water-to-air, water-to-water, air-to-air, and air-to-water. These heat pumps can usually be sourced through a local HVAC technician or sales representative.

The air-to-water heat pump that Ross saw being installed in upstate New York was an Advantage Air-to-Water Heat Pump manufactured by Enertech.

Expert assistance with this segment was provided by The Radiant Store, Air & Water Source Group, LLC, B&D Manufacturing, Inc., Taco Comfort Solutions, Axiom Industries Limited, and Caleffi North America, Inc.