Air Source Heat Pump FAQ

Basics

A heat pump, such as a common refrigerator, is a device that moves heat from one place to another but does not generate any heat of its own. Usually the flow of heat follows the temperature gradient from higher to lower temperature. However, a heat pump uses a compressor, heat exchangers, and other components to evaporate and condense a refrigerant at selective places to move the heat against a temperature gradient, much like a water pump moves water against the force of gravity. So it is called a heat pump.

A refrigerator uses a small heat pump to move heat from the inside of the refrigerator to the surroundings. Thus the food inside the refrigerator stays cool. An air conditioner (AC), which moves heat from inside a house to the outside against a temperature gradient, is another example. Similarly, an air source heat pump (ASHP) moves heat from inside to outside during the summer, functioning as an AC. In the winter, it has a mechanism to reverse the flow of heat and move heat from the cold outside to the warm inside, acting as a space heater.

A system has various components, including a compressor, outside and indoor heat exchangers, refrigerant (with associated piping), valves, and controls/wiring.

The most common forms of air source heat pumps are: 1. “split” systems that use air ducts inside the house and a central heat exchanger to condition the air flowing through the ducts, and 2. ductless “mini-split” systems that use one or more registers inside the house to condition the space directly without using air ducts.  There are also other variations, such as ducted mini-splits, that can sometimes offer advantages.

First, a heat pump provides not only space heating, but also air conditioning. By reversing the thermodynamic cycle, this one device can help keep you comfortable in both the winter and summer.  A heat pump is also more efficient because it’s designed to move heat, not create it.  Operationally, a heat pump takes longer to heat a home from a low temperature than does a natural gas furnace.  Accordingly, they’re generally used to keep a home at a more even temperature throughout the day.  

An air source heat pump has a distinct environmental advantage in most of the country since it operates on electricity (which can be generated with renewables) instead of relying on the use of a fossil fuel that contributes to climate change.  

It is advisable to use small setbacks with a heat pump since the heating process takes longer than with a natural gas furnace.  By maintaining a relatively uniform temperature, the temperature recovery time is reduced.

In general, mini-split systems are more efficient than split (ducted) systems.  They avoid the energy losses associated with ducting, which can be substantial in some cases. Also, the temperature setting of each register inside the house can be controlled separately, so the heating or cooling of the house can be more easily fine-tuned, thereby saving energy.

Economics

The installed costs of air source heat pump systems depend on a variety of factors, such as type of heat pump, home size, location, need for additional wiring, etc.  While they are typically more expensive than conventional heating or cooling systems, an advantage is that you only need one heat pump to cover both heating and cooling. 

This depends on the prices of natural gas and electricity in your area, your typical winter temperatures, etc.  In many cases, despite the fact that a heat pump system is more energy efficient than a gas furnace, the cost to operate a heat pump will be higher.

Maintenance

The maintenance needed is similar to an AC. It consists of cleaning the coils from time to time, changing the air filters in ducts or registers and recharging the coolant in case of leaks.

Covering both winter and summer means that a heat pump will log more operating hours than a traditional AC system.  Accordingly, it would normally have a shorter expected lifespan.

Environment

Air source heat pumps almost always help to reduce GHG emissions. (One exception is solar-thermal water heating.)  Since heat pumps are super efficient, they use less energy overall.  Also, when compared with water heaters fueled by fossil fuels (such as natural gas or propane) they have an inherent advantage because of the generally lower carbon content of electricity.

Steps to use

This will depend on the type of heat pump being installed.  A central (split-system) heat pump’s installation is generally similar to that of a central AC system.  A ductless mini-split will require the additional step of mounting one or more indoor registers inside your home. Depending on the complexity of the system, projects typically take 1 day to several days.

Mini-split systems are normally recommended if the house does not have air ducts or extensive rework would be needed on the ducts.  If existing ducts can be used, then a central system is often recommended.

In answering this question, a number of factors come into play such as house construction/orientation, number of rooms, and specific heating/cooling needs associated with each room. In some cases, a register will be needed in each room, but this is not always the case. Multiple registers (as many as eight) can be linked to an outside unit, and in larger homes, two outside units are sometimes employed.

Incentives

A $300 Federal Tax Credit is available through the end of 2021 for qualifying ASHPs installed at your principal residence. When applicable, this credit comes directly off the taxes you would otherwise pay. In addition, utilities, counties, Community Choice Aggregators, etc. sometimes offer rebates of various types.