1. What is a heat pump?

A heat pump 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 move the heat against a temperature gradient, much like a water pump moves water against the force of gravity. 

2. What are some everyday uses of heat pumps?

A refrigerator uses a small heat pump to move heat from inside your refrigerator to the surroundings, cooling your food. An air conditioner (AC), which moves heat from inside a house to the outside against a temperature gradient, is another example. Similarly, an air-to-air heat pump 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 outside of your home to the inside, acting as a heater.

3. What are the major components?

A system has various components, including a compressor, outside and indoor heat exchangers, refrigerant (with associated piping), valves, and controls/wiring.  These components work with a forced-air system, such as a blower that transfers the heat/cool to the interior of your home.  Some heat pumps have integrated blowers and some rely on separate blowers and ducting.

4. What types of air-to-air heat pumps are available?

The most common forms of air-to-air heat pumps are: 1. “split” systems that rely on 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. 

5. How does an air-to-air heat pump differ from a natural gas boiler?

First, an air-to-air heat pump provides not only space heating, but also cooling. 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 boiler.  Accordingly, they’re generally used in a way that keeps a home at a more even temperature throughout the day and night.  

When it comes to carbon pollution, heat pumps have an advantage over other types of space heaters because of their greater efficiency.  The amount of the carbon reduction depends on the carbon intensity of the electricity used to power your heat pump and the energy source (such as natural gas, electricity, etc.) you’re switching from.  If you add rooftop solar to your home, the carbon footprint of running a heat pump is drastically reduced.  

6. Can you set back the temperature when heating with an air-to-air heat pump?

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

7. Are mini-split systems more efficient than split systems?

In general, ductless mini-split systems are more efficient than split systems.  This is partly because 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.  The efficiency of a ducted mini-split system generally falls in between a split system and a ductless mini-split.

1. How expensive are air-to-air heat pumps?

The installed cost of an air-to-air heat pump depends on a variety of factors, such as the type of heat pump, home size, location, existence of a forced-air duct system, need for additional wiring, etc.  Whilst they are typically more expensive than conventional systems, a HP may have lower operating costs, depending on the energy source you’re switching from. Also, if you require home cooling anyway, an air-to-air HP has the advantage of combining both heating and cooling into one system.

2. How much can I save on energy costs with a heat pump?

It depends on a number of factors, especially the type of system you’re switching from. If you’re replacing an electric or oil heating system, you should be able to cut your costs substantially. When replacing a boiler fueled by mains gas, however, if there is any reduction in your monthly costs, it would likely be small. Pairing the HP with a rooftop solar system can generate significant savings since the solar power will be much less expensive than electricity from the grid.

1. What kind of regular maintenance is needed?

Efficient operation of your heat pump generally involves periodic cleaning of debris and dust that has collected in and around coils, cleaning/replacement of filters, and checking of the condensate drain. Some experts suggest annual maintenance visits by a professional to avoid issues before they become full-blown problems.

2. If the heat pump cools and heats year round, does it wear out more quickly than an air conditioner?

Covering both winter and summer means that a heat pump will log more operating hours than a traditional air conditioning system.  Accordingly, it would normally have a shorter expected lifespan, but it would depend on how often the cooling function of the heat pump is used. 

1. Do heat pumps reduce greenhouse gases?

Yes, it will reduce your carbon footprint.  The amount of the reduction depends on the carbon intensity of the electricity used to power your HP and the energy source (such as natural gas, electricity, etc.) you’re switching from.  If you add rooftop solar to your home, the carbon footprint of running a HP can become negligible. Additionally, as grid-supplied electricity becomes cleaner over time, heat pumps will have an increasing environmental advantage over conventional heating systems.

1. How is an air-to-air heat pump installed?

This will depend on your current heating system and the type of heat pump being installed. For homes that currently have a hydronic (“wet”) heating system, the most straightforward approach is to install an air-to-water heat pump (as discussed in the Water Heating with a Heat Pump FAQ).  Installing an air-to-air HP in such a home would likely require either a radical change to the home’s heating system or inclusion of a mini-split HP as a parallel (supplemental) heating system. 

For a home without a hydronic heating system, the most applicable HP type would depend on whether ducts are currently in place.  If so, a central air-to-air HP would likely be selected, requiring the replacement of the indoor unit and installation of a new outdoor unit. If no ducts are currently in place, a mini-split would likely be selected, requiring the installation of an outdoor unit and one or more indoor registers.

2. Is a mini-split heat pump better than a central (split) system?

Mini-split systems are normally recommended if a house does not have air ducts or if extensive rework would be needed on the ducts.  Without the need for ducts, which can introduce significant energy losses, mini-splits are normally more efficient.  Mini-splits are also often recommended if only a portion of the home, such as several rooms, require heating.  In this case, a heating/cooling register can be economically placed in each of the rooms and tied to a single outdoor unit. If heating/cooling is needed for the whole house and existing ducts can be used, then a central HP system is generally recommended.

1. What types of incentives are available for air-to-air HPs?

Unfortunately, the generous federal grants available for air-to-water HPs (which are much more common in Germany) aren’t available for air-to-air HPs.