What is the best air source heat pump system to choose?
The best type of air source heat pump (ASHP) for your home depends on many factors, including your existing heating and cooling set up, the fuel you use for heating, and your goals for the system. This post breaks down some questions you will want to ask yourself as you plan for an ASHP, and how the answers will impact your ASHP choices.
Does your home have ductwork?
Whether or not your home has ductwork is a key factor in determining what kind of ASHP is right for you. ASHPs come in two broad categories: ducted and ductless. Like a central air conditioner or forced air furnace, ducted ASHPs use ductwork to distribute conditioned air throughout the home. This makes them a good fit for homes that already have ductwork in place. A ducted heat pump can replace a central air conditioner to provide cooling during the summer months and heating during some or all of the winter season.
Ductless heat pumps are commonly called mini-splits and do not need ductwork to distribute conditioned air. Instead, ductless heat pumps directly condition the air of a room or area with an indoor head or cassette typically mounted high on a wall. This means they are ideal for homes that are heated by a boiler or electric baseboards and do not have existing ductwork.
What's your secondary heating source?
Most homes in Minnesota with an ASHP need a secondary source of heating to maintain a comfortable temperature on colder days. Heat pumps’ capacity to supply heat gradually diminishes as the temperature drops, so a secondary heating source helps maintain the thermostat setpoint when the temperature dips below a certain point. In many cases, your home’s existing heating system can be left in place to serve this purpose. An ASHP that covers all the cooling and part or most of the heating before switching to a natural gas or propane secondary system is called a dual fuel system. Another option is an all-electric system, in which the ASHP covers all the cooling and nearly all the heating with electric resistance heating as a supplemental source that activates in very cold weather.
Ducted ASHPs are typically installed with a forced air furnace (for a dual fuel system), or an air handler with electric resistance backup (for an all-electric system). Like a central air conditioner, a heat pump can be controlled by the same thermostat as the furnace or electric resistance heating. With a dual fuel system, the home’s heating source automatically switches from the ASHP to the secondary heating system at a set outdoor temperature, called the switchover temperature. Switchover temperatures are typically set by a contractor based on the ASHP’s efficiency and size and the prices of electricity and gas. Some ASHPs are designed to integrate with any existing furnace. A quality contractor will quote you options that are feasible for your setup. With an all-electric ducted system, electric resistance booster heating automatically turns on to supplement the ASHP during the coldest times of the year when the ASHP alone cannot deliver enough heat.
Ductless systems typically operate independently of the home’s secondary heating source. Most indoor units have their own thermostat and are individually controlled rather than integrated with any existing thermostats for a boiler or electric baseboards. Most homeowners with this type of system set the thermostat for their boiler or baseboards a few degrees cooler than the thermostat for the ASHP system when they want the ASHP to take on most of the heating load.
What are your goals?
Some homeowners are most interested in the operational cost savings an ASHP can offer over expensive propane or electric resistance heating. Others are more motivated by the opportunity to reduce their household’s carbon emissions by reducing their natural gas or propane use as much as possible. For homes heated with propane or electric resistance, these goals are aligned. In other words, running an ASHP to the lowest temperatures it can go both displaces the most fossil fuel and saves the most money on energy bills. You can read more about the benefits of ASHPs for homes heated with propane here.
For homes heated with natural gas, there is a tradeoff between reducing carbon emissions as much as possible and keeping costs low. This means that if you want to save on operational costs, a dual fuel system with a more moderate switchover temperature (25°F to 45°F) makes sense. Homeowners who are interested in reducing carbon emissions might choose a lower temperature to displace more natural gas.
Should you choose a cold climate ASHP?
Cold climate refers to a voluntary performance specification administered by Northeast Energy Efficiency Partnerships (NEEP) for ASHPs that can operate efficiently at low temperatures. ASHP products will also soon be tested and listed within a cold-climate specification developed by ENERGY STAR. Cold-climate ASHPs are inverter-driven, which means they can modulate their operation to best meet a home’s heating and cooling needs with less energy than a lower performing unit that can only turn on or off. Cold-climate ASHPs come with a higher price tag. For homeowners who heat with propane or electric baseboards, this higher upfront cost can be offset by lower annual energy bills. Inverter-driven ASHPs are also strongly recommended for homeowners seeking an all-electric system in a cold climate because they can operate efficiently at low temperatures, which reduces the need for expensive electric resistance backup heating on the coldest days of the year.
Homeowners who heat with natural gas likely won’t see the high upfront costs of a cold-climate ASHP offset by operational cost savings. In this case, it makes sense to go with a more affordable, lower performing unit. These types of ASHPs still cost more upfront than typical air conditioners, but in addition to providing efficient cooling, they offer the option to cost-effectively use electricity for heating during spring and fall. They can also provide a way to hedge against volatile natural gas prices.
When requesting quotes from contractors, you'll want to let them know whether your priority is to get the most affordable heating and cooling setup for the home or to reduce carbon emissions as much as possible.
For more information about different ASHP system types, visit our homeowner FAQ page and our homeowner resources page to view case studies, ASHP overviews, and buying guides. If you are interested in installing an ASHP, visit our Preferred Contractor Network for a list of trusted contractors in your area that can provide you with quotes. If you have further questions, you can contact us at info@mnashp.org.