What is the best air source heat pump system type 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 to determine which 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, 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 very cold days. Heat pumps’ capacity to supply heat gradually diminishes as the temperature drops, so a secondary heating source maintains the thermostat setpoint when the temperature dips below a certain point. In some cases, your home’s existing heating system can be left in place to serve this purpose. Dual fuel systems feature an ASHP that covers all the cooling and part or most of the heating before switching to a natural gas or propane secondary 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 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 day of the year when the ASHP alone cannot deliver sufficient 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 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 using as little natural gas or propane as possible. For homes heated with propane or electric resistance, these goals are aligned. In other words, running an ASHP to the lowest temperature it functionally operates both displaces the most fossil fuel and saves the most money on energy bills.
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 more interested in reducing carbon emissions than reducing costs might choose a lower temperature to displace more natural gas. lower temperature to displace more natural gas.
Should you choose a cold-climate ASHP?
The term “cold climate” is used to describe ASHPs that can perform well at below freezing (and even sub-zero) temperatures. Cold climate ASHPs have a high efficiency at five degrees, maintain heating capacity at lower temperatures, and feature a variable-speed compressor. Variable-speed ASHPs 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. Choosing a cold climate ASHP over a lower performing unit depends on the type of heating fuel you use and your goals for the ASHP. Variable-speed or cold climate heat pumps are a good fit for customers who
- Are interested in fuel flexibility. For natural gas customers, a dual fuel heat pump with a moderate switchover temperature is typically the most cost-effective option at current natural gas and electric rates. However, a variable-speed ASHP with increased capacity at colder temperatures offers fuel flexibility and resilience to the potential volatility of natural gas prices. The homeowner can adjust the system’s switchover temperature in the future to optimize savings as fuel prices change over the lifetime of the equipment.
- Have an expensive heating fuel. Center for Energy and Environment field research found that installing a variable-speed ASHP saved 30% on heating costs over a propane furnace and AC and 50% over electric resistance heating. You can read more about the benefits of ASHPs for propane customers here.
- Want to decarbonize as much as possible. The performance of variable-speed ASHPs at cold temperatures means the ASHP can run for longer before switching to a fossil fuel secondary system or an expensive electric resistance backup heat. You can read more about how to balance operating costs and carbon emissions reductions here.
The increased performance of a variable-speed ASHP does come with a higher price tag. However, current and future incentives like utility rebates, tax credits, federal and state rebates, and local cost-share programs can all be stacked together to greatly reduce upfront costs. Many of these programs require heat pumps to meet certain efficiency metrics, which means that variable-speed, cold climate equipment is more likely to qualify for a larger incentive.
Single and two-stage ASHPs are also widely available in the market. These do not modulate like a variable-speed ASHP and are mostly used for heating only in the shoulder seasons. A single-stage unit can only turn on or off. A two-stage unit can operate at a high speed and a lower speed. Single- and two-stage ASHPs are a good option for customers whose biggest priority is minimizing upfront cost and aren’t looking for better energy efficiency and performance than a standard air conditioner, especially if they heat with natural gas.
When requesting quotes from contractors, you'll want to let them know your motivations for installing the new system, so they can help you select the right equipment for your needs and goals.
Looking for more guidance?
To learn more about heat pumps, continue exploring News & Case Studies or visit the FAQs page. If you have further questions, contact us at info@mnashp.org.
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