TECHNOLOGY BASICS: What are Air-source Heat Pumps?

Air-source heat pumps (ASHPs) are all-electric building heating and cooling solutions that use electrical energy to move (not convert) thermal energy from a source to a sink. Unlike an air conditioner, a heat pump can switch between transferring heat out of the conditioned space (cooling mode) to transferring heat into the space (heating mode). Important specifications for an ASHP unit are its capacity, expressed as the rate at which the unit can add or remove heat from the space (Btu/h), and various measures of efficiency. ASHP installations also typically provide building ventilation, so the fan capacity and efficiency may be other important parameters to consider. Most manufacturer product lines include both air-conditioning (AC) only and heat-pump versions of a given design. The incremental cost of an ASHP relative to a comparable AC-only unit is typically not large.

ASHPs are regulated by the Department of Energy (DOE) Appliance Standards Program, and so they must meet minimum efficiency standards in both heating and cooling mode. Cooling efficiency when the unit is operating at full capacity is expressed by the energy efficiency ratio (EER), which is defined as the rate of heat removal (Btu/h) divided by the input power (W). The metric used by DOE to regulate efficiency depends on capacity. For ASHPs with capacities less than 65,000 Btu/h, the metric is seasonal energy efficiency ratio (SEER); for capacities greater than or equal to 65,000 Btu/h the metric is integrated energy efficiency ratio (IEER). Both SEER and IEER represent the average efficiency over multiple cooling loads and outside temperatures. As ASHPs are typically sized to meet the maximum expected load, they often operate under part-load conditions. Sophisticated ASHP designs utilize multi-speed fans and multi-stage compressors to optimize efficiency when the unit is not operating at full capacity.

The DOE metric used to regulate heating efficiency also depends on capacity. For ASHPs with capacities greater than or equal to 65,000 Btu/h, the metric is the dimensionless coefficient of performance (COP), defined as the rate of the produced heating effect to the net work input, when both are expressed in the same units. For smaller capacity ASHPs, the metric is heating seasonal performance factor (HSPF), which is an average of the COP over multiple heating loads and outside temperatures.

For all of these metrics (EER, IEER, SEER, COP and HSPF), the higher the value, the more efficient the ASHP.

ASHPs that just meet the required DOE standard are often referred to as baseline units. In addition to the baseline units, most manufacturers also offer higher efficiency models, which cost more to purchase but, due to higher efficiency, cost less to operate. The data presented on this website can help determine what the energy and cost savings might be when a high-efficiency model is purchased instead of a baseline ASHP.

Several factors can affect both the baseline operation of an ASHP and the impact of choosing a higher efficiency model:

• ASHP capacity and local climate – An ASHP is typically sized to meet the cooling load on the expected hottest day of the year, which depends on local climate. On the other hand, the total annual electricity consumption for cooling depends on the duration of hot weather. On the heating side, the ability of the ASHP to transfer heat into the space (i.e. heating capacity) decreases as the outdoor temperature decreases, so ASHPs operate best in climates that do not get extremely cold in the winter. Most ASHPs include an electric resistance backup heater to ensure that the heating load is met during very cold weather. Extensive use of this backup heating source reduces the effective efficiency of the ASHP.

• Building type and building activity – Building envelope features and occupancy schedule impact both the magnitude and duration of cooling and heating loads. Building occupancy also affects ventilation rates, and ASHP fans must provide adequate ventilation during occupied hours. Higher rates of outdoor air flow also affect the heating and cooling loads.

• Existing heating systems – Many buildings in California do not provide central heat. If an existing packaged air conditioner is being replaced, switching to an ASHP can provide central heating service at minimal extra purchase cost and significantly lower operating cost.

• Existing building systems – If the building has an existing building automation system (BAS) or energy management and information system (EMIS), integration of building controls with the ASHP control system may be an issue. If a fixed-speed system is being replaced with a multi-stage or variable-speed ASHP, a new thermostat may be required. In all cases, expert commissioning is required to ensure that the AHSP is set up to respond appropriately to the thermostat/controls.

• Installation – When replacing an existing packaged unit, the physical footprint of the unit may change. This requires installation of a conversion curb.