Choosing A Heat Pump
Heat pumps are rated for heating and cooling, each with different capacities and efficiencies. Capacity ratings are generally in British thermal units (Btu) per hour. Efficiency ratings are different for air-source and water- or ground-source heat pumps. An air-source heat pump can supply 2 to 3 times as much energy as it uses; a ground-source heat pump can supply 4 times as much energy as it uses. Gas-fired heat pumps, which are currently only available as air-source heat pumps, also use different efficiency ratings.
Ground-Source or Water-Source Heat Pumps: The heating efficiency of ground-source and water-source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in Btu per Btu of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit. Look for the ENERGY STAR label, which indicates a heating COP of 2.8 or greater and an EER of 13 or greater.
Air-Source Heat Pumps: Heating efficiency for air-source electric heat pumps is indicated by the heating season performance factor (HSPF), which is the ratio of the seasonal heating output in Btu divided by the seasonal power consumption in watts. Cooling efficiency is indicated by the seasonal energy efficiency ratio (SEER) which is the ratio of the seasonal heat removed in Btu per hour to the seasonal power consumption in watts.
To choose an air-source electric heat pump, look for the ENERGY STAR label, which is awarded to those units with SEERs of 12 or greater and HSPFs of 7 or greater. If you are purchasing an electric air-source heat pump and are uncertain whether it meets ENERGY STAR qualifications, look on the bright yellow Energy Guide label for an efficiency of 12 SEER/7HSPF or greater. For units with comparable HSPF ratings, check their steady-state rating at -8.3 degrees C, the low temperature setting. The unit with the higher rating will be more efficient.
The efficiency of gas-fired air-source heat pumps is indicated by their COP. COP is the ratio of either heat removed (for cooling) or heat provided (for heating) in Btu per Btu of energy input. Look for the ENERGY STAR label, which indicates a heating efficiency of 1.2 COP or greater and a cooling efficiency of 1.25 COP or greater. If you are purchasing a gas-fired heat pump and uncertain whether it meets ENERGY STAR qualifications, ask for a unit that meets these efficiency ratings.
Other factors to consider when choosing and installing air-source heat pumps:
Select a heat pump with a demand-defrost control. This will minimize the defrost cycles, thereby reducing supplementary and heat pump energy use.
If you're adding a heat pump to an electric furnace, the heat pump coil should usually be placed on the cold (upstream) side of the furnace for greatest efficiency.
Fans and compressors make noise. Locate the outdoor unit away from windows and adjacent buildings, and select a heat pump with an outdoor sound rating of 7.6 bels or lower. You can also reduce this noise by mounting the unit on a noise-absorbing base.