# Heating seasonal performance factor

Heating seasonal performance factor (HSPF) is a term used in the heating and cooling industry. HSPF is specifically used to measure the efficiency of air source heat pumps.

HSPF is defined as the ratio of heat output (measured in BTUs) over the heating season to electricity used (measured in watt-hours).[1][2] It therefore has units of BTU/watt-hr.

The higher the HSPF rating of a unit, the more energy efficient it is. An electrical resistance heater, which is not considered efficient, has an HSPF of 3.41.[3]

Depending on the system, an HSPF ≥ 9 can be considered high efficiency and worthy of a US Energy Tax Credit.[4]

For instance, a system which delivers an HSPF of 7.7 will transfer 2.25 times as much heat as electricity consumed over a season.[5] In Europe the term Seasonal Performance Factor ("SPF") is used to mean the same as the average COP over the heating season. A system which transfers 2.25 times as much heat as the electricity consumed is said to have an SPF of 2.25. A well designed ground source heat pump installation should achieve an SPF of 3.5, or over 5 if linked to a solar-assisted thermal bank.[6]

Example: For a heat pump delivering 120,000,000 BTU during the season, when consuming 15,000 kWh, the HSPF can be calculated as :

HSPF = 120000000 (BTU) / (1000) / 15000 (kWh)
HSPF = 8

The HSPF is related to the non-dimensional Coefficient of Performance (COP) for a heat pump, which measures the ratio of heat delivered to work done by the compressor. The HSPF can be converted to a seasonally-averaged COP assuming a lossless compressor and no heat loss by multiplying by the heat/energy equivalence factor .29307111 watts per BTU. HSPF, which is normally measured rather than calculated, is reduced by losses due to such things as electrical resistance in the motor and thermal resistance in the evaporator.