This part of EN 16905 specifies the calculation of the seasonal performance factor for gas-fired endothermic engine driven heat pumps for heating and/or cooling mode including the engine heat recovery, to be used outdoor. This document is used in conjunction with: a) the terms and conditions, EN 16905-1:2017 b) the safety, EN 16905-2:2021 c) the test conditions, EN 16905-3:2017 d) the test methods, EN 16905-4:2021 e) the heat pump standards, EN 14511-2:2018, EN 14511-3:2018 and EN 14825:2018. This document only applies to appliances with a maximum heat input (based on net calorific value) not exceeding 70 kW at standard rating conditions. This document only applies to appliances under categories I2H, I2E, I2Er, I2R, I2E(S)B, I2L, I2LL, I2ELL, I2E(R)B, I2ESi, I2E(R), I3P, I3B, I3B/P, II2H3+, II2Er3+, II2H3B/P, II2L3B/P, II2E3B/P, II2ELL3B/P, II2L3P, II2H3P, II2E3P and II2Er3P according to EN 437:2003 + A1:2009. This document only applies to appliances having: a) gas fired endothermic engines under the control of fully automatic control systems; b) closed system refrigerant circuits in which the refrigerant does not come into direct contact with the fluid to be cooled or heated; c) where the temperature of the heat transfer fluid of the heating system (heating water circuit) does not exceed 105 °C during normal operation; d) where the maximum operating pressure in the: 1) heating water circuit (if installed) does not exceed 6 bar, 2) domestic hot water circuit (if installed) does not exceed 10 bar. This document applies to GEHP appliances only when used for space heating or space cooling or for refrigeration, with or without heat recovery. This document is applicable to GEHP appliances that are intended to be type tested. Requirements for GEHP appliances that are not type tested would need to be subject to further consideration.