The performance of an inverter controlled air source heat pump unit for hydronic floor heating is measured at various conditions of outdoor temperature and humidity, remote control settings of outlet water temperature and load factor at steady state. In a temperature condition, the heat pump unit provides the highest COP at a load factor of 0.4 to 0.6, whereas it also shows decrease of COP at lower load factors below 0.28 due to on/off control. The partial load variations can be expressed by a function of load factor, outdoor temperature and water temperature supplied to heat pump. The use of the function may allow convenient performance evaluation for such heat pump systems.