Heat pumps work best above 7 degrees Celsius but will work down to minus 15 degrees provided they have good air flow. This is because the refrigerant in the heat pump boils at minus 26 degrees, so minus 15 is warm in comparison, zero degrees is hot and 7 degrees is very hot.
Ice will form on the evaporator coils below 7 degrees and will need to be defrosted occasionally. This requires the heat pump to operate in reverse sending the hot refrigerant gas to the coils to clear the ice rather than the heat exchanger to heat water. This process takes about 3 minutes and will occur two or three times during the heating cycle of a 315-litre domestic hot water storage tank at zero degrees.
Above 7 degrees the heat pump will continue in heating mode without the need for defrosting at all.
The efficiency of an air-to-water heat pump will vary with the air temperature and humidity. The warmer and wetter the air, the better the heat pump will perform. In fact, for every degree of temperature change, you can expect around 2.5% of performance increase if warmer or decrease if cooler. Therefore, good practice is to run your heat pump water heater in the afternoon. It will take 2- 3 hours of run time per day to complete the daily task of replenishing the hot water used for an average family of 4 people with average hot water use of around 160 litres per day. This will consume approximately 2-3 kWh of power per day which translates to around $220 per annum at 25 cents per kWh.
Heat pump water heaters are an ideal appliance to use with solar power. This is because they use about a third to a quarter of the power of an electric element water heater and can store hot water for 24 hours with little heat loss, just 2 degrees approximately depending on the air temperature and wind across the storage tank. This means a heat pump water heater can be programmed to run at any time in a 24-hour period, however it is best to run them in the afternoon when the air is warmest and power is available from solar panels. Even a small 3kW solar power system will provide ample power in cloudy conditions to run a 1kW heat pump water heater.
If you are located in an Alpine area, you may consider a heat pump water heater with electric element back up. This comes into play in extremely cold conditions, below zero, to assist the heat pump. The element will remain off as the air temperature rises above zero.
Another idea is to enclose the storage tank in a cabinet on the back porch, which may also serve as a drying cabinet for wet clothes, leaving the heat pump out at the top to cycle air through its evaporator. Better still, is to duct waste air from the roof to the evaporator coils to assist the heating process.