A critical thing to consider when selecting a heat pump is how it deals with defrost:
All air source heat pumps accumulate frost on their coils in cold temperatures, the water particles in the air passing through the coils has so much heat taken out of it, they change state and freeze on the coil.
The UK is an island and so our air is fairly humid, when the temperature drops to about 2°C, we have the perfect conditions for forming frost on the coils – it actually starts at about 6 degrees but is worst between 2°C and -2°C in the UK.
Below -2, the air starts to dry out (looses it’s humidity) and so less frost is accumulated at say -4°C than -2°C.
When frost accumulates, the efficiency of the heat transfer from the air to the refrigerant reduces, the frost on the coils creates resistance also reducing efficiency.
The more the coils get frosted up, the less heating output and efficiency is achieved.
For this reason, the heat pump will go into defrost mode, this is when the unit uses it’s 3 way valve to divert the compressor discharge gas away from the refrigerant to wate rheat exchanger and to the evaporator coils.
When it does this, the hot gas passes through the pipes in the air coils and melts the frost which has accumulated on the surface of the coils.
When it comes to defrost not all heat pumps are equal.
Conventional heat pumps will take heat from the heating water and therefore cool the water during the defrost.
If you have multiple heat pumps which have been started at the same time, they could all potentially go in to defrostat the same time – this could mean that in the worst case scenario, the heating water is being cooled for about 10 mins in every hour.
Many heat pumps have very simple defrost activation controls, once the ambient temp gets below say 6°C, the heatpump will automatically go into defrost for a pre-set amount of time – circa 10mins in every hour.
Better heat pumps will be reading the temperature of the coil & pressure drop though the coil and using this to determine when to go into de-frost and for how long.
This means that the heat pump is only in defrost for the amount of time that it needs to be.
Some heat pumps have what is called Smart or Intelligent defrost, these units will have some or all of the following functions:
1.) No reduction of water temperature during defrost – systems with multiple circuits, use control methodology to ensure that only 1 circuit is defrosted at a time, the other circuit(s) continue to heat and so the unit continues to deliver heat (perhaps at a lower temp) into the building.
2.) Self management of multiple heat pumps to ensure that only 1 heat pump defrost at 1 time – this ensures that full heat continues to be delivered into the building. It achieves this by monitoring multiple heat pumps and through the coil temp and pressure drop deciding which unit is worst effected – it will also bring units on and off to ensure that they are all at different stages of frosting up at the same time.
Having Smart /Intelligent defrost will ensure that there is un-interrupted heat to the building and that defrost is dealt with in the most efficient manor - insist on heat pumps with smart defrost!
