Why Does my Heat Pump Freeze?
Frosting Heat Pumps : An Overview
It is entirely normal for air source heat pumps to frost, especially in cooler weather (start and end of pool season). All air-source units have some form of defrost function for this reason.
Some units suffer worse than others, and this isn’t always due to a fault – this article aims to help you understand why it happens, minimise it, and identify if there is a fault with the unit.
How it works
An air source heat pump extracts a small amount of heat from the surrounding air and “amplifies” it to heat the pool. When working hard, the unit can pull up to 10°C from the air.
Under normal circumstances this is fine – for example if outside is 20°.
If the outside is much cooler than around 12° though, this can make the heat pump’s heat exchanger (the “evaporator”) drop to 5° or less. Add in wind chill, cold rain, a shadowy corner, and frosting becomes likely.
Use the heat pump on a cold morning, under 10°, and it’s inevitable.
Frosting isn’t a fault on its own. The unit can cope – unless it is poorly sited or overwhelmed by variations in climate.
Minimising Frosting
Before installation, bear a few things in mind:
Is it an indoor or all-year-round pool? The heat pump will be working hard in winter.
Location – ensure minimum clearances of 0.5m all round and 5m in front of the fan (even vertically if it’s a top-exhausting unit) to prevent air recirculation. The unit does not need a roof.
Check that the planned location is not too exposed and not permanently shaded.
Multiple units – don’t site them too close to each other, they can pull in cold exhaust air
After installation, keep the unit clean and free of obstructions, keep clear of vegetation etc.
The defrost cycle
Zodiac units are set to defrost at a default temperature (at the evaporator) of -12°C.
It will run this cycle for around 15 minutes and will then block the cycle from running again for around 5 minutes.
This is usually sufficient to clear ice, but if the weather remains very cold, or other factors are affecting the heat pump, it may not be enough to get rid of all the ice – which can then build up further.
Checking defrost operation
It’s possible to force the unit to defrost by telling it to cool the pool. This is done temporarily as a check.
First, turn on cooling mode. This is different across the range, but generally it means enabling the mode in the unit’s menu system.
Next, adjust the unit’s setpoint to a temperature well below the current pool temperature.
The unit should stop heating and switch into cooling mode – check the display status for confirmation.
Initially the fan will run, and after a short wait, the compressor will start to run too. Steam will rise as the ice begins to melt.
When you have finished, don’t forget to adjust the setpoint back to normal.
If this all works as expected, then you have proved that the unit’s physical and logical processes are operational. You know that the four-way valve works, that the PCB tells it to operate, and that the sensors are operating as they should.
If the unit does not defrost as expected, check you have properly enabled cooling mode. It’s possible there is a fault.
