defrost cycle for air source heat pump

I have a question about air source heat pump (ASHP) and defrost cycle.

During defrosting, the heat pump extracts heat from heating system to melt ice on the outdoor unit. This can cause the water temperature to drop.

My question is: If the water returning from the system reaches a low temperature, like 12°C, does that mean there’s not enough heat to defrost effectively?

Adjusting Defrost Cycle Settings is a good option ?
Estimate the defrost heat requirement ?
Synchronizing flow in the secondary side during defrost ?

How much water volume is generally needed in a heating system for efficient defrosting?

A larger system water volume /buffer can help maintain a more consistent water temperature during defrost

Anyone with ASHP experience, your insights are greatly appreciated!

A warm welcome to the community, Alex :slightly_smiling_face:.

Truth be told, defrost seems to be a somewhat black art. Controller defrost algorithms are very vendor-specific, but typically there is enough heat in the emitters and pipework to draw heat from for a complete defrost cycle. Don’t forget that much of the heat needed comes from the hot refrigerant vapour leaving the compressor when the circuit reverses itself. You shouldn’t normally need a volumiser to provide the additional heat (lots of folk on this forum manage to defrost OK without one).

Try using the search facility for specifics for your system (you don’t say which HP you have).


We can have a stab at it purely from an energy perspective. A defrost cycle for my 10kW pump requires around 1.2 kWh in heat that is used to thaw the ice (depending on outdoor conditions of course, but that’s the order of magnitude). Around 0.2 kWh of that come from electricity, the rest is extracted from the hot water in the system. 1 kWh is released when cooling 29 l of water from 30°C (my flow temp at 0°C) to 0°C. If you have twice the volume, you’ll only cool from 30°C to 15°C and so on.

From that energy you can thaw around 13 kg of ice which seems reasonable. Since it is not only thawed but actually ice heated to 0°C, then thawed, then water heated up it will actually be less mass you can heat up that way but the thawing itself is the by far largest fraction of it (the energy required to thaw the ice, i.e. transition from solid to liquid is the same energy that is required to heat water from 0 to 80°C!).

Taking all this into account I would say that anywhere upwards of 60 l would be a reasonable volume to allow safe defrosting for my specific setup. and I reckon it would be in similar ranges for other installations. You could do a survey on heatpumpmonitor to see how other defrost cycles behave in terms of total energy use and work you way backwards from there.

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