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This week has seen some icy mornings in Cambridge – a reminder that we are sliding into winter. Our heat pumps have been going, especially first thing in the morning to warm the place up a bit, and the cat has relocated his primary sleeping spot from a chair by a window to a rug closer to the unit.

We are able to see the outdoor unit of our downstairs heat pump from the dining room window. Tuesday morning, when I looked at it, there was something odd about it. It looked somehow different. The indoor unit then made some gurgling noises and went into defrost mode, and I realized what – it was iced-up. Instead of seeing the multitude of metallic fins on the back and side of the unit, I was seeing white ice. With the unit in defrost mode, this disappeared very quickly into a pool of water around the pump, the fins reappeared, and the heat pump started heating again.

This unfortunately is a problem with heat pumps. When it gets close to freezing outside, the fins of the outdoor unit drop below zero in temperature. That’s because the pump is pumping heat from the outside to the inside. The many highly thermally-conductive metal fins are there to provide good thermal contact area with the air, so that heat can be drawn from it as efficiently as possible. When they drop below zero, ice is going to form on them. How quickly this happens depends on a number of factors, but it is hard to prevent. (Aircraft have similar problems.)

Once the ice has formed, the efficiency of the machine is severely reduced. That’s because ice is a fair thermal insulator. It doesn’t allow heat to pass through it quickly, so the pump cannot suck heat from the air very well. Also, I noticed the ice forms between the fins, thus massively reducing the effective area of the pump in contact with the air. Overall, it is better for the machine to stop heating the house and instead heat the outdoor unit to remove the ice, then start again.

In the same way it’s better to periodically defrost your freezer – it will mean it will be able to keep the inside colder with less electricity cost. That might seem somewhat paradoxical – getting rid of the ice will help cool the interior – but the ice here is a consequence of the cold, not the cause of the cold – and it is insulating the inside from the cooling elements.