On 01/03/18 11:19, Andrew wrote:

My Ebac homedry has a circuit board with a 555 timer.

Once per hour it operates a solenoid valve in the gas stream
reversing the flow of warm refridgerant through the cooling
coils, so the ice crackles, melts and falls off. It does this
for about 3 mins and then reverts to normal operation.

QED, should work at very low temperatures.

I bought it 2nd hand in 1988, it's been around the world, to
the pacific for 2 years, and back. Still going strong.

You can buy special heating cable that is mains rated.

I wonder if anyone still makes a unit like that?

I'd prefer the safety of a refrigerant unit (based on what Andrew said
earlier) but the purpose is to keep a shed dry so it will get cold.