The mains never froze, only the pan and cistern and overground pipes
were
affected, if a small source of heat was left in the place it raised the
temperature enough to prevent freezing.

My dad used a small paraffin heater made for very small greenhouses.

That's a different application.

Different device, same application :-)

No, it's not to prevent freezing in a greenhouse but to maintain a
temperature several degrees higher than freezing.
Mary
--

Martin