Small heater for a plumbing valve enclosure

[whitmore]

I thought about heat tape but because of the irregular shape of the filter housings and the fact that they must be serviced, I was concerned that not having the heat tape in direct contact with the filter body it might not work.

As long as the thermostat is in a part of the enclosure that's not warmer than the filter body, the heat tape will
(assuming enclosure insulation and no cold drafts) warm the enclosure as a whole. The heat required to
keep an insulated box above freezing when temperature is 20 F is relatively modes, and warmed pipes
are just another kind of radiator.