Hi Nickjr,

A well sealed cold frame with polycarbonate glazing should be a good environment for using tubular heaters, I guess it could then be called a warm frame? Obviously you want to ensure the electric supply cables etc. don't get drenched. Most of the tube heaters are splash proof which should mean they'll be fine unless their constantly exposed to water.

I've got a wooden mini greenhouse with double-walled polycarbonate glazing, about 1.2m high, and 60cm wide and deep, which I've got a 90W tubular heater in.

My advice would be to seal all gaps between panels and around the glazing, to stop rain water and cold air getting in. I used clear silicone which worked very well. I also stapled bubble wrap along the frame in areas which don't get sunlight coming through, for a bit of extra insulation, and laid a double layer of bubble wrap on the ground.

As Nicos says moisture buildup can be an issue in a small grow house, but with my wooden greenhouse, a few hours with the lid open on a sunny day and the moisture evaporates. I also fitted a Bayliss XL greenhouse vent opener to the lid of mine, which does a great job in opening and ventilating the greenhouse when it gets hot. It could probably be adapted to fit on a cold frame too if you have a hinged lid. If it's cool and wet for a couple of days with no sun to dry it out I wipe the condensation off the panels.

For me, the 90W heater allows a 10 Celsius increase in temperature at night. I also use a cheap wireless thermometer with an alarm on it, so I can see if it's geting too hot or cold. The thermostats on the tube heaters aren't brilliant, so the temperature knob needs turning up on a cold night and reducing once it's a bit warmer outside to maintain a fairly constant temperature. I aim for 15C minimum for my chillies.