In article , Mike Deblis
writes

We have a load of tropical plants in the greenhouse - some fairly
hardy orchids, tamarinds, cactii etc.

Cacti on the whole are quite tough. They'll accept the very occasional
dip towards freezing if they are bone dry. Some are tougher than others.

Now I have some tubular greenhouse heaters - Can anyone recommend a
supplier for a good frost-stat? Will I need a fan to circulate the
heat? Would it be better to buy a small (water resistant) fan heater?
If so, where from?

There are plenty of greenhouse electrical heaters available from garden
suppliers. We use old storage heaters, because it's what we had
available free. I aim to keep the greenhouse above 45 deg F, but it does
dip below that, though never, so far, below 32.


Am I completely on the wrong track (I'm an electronics engineer, not a
gardener)?

As an electronics engineer, the most useful thing you could do would be
to link up a thermometer in the greenhouse with read-out in the house,
to alert you to those times when your heating isn't quite keeping up
with the temperature outside. Better still, feed the reading into your
computer and fit trends so you get early warning of sudden temperature
dips.
--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/garden/