In message , Troy
writes
We had our first sub-zero temperatures last night and I was interested to
see the performance of my new Parasene "Frost Shield" 2Kw fan heater. I
only need frost protection for the greenhouse and so set it for just that -
(should protect at temperatures of 5°C and lower). Very disappointed this
morning to find that the greenhouse temperature had dropped to -1°C during
the night. The heater had switched itself on and off a few times but the
total on time was less than 2 hours.

Where were you measuring the greenhouse air temperature though. If it
was against an outside wall then that might be accurate. Yesterday was
extremely cold down to -7C here so under powered heaters will struggle.

Until you have a feel for how well or badly it works you need a min-max
thermometer and a healthy safety margin of min 5C target to ensure it
all stays frost free even on the coldest nights.

Took it back to B&Q today and got a new one. This one seems to be working
the opposite way. It's a much warmer evening and it's (same settings)
already been on for over 4 hours with the greenhouse temperature currently
over 10°C.

Any ideas? Suggestions for a more reliable heater? I really don't want to
go back to paraffin :-(

Mine (much older version) worked perfectly for several years until the
greenhouse fell on it. The fan was still running after that but the
heating element didn't work.

A separate thermostat somewhere near the plants you most want protected
is the only half way reliable method. The one inside the heater is to
easy to satisfy (and some modern ones have it a daft location).

Regards,
--
Martin Brown