John Dann wrote:

I'm looking for an electric greenhouse heater, predominantly for frost
protection over winter. There are obviously lots available but most
that I've seen have only a vague setting for frost protection - it's
usually just a rather uncertain position on a continuously variable
control knob. And just a tiny adjustment either up or down seems to
make a major difference to when the heater comes on. There's obviously
a temptation to set it on the high side 'just to be safe' but the
result inevitably is an expensive waste of electric power.

What I'd like to find is a model that has a more accurate temperature
calibration, ideally one that has eg a positive click position just
for frost protection or alternatively one that has an accurate
temperature scale marked clearly in steps of eg 1-2 degrees so that
there is little ambiguity about exactly what temperature it should
come on at.

Anyone have any receommendation for a suitable make/model please,
preferably without spending a fortune. The greenhouse is 8x6 so the
power output doesn't need to be enormous.

TIA
John Dann
Afraid the kind of accuracy and repeatability you're looking for will cost several
times more than the heater. IIRC we paid around £150 for a nice digital one with 2
set points and a light sensor so we can set different temps for day and night. It
saves more than its cost in oil each year and helps us grow better plants - cheap
at the price. Economies of scale I'm afraid so not a lot of help to you. David's
idea of a room-stat is a good one but they again are crude bi-metal devices and
are only one or 2 steps better than the ones in the heaters. We use one for frost
protection on our irrigation pump. That thermostat is on the shed wall near the
pump box and turns on a tube heater in the insulated pump box if the shed gets
near freezing. We don't need great accuracy for that.
--
Rod
http://website.lineone.net/%7Erodcraddock/index.html
My email address needs weeding.