Troy wrote:

I'm still experimenting with my
(3rd) Parasene "Frost Shield" heater (another thread) and am quite amazed
at how inaccurate the thermostat is. For test purposes I have it connected
via a meter which records, amongst other things, the total time it's
drawing power. Last night was very warm for December .. 10°C+ and set to
"frost protection" the heater still managed to switch itself on for a total
of nearly an hour ! Over a quarter this would mount up to quite a few ££ on
my electricity bill. I wonder how many people use these heaters without
realizing what it's costing them in wasted power.

Yes, I'm afraid cheap answers to this are not very useful. If it is just frost
protection you need, you are looking for something like +6C (+/-2degrees) If you
remember from school, water behaves very strangely between +4C and 0C - it expands
and this can be damaging so it's desirable to stay above 4C. That range is quite
doable for a good thermostat but do remember you have to have some 'hysteresis' If
the 'stat was trying to switch on *and* off at say 5degrees it wouldn't know
whether is was coming or going and would just 'hunt' all the time and probably
damage the contacts. If you relaxed the spec and increased your energy costs you
could have 8C (+/-4degrees) Now that's probably getting near what could be
achieved with a mediocre thermostat. Probably move the range down a couple of
degrees if your plants aren't too sensitive. Another problem is the position of
the built in thermostat on those heaters - the air temperature next to the body of
the heater is irrelevant, what's important is the air temperature near the plants
and that's where the sensing element of your thermostat should be - as near as
practically possible anyway.
--
Rod
http://website.lineone.net/%7Erodcraddock/index.html
My email address needs weeding.