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

It has to be remembered that what ever thermostat you fit it will have a cut
on /off range of around 5f. so no stat is going to be very accurate.
You could get a room stat and hang it behind the heater with a poly bag
fastened to the cable above it, and the bottom open just to keep any stray
water off it.
It is always hard to get an even heat distribution in any greenhouse, the
larger the harder unless you can duct the heat around.
Always try to have enough heating capacity in hand for the one night when
the temp. droops down to -10 or -12.
If it looks like being really cold remember that a few sheets of newspaper
laid on top of the plants will help to keep the frost off.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk
***2004 catalogue now available***

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.

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.

Rod wrote:

David's idea of a room-stat is a good one

I'm seriously considering this option now. 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.
--
Regards,

Troy the Black Lab.

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.

"....... 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 ..........."

and if you were using paraffin or gas then the heater would be on all the
time and would cost a lot more than the electric as it would be on for
around 12 hours a night.
If you find an hour a night excessive then forget about heating, there will
be times when you could have it going for around 20 hours out of 24, and for
a couple of weeks.
You have to decide if the things you want to keep are really worth it.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk
***2004 catalogue now available***

In article , David Hill david@abacusn
urseries.freeserve.co.uk writes
"....... 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 ..........."

and if you were using paraffin or gas then the heater would be on all the
time and would cost a lot more than the electric as it would be on for
around 12 hours a night.
If you find an hour a night excessive then forget about heating, there will
be times when you could have it going for around 20 hours out of 24, and for
a couple of weeks.
You have to decide if the things you want to keep are really worth it.

And I presume you have adequate insulation? Our greenhouse is double
glazed, and I add a layer of bubble wrap in winter.
--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/edward/index.htm

"David Hill" wrote
You have to decide if the things you want to keep are really worth it.

I do not check my electricity bill from the before and after greenhouse on
purpose..........:~))
I'd rather live in ignorant bliss.
Jenny

On Sun, 14 Dec 2003 10:29:00 +0100, "JennyC" wrote:


"David Hill" wrote
You have to decide if the things you want to keep are really worth it.

I do not check my electricity bill from the before and after greenhouse on
purpose..........:~))
I'd rather live in ignorant bliss.

Huh! now we know the source of all those power cuts that emanate from
the R'dam area.

--
Martin