"David Hill" wrote in message
...
"........ But you could get the loss to a very low level by a heat
exchange
system. Seems a bit over-elaborate for a greenhouse though ;-)

True enough, but I have never seen a greenhouse application of that
principle. .........."

Frank.........try towards the end of the Electric heaters on the site I
posted
http://www.cmsgardens.co.uk/heaters.htm

They do one.

I must be blind. I saw no reference to a heat-exchanging vent in that URL.
By the way, surely electric heaters do not need a vent in the same way as a
paraffin heater needs one?

Franz