"........ 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.
--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk