Try this link. It is the same question.
http://forums.gardenweb.com/forums/load/tomato/msg0216110214127.html?19

oh ok thanks !

The thermostat in our greenhouse is set at 45 for the heat to come on. The real problem in a greenhouse is keeping the seedlings cool enough. ;)

Dave

Mine too Dave--Mine is set to come on at 10C which is just about what yours is set at. My greenhouse is 6'x8' and consists of mostly shelves on 3 sides,with a square on the north end for me to stand and work. I have a pullout rack under the shelves on the east side for impatiens and begonias and a few hooks for hanging baskets in the rafters. It is as airtight as I can get it, but still needs suplimental heat at night.

We live in Northern Ontario and get colder nights than many of you do. In the daytime if the sun is shining it can be below zero outside and I will have to open the vents to let out the excess heat. The reason I described mine is to say that it costs me about $50 for the season to heat.

I open it in mid April and things are all out of it by mid June. Most of my heating is done in the first month. I start most of my seeds in the greenhouse. I do have a cold frame which gets the overflow the last month. because my greenhouse is too small to hold all I plant, but by then I scarcely have to heat anyway

You are lucky you have until June oilpainter - I have to have everything out by middle of May at the latest even with 60% shade cloth on. ;)

Ours is 18x30 with 6 auto vents and 2 big exhaust fans on the ends and when the sun is out come June it is still 110 in there. So the sides come off and the screens go on. Until then, since we try to keep the max temp at 65F we spend far more on cooling than we do on heating.

Dave