[BlackBear]

I had an idea for my place in that the thermal mass (H20) is held in food grade barrels /Tubs ( at least 200 gallons to be worth while for a small situation)

50-60 gallons/barrel and that they would be able to "enhanced " with in dwelling

submersible fish tank heaters...... that could be adjusted to what you wanted

70 F.....or 60 F (if economize at end of season etc.) and only kick in when

needed at beginning and end of extended growing season. The enhanced warmed thermal mass water is also the source of thermal enhanced plant water (non hydroponic system).

If I really got it together I guess I could enhance the nutes in solution as well with the thermal enhanced water . ( I am sure some of you have a recirculating system to recycle thermal mass water with nutes etc. )

When the submersible heaters keep the tanks a certain temp . of course
they radiate and keep the inside greenhouse area air temp above freezing .

If you wanted to get all hard edged about it you could get a 12 volt system with an inverter as a safety back up ...I guess ....but also if you had an actual commercial operation you might also have a back up generator (rent) to supply electricity at the "sticky" part of the year if need be.... and preserve your hard won tomato venture . I have heard of some people with propane emergency systems (with proper venting) ...any time the weather gets close ...they apply some insurance heat
to tide the greenhouse over ......I have even seen the old woodstove with a

Big boiler system to enhance the Greenhouse at certain times of the season.

I guess however you do it ...you want the space above freezing Low by at least 6 degrees or more ... margin of safety to extend the season and utilize the high temps still available in the day .