Well in the winter Zem you can use the same systemto heat a bit
below the frost line world wide the ground is pretty much a constant 55 degrees.

So that being said, in the winter when its 2 below out you can warm the intake air a bit with the 55 degree water... now granted it wont get you to a pointwhere you can grow without heat, but it will save a bit on the bill and help out.

and to be honest if i were building it it would be built only to change the air every 1-1.5 hours. in a rush, a quik change, I would also get a temp control system and run CO2.

in the winter I would run the CO2 and vent less, to keep smell down id get a carbon scrubber and have it vent into the next room to promote air circ

As far as heat goes if i were haveing issues with the needing heat(light heat not being enough) i wouild put in a boiler. and fuel it off the main house either in gas or oil. dependong on how big yer set up is a really cant imagine needing a very big boiler. Here in the states you can get one for less than a grand. Happy growing mate, ill keep an eye on this thread i wanna see how it comes out .. post some pics too )