In zone 7 heating a greenhouse is relatively trivial. Radiant heat, ceiling mount, pointed down, or floor level using the "heat rises" all work. Biggest issue will be keeping the greenhouse cool. On a calm sunny January or February day, it is possible for the sun to heat a greenhouse into the upper 80's or low 90's very quickly. You will need fans and vents.
Fans should keep air circulating around the greenhouse 24/7, so no matter where you set the plants the leaves have a little motion to them. Mimic a gentle breeze. Circulation fans are separate from exhaust fans. Circulation fans are kept on continuously. This is important in keeping disease down, and avoids cold pockets and hot pockets in the greenhouse.
Second you need to be able to vent the greenhouse effectively on a warm day. A 12 x 12 x 10 foot greenhouse will have 1440 cubic feet of air. Ideally for summer you need to be able to exhaust the entire volume of air once every 3 minutes to avoid heat build up in direct sun. This means exhaust fan able to move 450 cubic feet per minute as a minimum. I've seen orchid greenhouses where they can exhaust the full volume of air every minute. In the 12 x 12 x 10 example that would be 1440 cubic feet per minute. This high exhaust rate allows the greenhouse to be used both winter and summer. Exhaust fans would only turn on when temperatures exceed a set temperature, for example maybe 85 F.
I have seen a number of orchid collections destroyed by power failures. If power goes out on a bitter cold day, you have hours to get it repaired or get a generator in place to get alternate heat running. Most interesting is I have seen more orchid collections destroyed by power failures in summer. In summer you only have minutes once the sun is out and beating on the greenhouse to get the air moving to cool the plants. An hour or two in a sun baked greenhouse and all plant life inside can be toast.
Roof vents should be installed. At least a portion should be automatic and not require electricity to operate. There are hydrolic and bi-metal operated hinges for opening vents that do not require electricity. These are key for a set up that survives a power outage.
I personally opted for growing under lights rather than trying to heat and cool a greenhouse in zone 5. If I had to do it over, I might have preferred to go with a detached greenhouse, but these days I have downsized my tropicals collection to the point now a greenhouse is unnecessary.
