I have a better idea. You can't restrict the vent in any way or you may get CO in your house. But instead of venting the CO into the greenhouse, what you can do, in a safer way, is to put a small air source heat pump right next to the vent. The hot waste air that comes out of the vent will heat the heat pump, and the heat pump will "move" the heat into the greenhouse. This way is more expensive(you get higher electricity bill), but much safer. You are not restricting the vent in any way. In a cold climate, the heat pump usually doesn't work so well, because it needs to run defrost cycles quite often. HOwever if you have this vent blowing into the heat pump, there's no need to run the defrost cycles because the hot waste air will do the defrosting for you. A lot of the heat will get transfered into the greenhouse by the heat pump. The greenhouse will not have that toxic CO either(it's all vented naturally to the outside).

This is our first winter with a wood framed version of a high tunnel/hoop house. It was less expensive for my husband to use a wood frame and make a peaked roof so snow will slide off. It will help you a lot to get one or both of the Eliot Coleman books on gardening year round. He market gardens in Maine. I keep re-reading so ideas sink in. Right now, the shorter days mean a dormant period. I see some new growth but not much. Even the outdoor kale shows some new leaves (I'm in NH). We have some nice spinach and beet greens inside. Some of the larger beets have formed tiny beets. The flavor of both the beets and the greens is incredibly sweet. The broccoli transplants I put inside were attacked by bugs but the plants survived putting out tiny heads. Since we have to keep the sides rolled up while it's warm, plenty of insects can come in. Last night was our coldest - 10 degrees. I think it got up to 60 before it clouded over (storm expected tonight). The ground hasn't frozen inside and that's really key. I have some seedlings that sprouted from a late October planting but they have hardly grown. I'm not sure they will survive. The spinach seedlings are larger. I should have had all seeding done by mid-September. I have the beds covered with Agribon fabric inside. The lettuce froze with temps got in the low 20s so next year I will try hardier varieties. Wish I had planted carrots and leeks inside. The carrots and spinach outside are now covered with more than 6" of straw. I'm not sure if your broccoli will come alive in March. It may depend on the variety. A neighbor got an early start on tomatoes in her hoop house and had tomatoes by July 1 so that is what I'm expecting next summer. Today I picked beets greens from inside. Some kale from outside. Any kale left later in the winter will end up as food for deer or other critters. I'm happy to be getting some food from the garden in the end of December and thrilled with a sweetness this time of year. Sure, it would be nice to have fresh tomatoes but we aren't going to spend money on heat.

I see also that DTD mentioned Tower Hill Botanical Garden in Boylston. This is another great place to go. In addition to the half price admission I believe if you are a AAA member or WGBH member there is 2 for 1 admission. Also many public libraries have passes for Tower Hill so check out your library. If in the Providence RI area, another neat indoor botanical garden is Roger Williams Botanical Garden center. I also visited there in 2008. If you want to see some photos to inspire you check them out at the link below. Here is a link that might be useful: Roger Williams Botanical Garden -- Providence RI

Finbar, good questions. I'm not sure it'll be warm enough consistently in the greenhouse. They will develop - but very, very slowly. I've just picked an Anna Russian which took until now to develop on the plant. I'm surprised it did develop at all and it ripened up. I'm sure it won't have that much taste, but will find out today or tomorrow. So, I guess I'll let them flower if they like. :) Flowr, I posted a list of tomato seeds on this forum a while ago. I think DerbyTas started the thread. Have a look if you find a tomato or two that you'd like to have seeds of and send me an email.