I live in a pretty strange basement apartment, built mid 1950s or so, that has a lot of strange heating issues that are costing me money. Before I get to my question, let me describe how this system seems to be set up.

My apartment is a 700 sq ft garden level, so I have nice windows in all of my rooms. However, my floor is a carpet covered concrete pad. My ductwork seems to be *below* the pad, since the heater vents are the kind of old fashioned ones that are up against the base of the wall, and I have found all the nice warm places in the floor via my heat-seeking cat. By the time the air leaves the ducts, it is perhaps 70 degrees--pretty tepid for just having come from the heater. There are two cold-air returns, a smallish one in the bedroom and a larger one in the living room. Both are near the ceiling, which I suppose makes sense--air is sucked through the top of the heater and blown out the bottom. Well, it makes sense as long as you don't believe in physics...

My problems are numerous, but the biggest one right now has to do with the big return. There is a TON of cold air that seeps out into my living room/workspace. If I have space heaters running, the temperature differential sucks up all my warm air that is accumulating near the ceiling, even if the heater isn't active taking in air (between cycles). Additionally, I shined a flash light into the grate to see what's going on...there is no definite ductwork, and I can see water pipes and floor beams, in addition to some ductwork-looking metal. I can't see what's happening in the small return because it is kind of dirty (I'm in the process of cutting through the many layers of paint so I can wash it and also continue my sleuthing). Either way, the small one doesn't seem to seep cold air in the same way that the big one does.

This is my question: Can I close off the bigger return so that I keep my space-heated air from being sucked away/close off the seeping cold air? The house is super drafty, so I'm not that worried about pressure problems. Also, I kind of assume that the heater can just suck its air from wherever all that dratted cold air is coming from.

This is a rental in a city with strict building codes, where the rentals are managed by companies with 1000+ properties that are mostly rented to students (i.e. service is bad), so I am kind of limited and I'm just trying to make do with what I have. I've already caulked up everything I can see in an attempt to halt as much cold air as possible! Thanks!