OT: heating vent in closet?

Hey folks,
I live in a second floor apartment with a gas forced air furnace. The furnace and water heater (both gas) are located in an interior closet. In addition to the water heater and furnace flues, there is another 4" pipe that comes through the ceiling and ends near the floor with only an open elbow (not connected to anything). I assume this is a combustion vent. Cut into the large duct coming out of the top of the furnace (is this the plenum? -- sorry for my ignorance of HVAC terminology), there is a large heating vent that serves only the closet. As a result, the closet interior (being fairly well closed off from the rest of the apartment except for about a 3/4" gap at the bottom), is always several degrees warmer (or cooler, if the AC is running) than the rest of the apartment. I have not measured the temp., but I would estimate that it gets to be about 75-80 degrees in there when the thermostat is set to 65. It seems to me that this is at best an inefficient use of heat, and I might get better performance by covering up the vent. Not being well versed in all things HVAC, however, I have a couple of questions:
First, is there a legitimate purpose for having that vent in there? For example, is it necessary for the combustion vent to function or something like that? Another way to put this would be "will I run into trouble by covering the thing up?" I have CO detectors installed, but I'd sooner avoid producing a situation where they might go off than have to figure out if they went off because of something I did.
Second, is covering the vent likely to have a noticable effect on: a) performance of the heater (which is already not bad),
or b) efficiency of the system (which I am not thrilled with. It is, according to the "EnergyGuide" label, near the bottom of the heap in its class, as one might expect from a unit in an apartment where the installer/landlord isn't paying the heating bill. Also, my heating bill for December was seemingly extremely high for a one-bedroom apartment, even considering an average temperature somewhere in the high teens or low twenties.)
Any help would be greatly appreciated. Thanks guys!
-AD
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I'll preface by stating that I am not a HVAC tech, but I am a mechanical and energy code inspector.
I sounds like you are unintentionally conditioning your attic. The combustion air setup you described is not up to current code. There should be 2 pipes coming from the attic. One should extend to 12 inches from the floor (like the one you described), and the other should be 12" from the ceiling, and both should extend 12" above the insulation level (about 2 feet total) into the attic, and be unscreened (so insulation or other debris cannot block it).
Also, the door to closet should be sealed (think of it as another part of your attic). I would weatherstrip the door, and block the bottom somehow. Sealing that room will almost eliminate the possibility of backdraft, and any CO problems you might have (keep the detectors).
It sounds like you have an upflow unit (return air on the floor). I would recommend sealing the wall cavity above the return.
When you have the closet door closed, the vent is creating positive pressure in the closet, and blowing all of your conditioned air through the pipe, and into your attic. You should be able to seal the vent, but get the proper combustion air before you do it. You will probably notice a nice difference in your bill.
Rob IECC 2004 IRC 2004

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