Double check the installation manual - if they didn't leave it, you may be able to find it on the web. If the thing has outside air and is vented to the outside, I don't think it needs the louvered door. Is the hot water tank an indirect one (i.e., it gets heated by the boiler)? If there is no other combustion device in there, it should be okay. Note I'm not a pro. For a device that draws air from the dwelling, you need about 1 sq in per 1000btu from what I read both at the bottom of the door, and at the top.