You can do this by attaching a piece of plywood on the back side of the door.
Furnace closet door requirements.
All penetrations must be air sealed as well.
So the florida building code fbc g2406 and international residential code irc do not allow furnace installation in a bedroom bathroom or an any adjoining closet unless it meets these specifications that essentially isolate the furnace from the room.
More well judgment call but be sure the draft isn t so strong it is creating an air curtain across the discharge into the draft hood more likely on the water heater in my experience.
Provide a door with a vented front large enough to provide the necessary air supply.
When locating the walls be sure to allow enough space at least 30 inches to move around inside the furnace room so that maintenance and repairs can be performed on the furnace.
A furnace malfunction can cause a deadly carbon monoxide co buildup.
Place the door across two sawhorses.
The door assembly shall be installed with a threshold and bottom door seal and shall meet the requirements of section 504 1 2.
The door must have weatherstripping and a threshold to prevent air leakage between the closet and the house.
This almost certainly means that you will need to add a basic vent to your closet so that it can bring in a flow of new air for combustion.
Close off vents in door.
Once your combustion vents have been installed and the ceiling in the closet has been finished out you will want to close off any vents in the door.
Less isn t good especially in winter when drafts tend to run higher.
The key is that you have to install air inlets so that the furnace or water heater gets the combustion air it needs when it s running.
Door assembly shall meet the requirements of section 504 1 1.
Keep checking the draft also for good measure open and close the garage door and an outside door on another side of the house note you want the draft to be between 0 01 and 0 02 water column inches.
Unscrew the hinges from the furnace room door with a screwdriver.
Combustion air for such installations shall be obtained from the outdoors in accordance with section 506 4.
In this arrangement the general rule is that your furnace room cannot be smaller than 50 cubic feet per 1000 btus of heat the furnace produces.
Cut off the bottom few inches of the door with a circular saw to create a gap that allows air to vent into the furnace room.
The closet shall be for the exclusive use of the water heater.
Be sure to caulk around the seams to make a tight seal.