We have a laundry room that also contains a natural gas furnace and water heater. Due to door placement and room layout, it is possible to frame in a 36" door in front of the furnace. However, we can't get more than an inch or two clearance from the face of the furnace to the framing. I know the door must help conduct air flow for the furnace with a vent or louvers.
Are there any other restrictions on the door, such as door material, thickness, clearance vs. material type, etc.? Also, what is the ratio or amount of 'open' area in the door that must be vented?
First, the furnace should have a label on it indicating required clearances. If not, it will be in the installation manual or available on their website. It is usually 6 inches in front.
Next, you must maintain access to BOTH appliances for service and replacement. It does not look like a 36 in door would provide that. If it is in front of the furnace, it will block access to the water heater and it will need replacement before the furnace.
If the space you are creating is less than 50 sq ft., which this will be, you are required to provide a minimum of 100 sq inches of combustion air supply from outside the space FOR THE FURNACE ALONE. There is a similar requirement for the water heater since it is also a gas burner. A standard 80 by 36 in door is 2880 sq in. Louvers de-rate the available area by 50% so you would effectively need at least 400 sq. in. of air inlet and it needs to be split with 50% in the lower 12 inches of the space and 50% in the upper 12 inches. It's doable but you are still blocking the water heater.
Time to rethink this project. Possibly some kind of folding door that could be easily removed? There is still the issue of clearance in front of the furnace and water heater as well.