Furnace clearance from rear closet wall

[Jeff Eastman]

...........

[Michael Thomas]

I would check manufacturer's instructions, can be zero.

[max kasten]

The building code requires 30" on all sides see bellow

Appliances shall be accessible for inspection, service,
repair and replacement without removing permanent construction,
other appliances, or any other piping or ducts not
connected to the appliance being inspected, serviced, repaired or
replaced. A level working space at least 30 inches deep and 30
inches wide (762 mm by 762 mm) shall be provided in front of
the control side to service an appliance. Installation of room
heaters shall be permitted with at least an 18-inch (457 mm)
working space. A platform shall not be required for room heaters

Max Kasten
Building code official

[John Arnold]

The building code requires 30" on all sides see bellow ...
...A level working space at least 30 inches deep and 30
inches wide (762 mm by 762 mm) shall be provided in front of
the control side to service an appliance....

I see where it says 30 inches in front. I don't see where it says 30 inches on all sides.