Monessen Hearth DFX24C User Manual

8

This heater shall not be installed in a confined space or unusually tight construction unless 
provisions are provided for adequate combustion and ventilation air.
The National Fuel Gas Code, (ANSI Z223.1/NFPA54), defines a confined space as a space 
whose volume is less than 50 cubic feet per 1,000 BTU per hour (4.8 m

3

 per kw) of the aggre-

gate input rating of all appliances installed in that space, and an unconfined space as a space 
whose volume is not less than 50 cubic feet per 1,000 BTU per hour (4.8 m

3

 per kw) of the 

aggregate input rating of all appliances installed in that space. Rooms communicating directly 
with the space in which the appliances are installed, through openings not furnished with doors, 
are considered a part of the unconfined space.

a)  walls and ceilings exposed to the outside atmosphere have a continuous water vapor 

retarder with a rating of 1 perm  
(6 x 10

11

 kg per pa/sec-m

) or less with openings gasketed or sealed;

b)  weather striping has been added on openable windows and doors, and
c)  caulking or sealants are applied to areas such as joints around window and door frames, 

between sole plates and floors, between wall-ceiling joints, between wall panels, at pen-
etrations for plumbing, electrical, and gas lines, and at other openings. 

The following formula can be used to determine the maximum heater rating per the definition 
of unconfined space:

(L

1

 + L

) Ft x (W) Ft x (H) Ft 

50

Consider two connecting rooms with an open area between, with the following dimensions: 

L

1

 = 15Z\x Ft., L

 = 1 Ft., W = 1 Ft., H = 8 Ft.

(15Z\x

 

 + 1) x (1) x (8)

50

If there were a door between the two rooms the calculation would be based only on the room 
with the heater.

(15Z\x) x (1) x (8)

50

x 1000

x 1000 = 5800 BTU/hr

x 1000 = 9,760 BTU/hr

BTU/hr = 

BTU/hr = 

BTU/hr = 

FP2439
combustion air

Room Divider

Fireplace

FP439