National Oilheat Research Alliance 7
3.3.14 Confined Space. For the purposes
of this standard, a space whose volume is less
than 50 ft3 per 1000 Btu/hr (4.8 m3 per kW) of
the aggregate input rating of all appliances
installed in that space.
3.3.60* Unconfined Space. Any space whose
volume is equal to or greater than 50 ft3 per
1000 Btu/hr (4.8 m3 per kW) of the aggregate
input rating of all fuel-burning appliances
installed therein.
So, any space that provides less than 50
cubic feet per 1,000 BTUs of total appliance
input is considered a confined space. Figure 7.
To determine the maximum total firing rate
allowable in a room, the total cubic feet must
be known.
To determine the total cubic feet of the room,
multiply the length times the width times the
height:
L x W x H = Total cubic feet
So, if you have a utility room that is 20' long,
24' wide with an 8' ceiling (Figure 8):
20 x 24 x 8 = 3,840 cubic feet.
Figure 8
To determine the maximum BTU firing rate
allowable in this room, multiply 3,840 x 1,000
BTU/50 (3,840 x 20)
3,840 x 20 = 76,800 maximum BTU input.
A gallon of #2 oil contains approximately
138,690 BTUs; to determine the maximum
allowable firing rate, divide 76,800 by 138,690:
76,800/138,690 = .55 GPH
The maximum firing rate for this room is
.55 GPH. Any unit—or units—with a higher
firing rate requires that additional air be made
available.
If the appliance, or appliances, installed in
this room have a firing rate greater than 55
GPH per hour, this would be considered a
“confined space.”
Chapter 5 of NFPA 31 details the require-ments
of the standard to ensure that adequate
air is available for the safe operation of oil fired
appliances:
5.3 Appliances Located in Unconfined
Spaces.
5.3.1* In unconfined spaces in buildings of
conventional frame, brick, or stone construc-
Figure 7