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5
For our example, we have chosen to add to our Total Volume of the Space by removing the door to an adjoining study
and increasing our Maximum Supportable Btu/Hr by the volume of the additional room. The new “space” calculations,
including the additional adjoining room are:
Living room/Dining room & Kitchen 3008 cu ft
Study (9' x 10' x 8') 720 cu ft
Total volume of space 3728 cu ft
The new
Maximum Supportable Btu/Hr
= (3728 cu ft ÷ 50 cu ft ) x 1000 =
74,560 Btu/Hr
74,560 Btu/Hr = Maximum Supportable Btu/Hr
-72,000 Btu/Hr = Actual Btu/Hr Used
2560 BTU/Hr = Remaining supportable BTU/H
Because the Actual Btu/Hr used is now less than the Maximum Supportable Btu/Hr, the space is considered an
Unconfined Space. No additional fresh combustion and ventilation air would be required.
**Unusually tight construction is defined as construction where;
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-m²) or less with openings gasketed or sealed;
b) Weather stripping 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 penetrations for plumbing, electrical, and gas lines, and at
other openings.
VENT FREE APPLICATIONS ● COMBUSTION AIR CALCULATIONS
COMBUSTION AIR CALCULATIONS WORKSHEET
1. Room Width Length Height W x L x H =Vol.(ft³)
1a.
1a.
1b.
1b.
1c.
1c.
1d.
1d.
2.
Total Volume (ft³) The Sum of Volume of all rooms (add lines 1a. thru 1d.).
2.
3.
Max Supportable Btu/Hr = Total Volume (ft³) ÷ 50 x 1000 (divide line 2. by 50 then multiply
by1000)
3.
4.
4a. Btu/hr 4b. Btu/hr 4c. Btu/hr 4d. Btu/hr
Actual Btu/hr used = The sum of Btu/hr of all fuel burning appliances inside the space identified
as rooms 1a. thru 1b (add line 4a thru 4d).
4.
5.
(Maximum Supportable Btu/Hr) minus (Actual BTU/hr Used) (Subtract line 4. from line 3. )
5. *
* If Line 5 is greater than zero, the Actual Btu/Hr Used is less than the Maximum Supportable Btu/Hr and the
space is considered UNCONFINED. No additional fresh combustion and ventilation is required.
* If Line 5 is zero or less, the Actual Btu/Hr Used is greater than Maximum Supportable Btu/Hr and the space is
CONFINED. You must either increase the Maximum Supportable Btu/Hr, decrease the Actual Btu/Hr Used or
operate as a Vented Appliance (See notes and warnings regarding vented installation on Page 7).
WARNING:
If the area in which the heater may be operated is smaller than that defined as an unconfined space or if the
building is of unusually tight construction**, provide adequate combustion and ventilation air by one of the
methods described in the National Fuel Gas Code, ANSI Z223.1 / NFPA 54, or applicable local codes.
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