6
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 one perm
(6x10
-11
kg per pa-sec-m
2
) or less with openings gasketed or sealed
and
b) weather stripping has been added on windows that can be opened 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.
If your home meets all of the three criteria above, you must provide additional fresh air. See
“
Ventilation Air From
Outdoors
”
(page 7). If your home does not meet all of the three criteria above, proceed to
“
Determining Fresh-Air Flow For
Heater Location: (next page).
DETERMINING FRESH-AIR FLOW FOR HEATER LOCATION
Determining if You Have a Confined or Unconfined Space
Use this worksheet to determine if you have a confined or unconfined space.
Space:
Includes the room in which you will install heater plus any adjoining rooms with doorless passageways or
ventilation grills between the rooms.
1. Determine the volume of the space Length
×
Width
×
Height= ________ cu.ft. (volume of space)
Example:
Space size 20ft. (length)
×
16ft.(width)
×
8ft. (ceiling height)=2560cu. ft. (volume of space)
If additional ventilation to adjoining room is supplied with grills or openings, add the volume of these rooms to the
total
volume of the space.
2. Divide the space volume by 50 cubic feet to determine the maximum Btu/Hr the space can support.
__________ (volume of space)
÷
50 cu. ft.=(Maximum Btu/Hr the space can support)
Example
: 2560 cu. ft. (volume of space)?0 cu.ft.=51.2 or 51,200 (maximum Btu/Hr the space can support)
3. Add the Btu/Hr of all fuel burning appliances in the space.
Vent-free heater
______________Btu/Hr
Gas water heater*
______________Btu/Hr
Gas furnace
______________Btu/Hr
Vented gas heater
______________Btu/Hr
Gas heater logs
______________Btu/Hr
Other gas appl ______________Btu/Hr
Total
= ______________Btu/Hr
*Do not include direct-vent gas appliances. Direct-vent draws combustion air from the outdoors and vents to the outdoors.
4. Compare the maximum Btu/Hr the space can support with the actual amount of Btu/Hr used.
Btu/Hr (maximum the space can support)
Btu/Hr (actual amount of Btu/Hr used).
Example
: 51,200 Btu/Hr(maximum the space can support)
56,000 Btu/Hr(actual amount of Btu/Hr used)
The space in the above example is a confined space because the actual Btu/Hr used is more than the maximum Btu/Hr
the space can support. You must provide additional fresh air. Your options are as follows:
a) Rework worksheet, adding the space of an adjoining room. If the extra space provides an unconfined space,
remove door to adjoining room or add ventilation grills between rooms. See
“
Ventilation Air From Outdoors,
”
page 7.
b) Vent room directly to the outdoors. See
“
Ventilation Air From Outdoors
”
, 7.
c) Install a lower Btu/Hr heater if lower Btu/Hr size makes room unconfined. If the actual Btu/Hr used is less than
the maximum Btu/Hr the space can support, the space is an unconfined space. You will need no
additional fresh air ventilation.
NOTE:
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, ANS Z223.1, Section 5.3 or applicable local codes.
Example:
Gas water heater
30,000
Btu/Hr
Vent-free heater
+ 26,000
Btu/Hr
Total
= 56,000
Btu/Hr
