11
CHECK 6 - DILUTION AIR.
If gas-fired appliances are to be vented into a clay tile liner,
a source of dilution air is required.
Dilution air cannot be obtained through:
•
Induced draft appliances
•
Natural draft appliances with vent dampers
Sufficient dilution air can ordinarily be obtained through the
draft hood of a natural draft appliance only if the appliance’s
vent connector does not include a vent damper.
If dilution air will not be available, the chimney must be
relined (Fix 4).
CHECK 7 - COMPLETE THE INSTALLATION.
If Checks 1 through 6 have been satisfactory, and the liner
is an acceptable size as determined by the tables in
National Fuel Gas Code NFPA 54/ANSI Z223.1 - latest
edition and in the National Standard of Canada, CAN/CSA
B149.1 and CAN/CSA B149.2 - latest editions and amend-
ments, then the clay tile liner can probably be used as a vent
for the gas appliances. However, the installer must keep in
mind the following factors which may render the tile liner
unsuitable for use as a vent:
•
Extremely cold weather
•
Long vent connectors
•
Masonry chimneys with no air gap between the liner
and the bricks. (In practice, this can be difficult to
detect.)
•
Exterior chimneys (The tables in National Fuel Gas
Code NFPA 54/ANSI Z223.1 - latest edition and in the
National Standard of Canada, CAN/CSA B149.1 and
CAN/CSA B149.2 - latest editions and amendments
assume interior chimneys.)
If, in the judgment of the local gas utility, installer, and/or
local codes; one or more of the above factors is likely to
present a problem, the chimney must be relined (Fix 4).
FIX 1 - LINER TERMINATION.
Any cap or roof assembly used with a liner must be
approved by the liner manufacturer for such use. The liner
and cap/roof assembly must then terminate above the roof
in accordance with the manufacturer’s instructions.
In some cases, a shorter extension above the roof may be
possible with a liner than would be required with a masonry
chimney.
For further information on relining, see Fix 4.
FIX 2 -CHANGE VENTING ARRANGEMENTS
If the masonry chimney has more than one channel, it may
be possible to vent the gas appliances into one channel and
vent the solid or liquid fuel appliance(s) into another
channel(s). Do not vent an Air Command 80 inside of a
metal liner with other appliances vented outside the liner.
Alternatively, the homeowner may agree to discontinue use
of the fireplace (solid fuel appliance). If so, the tile liner must
be cleaned to remove creosote buildup. The fireplace
opening must then be permanently sealed.
If oil-fired appliance(s) are being replaced by gas-fired
appliance(s), the tile liner must first be cleaned to remove
the fuel oil residue.
If none of the above options is practical, the Air Command
80 may need to be vented vertically with a B Vent.
Under some conditions an Air Command 90 or 95 could be
installed rather than an Air Command 80. The Air Com-
mand 90 or 95 can be vented horizontally or vertically
through PVC pipe.
FIX 3 - REBUILD THE CROWN.
If the chimney crown is damaged, a qualified mason must
repair it in accordance with nationally recognized building
codes or standards. One such standard which may be
referenced is the Standard for Chimneys, Fireplaces, Vents,
and Solid Fuel Burning Appliances, ANSI/NFPA 211.
FIX 4 - RELINING.
Relining options include B vent and flexible liners.
If the chimney has diagonal offsets, B vent probably cannot
be used.
If B vent is to be used, it must be supported adequately.
Supports (such as fire stops or thimbles) must be used to
prevent the B vent from coming into direct contact with the
tile liner or chimney walls. Direct contact would result in
higher heat loss, with an increased possibility of poor
venting system performance.
It is not acceptable to vent one appliance inside the B vent
and other appliances outside. The excess space between
the B vent and the chimney walls must be covered at the top
of the chimney by a weatherproof, corrosion resistant
flashing.
The B vent should then be topped with a listed vent cap. The
listed vent cap will, when installed per the manufacturer’s
instructions, prevent problems due to rain, birds, or wind
effects.
A B-vent installed as described in this section is considered
to be an enclosed vent system, and the sizing tables in
National Fuel Gas Code NFPA 54/ANSI Z223.1 - latest
edition and in the National Standard of Canada, CAN/CSA
B149.1 and CAN/CSA B149.2 - latest editions and amend-
ments may be used.
If a flexible liner is to be used, it must be made of the proper
materials:
•
For most residential applications, an aluminum liner
should be acceptable.
•
If the combustion air supplied to the furnace will be
contaminated with compounds containing chlorine or
fluorine, a liner of AL294C stainless steel should be
