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For additional chimney design and sizing information,
consult the ASHRAE HVAC Systems and Applications
Handbook, "Chimney, Gas Vent, and Fireplace
Systems," of the National Standard for Chimneys,
Fireplaces, Vents and Solid Fuel Burning Appliances,
ANSI/NFOA 211.
2. A draft regulator is recommended with or without
a neutral pressure point adjustor in all cases where
there is chimney draft present. When flue is under
positive pressure, such as with a through-the-wall
vent, a draft regulator is not used. When a draft
regulator is used, it should be the same diameter as
the smoke pipe.
For gas units, a double-acting draft regulator is
preferable to a draft diverter. All vent pipe joints and
seams operating under positive pressure must be
sealed gas tight with minimum 400 F RTV Silicone
Rubber Adhesive Sealant. Category III Special
Venting System is required.
3. Where draft is present, adjust draft regulator
to maintain .01 in breeching. See "Controlling
Excessive Draft" and Figure #3 on this page for
instructions on controlling excessive draft by means
of a neutral pressure point adjuster.
4. Slope pipe up from boiler to chimney not less
than ¼" per foot.
5. Run pipe as directly as possible with a minimum
number of elbows.
6. Do not connect to fireplace flue.
FIG. 3 – CONTROLLING EXCESSIVE DRAFT WITH A NEUTRAL PRESSURE POINT ADJUSTER
7. The end of flue pipe must be flush with the inside
face of chimney flue.
8. Support the flue pipe rigidly with substantial
hangers plus three sheet metal screws at each pipe
joint.
9. The chimney should extend at least two feet
above any object within 10 feet and extend at least
three feet higher than roof at point of exit.
10. Particular attention should be paid to the point
where the flue passes through a wall or ceiling. Such
penetration should always be made in accordance
with prevailing code requirements.
CONTROLLING EXCESSIVE DRAFT
A neutral pressure point adjuster will reduce excessive
draft-regulator air flow which can cause condensation
and chimney damage. If draft regulator must be more
than slightly open to maintain draft at .01 at breeching,
a neutral pressure point adjuster should be installed
between the boiler flue collar and the draft regulator.
Insert adjuster so that draft regulator is not more than
slightly open to maintain draft at .01 in breeching. The
adjuster should be constructed so that it is permanently
locked in place and cannot be moved accidentally in
a manner that will interfere with normal operation of
the burner. Do not lock adjuster in place until burner
firing rate has been established and burner has been
adjusted to normal operating conditions.
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