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Hydronic Heating Boilers and
Domestic Water Heaters
16
INSTALLATION
Continued
1. Conventional Negative Draft
Venting
FIG. 17 Conventional Negative Draft Vertical Venting with
Combustion Air Louvers
This option uses Type-B double-wall flue outlet piping. The
blower brings in combustion air. The buoyancy of the heated flue
products cause them to rise up through the flue pipe. The flue
outlet terminates at the rooftop.
Negative Draft
The negative draft in a conventional vent installation must be
within the range of 0.02 to 0.08 inches w.c. to ensure proper
operation. Make all draft readings while the unit is in stable
operation (approximately 2 to 5 minutes).
Connect the flue vent directly to the flue outlet opening on the top
of the unit. No additional draft diverter or barometric damper is
needed on single unit installations with a dedicated stack and a
negative draft within the specified range of 0.02 to 0.08 inches
w.c. If the draft in a dedicated stack for a single unit installation
exceeds the maximum specified draft, you must install a
barometric damper to control draft. Multiple unit installations
with combined venting or common venting with other Category I
negative draft appliances require each boiler to have a barometric
damper installed to regulate draft within the proper range.
Do not connect vent connectors serving appliances vented by
natural draft (negative draft) to any portion of a mechanical draft
system operating under positive pressure. Connecting to a positive
pressure stack may cause flue products to be discharged into the
living space causing serious health injury.
Flue Outlet Piping
With this venting option, you must use Type-B double-wall (or
equivalent) vent materials. Vent materials must be listed by a
nationally-recognized test agency for use as vent materials. Make
the connections from the unit vent to the outside stack as direct as
possible with no reduction in diameter. Use the National Fuel Gas
Code venting tables for double-wall vent to properly size all vent
connectors and stacks. Follow the vent manufacturer’s
instructions when installing Type-B vents and accessories, such as
firestop spacers, vent connectors, thimbles, caps, etc.
Provide adequate clearance to combustibles for the vent connector
and firestop.
When planning the venting system, avoid possible contact with
plumbing or electrical wiring inside walls, ceilings, and floors.
Locate the unit as close as possible to a chimney or gas vent.
Avoid long horizontal runs of the vent pipe, 90° elbows,
reductions and restrictions.
No additional draft diverter or barometric damper is required on
single unit installations with a dedicated stack and a negative draft
maintained between 0.02 to 0.08 inches w.c.
Common Venting Systems
You can combine the flue with the vent from any other negative
draft, Category I appliance. Using common venting for multiple
negative draft appliances requires you to install a barometric
damper with each unit. This will regulate draft within the proper
range. You must size the common vent and connectors from
multiple units per the venting tables for Type-B double-wall vents
in the latest edition of the National Fuel Gas Code, ANSI Z223.1
and/or CAN/CGA-B149 Installation Code.
Common venting systems may be too large when an existing unit
is removed.
At the time of removal of an existing appliance, the following
steps shall be followed with each appliance remaining connected
to the common venting system placed in operation, while other
appliances remaining connected to the common venting system
are not in operation.
1. Seal any unused opening in the common venting system.
2. Visually inspect the venting system for proper size and
horizontal pitch. Make sure there is no blockage or restriction,
leakage, corrosion and other unsafe conditions.
IMPORTANT: Before installing a venting
system, follow all venting clearances and
requirements found in the Venting, General
Information section, page 11.