Dear
Inspector,
This
month I would like to clear up a common misconception regarding
gas appliances. I have many inspectors ask if it is okay
to vent a water heater with draft hood and an induced draft
furnace into the same vent.
A
fan assisted furnace (AKA induced draft and/or a "plus-80
furnace") does not pressurize the flue pipe and can
be vented like/with natural draft appliances. The draft
inducer fan places a slight negative on the heat exchanger
to keep the products of combustion moving, it does not pressurize
the flue or vent. These heat exchangers are serpentine and
slow down the products of combustion so much they need a
little help.
For
further assistance with gas appliance venting, consider
the following:
All
gas appliances are categorized by the same system, which
is also referenced in all mechanical codes:
VENTED
GAS APPLIANCE CATEGORIES.
Appliances that are categorized for the purpose of vent
selection are classified into the following four categories:
Category I.
An appliance that operates with a nonpositive vent static
pressure and with a vent gas temperature that avoids excessive
condensate production in the vent.
Category II.
An appliance that operates with a nonpositive vent static
pressure and with a vent gas temperature that is capable
of causing excessive condensate production in the vent.
Category III.
An appliance that operates with a positive vent static
pressure and with a vent gas temperature that avoids excessive
condensate production in the vent.
Category IV.
An appliance that operates with a positive vent static
pressure and with a vent gas temperature that is capable
of causing excessive condensate production in the vent.
The
category of every appliance is marked on the nameplate.
Water heaters with draft hoods, and mid-efficiency induced
combustion furnaces, are both category I. The installer
selects the vent based upon the GAMA (Gas Appliance Manufacturer's
Association) tables provided with all gas appliances.
Mike
Casey
Kaplan Professional Schools
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