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Figure 7 - Typical Installation Using Zone Valves
By Federal Codes, gas appliances are categorized by the
pressure and temperature of the flue gas vented from the
appliance.
• Category I and II appliances are natural draft (draft
hood) vented, with high flue gas temperatures
(Category I), or low flue gas temperatures (Category
II).
• Category III and IV appliances are fan forced vents with
high temperature (Category III) or low temperature
(Category IV) flue gasses. Appliance efficiency is directly
related to flue gas temperature. Higher efficiency
appliances remove more heat from the gas, so they will
have lower temperature flue products.
• When flue gas temperatures are lowered, corrosive
condensates may form in the gas vent or in the
appliance. Condensates may form in Category II, III, IV
appliance vents, so special corrosive resistant venting
systems are required for higher efficiency appliances.
CONNECTING SUPPLY AND RETURN PIPING
GENERAL INFORMATION GAS VENTS AND APPLIANCES
• Proper operation of the vent system and appliance is
dependent upon the use of all parts specified by the
manufacturer for use in the particular installation.
Appliance and vent system performance may be affected
by improper assembly.
• Vents for Category I appliances may not be suitable
for use with Category II, III, or IV appliances because
condensate may corrode the vent.
• Vents for Category III appliances may not be suitable
for use with Category I appliances because flue gas
temperatures may be too high.
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