Venting listed category i gas fired appliances listed appliances equipped with a draft hood and appliances listed for use with type b gas vents are 1 factory built chimneys 2 listed chimney lining systems for gas venting in masonry chimneys and 3 type b gas vents.
Category 3 appliance venting.
Category iii refers to positive vent pressure which is created by exhaust blowers in unit heaters.
Category iii vent pipe al29 4c for use in boilers tankless water heaters pool heaters and other appliances requiring category iii stainless vent pipe or category 3 vent pipe.
In short it is a us standard for gas venting systems required in category iii applications.
These appliances operate with positive vent pressure and therefore cannot be adjoined to a gravity vented water heater.
Category iii appliances in residential use are mostly tank less water heaters that are vented through the wall with stainless steel.
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This stuff is gastight stainless steel and hideously expensive.
Newer ul standards however only permit the use of specialized category iii appliance vent material for residential installations that are sidewall vented.
When venting two appliances separately into a common chimney always install the smaller flue pipe appliance with lowest gph input at a higher point into the chimney than the larger flue pipe for the appliance with the largest gph input.
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It s acceptable for a category iii appliance to be sidewall vented and the vent can even pitch downward.
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Use only special condensing service vent as specified by manufacturer.
Chapter 13 sizing of category i venting systems vent connectors slope in ft upward to chimney vent not smaller than appliance flue up to 2 sizes larger than appliance flue length depends on type for single appliance single wall 75 of vent height b vent 100 of vent height 18 inch diameter for multiple appliances 34.
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Refer to figure 2.
Category iii is defined as a gas heating appliance with a flue loss equal or greater than 17 non condensing with a positive vent static pressure.
