Category iii refers to positive vent pressure which is created by exhaust blowers in unit heaters.
Category 3 appliance venting.
In short it is a us standard for gas venting systems required in category iii applications.
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Refer to figure 2.
Use only special condensing service vent as specified by manufacturer.
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.
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.
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Use only pressurizable vent as specified by manufacturer.
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 iii appliances in residential use are mostly tank less water heaters that are vented through the wall with stainless steel.
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These appliances operate with positive vent pressure and therefore cannot be adjoined to a gravity vented water heater.
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In a category iii venting system if there is a.
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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.
It s acceptable for a category iii appliance to be sidewall vented and the vent can even pitch downward.
