Mr Heater | sku: 3578155 Mr. Heater Horizontal Vent Kit for Unit Heaters The category 3-horizontal 4 in vent kit provides class 3-single wall ducting for use with Mr. Heater Big Maxx unit heaters, ranging from 50,000, 80,000, and 125,000 BTUs. Complies with US standard (UL1738) and Canadian standard (S636). Designed for... Log in for pricing