When designing a vent system for a furnace with 120,000 BTU/hr and 80% efficiency, what is the minimum size vent system required?

To determine the minimum size of the vent system required for a furnace with a rating of 120,000 BTU/hr and an efficiency of 80%, it is important to consider how vent sizing correlates with the amount of exhaust gas produced and the efficiency of the unit.

With 80% efficiency, the furnace converts 80% of the input energy into usable heat while the remaining 20% is lost, which in turn influences the flue gas volume and temperature. The total volume of flue gases emitted must be accommodated properly, and the vent must be sized accordingly to effectively carry away those gases without causing backpressure or excessive cooling.

For a furnace running at 120,000 BTU/hr with the given efficiency, tables or guidelines can be referenced that indicate vent sizes based on the input capacity (BTUs). These guidelines offer specifications for vent sizes needed for different input capacities, factoring in type of fuel, vent height, and material.

In this case, the guidelines suggest that a 4-inch vent diameter is appropriate for appliances in this input range, as it can handle the exhaust gases adequately and allow for proper flow. Therefore, the choice of a 4-inch vent system aligns with standard practices and efficiency considerations in venting for gas