For a common vent system, what is the recommended vent connector diameter for both a 35,000 BTU/hour water heater and a 100,000 BTU/hour furnace?

• A. 6 inch
• B. 5 inch
• C. 4 inch
• D. 3 inch

Answer: (C)

In a common vent system, the sizing of the vent connector is crucial for ensuring the safe and efficient operation of gas appliances, such as a water heater and furnace. When determining the appropriate diameter for vent connectors, various factors such as the BTU input of the appliances involved must be considered.

For a 35,000 BTU/hour water heater and a 100,000 BTU/hour furnace, using a 4-inch diameter vent connector is appropriate as it provides sufficient capacity to handle the total venting needs of both appliances. The 4-inch size accommodates the combustion gases produced by these appliances, minimizing the chances of backdrafting and ensuring proper venting.

Larger diameters, such as 5 inches or 6 inches, may be more than necessary for this combination of appliances, and using a smaller diameter, like 3 inches, would likely be inadequate. Correct sizing is essential to maintain proper airflow, combustion efficiency, and to reduce the risk of dangerous gas buildup in the venting system. Therefore, the 4-inch vent connector is the recommended choice in this scenario.