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?

The recommended vent connector diameter for a common vent system involving both a 35,000 BTU/hour water heater and a 100,000 BTU/hour furnace typically needs to be sufficient to handle the combined flue gas output of both units without exceeding the vent system's capacity.

In general, the standard practice for sizing vent connections for gas appliances is to refer to applicable codes and standards, which often suggest that a 4-inch diameter is adequate for lower BTU inputs, such as those given in this scenario. A 4-inch connector would efficiently accommodate the venting needs without risking backdrafting or excessive pressure drop, and it keeps within industry recommendations for such BTU ratings.

The larger diameter options, while potentially able to vent efficiently, may not be necessary for the specific BTU ratings given. A 3-inch diameter, on the other hand, would likely be too small for the total BTU output, leading to possible venting issues. Therefore, the choice of 4 inch as the correct answer aligns with safety and performance guidelines for vent connections in HVAC applications.