When common venting a 100,000 BTU/h furnace with a 35,000 BTU/h water heater, what should the water heater vent be in relation to the furnace vent?

When common venting a furnace and a water heater, the vent for the water heater should be shorter than the vent for the furnace. This is primarily due to the differences in the BTU input values and the necessary venting requirements for efficient operation and safety.

The furnace, with a higher BTU rating, produces more combustion gases. As such, its vent must be able to effectively handle and safely expel these gases. The water heater, having a lower BTU output, requires less venting capacity, leading to the recommendation that its vent be shorter. This ensures that the drawing pressure and flow dynamics within the common venting system don't become compromised.

Additionally, shorter vents for the water heater help in maintaining proper draft conditions, which is crucial for efficient operation and to prevent backdrafts or spillage of combustion gases. Proper configuration contributes to an effective and safe venting system, ensuring that both appliances can operate harmoniously without affecting each other adversely.