Customer has existing nat gas furnace with 90k input and existing hot water heater....don't know input, but assume roughly 40k input.

Same customer is putting in a new tankless Bosch...with output of 91k btus. They are considering new furnace with output of 80k btu max. So total output with the new tankless of 171k btus.

Is 5" b-vent sufficient...assuming it's verticle height is 20' or less?