Must the vent connector from an oil-fired furnace connect to the chimney above the vent connector from the water heater?

The correct understanding is that the vent connector from an oil-fired furnace does not necessarily have to connect to the chimney above the vent connector from the water heater. In many cases, local codes provide specific guidelines for venting arrangements, but the general principle remains that the venting systems for different appliances must function effectively and not create backdrafts or other issues.

The positioning of the vent connectors is designed to ensure proper exhaust flow and to take advantage of the natural draft created in the chimney. If the vent from the oil-fired furnace were installed above the water heater's vent, it could potentially cause combustion gases from the furnace to be pulled into the water heater's venting system, leading to dangerous situations, including the risk of carbon monoxide backdraft or improper ventilation.

Therefore, for safety and to ensure compliance with typical venting regulations, it is acceptable for the vent connector from the oil-fired furnace to be located below the vent connector from the water heater, provided that all other venting requirements and best practices are adhered to.