If a mid-efficiency, induced-draft gas furnace is manifolded with a natural-draft water heater, is it a concern?

When a mid-efficiency, induced-draft gas furnace is manifolded with a natural-draft water heater, there are critical ventilation and pressure considerations to take into account. In the case of an induced-draft furnace, the venting system is designed to create a positive pressure to expel combustion gases effectively from the furnace. This means that if the vent pressure is positive, it can potentially cause backdrafting, where gases could flow back into the combustion appliance instead of venting safely outside.

In contrast, a natural-draft water heater relies on the natural flow of hot gases upwards through its vent and is designed to operate with negative pressure in its venting system. When both appliances are connected to the same venting system, there may be an issue with the induced pressure from the furnace disrupting the natural draft of the water heater, posing a risk of backdrafting and incomplete combustion. Therefore, the concern in this situation arises primarily due to the positive pressure created by the furnace affecting the correct functioning of the natural-draft system for the water heater.

This understanding highlights the necessity of carefully assessing the compatibility of different venting systems to maintain safety and efficiency in gas appliance operations.