The pressure inside an induced draft heat exchanger for a mid-efficiency furnace is approximately how much?

In a mid-efficiency furnace, an induced draft heat exchanger typically functions under a positive pressure condition. This means that the pressure inside the heat exchanger is usually above atmospheric pressure, which is indicated by a positive measurement in water column (W.C.). A reading of approximately +2" W.C. signifies that the pressure is sufficient to support the combustion process and effectively move exhaust gases through the venting system without backdrafting.

The positive pressure also helps in pushing combustion gases out of the heat exchanger and into the vent, ensuring that the appliance operates efficiently and safely. This is crucial for maintaining optimal combustion and heat extraction, as well as preventing any potential issues such as spillage of combustion gases into the living space.

Understanding the operational pressures is essential for assessing the performance and ensuring the safety of heating appliances in HVAC applications. In contrast, negative pressures or zero pressures could lead to inefficiencies and hazardous situations, where flue gases might not be properly vented.