On a copper fin boiler like lochinvar or RBI dominator how does the air proving switch measure a differential when one side of the air flow switch sits in the burner compartment and the other side just plugs on top of the burner. I realize it takes into account the Venturi effect but the blower blows right into the burner compartment pressurizing it where the one side is measured and the other sees the air pressure being "pushed" through the burner. How does an externally fouled heat exchanger lessen that differential causing the pressure trip not to close?