The heater bypass valve controls the coolant flow through the heater core. Coolant bypasses the heater core through a spring-loaded valve inside the housing. The valve is normally closed at low coolant flow rates so that all coolant passes through the heater core.

At increased engine speeds, the valve begins to open and limits the maximum flow through the heater core to 5-6 gal/min by bypassing the excess. The amount is sufficient for all heat requirements and prevents long-term erosion and corrosion of the heater core due to excessive flow rates.