Abstract: A fluid flow control valve, e.g. a pressure vacuum breaker, includes a thermal expansion relief valve for protecting the control valve against damage due to freezing. The control valve has a valve body of rigid material and defines a main conduit. The thermal expansion relief valve has a relief valve body mounted to the main valve and defines a relief conduit in fluid communication with the main conduit, a valve element the moves within the relief conduit between a first sealing position in engagement with a valve seat to resist flow and a second position spaced from the valve seat to allow flow from the main conduit through the relief conduit, and a spring urging the valve element toward sealing position with a predetermined force selected to exceed normal operating pressure of the main valve and to lie below pressure damaging to the main valve. A thermal expansion valve for protecting fluid flow control valves, e.g. by retrofit, is also described.