Abstract: A heater circuit in a vehicle engine cooling and passenger compartment heating system comprises a control valve (26), and a supply line (22) and a return line (24) both of which are separate from the radiator circuit of the system. A bypass passageway (48) extends between the supply line (22) upstream of the valve (26) and the return line (24) downstream of the valve (26). A bypass valve normally closes the passageway (48) and opens in response to a pressure differential between the supply line (22) and return line (24) exceeding a predetermined magnitude. The passageway (48) has first and second detachably connected portions that communicate with the supply and return lines (22, 24) via radial openings (44, 46). A spring (72) abuts a shoulder (54) in the second portion and a valve element (62) that seats against a flat surface surrounding the radial opening (44) in the supply line (22). The element (62) has laterally extending guide teeth (68) and a rear reduced diameter lug (70) engaged by the spring (72).