Vehicle cabin heater

A water circuit arrangement of an engine having a water jacket therein, comprising:

• an engine cooling circuit including a thermostat valve, a radiator and a water pump connected in series by means having one end connected to an outlet of said water jacket and the other end connected to an inlet of said water jacket;
• a bypass passage bypassing both said thermostat valve and said radiator;
• a vehicle cabin heating circuit including a passage having one end connected to the outlet of said water jacket and the other end connected to the passage of said engine cooling circuit at a position just upstream of said water pump, and a heater core unit arranged in the passage of said vehicle cabin heating circuit to receive water from the outlet of the water jacket flowing through said heater core unit before reaching said water pump;
• valve means arranged in said bypass passage for selectively opening and closing said bypass passage;
• temperature sensing means for measuring the temperature of water circulating through said vehicle cabin heating circuit;
• and
• control means for actuating said valve means to close the bypass passage when said temperature sensing means senses a water temperature in the vehicle cabin heating circuit below a predetermined level, and to open said bypass passage when said temperature sensing means senses a water temperature in the vehicle cabin heating circuit above the predetermined level.