Heating circuit, function

P83.20-0273-06

1 Cap
2 Expansion reservoir
3 Radiator
4 Thermostat
5 Coolant pump
6 Oil filter with heat exchanger
8 Heater core

Y21 Duovalve

Y21y1 Left duovalve

Y21y2 Right duovalve

A Coolant return

B Coolant feed

C Bleed line

D Short circuit operation at < 85 °C

E Mixing operation at > 85 °C and
< 94°C

F Cooling operation at > 94 °C

M Engine

S Windshield washer reservoir

Function
The coolant pump (5) pumps cold coolant through the engine (M). Here the coolant absorbs the engine heat and is warmed up. Then the heated coolant flows through the heater core (8) where the heat is transferred to the passenger compartment air. The quantity of coolant flowing through the heater core (8) is controlled by the duovalve (Y21). The quantity of coolant can be regulated separately for left and right sides.

The coolant then travels through the coolant return (A) to the coolant pump (5). A coolant circulation pump may be installed in the return between the duovalve and coolant pump. The major portion of the coolant flows through the engine (M) rather than through the heater to the thermostat (4). The thermostat allows water to flow through the radiator (3) or directly back to the coolant pump (5) depending on the water temperature.