
ENGINE COOLING AND VEHICLE HEATING 113
In the case of the simple permanent bypass the
transfer connection is no more than a bleed hole, since
it must not be so large as to reduce the normal flow of
coolant through the radiator when the thermostat con-
trol valve is open (Figure 5.23a). With a variable bypass
the transfer connection takes the form of a large-bore
duct, which is controlled by the lower of two thermostat
control valves that operate in unison (Figure 5.23b).
Therefore as the upper valve opens to the radiator so
the lower one closes the duct to the pump, the net
effect of this gradual transition in coolant flow being to
maintain