Abstract: A heat exchanger or boiler has a bypass between the inlet and the outlet. The outlet includes a chamber having a partition with a thermostatically controlled valve, which controls flow of heating medium from the heat exchanger to the outlet connection. The thermostat maintains the temperature at the chamber. The bypass is from an inlet chamber to the space beyond the thermostat. The thermostatic valve, itself, is of a typical known construction consisting of a valve member actuated relative to a port in a plate by means of a thermostatic element axially positioned relative to the valve member. The plate forms part of a partition formed in a header member which forms a chamber at the outlet of the heat exchanger from which the heating medium flows through the thermostatic valve, to the outlet conduit. The bypass is controlled by a plate partially obstructing a passageway and it may be flexible to respond to flow of medium to have a throttling effect.