Abstract: A hot water heat control system for a building having a given boundary layer heat loss characteristic is described. The heating system, which includes a water heating device, a water circulation loop, and a mechanism for varying the amount of heat to the loop, is controlled by a unique valve arrangement. In the best mode, the valve arrangement controls the water temperature supplied to the perimeter heater in response sensed changes of ambient environmental conditions. In one system configuration, a controller is used to control the flow of heated circulating water in response to the sensed weather parameters. The valve controller has a flow timer for variably establishing flow periodicity between a minimum flow time and a maximum flow time. The valve controller also has a base cycle timer for variably establishing base cycle periodicity between a minimum base time and a maximum base time.