Andrew rightly identifies on/off solenoid switched valve as easiest. At first, I thought this would be no good because of the thermal inertia involved. It would never settle down to the desired 79 degrees. HOWEVER, if there are 2 valves in PARALLEL, one manually adjusted and one on-off (ex-washing machine?) solenoid, then the coolant flow would be a constant plus/minus the solenoid discharge. The manual one can be adjusted according to the rate of heating that day, and the solenoid can make all the fine adjusments needed to stabilise temp. What we need now is a circuit including temperature probe (thermistors are a lot cheaper than thermocouples please!), amplifier and mains-rated relay to drive the washing machine valve. Maybe an adjustable 555 timer to damp down any hunting. PLEASE.
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