The Tri-port value assembly is a remarkable piece of engineering that allows one value to control heating and hot water requests via an actuator.

It is a tri-state machine - steady in any of the three states - this is managed by using different mechanical/electronic implementations. It switches constantly between the states depending on the application criteria.

There are micro switches between Closed-Midway and Midway-Open.

Its positions are:-

• Closed - Hot water only - when no power is applied to the actuator a spring pulls the value back to this position.
• Midway - Hot water and Central Heating - the value is held in this position by switching in a diode at the mid point of the actuator whilst it is still being powered.
• Open - central heating only - the actuator is driven against its end stop.

The timer provides

• Central Heating Only - 'Hot Water' and 'No Hot Water' are live'
• Central Heating And Hot Water - 'Hot Water' and 'Central Heating' are live'
• Hot Water Only - 'Hot Water' is live

The water circuit is pretty standard