This article is about motorised valves used in Central Heating systems. Motorised valves are a type of electrically actuated valve (others include solenoid valves and valves using electrically-generated thermal effects).
Motorised valves generally comprise:
- a synchronous electric motor
- gears to reduce the speed and increase the torque output of the motor
The motor and gears are often combined in a package known as a Synchron motor.
- a linkage from the output gear to the hydronic part of the valve
- one or more switches (usually microswitches) actuated by, for example, a cam attached to the output linkage
- a spring: see below
- the above components usually comprise one module, often known as the 'head', which may be detatched from the parts below for repair or replacement.
- a valve controlling flow of water in the hydronic system, actuated by the head: this part is often known as the 'body'.
Configurations of motorised valve
Valves are commonly found in the following configurations:
The valve has 2 pipe connections (ports) and permits or blocks flow between the ports depending on whether it is actuated or not. Flow is permitted when the valve is actuated and blocked in its resting state.
The valve has 3 pipe connections, in a T formation. The centre leg of the T is the inlet and the two short arms are the outlets, usually referred to as ports A and B. Actuation controls whether the flow from the inlet passes to the A or B port. In the resting state flow is to the A port.
There are two types of 3-port valve:
- Diverter valve
- This is a simple change-over version of a 2-port valve: when the valve is actuated all flow is diverted to the B outlet.
- Mid-position valve
- In this type of valve there is a stable state in which flow from the inlet can pass to both (A and B) outlet ports simultaneously. This type of valve is commonly found in domestic central heating systems where it controls flow of hot water from a boiler to hot water and radiator circuits in a Y-plan system configuration.
Types of motorised valve
Spring return valves
In these types when power is applied the motor and gears act against the force of a spring to operate the valve. When power is removed the spring returns the valve to its resting position. In a 2-port valve this is the closed position. In a 3-port valve the resting position is where flow from the inlet passes to the A port.
These types of valve consume power whenever they are open (and, in the case of 3-port valves, often even when they are not). In 2-port valves the motor runs until the valve is fully open and then stalls as the actuator mechanism cannot move any further. 3-port valves are more complicated: a description can be found in the uk.d-i-y FAQ and here is a diagram showing a 3-port valve in a Y-plan CH system
Motor On / Motor Off (MOMO) valves
Since the motor only runs whilst the valve is changing from one state to another there is a saving in energy compared to valves where the motor runs continuously while the vale is open. Such valves may also be more reliable since the motor and surrounding components will run cooler.
ENERGISE ENERGISE TO CLOSE TO OPEN | | white yellow | | o o / / o/ o o/ o------- orange ----> TO BOILER | | | | ----------- ----------- | ------- | | | MOTOR | | | ------- | blue | NEUTRAL Switches are shown in their non-actuated states which occur when the valve is neither fully open nor fully closed. The switch on the left operates when the valve reaches its closed position. The switch on the right operates when the valve reaches its open position.
These types of valves are occasionally found in domestic central heating systems: usually as Satchwell or Sunvic valves with grey plastic heads. Sunvic manufacture both MOMO and spring-return valves in similar casings.
If a motor-open/motor-close valve is replaced by a spring-return valve, or vice-versa, the controls wiring must be modified to suit. In the case of replacing a spring-return with a MOMO valve this requires change-over contacts at the room thermostat (or progstat) and an extra wiring conductor between the 'stat and the valve.
|This type of valve is also found in non-domestic installations.|
Sunvic's SDMV series 3-port valves are motor-open/motor-close types but are wired identically to spring-return valves.
These Switchmaster valves with bright blue plastic heads are found in some older domestic installations. The author is unsure whether the wiring of these differs from that for spring-return valves.