These are used to control the flow of heated water from the boiler, which would typically flow to the hot water cylinder, radiators or both.
The simplest type of valve, with two plumbing connections (in and out). On most types, a spring holds the valve closed. When power is applied, the internal motor opens the valve allowing water to flow. When fully open, a switch inside the valve activates. This switch is normally used to activate the boiler.
To close the valve, power is removed. The spring will close the valve and the internal switch will be off, so that the boiler is also off.
A video showing the internal construction of a typical valve actuator is available in the product teardowns section.
These have three plumbing connections - flow in and two outputs (A and B). On most systems, the hot water cylinder is connected to output B and the radiators to output A.
There are three possible positions. With no power applied, a spring holds the valve in position B.
When power is applied to one wire, the motor moves the valve to the middle position, so both outputs are open.
When power is applied to a second wire, the motor moves the valve to position A
Disconnecting the power results in the spring returning the valve to position B.
As with the two port valve, there is a switch inside which provides an output to activate the boiler.
Internal construction and operation of a 3 port valve. View on Youtube.
Some valves do not have a spring, but instead use a motor to open and also close the valve. These have the advantage that the motor is only powered when the valve is moving, but at the expense of increased complexity and cost.
Three port valves are available in a 'diverter' format. These appear physically identical to the 3 port valve described above, but have two operating positions - A only or B only. There is no mid position setting. If one of these is fitted, it means you can have hot water or heating - never both at the same time.
All of these valves have a manual lever on the side which can be used to open the valve even if no power is applied. This is used when filling the system with water, and the lever must be returned to the 'auto' position before the system is used.
As the motor is powered continuously while the valve is open, motor failure is a common problem. This results in the valve being closed permamently and the system will not operate at all, as the switch in the valve is never activated. The motor can usually be replaced without removing the whole valve - the top section can be removed separately.
The switches inside the valve can fail, which also results in the system not working at all.
Less common is the valve itself sticking or failing in some other way. If this happens, the system may operate but incorrectly - radiators may heat up when not required, or water isn't heated even through the boiler operates normally. In these situations, the inner workings of the valve or even the entire valve itself must be replaced which involves draining the water from the system.