Most domestic houses in the UK have one of two heating control systems. The ‘S’ plan and the ‘Y’ plan. Both are designed to closely control your heating system. Here we talk about the ‘Y’ plan.

The benefits:

• Close temperature control on hot water system and heating system
• Timed operation of all heating components
• Saves money on fuel

​The drawbacks

• Dilutes and restricts flow in certain systems

Overview of a Y-Plan Heating System:

The reason this system is called a ‘Y’ plan is because of the shape of the 3 port valve. There are many 3 port producers including Honeywell, Horstmann and Danfoss to name but a few.

Heated water from the boiler is diverted to the hot water coil or the radiators depending on which service the programmer is asking for and which thermostat is calling for heat. The 3 port valve also acts as a switch to turn on heating system components. Therefore you need a certain sequence of switches to work in order for the system to operate.

For example with radiators:

1 The programmer reaches a timed period where the radiators require heat.

2 The programmer activates a live supply to the room thermostat.

3 If the room is cold then the thermostat will switch this live supply to the motor in the three port valve.

4 The three port valve will motor over to open the radiator water outlet. (Takes about 10 seconds)

5 When the valve opens it switches a separate live supply which feeds the boiler and pump.

6 Boiler and pump receive live supply and operate. Soon you should feel some hot water in your radiators.

The three port valve has 2 live supplies to motor it one way or the other depending on what the system is calling for: hot water or hot radiators or both.

Problems with Y Plan Heating Systems

Over time the valves can get stuck (especially after a summer of being in one position). When the motor head goes to move the valve position it burns the head out. This will require a new 3 port valve. You can buy a new one from here.

Symptoms of this can be only hot water and no central heating or vice versa. You can test to see if the valve is stuck by moving the manual arm on the side of the valve. When this is latched open it will divert heated water to both hot water coil and radiators.

How to change a 3 port valve:

Turn off all heating components electrically (remove fuses and test)

1 Isolate water supply to heating system (in loft or mains)

2 Latch the valve arm open (on the side of the valve) and drain heating system down (find a drain-off below the level of the 3 port valve)

3 Remove 3 port valve making a note of the wiring (you should have a switched live from the room thermostat to the valve motor. Also there will be a hot water off live, fed through the cylinder thermostat. This allows the system to work on heating only. Also there will be a neutral and earth. Then you should have a permanent live that is switched by the 3 port valve itself that goes off to the boiler and pump. Beware of terminal boxes for electrics in heating systems. There are often live wires there even if you pull the fuses. Always double check! Generally they are a nightmare to figure out without a solid knowledge of how these systems work)

4 Fit the new valve and follow the instructions on how to wire it up. Don’t just go by the old colours of the old valve. The manufacturers often change the colours of switched wires etc).

5 Fill the heating system and vent any air out.

6 Restore power to the system and test operation.

Here is the usual wiring for a Y plan system: