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, diverting heated water to where it is needed according to thermostats and controls. Here we talk about the ‘S’ plan.

If you want full and comprehensive advice then watch the instructional video below!

The benefits:2 port valve

  • Close temperature control on hot water system and heating system
  • Multiple zone control for more than one hot water cylinder, radiator or under-floor system
  • Timed operation of all heating components
  • Money saving on fuel
  • No reduction in heating water flow


Overview of an ‘S’ plan heating system:


The reason this system is called a ‘S’ plan heating system is because of the shape of the system when drawn schematically. 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 2 port valves serve one service each. For example there is only one 2 port valve for the hot water cylinder and a separate one for the heating system. 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 2 port valve.

4 The 2 port valve will motor open and allow water flow to the radiators. (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 2 port valve has one live supply to motor it open when called to by the particular service it supplies.
A great advantage of S plan systems over Y plan systems is the opportunity to have more than two zones. Instead of just having one zone for the hot water cylinder and one for the radiators, you can use another 2 port valve with its own controlling thermostat and power. This is ideal if you have an under-floor heating system that you want to control separately.

Where to buy 2 Port Valves:

So we’ve had a quick look at how 2 port valves and how s plan systems work. What if you’re looking to buy new or replacement components? Most customers find it easier to allow their installation engineer to buy the parts but this is not always the cheapest option. If you buy the boiler and get the engineer to fix it for you, then you avoid their mark up and save money. If you aren’t sure which part to buy then ask you engineer what make and model you need.

Buying 2 port valves online is relatively simple. You can buy one here.

Problems with S 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 2 port valve. You can buy a new one from Screwfix. Symptoms of this can be only hot water and no central heating or vice versa. Sometimes both can go wrong. You can test to see if the valve is stuck by moving the manual arm on the side of the valve. You can manually latch the valve open while you wait for a replacement valve to be fitted.

How to change a 2 port valve:

1 Turn off all heating components electrically

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

3 Drain heating system down (find a drain off below the level of the 2 port valve)

4 Remove 2 port valve making a note of the wiring (you should have a switched live from the thermostat to the valve motor. A neutral and earth. Then you should have a permanent live that is switched by the 2 port valve itself that goes off to the boiler and pump. Beware the terminal boxes for electrics in heating systems. Generally they are a nightmare to figure out without a solid knowledge of how these systems work)

5 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).

6 Fill the heating system and vent any air out.

7 Restore power to the system and test operation.

Here is the usual wiring for an S plan system:

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