S and Y Plans Explained Re March 30, 2021

S and Y Plans Explained

No heating or hot water

S and Y Plans are still used to form the basics of most domestic heating systems, however, there is often confusion between both plans and how they work. This blog aims to explain the difference between S and Y plans so that you have a better understanding of your heating system. We will also touch base on older heating system plans that are no longer used. 

We have also included a step-by-step guide on how to wire a Google Nest Thermostat and Hive Thermostat.

Central heating system designs can differ from house to house, based on the requirements of the property. However, the majority of properties in the UK use either an S Plan or a Y Plan heating system. 

S and Y plans have been designed to give you close control of your central heating system, ensuring that your heated water is diverted to the correct areas. S and Y plans are also a way to ensure that your boiler interlock works. This is when your boiler automatically switches itself off when there is no demand for central heating of hot water in your home. 

What is the difference between an S and Y Plan Heating System?

The main difference between an S and a Y plan is that a Y Plan has a single, three-port valve supplying both the central heating and/or hot water. This three-port valve is shaped like the letter ‘Y’ when you look at it from a schematic layout of pipework point of view, as seen below. This is where the name ‘Y’ comes from.

The style of system chosen for each household can be influenced by many factors including the size of the property, the provision of an unvented cylinder and space, such as a compact airing cupboard, which may not have room for the system bypass required for S plan designs, and so may lend itself to a Y Plan. 

What is a Y Plan Heating System?

The water comes through the bottom of the valve. This valve then sends this flow of water to either Port A, which is the heating port, or Port B, the hot water port, or through both, to work both the central heating and hot water simultaneously.

A Y Plan Heating System has fewer components than an S Plan Heating System, making it a more compact and easier installation. 

As mentioned, a Y Plan includes a three-port valve, which is called a mid-position valve.

This valve will direct the primary flow through the central heating and/or hot water circuit.

A Y Plan Heating System is often still found in older properties and is still suitable for use if it’s only a part replacement or a small repair that is needed. For example, if only the hot water cylinder needs to be replaced, it can still be retained.

A Y Plan Heating System is built with one three-port motorised valve which controls both the central heating and the hot water at the same time, it also has a room thermostat, a cylinder thermostat, and a programmer. 

Depending on what service you’re asking your heating system programmer to provide, whether it be hot water or heating, the heated water from your boiler will be diverted to either the hot water coil or your radiators. 

There are benefits to having a Y Plan Heating System, which includes:

  • Close temperature control on the hot water system and heating system
  • Timed operation of all heating components
  • Saves money on fuel
  • Easier to install due to fewer parts

A Y Plan heating system is still found in many homes, so it’s important for any qualified engineers or installers to understand how it works. However, there are also some downfalls with having a Y Plan Heating System. 

The main limitation is that there is only one heating zone that is in control of both the central heating and hot water, meaning that the flow rate has to be shared between them both. Another limitation is you are relying solely on that one valve, meaning that if it fails, you will be without both central heating and hot water until it is replaced. 

In addition to these limitations, there are also a few common problems that many find with having a Y Plan Heating System. One is that when the demand for hot water and central heating in your home is considerably less, the valve can get stuck due to not being used as much. As a result of this, the head of the motor can get burnt out when the motorhead goes to move the position of the valve. 

Now that we’ve covered what a Y Plan Heating System is, let’s move on to what an S Plan Heating System is.

What is an S Plan Heating System?

An S Plan Heating System has two separate motorised valves, one that controls the heating, and the other controls the hot water. These valves can be used individually to control the heating system. It is also possible with an S Plan that further zones can be added by adding additional thermostats and motorised valves to each circuit. 

Whenever either the central heating or hot water port is needed, 230V is sent down the Motor Live wire (usually the brown wire)

The motor inside then turns the valve, opening the body of the valve, allowing the heating pump to push water through. Once the valve is pushed all the way open, it also pushes down a microswitch, activating the circuit between the Permanent Life (usually a grey wire) and the Switch Live (usually orange in colour). Once the 230V hits the Switch Live, the boiler ignites. 

Once the demand for either hot water or central heating is fulfilled (either the timer is turned off, or the thermostat reaches the desired temperature) the power to the Motor Live (Brown) wire is lost, so the valve body springs back and shuts off the flow through the valve. At the same time, the microswitch making a circuit between the Permanent Live (Grey) wire and the Switch Live (Orange wire) is opened, resulting in no power being sent through and the boiler will stop running. 

S Plan Heating System

A typical S Plan Heating System has two separate motorised valves which control the temperature of the central heating and the hot water individually. It also has a cylinder thermostat, a room thermostat, and a programmer.

You will often find that an S Plan Heating System is used in modern properties.


It is also more often than not, the preferred choice of system. This is because it is easier to wire, commission and fault find as it has the ability to isolate both the heating and hot water circuits. 

Having an S Plan Heating System installed means that you have the advantage of including additional heating zones. Therefore, if you decide to have a conservatory extension or simply want more control over individual room temperatures, then adding more heating zones is possible. Zoned heating systems give homeowners the greatest level of flexibility, comfort, and energy efficiency. 

An S Plan Heating system is also better suited to larger properties with higher hot water demands, as it provides superior flow rates over a Y plan. Other benefits of having an S Plan Heating System are below:

  • Close temperature control on the 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
  • Helps you save money on fuel
  • No reduction in heating water flow

A common fault with an S Plan Heating system is that the electrical micro-switch in the actuator starts to stick. This causes the boiler and the pump to continuously run. This can lead to other faults rising, such as the boiler overheating.

Similar to a Y Plan Heating System, there is the possibility that the valve can stick after not being used for some time. The vale sticking can also be a result of magnetite sludge that can foul at the seat of the valve, causing the valve spindle to seize or stiffen. However, in contrast to a Y plan, you are not relying on one valve to produce your hot water and central heating, so the chance of being without both at the same time is unlikely. 

For professional installers and gas engineers, wiring either a Y or S Plan Heating System should be relatively uncomplicated, which is why it is always best to leave it to them. However, if you are interested in learning how to wire these systems yourself, there are plenty of videos online.

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What is a C Plan Heating System?

The C Plan is not used in heating systems anymore but can still be found in older systems. Here’s a little bit about C Plan Heating Systems just in case this applies to your boiler heating system.

The C Plan were used with pumped central heating and gravity hot water systems to provide full independent temperature control over both circuits in your home. With the C Plan central heating design, the hot water heat recovery is slow because the hot water circuit is gravity circulated.

With pumped heating and gravity hot water the ‘C Plan’ components are:

  • One 28mm 2 port motorized valve for the domestic hot water secondary circuit
    Note: the water must flow in the direction indicated on the valve body
  • One Room thermostat for space heating temperature control
  • One Cylinder thermostat (strap-on) for hot water temperature control
  • An Automatic by-pass valve – especially when using Thermostatic Radiator Valves (TRV’s)

Time control for the boiler and pump in this central heating design is by the cylinder thermostat, timer and 2 Port valve for hot water, and by the ‘timer’ or ‘programmer’ and pump for the central heating. When the demand for heat has been satisfied by the hot water cylinder the 2 Port will close, if there is no demand for space heating the boiler will shut down.

What is a W Plan Heating System?

Similar to the C Plan Heating System, the W Plan Heating System is not used in newer systems, but can still be found in older systems.

The hot water and central heating design for a W Plan is different to that of the S Plan and S Plan Plus. The W Plan uses a three-port motorized valve instead of the two-port valve(s) used in the above plans; C, S and S Plus.

The three-port valve that is used in the W Plan is a ‘two’ position diverter valve. The default position of this valve is to the hot water circuit. This plan is not recommended in your home if there is a high use of hot water.

Basically, this valve gives priority control to hot water demand. So if you are going to use a lot of hot water in the winter month you may experience a drop in the space heating temperature, because this three-port valve is a diverter valve and not a mid-position valve it cannot serve hot water and heating at the same time.

With a fully pumped heating and hot water the ‘W Plan’ components are:

  • One 22mm 3 port motorized diverter valve, serving both the domestic hot water circuit and the central heating circuit
    Note: the water must flow into the A-B inlet Port. Port A outlet connection is for the central heating primary circuit. Port B outlet connection is for the domestic hot water secondary circuit
  • One Room thermostat, for the space heating temperature control zone
  • One Cylinder thermostat (strap-on) for hot water temperature control
  • An Automatic by-pass valve – especially when using Thermostatic Radiator Valves (TRV’s)

Time control for the boiler and pump in this arrangement can be from either the hot water cylinder’s thermostat or room thermostat, or ‘timer’ or ‘programmer’. Once the demand for hot water and/or heating have been satisfied the boiler and pump will shut down. Remember that with the W Plan heating will remain off until the hot water cylinder’s thermostat has been satisfied.

A guide to wiring your Google Nest Thermostat

Smart thermostats, such as the Google Nest are another great way of controlling your home’s central heating. Here’s a step by step to help you to wire your Google Nest thermostat correctly. 

1. Switch off mains power
You’ll be working with mains voltage, so to ensure that you and others around you are safe, as well as protecting your heating system and the thermostat, turn your power off. 

2. Connect to power and the control circuit
Check the heating system’s wiring to determine what type it is.

Remove the Heat Link cover and connect the L (live) and N (neutral) terminals on the Heat Link to the L and N circuit on the boiler or junction box. This will power the Heat Link.

  • Connect the heating control circuit to the Heat Link 1 (normally closed/satisfied), 2 (common), 3 (normally open/call-for-heat) as needed.
  • If the system has a hot water control circuit, connect it to 4 (normally closed/satisfied), 5 (common), 6 (normally open/call-for-heat) as needed.
  • If you have an OpenTherm boiler, connect its control circuit to the OT1 and OT2 terminals.

3. Connect to the thermostat wires
Skip to the next step if you’re installing the Nest Thermostat on the stand and are using the USB cable for power.

Take two wires from the wire bundle that goes to the current thermostat and attach them to the T1 and T2 terminals on the Heat Link.

4. Mount the Heat Link
Use the screws to mount the Heat Link near the boiler or junction box, then put the Heat Link cover back on. To ensure a good wireless connection to the thermostat:

  • Leave at least 30 cm of space on each side.
  • Install it within 30 m of the thermostat.

5. Install the Nest base Remove the old thermostat if there is one. You can attach the Nest base directly to the wall or use the trim plate included to cover up any marks. If you’re using the trim plate, snap it onto the base first, then screw them to the wall as one piece. When installed on the wall, the Nest Thermostat should be 1.2-1.5 m above the floor.

6. Connect the wires
Connect the wires coming through the wall to Nest’s T1 and T2 terminals. It doesn’t matter which wire goes into which connector.

7. Install the Nest Stand
Follow the instructions included to attach the thermostat base to the stand.

Connect the power cable to the base and plug it into a wall socket. Stand-mounted thermostats should be placed on a table or shelf 0.75–1m above the floor and no further than 1m from the edge.

8. Attach the display.
The thermostat display should click straight into place, onto the stand. Once you turn your main power back on, your Nest should power up and begin setup. 

9. Setup
The Nest Thermostat will turn on and take you through the necessary steps to set up. The Nest Thermostat doesn’t have a touch screen. Turn the thermostat display and press it to select.

10. Control it with the Nest app
You can now control your thermostat with your mobile! Download the free Nest app from Google Play or the iTunes App Store. Then create a free Nest account with the app and off you go! 

A guide to wiring your Hive Thermostat

To successfully set up your Hive Thermostat, there are three components, the hub, the receiver and the thermostat. Here is a step-by-step guide to help you set up each part correctly. 

You may already have the Hive Hub, if you do, then all you need to do is put your hub into pairing mode. You can do this by pressing the button at the back of the hun, hold it down for 1 second and then release it. 

The middle light should now be flashing amber. This means that the hub is in pairing mode. 

Installing the Hive Hub

If you do not have a Hive Hub, then there is how to install and set it up:

  • You will need a broadband connection
  • You will need a spare network port and power socket nearby

1. Connect the hub to the broadband router with the cables provided

2. Fit the power cable into the hub and plug it into the socket

3. Wait for the middle light to start flashing amber. Set-up normally takes 5-10 minutes.

Installing the Hive Receiver

Before fitting the Hive receiver, you will need to need to isolate the mains electricity supply to the central heating system. It is important that the Hive receiver is installed by a qualified electrician. Your wiring must comply with the current edition of BS7671 (The IET Wiring Regulation), and the appropriate Building Regulations or Standards in place. 

This product is designed for fixed wiring installation only. It must be supplied via a switched fused spur with a minimum contact separation of 3mm (both live and neutral) and fitted with a 3A fuse.

The Hive Active Heating Thermostat has two different receivers; a single channel and a dual-channel receiver. 

Make sure that the receiver is at least 30cm away from any large metal objects such as the boiler or hot water cylinder, as these can interfere with the radio signals.

1. Loosen the screws on the underside of the receiver

2. Remove the backplate by pulling the bottom of it away from the front panel 

3. Fix the backplate to the wall 

4. Run the cable provided from the boiler or wiring centre. We have included diagrams below to show the writing of a single and dual-channel receiver

5. Test the boiler:

  • Visually check the wiring to confirm it’s correct
  • Confirm it is safe to commission the boiler 
  • Check the middle light on the Hive Hub is flashing amber, if it’s not, you will need to go back to step one
  • Restore the power in your central heating system.

Once you’ve completed these steps, if your status light is flashing, then you are ready to connect to the Hub and Thermostat. 

We have attached below a list of the receiver lights and what they mean: 

Central heating Green Solid Heating is on. 
Central heatingGreenFlashingCommands queued. 
The receiver has received 2 or more commands. 
The Hive receiver protects the boiler from damage that may occur if it’s switched on and off very quickly. Once the boiler has been switched on (or off), it will not change state again for 1 minute as a protective measure
Hot waterGreenSolidHot water is on.This light is only present on dual-channel receivers.
Hot waterGreen FlashingCommands queued. The receiver has received 2 or more commands to switch the boiler on or off within 1 minute.See ‘Central heating’.
StatusGreenSolidNormal operation. 
Status BlueSolidThe system is in gravity-fed mode and is connected to the rest of the Hive system. 
Status AmberFlashing Installation. The receiver is trying to connect to other Hive devices.The receiver will search for devices for 40 minutes at a time.
Status White FlashingThe receiver is in stand-alone mode and is actively seeking to connect to a Hive thermostat.The receiver will search for a thermostat for 40 minutes. If no thermostat is found within this time it will stop searching and the status light will turn solid WHITE.
StatusAmber or White SolidInstallation. The receiver has paused its search for other devicesTo restart the search, simply turn the receiver off and then on. This will return it to the AMBER or WHITE flashing state.
StatusRedSolidError. The receiver has lost its wireless connection to the thermostat.Whilst in this state heating and hot water will default to off. They can be switched on manually by pressing the ‘heating’ and ‘hot water’ buttons on the receiver (hot water is only available on dual-channel receivers). For help call Hive Technical Support.

*Receiver light information is taken from Hive Heating Installation Guides. 

Installing the Hive Thermostat

Your Hive Thermostat should be fixed to an internal wall, around 1,5m from the floor. It is important that your Hive is located well away from any heat sources that could affect the operation. 

To ensure that the thermostat takes accurate temperature readings, it needs a free flow of air. Also, do not install your Hive Thermostat in the same room as a radiator that is currently being controlled by a  radiator TRV. If you’re not sure what a TRV is, we have a blog that covers this. 

1.Remove the backplate of the thermostat by pressing the lever at the bottom and pulling away from the front of the device. Once you’ve found a suitable location, fix the backplate to the wall using the appropriate fixings. The backplate is desi ned for 3.5mm/size 6 or size 8 screws. Be careful to avoid any cables and pipes that may be buried in the wall.

2. Check the Hub is flashing amber, which means it’s ready to connect. Insert the batteries provided into your Thermostat 

3. Your thermostat will display will show ‘Search’ meaning that is searching for the Hub and Receiver. 

4. Your receiver will display a solid green status light

5. Your Thermostat should now show a ‘Welcome’ page and you’re all set!

6. Hook your thermostat back onto its backplate.

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