Underfloor Heating Wiring Diagrams

by Editor on October 18, 2016

Here are wiring diagrams for connecting underfloor heating zones to S Plan and Gravity Hot Water systems. Also how to connect UFH Control Centres such as Wunda, Pipypipe and Honeywell.

How to connect UFH to an S Plan Central Heating system

This schematic wiring diagram shows how to connect a single zone Underfloor Heating to an S Plan. This is a fully pumped system , which means that the pump is in operation for both Hot Water and Central Heating modes. The system pump is controlled by boiler and has a timed “over run”. You’ll often hear the pump circulating through the bypass loop and ABV (Automatic Bypass Valve) after the boiler has turned off and all the zone valves have closed.

As the system pump is controlled automatically from the boiler it makes the wiring of additional underfloor heating controls quite straight forward. A typical UFH system has it’s own independent manifold pump.

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Zone Valves and Relay Switches

An S Plan system uses 2 port zone valves connected to the pipework. One for HW and one for CH. These valves can open and close to allow circulation when heat is required by either the HW cylinder thermostat or CH room thermostat.
Under floor heating zones can be connected to an existing S Plan system via additional 2 port valves which can be controlled by local room thermostats.

A 2 port zone valve such as the V4043 has its own internal relay switch which sends a feed to the boiler (on the orange wire) when the valve is open and prevents a backfeed when the valve is closed.

Wiring colours for 2 Port Valves:

  • Brown- Supply to motor
  • Blue- Neutral
  • Grey- Feed to Relay Switch
  • Orange- Feed from Relay Switch to Boiler/Pump
  • White- Not used
  • G/Y- Earth

Find Zone Valves for additional UFH zones- V4043 2 Port Zone Valve

S Plan Plus

Here’s a wiring diagram for the Honeywell S Plan Plus 3 zone system which can be used for connecting additional zones for radiators or UFH.


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3 Channel Programmer

It’s standard practice to use a 2 channel programmer to control the HW and a single (CH) heating zone. Additional heating zones can be controlled with separate time switches or programmable room thermostats.
An option that keeps all the time controls in one place would be to use a 3 channel programmer for adding UFH zone control- such as the Horstman H37 for S Plan Plus. The Series 2 H37XL is a direct replacement for the H37 and fits on the same backplate.

The live feed wiring connection to an UFH room thermostat would be connected to the 3rd channel output of the programmer rather than from the permanent live at the wiring center.

Can an Underfloor Heating Zone be connected to Y Plan?

A Y Plan heating system uses a 3 port “Flow Share” valve to give priority to either the hot water or heating as required. The valve never actually shuts off, so to connect an underfloor heating zone to a Y Plan you’d need to install two 2 port zone valves. One to feed the UFH before the 3 port and one to close off the supply to the Y plan.

There is a good deal of work involved in altering pipework to accommodate the extra zone valves, so I’d recommend converting a Y Plan to an SPlan “plus” an additional zone (as described above) for connecting underfloor heating.

How to connect UFH to a Gravity HW system

If you need to know how to connect Underfloor Heating System to a Gravity Hot Water central heating boiler then these wiring diagrams may help.

The problem with connecting an under floor heating circuit to a older gravity hot water system is that there are often no pipework valves and the pump is only used for circulating to the central heating radiator circuit.

Gravity System Without UFH Zone Valve

This wiring diagram shows an underfloor heating zone connected to a system without a 2 port UFH valve. A 2 port valve has been fitted to the CH zone, however a relay is required to bring the boiler on for UFH operation. This system is controlled from a basic old style Randall 1020 programmer.


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Gravity System With UFH Zone Valve

A much simpler way of connecting an underfloor heating zone to a system with gravity HW is to install a 2 port zone valve with an internal relay switch to the UFH pipework.


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As you can see from the wiring diagram a 2 port valve is installed to the central heating radiator circuit. This opens when the room thermostat calls for heat and turns on the central heating pump. The relay switch of the UFH zone valve is used to turn on the boiler when needed for underfloor heating operation.

How to connect Multi Zone UFH systems to S Plan and Zone

With larger underfloor heating installations there is a need to separate the UFH pipework runs into smaller zones using an UFH manifold, wiring control centre and often a slave unit for additional zones.

The UFH Wiring Centre enables control of individual pipework zones with room thermostats and zone actuators. These may be ELV or 230 volt and the wiring centre will often contain a transformer for this.  Most slave systems limit to 4 actuators per zone thermostat.

An UFH wiring control center may seem confusing in relation to the S Plan or Combi wiring.  The systems vary slightly but I’ve found that Wundafloor, Polypipe, Honeywell etc. control centres are similar. Basically the job of the Control Centre is to switch on the manifold pump and open the S Plan zone valve (which in turn triggers the boiler) when required by the room thermostats. The zone actuators also open and close depending on demand from the room thermostats.

UFH to S Plan Wiring Info

  • Typically you’ll need to run 3 cores and earth from the S Plan wiring centre to the UFH wiring centre as per the diagram.
  • I would recommend installing a 3 pole switch to this cable to make it easy to isolate the UFH from the rest of the system.
  • You would normally connect the S Plan zone valve flex directly to the UFH Control Wiring Centre.
  • Some Control Wiring Centres will require you to add or remove links depending on the system you’re using.

Here is an example of a wiring diagram for how to connect an UFH Control Wiring Centre to an S Plan. It’s typical of Wunda, Polypie and Honeywell UFH connections.

UFH-control-wiring-centre-to s-plan-diagram

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NB- Please note that Wundafloor controls has a 2 minute delay after calling for heat before the controls switch.

Connecting to Combi Boiler Single Zone

A single zone wiring control centre will normally have connections for manifold pump, S Plan two-port zone valve, boiler switched live connection (230V or volt free) and a programmable room thermostat.

For wiring a single under floor heating zone to a Combi boiler without an UFH Wiring Centre you’ll simply need to connect the switched output from the room thermostat / programmer to bring on the UFH Pump and to open the UFH motorized zone valve.

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