Central Heating Design - Sundial Plans

Central heating design for heating control can differ from house-to-house based on your requirements; and whether you are building a 'new' house, or upon the 'design' you inherited (already installed) in a ready built house.

There are a number of heating control configurations you can use in your central heating design today. In the UK over 30 years ago Honeywell introduced a number of 'Sundial Plans' to enable time and temperature control for heating and hot water.

A Sundial Plan identifies the components that are used to independently control the temperature and heating periods for both space heating and hot water provision.

Regardless of the central heating design the 'Sundial Plan' is a system that controls the boilers operating periods and temperatures for space heating, and/or hot water.

This simply means - the time the boiler starts up and for how long - how much heat it provides for either space heating, or for domestic hot water, or both.

Below is a list of Sundial Plans to enable independent control for heating and hot water:


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C Plan

The C Plan can be 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)
sundial C Plan control design
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. However, if there is a demand for space heating the boiler and pump will shut down when the room thermostat has been satisfied.

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S Plan

The S Plan is used in fully pumped central heating and domestic hot water systems. With this system you have full independent temperature control for both heating and hot water. Hot water heat recovery is far quicker with the S Plan central heating design because heated water is pumped to the hot water cylinder as opposed to a gravity system like the C Plan.

With a fully pumped heating and hot water the 'S Plan' components are:

  • Two 22mm 2 port motorized valves - one for the domestic hot water secondary circuit, and one for the central heating primary 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)
sundial S Plan control design
Time control for the boiler and pump in this central heating design 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.

image source Honeywell UK

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S Plan Plus

The S Plan Plus design is recommended if your central heating design heating zone (floor area) is greater than 150m2. This plan adds additional heating zones to above S Plan design. Each heating zone will be served by it's own 2 port motorized halving valve, room thermostat and timer or programmer, enabling full space heating temperature control.

With a fully pumped heating and hot water the 'S Plan Plus' components are:

  • Three 22mm 2 port motorized valves, 1 for the hot water secondary circuit and 1 for each of the central heating primary circuits
    • Note: the water must flow in the direction indicated on the valve body

  • Two Room thermostats, one for each 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 central heating design 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.

sundial S Plan Plus control design

image source Honeywell UK

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W Plan

The hot water and central heating design for the 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)

sundial W Plan control design
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.

image source Honeywell UK

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Y Plan

Although the Y Plan uses a three port valve like the W Plan, the difference is that the three port valve that is used in the Y Plan is a motorized mid-position valve unlike the 'diverter' valve that is used in the W Plan.

This means simply that a three port mid-position valve can distribute hot water to both the domestic hot water circuit and the heating circuit at the same time.

With a fully pumped heating and hot water the 'Y Plan' components are:

  • One 22mm 3 port motorized mid-position 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. When the boiler has been shut down, or when there is no demand for heat from the boiler the 3 Port valve will remain in the last position of operation when the timer - or programmer is 'on'.

sundial Y Plan control design

image source Honeywell UK

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