08 Oct 2018

Revisiting S and Y Plans

If you’re a veteran heating engineer, chances are that Sundial plans formed a major part of your training and, although they’ve been around for more than 30 years, they still form the basis of most domestic heating systems in the UK.

Conventional Sundial S and Y control plans ensure boiler interlock is achieved – that the boiler is switched off when there is no demand for heating or hot water – which is a requirement under part L1A of the Building Regulations.

The fundamentals may not have changed, but the rest of the industry has, which in turn has affected the application of traditional wiring layouts. One of the key differences is changes to best practice guidelines, such as the recommendation that homes with a floorspace greater than 150m2 should have multiple heating zones which are time and temperature independent.

So, whether you’re looking to learn new skills or refresh your knowledge of Sundial plans, here’s what you need to know:

Y Plan

A Y plan is more compact than the ‘S’ and is made up of one three-port motorised valve for both heating and hot water, a room thermostat, cylinder thermostat and programmer for control of a single heating zone.

This type of system is quite outdated as it doesn’t cater to the requirements of many modern homes. That said, it is still commonly found in older properties and is suitable for use when conducting part replacements or repairs. For example, if you’re just changing the hot water cylinder, it’s fine to keep the Y plan in place.

It does have some drawbacks though, as it can’t supply more than one zone and the maximum flow rate through the valve is for the combined heating and hot water supply, so it’s not recommended for properties with multiple bathrooms. This also means that if only the valve fails, both the heating and hot water supply will be unavailable.  

S Plan

Unlike the Y plan, a typical S plan has two separate motorised valves, with independent time and temperature control of heating and hot water; a cylinder thermostat; a room thermostat; and a programmer. 

It’s the easiest plan to wire in, commission and fault find because you can isolate the heating and DHW circuits and usually gives better flow rates than a Y plan.

It is also the most flexible, with the possibility of adding further heating zones to set up what is known as an S Plan+.

S Plan +

For larger homes (greater than 150m2), it’s a good idea to make the heating zones time and temperature independent, so each zone has its own timing, its own temperature control and its own zone valve. This is known as an S Plan+.

Even if multiple zones are not initially required, installing an S plan+ gives the advantage of future-proofing the system so that additional heating zones can be added later. For example, the customer might have a conservatory extension, or simply decide that they want more control over individual room temperatures.

Multi-zone systems are great for energy saving as rooms that are not in use can be ‘zoned off’, so that heating is only supplied to certain areas rather than the entire property. They also work well in multi-occupancy premises such as HMOs because each tenant can have their living space at their ideal temperature.  

Zoned systems used to be fiddy and time-consuming to install but thanks to the evohome system, this is no longer the case. evohome is a wireless control that provides zoning via smart TRVs instead of motorized zone valves. It can be fitted easily to most radiators and, when there is an existing TRV in place, the system does not need to be drained down.

If you still need more information on Sundial Plans, check out the Honeywell professional zone where you can find the latest edition of the wiring guide, as well as diagrams for individual scenarios, such as installing a pump overrun on an Y, S and S Plan+. 

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