Neil Macdonald, Technical Manager at the Heating and Hotwater Industry Council, breaks down the benefits and common issues found with S and Y plan heating systems.

Central heating system design will differ from house to house, based on the requirements of the property, but the vast majority of domestic properties in the UK will incorporate either a ‘Y plan’ or ‘S plan’ style of system.

By way of a reminder, a Y plan uses a mid-position valve to direct the primary flow through the heating circuit, hot water circuit, or both, while the S plan utilises at least two separate zone valves for both the heating and hot water functions.

The style of system design chosen can be influenced by many factors. This includes the size of the property, with Part L requiring at least two space heating zones where dwelling floor area exceeds 150m2 (often lending itself to an S plan), the provision of an unvented cylinder (typically requiring a spring return zone valve for safety), or even space, such as a compact airing cupboard, which may struggle to accommodate the generally required system bypass for S plan designs, and so may lend itself to a Y plan.

Although many modern boiler designs incorporate an internal bypass, this is often only to help prevent damage to the appliance in ‘no flow’ conditions, and is not a substitute for a correctly installed system bypass, to dissipate residual heat around the circuits once a demand ends and a pump overrun period begins.

Y plan systems typically always provide a circuit for this purpose, the valve open to CH, HW, or mid-position at any given time. However, it is important that at least one radiator is fitted without a thermostatic radiator valve, and left on, to provide a return through the heating circuit at all times.

It is also important to check the boiler manufacturers’ instructions for any other specific requirements with regards to system design criteria. It’s worth noting that Part L requires an automatic bypass to be fitted where the appliance manufacturer specifies that a bypass arrangement is necessary, typically S plan design.

Historical system bypass arrangements, such as the use of a gate valve, can be prone to incorrect setting or maladjustment, risking an unbalanced system with large amounts of heat ‘short circuiting’ between flow and return.

There are some older style ‘motor open, motor closed’ S plan systems still in existence, characterised by a ‘satisfied’ wire from the respective controls (room thermostat or cylinder stat). Typically, a spring return zone valve, incorporating a permanent live supply to the actuator, is now used.

When installing or replacing the zone valves in an S plan system, care must be taken to ensure the valve body is installed in the correct orientation, typically ‘A’ for the inlet and ‘B’ for the outlet.

If not, the system will generally still work correctly, but a clunking noise may be heard upon valve closure, which can lead to consumer complaints and revisits.

With the Y plan, unless the component manufacturer says otherwise, the valve should be installed with the ‘B’ port supplying the hot water cylinder (“B for bath”, as I was always taught).

A common fault with an S plan system is the electrical micro-switch in the actuator ‘sticking’, causing the boiler and pump to run continuously. This can be a baffling and somewhat disconcerting experience for the consumer. It can also give rise to other faults, such as overheating of the boiler if firing around the bypass circuit only.

The valve components themselves are generally highly reliable, and frequently it is underlying water quality issues which can give rise to such events. Magnetite sludge can foul the seat of the valve, causing the valve spindle to seize or stiffen, and the actuator to operate with restricted movement.

Water quality also often accounts for another common problem, usually noticed by consumers in the summer months when the heating is no longer on.

Primary heat can continue to pass through the port of the valve, which should be closed, under pressure of the circulator. This could mean the radiators heating whenever there is a hot water demand.

Depending on the valve manufacturer, the valve may have serviceable parts, which can be safely dismantled, cleaned or replaced, before reassembly. If not, then a new valve may be required if faulty, as well as addressing any underlying cause, such as recommending a thorough cleanse and flush of the system, if sludge and/or corrosion are evident.