There is often great confusion regarding the wiring of central heating systems.
The two most common are known as S Plan and Y Plan. The basic differences are the Y Plan has one three port valve supplying heating and/or hot water. The three port valve can look like a Y shape when looking at the schematic layout of pipework, as shown below.
The S Plan has two SEPARATE motorised valves, one for heating and one for hot water. These two valves can be independently controlled and switched. It is also possible to add further zones by adding additional thermostats and motorised valves to each circuit.
The wiring of both systems is reasonably uncomplicated if the following diagrams are followed. I have put these together with reference to most popular manufacturers instructions. Trying to mix and match different manufacturers equipment and instructions can be very confusing, and time consuming. The following diagrams are based on a standard 10 way wiring centre. I have included CPC details for all conductors for completeness.
The S Plan diagram is shown below.
The plan above includes provision for a second circulating pump. This may be the case in a larger property where pumped hot water is needed to avoid drawing off too much cold water on long pipe runs from the boiler.
The Y Plan system is less complicated as there is only one three port valve needed.
The cost savings from installation of thermostatic and timer controls must not be under estimated. I have had Customers who have saved £20.00 PER WEEK in times of peak demand. It should cost between £125.00 and £350.00 to upgrade the heating and hot water control systems. Money well spent, and should be recovered within 12 months due to reduced gas consumption.
Please contact me if you need any further advice or have ideas to improve this article.
I have PDFs for the two wiring centre diagrams. E-mail me at email@example.com if you would like copies.
I’ve also included a link to the Honeywell download page. The wiring guide PDF shows details of all available heating/hot water system configurations.
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