It has been noted that wiring of boilers not of the combination type is causing problems as there are less and less found. boilerC3There are four main wiring systems, W, S, Y, and C. The latter I became interested with for my fathers system and the Y system for my daughters house. It is easy to design a system as shown. But it can be seen the change over contacts in the valve are very important. The programmer and tankstat only control the valve and the valve controls the boiler. So step by step here's how it works.
When hot water only is called for the tankstat will activate the valve as the cylinder cools down this will open (to stop feed to pump) the connection from the roomstat and close to connection to boiler and thermo siphon will then heat hot coil and in turn domestic hot water.
When central heating only is called for the Roomstat will directly feed pump and through contact in valve (which is closed) to boiler.
When both are called for then the boiler is feed through the valve and the pump from the roomstat.
As shown the motorised valve is open and only hot water is required. This of course could be expended upon. For example one could add zones so that bedrooms and living rooms can be heater independently the supply is taken through the valves so should they fail the boiler will not work. Combining the S and C plans together.
Should one not want to fit a valve and tankstat on the hot water on the hot water supply it would need replacing with a relay. This is the only way to separate the pump and boiler supply.
There are problems in removing the tankstat in that the boiler will cycle when ever hot water is called for and the water is hot. Also the domestic water will become the same temperature as the central heating water which at 80°C is really too hot. But it is sometimes done.
Roomstats are often left off as the TRV's will control the room temperature and only as summer mouths are approached would it be required to turn off a variable output boiler. If they were fitted then they should be placed were the central heating has very little effect such as a hall way as they are only required to turn it off in the summer months and normally would not open as the TRV's do all the work. In theory one before the programmer supply to zones would be better but the design of the programmer will not allow that option. boilerC2
The W and Y plans use three port valves, the Y plan having a mid-position. The W is odd one out having no contacts used within the valve. The Y plan again supplies the boiler through the valve. But unlike the other valves the micro-switches are connected to the supply to the valve. Also they are exactly not as shown in the diagram and some odd results can be found when fault finding. When the switches become faulty it can stop the central heating working. Moving the manual leaver which allows the setting to central position for bleeding can start the system running until it turns off then again will not re-start. Because the diagrams only represent the real switches and is not exactly as it works in reality this can cause problems. Wish I had kept old unit now basically the three micro switches not only switched the supply but also maintained the valve in the mid position.
The C plan diagram has been altered to show the junction box connections that Honeywell recommend. To follow anything else could cause confusion latter. The second diagram adds zones and with the advent of TRV's is rarely used now.
Central heating picture