Here we start with the diagram for the C Plan. The small black circles are showing each cable. It is assumed the controller and Boiler will be right next to each other and either the boiler will be wired through controller or controller wired through boiler giving one three core and one two core cable between the pair and the junction box near the cistern. The junction box is normally close enough to the motorised valve for the cable that comes with the valve to go directly into the junction box. The same with the Pump which is normally very close to the valve only the room stat will have more than a few meters of cable fitted. Often the room stat will also have a neutral feed so will be three core cable. The earths are not shown for clarity.

Although systems using the Y or any of the other Honeywell are slowly become less and less this is likely to reverse as renewable energy sources become more popular. Because the pre-combi boiler systems used a water storage tank the basic system lends itself to the use of multi heat sources.

The position and purpose of the room thermostat is also changing with the introduction of radiator thermostats and instead of being the main control they now tend to be more an anti-cycle device which switches off the central heating in the summer months.

The location has also changed moving away from the main room and sometime are just fitted into a non heated or poorly heated room. In some cases clever positioning in window spaces where the morning sun enters can prevent the central heating from turning on when the day is likely to warm up so keeping house cool in summer months.

Very clever systems may have multi room stats and measure outside temperature as well as inside temperature. The room thermostats now often have timers combined and can set different temperatures at different times of day. Here we start to return to the thermostat controlling the room again with more motorised valves. The diagram to right is a modified C system with two motorised valves. Using this one could easy have 10 motorised valves one for each room. As the control grows one may find home versions of program logic controllers appearing on the market. The problem is a balance between cost of control and cost of fuel and as fuel prices rise so the relative cost of control will result in more electrical controls being added.

DIY
It can be seen how complex heating systems can become and there is very little electrically the DIY can do. If the central heating does not work one would not suspect the thermostat fitted to the hot water tank could have any affect. But with the Y plan this can actually be the case. At the top centre of the drawing to left it shows three connections to the tank thermostat. Here the motorised valve heats the domestic water if not powered and if the central heating was allowed to run before the valve moved the boiler may over heat. The motorised valve is shown as having one micro switch where in real life it may have two or three some used to insure it holds in the mid position when both domestic hot water and central heating is required. These extra switches can cause unexpected back feeds and even an electrician can easy be mislead. The combination boilers often have the same system inside the boiler and really the only option is to call an expert and today that is in the main neither electrician or plumber but someone who does nothing else but work on central heating systems.