With a combined boiler, heating controls are very simple. Hot water doesn't count – it's made as soon as the tap is opened. Usually, there is only one programmer and one room thermostat or one room thermostat with a built-in programmer. There are three forms of central heating controls for conventional boilers and system boilers: gravity supply, Y-Plan, and S-Plan.
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The gravity feed system is usually a pump that removes heat from the boiler around the cooling circuit when the room thermostat needs heat. To do this, the boiler for hot water must be turned on to the programmer. The cylinder is heated by an increase in gravity and a decrease in the output of the boiler without a pump. New installations cannot use this method and must be "pumped" for hot water and central heating.
The Plan Y system consists of a 3-way valve (often referred to as a reversing valve) which diverts hot water generated in the boiler either to the indirect cylinder circuit or to the radiator or in the "centre" position and supplies both.
The Plan 'S' system consists of 2 or more 2 port valves (often referred to as 'zone valves') which allow water to pass when the thermostat controlling it requires heat and the programmer is ready. "including".
According to current building regulations L, at least a thermostatic radiator valve must be installed in the sleeping area and the system must have an automatic bypass valve.