London Heating Services

  • Central Heating Guide
  • C/H Controls Introduction
  • C/H Thermal Controls
  • C/H Valve Controls
  • C/H Function Controls
  • C/H System Replacement
  • C/H Controls Installation
  • Energy efficient home
  • Heating appliance valve controls

    This section explains the range of valve controls that are commonly available, what they do and why they are important. Valve controls are only for wet heating systems, and are fitted away from the heating appliance.

    Used to limit temperatures in individual rooms and so reduce energy consumption. Provides an upper limit to room temperatures and so can usefully prevent overheating due to solar and incidental gains.

    Self-acting valves without motors that are used to limit hot water temperature in domestic hot water cylinders. Units are available that sense the primary water (boiler) temperature and also with a separate remote sensor that can sense stored water temperature. Cylinder controls should not be used unless they also operate an electrical switch to provide boiler interlock, otherwise the boiler will cycle unnecessarily.

    Used to control water flow from boiler to heating and hot water circuits. Two-port valves can also be used to provide zone control, eg lower temperatures in sleeping area or different heating times. See ‘Definitions of controls’ on page 22 for an explanation of different types.

    This is not a control device but a wiring arrangement to prevent the boiler firing when there is no demand for heat. The boiler can be said to be ‘interlocked’ when the boiler is switched ‘on’ and ‘off’ by the operation of a room or cylinder thermostat (or a boiler energy manager). In many cases the interlock will also apply to the pump operation but any requirements for pump overrun must be observed. Without an interlock, the boiler is likely to cycle on and off regularly and waste energy by keeping the boiler hot when it is not necessary. For regular boiler systems, the interlock is usually arranged so that the room or cylinder thermostat switches the power supply to the boiler (and sometimes the pump) through the motorised valve ‘end’ switches. For combi boilers interlock is usually achieved by using a room thermostat.

    This device controls water flow in accordance with the water pressure across it, and is used to maintain a minimum flow rate through the boiler and to limit circulation pressure when alternative water paths are closed. A bypass circuit must be installed if the boiler manufacturer requires one, or specifies that a minimum flow rate has to be maintained while the boiler is firing. The installed bypass circuit must then include an automatic bypass valve (not a fixed position valve). The use of an automatic bypass is important where the system includes a large number of thermostatic radiator valves (TRVs). When most TRVs are open, the automatic bypass remains closed, allowing the full water flow to circulate around the heating system. As the TRVs start to close, the automatic bypass starts to open, maintaining the appropriate water flow through the boiler. The use of an automatic bypass is also likely to reduce noise in the system due to excess water velocity. An automatic bypass is always preferable to a fixed bypass. With a fixed bypass there is a constant flow of hot water coming out of the boiler, which is fed directly into the return at all times. This allows the boiler to operate at a higher temperature, which reduces efficiency and restricts the amount of heat transferred to the system.

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