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
  • Central Heating Installation - INSTALLING CONTROLS

    This section describes the range of central heating system controls and how to install the safely.

    Ensure that the unit is suitable, because time switches can only switch one circuit (eg heating for combi boilers). Programmers can switch two circuits (eg heating and hot water). They should be installed so they can be read and easily reached and altered. Do not install in positions that are inconvenient for the householder (eg in an airing cupboard).

    A room thermostat should be sited in a regularly heated area that allows free circulation around it. It should not be exposed to draughts, and should be away from internal heat sources and direct sunlight. It should also be in a position that is readily accessible to the householder, eg not in cupboards or behind furniture. The thermostat should be sited about 1.5 m above floor level, except where the occupants include a wheelchair user. In this case a suitable height in excess of 1 m should be agreed with the householder. The room thermostat should not be sited where supplementary room heating (eg gas, electric, solid fuel fire) will affect it. Appropriate positions will be in the hall or other living room (ie without supplementary heating). It should not be sited in a kitchen or combined kitchen/dining room and should only be sited in a main living room where it is certain that supplementary heating is not used.

    This usually straps onto the cylinder at a height of about one-third up from its base the strap should be tight for good thermal contact. It should be adjusted to around 60C. If set too high it may give rise to scalding, and if set too low it will increase risk of the growth of legionella bacteria, which could lead to serious health problems.

    Two- and three-port valves are most commonly used and installation depends on pipework layout and preference. For example:
  • three-port valves are suitable for providing separate heating and hot water circuits; most three-port valves provide a mid-position so that shared flow is possible
  • for more than one heating zone, in addition to a hot water zone, use a separate two-port valve for each zone
  • 22 mm valves are usually suitable for boilers up to around 20 kW; for larger boilers, and when fitting a motorised valve on a gravity hot water circuit, 28 mm or larger should be used
  • the motorised valve is not to be positioned in the line of the open safety vent pipe or feed and expansion pipe.

    They should be installed together with a room thermostat to provide boiler interlock. Many TRVs can now be installed in either the flow or return to the radiator and many modern ones are bi-directional if not, it is necessary to ensure that the water flow is in the correct direction. If more than half of the radiators in a dwelling are fitted with TRVs, an automatic bypass will usually be necessary.

    The valve should be installed between the boiler primary flow and return, noting the direction of flow. Ensure that the valve has adequate flow capacity. It should be set correctly so as to ensure adequate flow rate through the boiler when all motorised valves and/or TRVs close.

    Where both air and pipe thermostats are used, the contacts should be wired in series from a live supply that is not switched by time switch/ programmer or thermostats, ie protection needs to be available 24 hours a day. Note that some boilers already include their own frost protection, but it should be checked to confirm that the dwelling is still adequately protected.

    Where a unit includes an external sensor it is important to position the sensor on a north-facing wall, out of direct sunlight and away from other heat sources.

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