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Boiler, central heating and hot water cylinder controls

The controls fitted to a domestic boiler and wet central heating system should allow you to control when your home (and hot water if appropriate) is heated and the temperature it is heated to. Using the controls effectively will improve the energy efficiency and minimise running costs without affecting comfort.

The basic controls are:

  • A Timer or Programmer (usually 2 timers where a hot water storage cylinder is used).
  • A Room Air Thermostat (RoomStat)
  • Thermostatic Radiator Valves (TRVs).

Modern central heating systems are fitted with a timer, room thermostat, and thermostatic radiator valves (TRVs) on each radiator; older systems tended just to use a timer and room thermostat.

Each of the controls does exactly what their name suggests:

  • A timer or Programmer sets when the central heating and replenishing the hot water cylinder are ‘ON’,
  • A room thermostat sets how warm the air will be,
  • Thermostatic radiator valves set the temperature of each space heated by a radiator.

Where the system has a hot water storage cylinder (i.e. not a combi), a cylinder thermostat will also be installed together with a motorised valve so that the boiler heats the cylinder only when required.

The Timer/Programmer

Mechanical Timer
Mechanical Timer
Simple Programmer

The timer allows the user to set the time when the central heating and hot water (where a cylinder is used) switches ‘ON’ and ‘OFF’. A separate timer is used for the hot water but the basic principle is the same other than usually there is a ‘boost’ override on the hot water timer which allows it to be heated on demand outside the set times.

Simple timers use just a 24 hour clock which is applied to every day, other timers (usually referred to as Programmers) provide the timers for both central heating and the hot water, and usually offering multi ON/OFF periods on different days.

Programmers generally provide a number of ON/OFF periods per day; ranging just three to six or more – often only 2 central heating periods will be required on workdays; one in the morning and one in the evening. With modern highly insulated hot water cylinders, only one period of hot water heating may be required. They can also provide the ‘boost’ control for the hot water.

Programmers vary, they may offer ‘5/2’ which allow different ON/OFF timings for weekdays (i.e. the ‘5’) from those set for the weekend (i.e. the ‘2’). More complicated programmers offer ‘7 day’ programming which allow the ON/OFF periods to be set differently for each day – thankfully, these ‘7 day’ programmers usually have a ‘copy’ facility where the setting from one day can be just copied to other days.

More complicated Programmer

Programmers are often connected to the boiler by wire, however, Wireless Programmers are available which have a mains powered ‘receive’ box connected to, and fitted next, to the boiler while the actual Programmer is battery powered and can be located almost anywhere in the property. Wireless Programmers can make updating an existing system, or fitting a new system in an existing property, easier as there’s no need to run cables between boiler and Programmer.

Thoughtful use of the programmer to match the household life style should minimise the energy used and save money – i.e. if everyone is out at work during the day, set the timer to switch off the central heating just before the normal ‘going to work’ time and to switch it back on just before the normal ‘arriving home from work’ time.

Combi boiler systems just use one timer/programmer as the hot water is heated as required.

Room thermostat (or RoomStat)

Mechanical Room Thermostat
Mechanical Room

A RoomStat is set to whatever temperature you want and switches off the boiler and central heating pump (if the central heating is programmed to be on) when the air temperature reaches the setting. When the temperature drops below the setting, the thermostat switches the central heating back on (again, providing it’s programmed to be on).

Where a single RoomStats is used, it is usually positioned in the hall, on the stairs or the landing so that it measures the temperature of the main space. It shouldn't’t be fitted near a radiator, in a main room or where it will be subjected to undesired heating (such as where direct sunlight can fall on it) as you will find that the central heating switches off when the house generally is at a lower temperature than required.

Any radiator in the space with the RoomStat should not be fitted with a TRV, or, if it is, the TRV should be kept at its highest settings; this will avoid the TRV and RoomStat ‘fighting’ over the control of the central heating.

Wireless Programmer
Wireless Programmable
Room Stat

A programmable RoomStat allows you to choose the times you want your home to be heated and the temperature you want it to reach while it is on. It allows different temperatures to be set for different times and even days of the week.

RoomStats vary from simple mechanical thermostats, which need to be wired to the boiler/programmer, to digital wireless RoomStats which, like the wireless Programmer, has a mains powered ‘receive’ box fixed next to the boiler and a battery powered RoomStat which can be fixed anywhere.

Thermostatic Radiator Valves (TRVs)

Thermostatic Radiator Valve (TRV)
Radiator Valve

Thermostatic Radiator Valves (TRVs) are fitted to individual radiators to control the temperature in each heated space –it is now a Building Regulation requirement that all new, and updated, installations have a TRV fitted to every radiator.

A TRV senses the air temperature and adjusts the flow of hot water flowing through the radiator to maintain the set temperature of the room. Using TRV allow different temperatures to be set in different rooms (i.e. lower in bedrooms than in a lounge).

Unlike RoomStats, TRVs do not control the boiler, just the flow of heated water through each radiator; without a RoomStat the boiler and pump will be on even if all the TRVs have closed once the space has reached the set temperatures.

A Rondostat is an electronic TRV. It is battery powered and programmable so that a user can precisely control the temperature of a room for set periods of the day – i.e. a lower temperature can be set for living areas during the night or bedrooms during the day. A Rondostat is fitted to a radiator just like a TRV, in fact some are designed to simply replace the body of a TRV so there’s no need for any drain-down of a system where conventional TRVs are fitted – just unscrew the TRV body and screw on the Rondostat.

Note - RoomStats and TRVs need to have a free flow of air around them to accurately sense the temperature. They should not be covered by curtains or blocked by furniture. Also, undesirable sources of heat such direct sunlight or lamps will also prevent them from working correctly.

Hot water cylinder thermostat

A hot water cylinder thermostat is (as the name suggests) fixed to the cylinder and measures the temperature of the water in it. The boiler should only run to heat the hot water when the hot water timer is on and the temperature of the water in the cylinder is below the thermostat setting.

A motorised valve is usually connected to the outlet of the boiler so that the boiler heated water only flows to either the hot water cylinder or the central heating – never both at the same time. The hot water cylinder normally has the priority – i.e. when the timers for both the central heating and hot water are on, the boiler will heat the hot water cylinder until it reaches its temperature and then the motorised valve will operate so that the boiler feeds the central heating.


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