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TYPES OF TIME CONTROLS

Domestic central heating time controls are essential components of the ‘Minimum Set’ recommendations in the

Government’s new Good Practice Guide 302 - Controls for domestic central heating and hot water – guidance for

specifiers and installers. They fall into four categories

- Time Switches

- Mini- Programmers

- Standard Programmers

- Full Programmers

Incorporating either electric or electronic timers, most models require mains power input. At desired times each

day, these timing mechanisms operate the units’ output relays to switch the heating system on or off. Usually, the

regular switching programme can be overridden temporarily if required to cope with unexpected household events.

Both electro-mechanical and electronic time controls usually provide 2 or 3 On/Off settings a day. They are easily

programmable for either 24-hour, weekday/weekend (5/2 day) or 7-day periods; and some models offer all these

options in one unit. Electronic models incorporate battery back-up to retain programme memory in the event of

power cuts.

Many time controls feed mains electricity to the control system through their output relays. If the control circuit

operates on a different voltage, however, output contacts need to be electrically isolated (voltage-free) from the

timer’s input.

Programmable room thermostats, combining the functions of programmer and room thermostat, will be explained

in a later article. Usually battery powered, these controls are placed in the ‘Best Practice’ category in Good

Practice Guide 302.

Danfoss Randall is the UK market leader for time controls and programmable room thermostats, offering

comprehensive electro-mechanical and electronic product ranges for all applications.

TIME SWITCHES

Time switches, electro-mechanical or electronic, have just one single ‘On’ output or a changeover ‘On/Off’ output

(Fig. 2). They are for controlling single circuits such as separate heating zones or the heating from combination

boilers.

Fig. 2. Typical time switch configuration with changeover output contacts

TIMER

1 2 3

5 6

COM

N/O N/C

COM

OFF

Voltage Free Outputs

Mains

1 2 3 4 5 6

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