Danfoss DOMESTIC HEATING CONTROLS ARE EASY Fact sheet

DOMESTIC HEA TING CONTROLS ARE EASY
P ART 2 - TIMERS & PROGRAMMERS

T oday’s inescapable need to improve energy ef ficiency in UK homes has brought installers many opportunities to
make extra earnings through the upgrading of heating and DHW controls. Often, however , misunderstandings and
technical ‘mystique’ surrounding some of these products inhibits installers from fitting the most appropriate energy
saving devices that can cut household heating bills, improve comfort levels and reduce carbon dioxide emissions
into the atmosphere.

This series aims to remove some of the doubts and show that domestic heating controls really are easy .

TIME CONTROLS

Running a domestic central heating/hot water system continuously is prohibitively expensive for most users. One
certain way of holding down a home’s heating bills is to switch the system off as often as possible. Of course, this
can be done only when room and domestic water heating are not needed.

Time controls on most domestic heating/hot water systems are among the first essentials for improving home
comfort and reducing running costs. They trigger the controls ‘chain of command’, sending automatically timed
On and Off instructions to other system components. Each user-set, timed switching programme helps maintain
desired comfort levels and prevents excessive fuel consumption by inhibiting unnecessary boiler firing.

Winter temperatures in the UK are rarely low enough to keep the heating on overnight, so we usually cut heating
costs and sleep better in a cooler room by using a time control to automatically switch off the system late evening
and on again early morning. Also, we use the same timer to set Off and On periods during the daytime for
additional benefit.

Fig.1 shows a simplified typical heating controls wiring sequence. The time control governs when the heating
system operates whilst the thermostats and valve auxiliary switch provide the electrical interlock that prevents the
boiler firing if there is no demand for heat.

TIME

CONTROL

THERMOSTATS

(ROOM OR

CYLINDER)

RELEVANT
MOTORISED
VALVES

BOILER

AND

PUMP

Sends timed ON/OFF

signals to thermostat

If heat demand exists,

switches close and

pass signal to valve(s)

Motor opens valve(s),

activates auxiliary switch

to boiler/pump

Boiler can fire only

when a demand for heat

exists

Fig.1

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 of f. 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 of fer 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 contact s 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.

COM

TIMER

N/O N/C

E

5 6

N

Mains

L

1 2 3

ON

Voltage Free Outputs

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

OFF

COM