Page 497 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 7 Central heating systems

2 A standard programmer: these use the same time settings for space heating KEY POINT
and hot water.
3 A full programmer: allows the time settings for space heating and hot water to Document L was
implemented to save
be fully independent. Some will allow seven-day programming of both heating energy and power.
and hot water so that the two circuits can be used individually or both together. A great deal of heat
is lost from a building
Programmers are often fitted to the front fascia of the boiler and integrated into through not insulating
the boiler design. This, however, is not always convenient, especially if the boiler properly and not having
is sited in a garage or roof space. the necessary controls
on a heating system
Room thermostats to prevent the wastage
of fuel. Document L
A room thermostat senses air temperature. It is simply a temperature-controlled prevents this by ensuring
switch that connects or breaks an electrical circuit when either calling for heat or that even existing systems
shutting the circuit down when the correct temperature has been reached. Most are brought up to date.
room thermostats contain a very small heater element called an accelerator, It is vital for energy
which ‘tops up’ the heat to the room thermostat by 1°C or 2°C, smoothing out conservation that we
the temperature cycle, preventing the boiler from ‘cycling’ when it isn’t required. follow the rules it lays out.

Programmable room thermostats allow different temperatures to be set for
different days of the week. It also provides a ‘night set-back feature’ where a
minimum temperature can be maintained at night. Some units also allow the
time control of the hot water cycle.

Cylinder thermostats

A simple control of stored hot water temperature, usually strapped to the side
of the hot water cylinder about a quarter of the way up from the bottom. It is
used with a motorised valve to provide close control of water temperature and
should be set to 55°C.
p Figure 7.56 A room thermostat
Frost thermostats and pipe thermostats

The purpose of the frost thermostat is to stop the boiler and any other vulnerable
parts of the system from freezing in extremely cold weather. It is wired in to the
system to override all other programmers and thermostats. It should be set to
between 3°C and 5°C, and should be placed close to the vulnerable parts of the
system, especially if they are fitted in unheated garages and roof spaces.
Frost thermostats are much more effective when installed alongside a pipe
thermostat.
A pipe thermostat is strapped to vulnerable pipework and senses water
temperature. It is designed to override all other controls when the temperature
of the water is close to 0°C and works in conjunction with the frost
thermostat. The pipe thermostat and frost thermostat should be wired in
series (see Chapter 3, Scientific principles, page 190).

Motorised valves

We have already seen that both the two-port zone valve and the three-port mid-
position valve are key controls for the S-plan and Y-plan fully pumped systems,
and the C-plan plus semi-gravity system. To recap the key points of these valves:

485

9781510416482.indb 485 29/03/19 9:03 PM