Page 461 - 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

Should the valve be inadvertently closed, a dangerous situation could develop,
especially if the vent is also blocked as the pressure will build up in the system,
raising the boiling point of the water to dangerous levels. Both the cistern and
any float-operated valve it may contain must be capable of withstanding hot
water at a temperature close to 100°C.
Primary open safety vent
The purpose of the open vent pipe is one of safety. The open vent is installed to:

l provide a safety outlet should the system overheat due to a component failure
l ensure that the system always remains at atmospheric pressure limiting the
boiling point to 100°C.

In a fully pumped system, the height of the open vent should be a minimum of
450 mm from the water level in the cistern to the top of the open vent pipe.
This is to allow for any pressure surges created by the circulating pump. The
minimum size of pipe for the open vent is 22 mm and this, like the cold feed
pipe, should not be fitted with any valves.

450 mm

Open vent pipe

p Figure 7.17 Height of the open vent pipe
KEY TERM
Sealed (pressurised) heating systems
Sealed heating systems are those that do not contain a feed and expansion Sealed heating systems:
cistern but are filled with water direct from the mains cold water supply heating systems that are
sealed from the atmosphere
via a temporary filling loop. Large systems would be filled via an automatic and operate under pressure.
pressurisation unit. The expansion of water is taken up by the use of an expansion They do not contain a feed
vessel and the open vent is replaced by a pressure relief valve, which is designed and expansion cistern.
to relieve the excess pressure by releasing the system water and discharging safely Instead, they have an
to a drain point outside of the dwelling. This is vital as the water may be in excess expansion vessel to take
of 80°C. A pressure gauge is also included so that the pressure can be set when up water expansion and a
filling loop to fill the system
the system is filled, and periodically checked for rises and falls in the pressure as from the cold water main.

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