Page 491 - 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

Boiler management control

Zone
controller

Zone
controller
Outdoor
sensor
Zone
valves
Shunt
pumps
Primary circuit Secondary circuit

Hot water cylinder
Boiler shunt pumps Low loss
header

Expansion
vessel
Boiler Boiler Boiler
Pressurising unit

p Figure 7.52 A multiple boiler installation with a low loss header

Water velocity is just part of the problem. Water temperature is also important.
There are two potential problems here:
1 If the difference in temperature between the flow and return is too great, it
puts a huge strain on the boiler heat exchangers because of the expansion
and contraction. This is known as ‘thermal shock’.
2 For a condensing boiler to go into condensing mode, the return water
temperature must be in the region of 55°C. In some instances, temperature
sensors are fitted to the low loss header to allow temperature control over
the primary circuit.
The low loss header is ideal for use with systems that have a variety of different
heat emitters. It is the perfect place for installing an automatic air valve for
removing unwanted air from the system. Drain points can also be fitted for
removing any sediment that may collect in the header. Both of these features
are usually fitted as standard on most low loss headers.

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