Page 393 - 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 6 Hot water systems

Comparisons between open vented and unvented hot water
storage systems
There are important differences between these two types of system.
Table 6.10 compares open vented and unvented hot water storage systems.

 Table 6.10 Vented and unvented storage hot water systems: a comparison
Advantages Disadvantages
Open vented systems
Storage is available to meet demand at peak times Space needed for both the hot water
Low noise levels storage vessel and the cold water
Always open to the atmosphere storage
Water temperature can never exceed 100°C Risk of freezing
Increased risk of contamination
Reserve of water available if the mains supply is
interrupted Low pressure and, often, poor flow rate
Low maintenance Outlet fittings can be limited because of
Low installation costs the low pressure
Unvented systems
Higher pressure and flow rates at all outlets, giving No back-up of water should the water
a larger choice of outlet fittings supply be isolated
Balanced pressures at both hot and cold taps If the cold water supply suffers from
Low risk of contamination low pressure or flow rate, the system
will not operate satisfactorily
The hot water storage vessel can be sited almost
anywhere in the property, making it a suitable There is the need for discharge pipes
choice for houses and flats alike that will be able to accept very hot
The risk from frost damage is reduced water and there will be restrictions on
their length
Less space required because cold water storage is
not needed A high level of maintenance is required
Installation is quicker as less pipework is required Higher risk of noise in the system
pipework
Smaller-diameter pipework may be used in some Initial cost of the unvented hot water
circumstances
storage vessel is high

Gas-fired instantaneous multipoint hot water heaters
With this type of hot water heater, cold water is taken from the water
undertaker’s main and heated in a heat exchanger as demand requires before
being distributed to the outlets. As long as a tap is running, hot water will
be delivered to it. There is no limit to the amount of hot water that can be KEY TERM
delivered. There is no storage capacity. Bernoulli’s principle: when
a pipe reduces in size, the
Expansion of water due to being heated is accommodated by back pressure pressure of the water will
within the cold water main. However, if this is not adequate or the cold water drop but the velocity of
system contains pressure reducing valves or check valves, then an expansion the water increases. When
vessel must be fitted. the pipe increases back to
its original size, then the
The heater works on Bernoulli’s principle by using a venturi tube to create a velocity will decrease and the
pressure differential across the gas valve when the cold water is flowing into the pressure will increase almost
heater. to its original pressure.

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