© Baxi Heating UK Ltd 2008
24
3.10.6 Sealed Systems
System Components
All components used in the sealed system must be
suitable for operation at 110°C and at the maximum
pressure allowed by the pressure relief valve (normally
3 bar).
Pressure Relief Valve
A pressure relief valve, set to a maximum 3 bar
opening pressure must be fitted.
Pressure Relief Valve Discharge Pipe
The pressure relief valve discharge pipe should be not
less than 15mm diameter and run continuously
downward. It should be routed in such a manner that
no hazard occurs to occupants or damage caused to
wiring or electrical components.
Consideration must be given to the possibility that
boiling water/steam could discharge from the pipe.
The discharge must be outside the building, but must
not be above a window, entrance or other public
access.
The end of the pipe should terminate facing down,
towards the wall and preferably over a drain.
Pressure Gauge
A pressure gauge of minimum range 0-4 bar with a fill
pressure indicator must be connected to the system,
preferably at the same point as the expansion vessel in
a position easily visible from the filling point.
Expansion Vessel
An expansion vessel complying with the requirements
of BS 4814 must be connected to the system close to
the inlet side of the circulating pump in accordance with
the manufacturers instructions, the connecting pipe
being unrestricted and not less than 15mm nominal
size.
The volume of the vessel should be suitable for the
system water content and the nitrogen or air charge
pressure should not be less than the system static
head.
Calculation
To size the expansion vessel it is first necessary to
calculate the volume of water in the system in litres.
The following volumes may be used as a conservative
guide to calculating the system volume.
Boiler Heat Exchanger:
2.7 litres
Small Bore Pipework:
1 litre per kW of system output
Micro Bore Pipework:
7 litres
Steel Panel Radiators:
8 litres per kW of system output
Low Water Capacity Radiators: 2 litres per kW of system output
Hot Water Cylinder:
2 litres
The vessel size can now be determined from the
information in Table 3 where V = System size in litres.
Where a vessel of the calculated size is not available, the
next available larger size should be used.
If the system is extended, the expansion vessel volume
may have to be increased unless provision has been
made for extension.
Filling Point
A filling point connection on the return pipework must be
provided to facilitate initial filling and pressurising and any
subsequent water loss replacement/refilling.
The filling method adopted must be in accordance with all
relevant water supply regulations and use approved
equipment. Your attention is drawn to, in GB, Guidance
G24.2 and recommendation R24.2 of the Water
Regulations Guide. and IE, the current edition of I.S. 813
“Domestic Gas Installations”.
The system may be filled or replenished by means of a
temporary connection between the circuit and a supply
pipe, provided a ‘Listed’ double check valve or some other
no less effective backflow prevention device is
permanently connected at the inlet to the circuit and the
temporary connection is removed after use - See Fig. 32.
Further Details on Sealed Systems
Further details of sealed system design are shown in
BS5449, BS6798 and BS7074.
1.5
1.5
V x 0.125
Vessel Charge Pressure (bar)
Initial System Pressure (bar)
Expansion Vessel Size (litres)
0.5
1.0
V x 0.11
Table 3
