Fill And Vent The System - Grant COMBI 70 MkII User, Installation And Servicing Instructions

Expansion vessel pressure
5.1
The expansion vessel fitted is supplied with a
charge pressure of 1.0 bar (equivalent to a max.
static head of 10.2 metres). The charge pressure
must not be less than the actual static head at the
point of connection (see Fig. 12). Do not pressurise
the vessel above 1.5 bar.
The air pressure in the vessel must be checked annually.
The central heating system volume, using the
expansion vessel as supplied, must not exceed 87 litres
(Combi 70) or 117 litres (Combi 90). If the system
volume is greater, an extra expansion vessel
(complying with BS 4841) must be fitted as close as
possible to the central heating return connection on the
boiler. The charge pressure of the extra vessel must be
the same as the vessel fitted in the boiler. Refer to BS
7074:1 for further guidance.
The air charge pressure may be checked using a tyre
pressure gauge on the expansion vessel Schraeder
valve. The vessel may be re-pressurised using a
suitable pump. When checking the air pressure the
water in the heating system must be cold and the
system pressure reduced to zero.
A simple test to check if the expansion vessel size is
adequate, is to fully heat the system and if the pressure
rises no more than 2.3 bar the vessel is adequate. A
higher figure indicates that an extra vessel is required.
Fill the system
5.2
1 An automatic air vent is fitted to the top of the
boiler (see Fig. 17 or 19). Check that the small cap
on the top of the air vent is screwed on fully, then
unscrew it one complete turn - the cap remains in
this position from now on.
2 Ensure the primary return isolating valve (see Fig.
37) is fully open. The valve is open when the
operating lever (yellow) is in line with the valve.
3 If the flexible filling loop is used to fill the system,
ensure it is connected and that the valve connecting
it to the boiler is open and the valve at the front is
closed. A valve is open when the operating lever is
in line with the valve, and closed when at right
angles to it.
4 Ensure the mains cold water supply valve (just
below the DHW heat exchanger - see Fig. 37) is
open (operating lever in line with the valve), then
turn on the mains cold water supply and gradually
open the front valve on the filling loop until water
is heard to flow.
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5 - FILL AND VENT THE SYSTEM

5 Vent each radiator in turn, starting with the lowest
in the system, to remove the air.
6 It is important that the pump is properly vented to
avoid it running dry and damaging its bearings.
Remove the cap then unscrew and remove the plug
from the centre of the pump, using a suitable
screwdriver rotate the exposed spindle about one
turn. Replace the plug and cap.
7 Check the operation of the safety valve (see Fig. 17
or 19) by turning the head anticlockwise until it
clicks. The click is the safety valve lifting off its
seat allowing water to escape from the system -
check that this is actually happening.
8 Continue to fill the system until the pressure gauge
indicates 1.0 bar. Close the fill point valve and
check the system for water soundness, rectifying
where necessary.
Water may be released from the system by
manually operating the safety valve until the
system design pressure is obtained.
9 The system design pressure (cold) should be
between 0.5 and 1.0 bar. This pressure is
equivalent to the maximum static head in bar + 0.3
(1 bar = 10.2 metres of water). See Fig. 12.
Set the adjustable pointer on the pressure gauge to
the system design pressure.
10 Close the valves either side of the filling loop and
disconnect the loop.
Combi 70 & Combi 90 Oil Boilers