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Pre-Installation
The table below is based on copper tube. Plastic pipes may be of different bore and resistance. Sizes and maximum lengths of plastic should be
calculated using data prepared for the type of pipe being used.
Valve outlet size
Minimum size of discharge
G1/2
G3/4
G1
1) see Fig. 16
Table 8 Sizing of copper discharge pipe "D2" for common temperature relief valve outlet sizes
Worked example:
The example below is for a G1/2 temperature relief valve with a
discharge pipe (D2) having 4 No. 22 mm elbows and length of 7 m from
the tundish to the point of discharge.
From Table 8:
Maximum resistance allowed for a straight length of 22 mm copper
discharge pipe (D2) from a G1/2 temperature relief valve is: 9.0 m
Subtract the resistance for 4 No. 22 mm elbows at 0.8 m each = 3.2 m
Therefore the maximum permitted length equates to 5.8 m which, is less
than the actual length of 7 m therefore calculate the next largest size of
discharge pipe (D2).
Maximum resistance allowed for a straight length of 28 mm copper
discharge pipe (D2) from a G1/2 temperature relief valve is: 18 m
Subtract the resistance for 4 No. 28 mm elbows at 1.0 m each = 4 m
Therefore the maximum permitted length equates to: 14 m
As the actual length is 7 m, a 28 mm (D2) copper pipe will be
satisfactory.
4.8
Cleaning primary systems
NOTICE
Risk of damage to system or appliance!
Debris from the system can damage the appliance and reduce efficiency.
Failure to comply with the guidelines for the use of water treatment with
the appliance will invalidate the appliance guarantee and contravene the
Building Regulations.
▶ It is a requirement of the Building Services Compliance Guide which
is a second tier document to the Building Regulations to flush and
inhibit the primary water system in accordance with BS 7593.
▶ The Building Services Compliance Guide recommends that a primary
water cleanser is fitted to the system.
▶ The filter fitted to the appliance will help remove both magnetite and
non-magnetic debris.
NOTICE
Risk of damage to system or appliance!
▶ Salt based, artificially softened water that adjusts the pH value MUST
NOT be used to fill the system.
▶ Alternatively there are water softening/treatment products that do
not adjust or alter the pH levels of the water.
With these products it may not be necessary to provide an untreated
water by-pass to the primary water filling point of the heat system
16
Minimum size of discharge
1)
pipe D1
pipe D2 from tundish
15 mm
22 mm
28 mm
35 mm
22 mm
28 mm
35 mm
42 mm
28 mm
35 mm
42 mm
54 mm
Maximum resistance
allowed, expressed as a
length of straight pipe (i. e.
no elbows or bends)
up to 9 m
up to 18 m
up to 27 m
up to 9 m
up to 18 m
up to 27 m
up to 9 m
up to 18 m
up to 27 m
Before cleaning the system:
▶ Ensure that the system and pipework is in good working order.
▶ Where possible keep the existing appliance/circulating pump in
place when flushing the system before installing the new
appliance. Alternatively use an after market power flushing
system or similar to remove magnetite and other debris from the
central heating system.
– Even new central heating systems contain contaminants that
need to be removed before treating the system with inhibitor/anti-
freeze.
– When an appliance is installed onto a new system then it is
possible to use the appliance whilst cleaning the system. A system
filter must be fitted to the return to reduce the risk of
contaminants entering the appliance.
▶ Follow the guidance of BS7593.
4.8.1
Flushing the system
Flushing the system using existing appliance/circulating pump or new
appliance.
Existing appliances/circulation pumps
▶ An existing appliance/circulating pump should be used to flush the
system before the new appliance is installed.
New appliances
▶ A new appliance must only be used to assist the cleaning of a newly
installed system.
▶ Fill the system with cold water and check for leaks.
▶ Open all drain cocks and drain the system.
▶ Close drain cocks and add a suitable flushing agent at the correct
strength for the system conditions in accordance with the
manufacturer's instructions.
▶ Mark the position of the lockshield valves and open them fully.
▶ Fully open all TRVs.
▶ Circulate the flushing agent before the appliance is fired up.
▶ Run the appliance/system at normal operating temperature as
directed by the manufacturer of the flushing agent.
▶ Drain and thoroughly flush the system to remove the flushing agent
and debris. Do not drain the system through the PRV of new
appliances.
Pre plumbed cylinder – 6 721 852 928 (2023/04)
Resistance created by
each elbow or bend
0.8 m
1.0 m
1.4 m
1.0 m
1.4 m
1.7 m
1.4 m
1.7 m
2.3 m
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