Extract from 'Central Heating - Fault Finding & Repair'
Introduction
The position of a circulating pump is extremely important
and if not correct can cause major problems with
both the pump and the heating systems. The pump must
be located correctly within the system and must also
be correctly sited at that position.
Water Pressure
In many central heating and hot water systems the pump
will be contained within the boiler housing as an integral
part of the boiler. In systems where the pump is installed
in the pipe work it is critical that this location is
correct for that system.
In a typical open vented system that uses a feed and
expansion cistern, a cold feed and expansion pipe and
an open safety vent pipe there will be a pressure created
by the column of water in the feed and expansion cistern
and pipe work. This pressure is referred to as the ‘static
head’ and is usually measured in metres. It is
the vertical distance between the level of water in the
cistern down to a given point in the system. Where the
cold feed pipe connects into the system pipe work is
the ‘neutral
point.’ In systems where the cold feed and vent
connect at the boiler then the boiler is the ‘neutral
point.’
When the circulating pump is running it will circulate
water around the system. The static pressure should not
change but pressure changes will occur within the system
pipe work. At the pump outlet will be positive pressure
and at its inlet will be negative pressure (suction).
Therefore, somewhere in the system there must be a point
when the positive pressure created by the circulating
pump changes to negative pressure. This point is the
neutral point. It will always be at the point where the
pressure cannot physically change. In open vented systems
it is where the cold feed connects and in a sealed system
it is where the expansion vessel is connected into the
circuit pipe work. The pressure at the neutral point
is either the static head generated by the column of
water in the feed and expansion cistern and pipe work
or the charge pressure in the expansion vessel.
From the circulating pump to the neutral point the system
operates at a pressure greater (positive) than the static
pressure at the neutral point. From the neutral point
to the circulating pump inlet the system operates at
a pressure less (negative-suction) than the static pressure.
So, locating the circulating pump in the flow pipe will
have a different effect than locating it in the return.
Different problems can arise depending on system design
such as:
a) ‘Micro air leaks’ due to excessive negative
pressure drawing air in through joints and fittings
b) Air ingress via the open safety vent pipe due to excessive
negative pressure
c) Pump over through the open safety vent pipe due to
excessive positive pressure
Circulating Pump Locations
Position 1
The circulating pump in the return pipe before the cold
feed with the open safety vent in the flow (many older
installations were designed this way), a modern replacement
circulating pump may cause an increase in negative pressure.
This may draw air into the system through micro-leaks
in joints and fittings. This air will cause damage and
corrosion to the system and its components.
 Position 2
The circulating pump in the return pipe after the cold
feed with the open safety vent in the flow. This may
lead to ‘pump
over’ unless the vent pipe has a minimum height
of 1.33 x pump head. So a circulating pump head of 5
metres needs a vent pipe of at least 6.67 metres to be
sure of avoiding this occurrence.
When ‘pump over’ occurs water is forced through
the open safety vent pipe, due to the extra pressure
created, into the feed and expansion cistern. This causes
greatly increased oxygenation of the system water and
rapidly leads to severe corrosion (the water is coloured
rust red), system breakdown, possible water damage from
overflow and increased scaling.
 Position
3
The circulating pump in the flow before the open
safety vent pipe with the feed and expansion pipe
in the return. Again, increased pressure may cause ‘pump
over.’
 Position 4
The circulating pump in the flow after the open safety
vent pipe with the feed and expansion pipe in the return.
If the open vent pipe is high enough (1.33 x pump head)
this pump location should work without problems developing.
However, if the open vent pipe is not high enough see-sawing
may occur.
 Position 5
The circulating pump in the flow after the open vent
pipe with the cold feed and expansion pipe also in
the flow and before the vent pipe (but within 150mm).
This is the best location and pipe layout and it is
worth changing positions 1 to 4 to position 5 for increased
system performance and maximum components working life.
Position 6
Where the boiler manufacturer permits (or recommends)
a close coupled vent and feed arrangement. This is
the best layout for modern automatic boilers. In this
case the open safety vent pipe is in the flow before
the circulating pump with the cold feed and expansion
pipe between it and the pump. The gap between the vent
and feed must be no more than 150mm (see Domestic Central
Heating – 1.8). Again, always follow the boiler
manufacturers recommendations.
In sealed systems the
expansion vessel connection (neutral point) should
be as close to the pump inlet as
possible between the pump and the boiler in the flow.
This will place most of the system under positive pressureas
in position 5 above. Always follow the manufacturers
recommendations.
Circulating Pump Siting
Avoid siting the circulating pump:
a) At the lowest point in the system where sediment
tends to accumulate. This siting may quickly lead to
circulating pump failure and unless the pump is re-sited
will continue to be a source of breakdown
b) Where it touches any flammable surface or material
- allow a 25mm gap
c) Where any slight vibration could cause noise
d) Close to sharp bends in the pipe work which may
cause flow noise or pipe work vibration
e) In pipe work that is under tension as the tension
can create noise when the circulating pump is running
Ensure the circulating pump is sited and positioned
so that:
a) The terminal box is easily accessible for maintenance
b) The terminal box is not under pipe work where a
leak may occur
c) The connecting pipe work is well supported with
fixing brackets. This will avoid noise and possible
union joint leaks
d) Connecting pipe work has a cushioning material (e.g.
small squares of felt) between it and any structure
it may be touching (e.g. beams, joists etc)
Extract from 'Central Heating - Fault Finding & Repair'
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