Extract from 'Central Heating - Fault Finding & Repair'
Introduction
Most modern circulating pumps designed and manufactured
for domestic central heating and hot water systems
are suitable for either indirect open-vented or sealed
type installations. There are many types of pumps with
even more applications but in the domestic central heating
environment there are generally just three types used.
 Fixed Head Circulator
This is a circulating pump which is designed and installed
to operate an often generalised type of central
heating and hot water system. It operates at one fixed
speed with no adjustment available. Its operating
tolerances against head (pressure) is quite wide so the
addition of an extra radiator should not cause too much
of a problem.
However, an extension to a property may increase the
pressure beyond its design rating. This may result in
slow heat up and possibly pump failure. System noise
can also be caused by one speed pumps.
Three Speed Fixed Head Circulator
The most common (currently) circulating pump is the three
speed. Still with a fixed head it has a speed control
for energy efficiency and more precise system matching
with the consequent reduction in system noise.
Most domestic central heating boilers will not exceed
120,000 btu’s/hr (about 35 Kw) and therefore a
standard 3 speed circulating pump can be used. In large
domestic properties and central heating systems using
micro-bore pipe work a circulating pump with a higher
head should be used.
These circulating pumps tend to be set at the highest
speed unless ‘pumping over’ or noise is a
problem.
Automatic Variable Speed Circulators
This type of circulating pump will automatically adjust
its speed to meet the pressure fluctuations caused by
differing heating demands. So as thermostatic radiator
valves close in response to room temperatures the pump
speed is automatically adjusted and reduces the differential
pressure as the resistance of the system rises. This
saves energy (electricity) and reduces the possibility
of radiator valve noise.
It should be noted that circulating pumps which have
an automatic speed control should not be used in central
heating systems with thermostatic radiator valves fitted
on every radiator unless the pump and system design and
initial set up at installation ensures that the minimum
flow rate through the boiler is maintained (see boiler
manufacturers instructions). This is especially important
as should all the thermostatic radiator valves close
the pump will slow and the flow of water over the heat
exchanger may not be sufficient to remove the latent
heat. This can cause damage to the boiler heat exchanger
or may cause noise in the boiler (kettling).
Automatic variable speed circulators must not be fitted
to combination or condensing combination boilers. There
are now several boilers available (fully modulating condensing
boilers) that have these pumps pre-fitted by the manufacturer
within the boiler casing. If this is the case do not
fit an automatic by-pass. However...“Always follow
the manufacturers instructions.”
 Automatic Variable and Fixed Speed Circulators
This design of pump is suitable for all domestic central
heating systems. It can replace most other pumps, it
is easy to install and is suitable for one and two pipe
radiator systems and under floor heating. It can be used
with fixed or automatic by-pass valves.
Electrical Connections
There is no need to open the terminal
box as they are supplied with a plug, which once wired
up, attaches to the pump. Fault finding is made easier
with an indicator light showing correct supply and wiring.
Automatic De-blocking
A special start up sequence of starts and stops dislodges
contaminants and particles from within the pump
eliminating pump jamming.
Energy Efficiency
The pump automatically provides correct performance
matching its speed to the varying heat requirements
ofthe central heating system. This reduces running
costs by up to 39%.
Speed
This circulating pump is able to operate at both automatic
or fixed speed. Its operation is similar to the automatic
variable speed circulators (see above) i.e. as the
demand for heat decreases it automatically adjusts
to the actual speed required, regulating the flow and
pressure throughout the system. This reduces
power consumption, noise and wear and tear on the pump
and system.
The pump adjusts itself to the thermostatic radiator
valves required water output eliminating radiator valve
noise. The fixed speed settings can be used to:
a) Help purge the central heating system of air when
set on the highest fixed speed
b) Set up any fixed (manual) or automatic by-pass values
when set on the lowest fixed speed
Operating Temperatures
It is especially important to ensure these automatic
and fixed speed pumps are not covered with insulation/linen
etc as they have a maximum ambient temperature of 35°C
at 90°C system water temperatures.
DO NOT use this type of pump with combination or condensing
combination boilers always follow the
manufacturers instructions.
Twin-head Circulators
Interestingly, there are now domestic circulating pumps
with twin heads. They effectively replace the 3 port
midposition/3 port fully diverted/2 x 2 port zone
motorised valves (Y plan, W plan, and S plan respectively)
system type installation.
 The pump has a separate wiring centre which is connected
to the boiler and controls (programmer, room and cylinder
thermostats) and will simultaneously circulate water
around the central heating and primary hot water systems
or operate individually to meet heat demands. Each
pump head has individual speed settings which means
they can be set up to match the individual flow and
pressure requirements of the central heating and domestic
hot water system.
Extract from 'Central Heating - Fault Finding & Repair'
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