To ensure an even distribution of the hot water to the radiators
in a system you’ll need to do what’s known as balancing the
system. Because the water is pumped via a pipe, and branches to
feed each radiator, the ones at the beginning of the ‘run’ tend
to get more than their fair share.
The way to do this is by using the lock shield valve on each to
regulate the flow of water to the radiator. By partial closing
of these valves the radiators nearest the pump can be restricted
more than those further away.
best way to get this right is by using radiator thermometers,
which can simply be clipped to the flow and return pipe at
either end, and measuring the difference. You won't often see a
plumber using these though - experience enables them to adjust
the valves without measuring the temperature.
Remember, the pipe to each radiator is a branch from the main
flow pipe. Near the furthest radiator, the main flow pipe turns
back towards the boiler and becomes the return pipe. The pipe
from each radiator then feeds back into this.
water does not go into the first radiator then out and on to the
next etc. It actually flows from one main pipe feeding all of
them, and then flows back out into the return coming back from
all of them.
the heating off well in advance of wishing to balance the
system, so that the water has a chance to cool right down. Each
radiator has a control valve – the one you use to turn it on or
off, and a lockshield valve – the one with a cover which you
don’t normally turn. To open or close this lockshield valve,
you need to remove the cover and use a pair of pliers. Open both
valves on all the radiators.
turn the heating back on and go to the first radiator. You are
going to adjust them in the order in which they are served, so
if you don’t know this, make a note of the order in which they
the radiator thermometers to the flow and return. Turn off the
lockshield valve then open it gradually again until the
difference between the two thermometers is about 50 degrees
on to the second radiator on the system and do the same. If you
repeat this for all of them in order, you should have a
perfectly balanced system so that all radiators heat up
efficiently. You may find the last one will need the lockshield
valve fully open.
A new system should always be balanced. The aim being to achieve
a 11% drop between radiator flow and return pipes.
On a normal fully
pumped two pipe system each radiator has two valves called lock
shield valves. These are used to open and close the flow of
water through the radiator. This means that the more the valves
are closed the greater the cooling off factor. To reduce the
water temperature at the outlet pipe, the lock shield is closed;
to increase the temperature the valve is opened up.
You will need 2
clamp-on thermometers which have springs which hook around the
inlet and outlet pipes of a radiator to clamp them on (they can
be purchased from plumbers merchants or d.i.y stores).
Ensure all timers
and room stats, trv's,
(thermostatic radiator valves) are all on so the system will not
switch off during balancing period.
Attach the clamp-on
thermometers to the boiler flow and return pipes, both
equidistant from the boiler and far enough away that the radiant
heat of the boiler does not upset the thermometers.
Open all control
valves and all lock shield valves.
Balancing is done
using a differential setting: that is, the absolute temperature
is unimportant; what you are setting is the difference between
the flow and return. The usual setting is 11 degrees C (approx
20 degrees F).
Allow the house to
get to its typical room temp, then adjust the flow rate by
altering the circulating pump speed until you get this 11
degrees difference between flow and return. If the pump is not
powerful enough, or in the case of a variable head pump it is
set at too low a speed, even with all the lock shields fully
open there will still not be enough flow to keep the temperature
gradient shallow enough. With the speed on maximum, if the drop
is greater than 11C, the pump will have to be replaced with a
Remember to allow
sufficient stabilizing time after each adjustment.
Once this is
correct, you then measure the temperature drop across each
radiator. You will find that one has a much higher drop across
it than the rest, and this is known as the index radiator (or
index loop, for a single pipe system). This will nearly always
be the radiator furthest away from the pump, and its lock shield
valve should be left fully open.
The other radiators
should now be adjusted to achieve 11 degrees drop across each,
if possible working backwards towards the boiler. This will take
time and patience, because of the stabilizing period (at least
15 minutes) between adjustment and measurement, and because the
adjustments will be interdependent. But you should soon get
quite good at estimating the effect of a certain degree of
adjustment. Most lock shield valves need to be opened and closed
about 50% from current position to have any effect on the
When the radiator
temperature differences are getting somewhere near the correct
values, and before any final fine adjustments, the temperature
drop across the index radiator should be considered. If this is
considerably different from the design figure ( 11 deg. C ) then
the pump needs to be adjusted. If the temperature drop is
greater than the design figure the pump setting needs to be
increased in speed, and the converse for too little a
temperature drop. If it is not possible to decrease the pump
speed any lower, its isolating valves can be used to restrict
the flow further.
Having done this the
lock shields should then have their final fine adjustments.