Central heating radiators
Radiators work by heating the air which
flows past them. Warm air rises from the radiators and
colder air in the room falls. This circulation develops a
flow of air around the room sending warm air from the
radiator and delivering cooler air back to be heated. So,
for radiators to work well there must be adequate clearance
around them so that air flow isn't restricted. Now you know
why radiators are mounted off the wall a little and above
the floor.
Radiators, as the term is normally used, are simple heat
exchangers which distribute the heat by natural air
circulation (hot air rises, so the heated air next to the
surface of a central heating radiator rises pulling cooler
air up from the floor level). They are simple (very little
can go wrong), easy to install and operate.
It is worth remembering that any room can have more than
a single central heating radiator, with rooms greater than 6 metres (18 ft) in any one direction, it is worth considering
distributing a number of radiators to minimise the thermal
gradient within the room.
Normally the manufacturer's data sheet will quote the
output for when there is a temperature difference of 56 °C
(100 °F) between the water in the radiator and the air in
the room. Where the temperature difference is not 56 °C, the
following correcting factors are necessary to determine the
actual anticipated output from the radiator.
Different manufacturers will quote
slightly different outputs for seemingly similar sized
central heating radiators, so where possible always use the
figures from the manufacturers data sheet.
Balancing central heating radiators
With a 'feed and return' central heating system, the
radiators near to the boiler/pump would tend to be warmer
than the radiators further away. To avoid this, the outlet
of each radiator is fitted with a 'lockshield valve' (shown
right) which needs to be adjusted when the system is first
installed. The 'lockshield valve' is normally covered by a
push on cover to hide the adjustment.
The intention is to even out the flow of water through
each radiator so that with the system in its normal
operating condition, the temperature drop across each
radiator is about 20°F (12°C). Problems with setting up the
system may be experienced if the outside temperature is
above the design operating temperature - if the outside
temperature is above the system design value, the heat
dissipated by each radiator will be less than intended by
the design, and the temperature drop across each radiator
will be less than 20°F. If balancing on a hot summers day,
adjust to achieve a lower temperature difference.
Once the central heating radiators in a system have been
balanced, the valves should not need to be adjusted again
unless the pipework or radiators are changed. The easiest
way for a diy'er to measure the temperature drop across the
radiators is to use a pair of radiator thermometers, they
can be purchased or better still hired or borrowed. These
thermometers just clip onto the pipework and indicate the
temperature of the pipe (which is, effectively, the
temperature of the water flowing within it). Most
professional plumbers do not use these thermometers, years
of experience has enabled them to balance a system just
using a hand to check the temperature differences.
