Thanks for that - much clearer now. I can see that this is going to be my
next obsession, now that I've got over my last one about oiling Ikea wooden
kitchen worktops...

Jonathan



"Andy Hall" wrote in message
...
On Mon, 11 Aug 2003 17:04:21 +0100, "Jonathan"
wrote:

Hi All,

Just reading the FAQ on central heating at
http://www.cucumber.demon.co.uk/diy_test/Q2.12.html

It says "After installation and commissioning, a new central heating
system
should be balanced." And says: The "...aim of balancing is to achieve
the
correct temperature drop across the boiler flow & return pipes (11°C)."

I assume that in layman's terms this means, "... so that the water that
goes
back to the boiler is cold enough to need re-heating, yet warm enough to
have delivered sufficient heat to the last radiator in the chain? Is that
right?

Sorry if this is a stupid question...

Jonathan



It's that and also to achieve the correct flow through each radiator.
When a system is designed, the radiators are sized to compensate for
the heat loss through the surfaces and to changes of air. The
radiator output is specified for a certain temperature drop across the
radiator and this will imply a certain flow rate (dependent on the
size of the radiator).

The plumbing is normally also designed to provide enough flow rate to
all radiators, but because of different pipe lengths and sizes there
will naturally be incorrect flow rates on some radiators.

In modern systems, the radiators are not in a "chain" as such but
connected from the boiler flow pipe to the return. If you compare
with an electrical circuit, this is a parallel connection not a series
one.

If you consider a small radiator quite close to the boiler and a large
one much further away, naturally speaking the flow would tend to go
through the nearer one because it is the line of least resistance.

Balancing basically entails reducing the flow through these lower
resistance radiators so that more water flows through the larger or
higher resistance ones. This is an iterative process to some extent
because adjusting each radiator, affects the others to a degree.

The methods in the FAQ are how to do it properly with a thermometer,
so that you get the correct flows everywhere. It is time consuming,
but worth doing.





.andy

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