On Sat, 25 Sep 2004 06:41:12 GMT, Peter wrote:

On Thu, 23 Sep 2004 13:45:07 GMT, Phil Addison
wrote:

I'm being dim, but I can't see the point of balancing radiators that
are fitted with TRVs?

Sure, you can run the whole system flat out with the TRVs open, but
surely as soon as they cut in the system will go out of balance and
undo all ones hard work?

Balance is the word causing trouble here and it may not be the best
choice, but we are stuck with it.

The objective of balancing is to restrict flow in the radiator pipework
such that no radiator takes an excessive flow. If that was not done, the
radiators on more restricted flows would be starved and not get properly
hot.

If the system is correctly designed the pump will be just powerful
enough for the most restricted radiator (usually the one on the longest
run) to get sufficient water to fully heat it when its valves are fully
open. However, if a radiator closer to the boiler has its valves fully
open, it will allow loads of water through itself, bypassing the more
restricted distant one. The effect of this extra flow is to reduce the
pump pressure that is available to push the water to the furthest
radiator, which will then fail to heat properly.

The solution is to restrict the nearer one, and also the intermediate
ones, so that they only takes the water needed. That way a high pump
pressure is maintained and each radiator gets only its proper share, and
the system is said to be balanced.

You ask what happens if a TRV now starts to close down? The answer is
that the TRV radiator just takes less water. Because the pump pressure
is nearly equally applied to all radiators, the others are unaffected.
Actually the pump pressure will rise slightly due to the reduced total
flow, and this will push a very small extra amount equally through the
remaining radiators.

Another way of looking at this is to consider what happens when one
radiator TRV closes down. Once it is closed completely the system just
becomes a system with one less radiator, the remaining ones still being
in balance with each other.

As you extend this process to all the TRVs closing down we find that the
water flow through the radiator circuits is much (or completely)
reduced. This will cause the pump pressure to rise to its maximum value,
and it is this higher pressure that can open an automatic bypass valve
(if fitted) to maintain a minimal flow through the boiler.

Phil
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