On Tue, 12 Aug 2003 23:51:40 +0100, "IMM" wrote:



Install a micro-bore system with manifolds. They are self balancing if
the
rads are sized reasonably correct. Then install an auto variable speed
Grundfos Alpha pump.

Best is a heat bank with the CH manifolds taken off the bottom of the
cylinder. The boiler will be self balancing and so will the rads. A win,
win situation all around in DHW response, commissioning and simplicity.

Just to add:

Plumbing Mechanical Service Book 2, a NVQ plumbing book. This is not a DIY
book so please don't buy.

The recommendation not to buy is a reasonable one, but not because it
is an NVQ book but because the information is obviously wrong. If it
is as incorrect in other areas as it is in this it could well be
dangerous.


It has a section on heating, and a sub section on
Mini-bore on page 170. It says:

"If the manifolds can be situated in such a way that the branch flow and
return to each radiator is approximately the same length, the frictional
resistance will also be approx the same, making the system self balancing".

That is incorrect, even if it could be done anyway. For this to work,
the radiators would also have to be identically sized.

For example, a living room might require a 3kW radiator, while a small
bedroom only 750W. This immediately implies that the living room
radiator requires four times the flow of water as the bedroom one.
If they are connected through equal lengths of pipe from the same
manifold, then the flow rates would be equal. That is not what is
required, and the system is not self balancing.




And on page 171:

"In a well-designed system balancing should not be necessary as the aim is
to keep all pipe runs to heat emitters at, or near, as possible, the same
lengths."

That is also clearly wrong because again it would only work for
radiators of the same size.



And on page 172, there is a cut-away of a twin entry rad valve without
lockshield adjustment because in properly designed mini-bore systems, which
is easy to do, balancing is not required.


It is easy to do, but that is not the way to do it. These twin entry
valves are not commonly found at heating suppliers or on their web
sites, presumably because they are not popular. If this is the basis
of their use, I can see why.


I have a system connected mainly with 8mm pipe to manifolds. There
were a couple of radiators in a living room which were over the limit
for the size of pipe. During my refurbishment to allow lower
temperature operation, these were replaced with larger capacity
radiators and replumbed in 15mm.

It is certainly true that microbore tube helps a bit with the
balancing in the sense that the sensitivity of the lockshield valve is
reduced, making it easier to adjust.

This is fairly obvious behaviour and analogous to an electrical
circuit with a fixed and variable resistor in series. The
adjustability of the current through the load is reduced as compared
with having a variable resistor only.

There are advantages of microbore, that is true, but self balancing is
not one of them.







---

..andy

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