On Fri, 18 Jul 2003 10:22:16 +0100, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Fri, 18 Jul 2003 00:30:58 +0100, "IMM" wrote:



Because you are suggesting that a single
box combi solution is the most appropriate
for a DIYer on the basis that it is apparently
simple.

You are getting there. It is simple. And..will
most likely fit the bill.

But doesn't answer the question which
was asked, which was how to go
about designing the complete system.

It does answer the Q. Most of the design is inside the box, done by
experts.

Sorry I don't buy that. From the high level functional point of
view, the combi is simply bringing into the box the pump and an
alternative means of providing the hot water. Granted, it does
reduce the component count vs. a cylinder system.

However, from a *design* perspective, there is little or nothing
involved in provisioning a vented cylinder, motorised valve(s) and
pump. One is going to choose a suitably sized cylinder for the
application, hopefully of a fast recovery type. The only design
decision there is to try to make sure that the boiler is adequately
sized to provide enough heat to recover the cylinder in a sensible
time. This will imply a certain flow rate and pipe size for the
primary connections to the cylinder - normally 22mm.

Motorised valves are completely standard, so no design decision there.

If the boiler is not a system one and does not have an internal pump
then in a proper design, one would normally work out the required flow
and head in order to select the right pump type and setting.
However, with electronically controlled pumps such as the Grundfos
Alpha this is largely taken care of anyway.


The only design is the rad sizing and the pipe run. Not
difficult.

No it isn't, but that is where most of the *design* work is - not in
the boiler.




That certainly is not a case of trotting
along to B&Q, buying a combi
boiler, a few radiators and plumbing bits
and sticking it all together
and hoping.

Not hoping. It is not far off what you say. Just a combi with the correct
flowrate and size the rads and buy them.

.... and checking the provisioning of services, adequacy of plumbing
and so on.


Radiator and pipework sizing need to
be done properly whichever
solution is chosen.

That is why I said get back to me.

For a combi boiler of sensible capacity
to be useful, it is often necessary to upgrade
the gas supply from the meter - this may be non
trivial.

One pipe run is trivial, when looking at the whole system.

That depends on where it has to run



The water main may need to be upgraded,
also non-trivial.

It may not need an upgrade.

Agreed. I am simply pointing out that it may do and that can be
difficult and expensive, which in turn may be a decision factor in
what to do. It is better to point this out up front so that it can
be checked, rather than glossing over and potentially having a nasty
surprise.



If there is existing internal pipework
for hot and cold water, it is
likely to need modification and re-routing
in order for the relative
flows to work reasonably well.
Flow restrictors may need to be
fitted, etc. etc.

It may only require a small modification. Flow restrictors on all taps are
easy and cheap to fit.

Fine, but it should all be taken into account.


A conventional boiler and system
may be more suitable

Maybe, but unlikely.


We have no data to say one way or the other.






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..andy

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