On Fri, 29 Aug 2003 11:15:36 +0100, "David W.E. Roberts"
wrote:

Hi,

just catching up on the NG after a few days away.

Currently there seem to be several threads about problems providing hot
water at a decent flow rate with combi boilers.

Now I can see that with a good high water pressure and flow rate, and a
limited number of outlets, the combi boiler allows you to install a domestic
hot water system with a much reduced component count, saves space previously
used by header tanks and hot water cylinders, and can give you a nice hot
shower without needing a pump.

True, but there is nothing magically new - it is really the 21st
century version of the Ascot. The principle, benefits and
limitations are the same.


However with low water pressure and/or a large number of outlets (and
according to the TV shows a house is not a house without at least one
en-suite in addition to the family bathroom) the combi starts to struggle.

Fixes suggested include installing a header tank and pump for additional
cold water pressure/flow and adding a hot water storage tank to provide
enough hot water at peak times.

Once you start down that route I must ask "Why was a combi fitted in the
first place"?

Seems to me that at the moment the automatic design for a new system is to
use a combi boiler.

Surely this is best for flats and small houses, and any reasonable sized
house should be designed with hot and cold water storage because the peak
demands cannot be supplied by heating water 'straight from the main'.

Or am I missing something :-)
[Hmm...cheaper option for the developer?]

I think that that is one point, as is the ability to use less skilled
labour because the pipework is simplified to an extent.

Provided the mains water and gas supplies are adequate (which should
be possible on a new build and the boiler is specified properly, there
should be no problem, although as you've spotted the amount of heat
required to raise the temperature of a flow of water 'X' by 'Y'
degrees can exceed the ability of a domestic gas supply if X and Y are
high and within the requirements of a medium to large house.

The developer can claim to have provided the hot water service in the
same way that they can claim to have fitted enough power sockets.
They are there but may well not be adequate for the purposes of the
occupants.

The more problematic area is with replacements in houses already
there. In mine, which is only 18 years old, the water supply would
not be adequate for a worthwhile mains fed HW system without upgrading
the communication pipe from the main - a cost of about £5k.
People are sold mains systems, and especially combis on the argument
that they can have high pressure and flow showers etc. etc. without
the obvious service checks being made. Then they find that the
results are not as good as they had with their storage system.






Cheers
Dave R

[Also noted comments about 'high output' combis which need so much gas that
they need a larger gas pipe and can compromise pressure for other customers.
In this situation a slower heating coupled with storage of hot water seems a
more sensible option. Peak flow of gas and water must limit to the amount of
hot water you can supply through an 'instant heater'. Then again, how about
a pumped gas storage tank to supply extra gas at peak demand without
compromising the main supply - design for a mini-gasometer anyone?
Hmm...compressor fed from the mains to fill a high pressure tank with the
outlet pressure reduced back down to mains gas pressure but through a large
bore pipe?]

..andy

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