"Pete C" wrote in message
...
On Wed, 6 Oct 2004 00:40:02 +0100, "John Aston"
wrote:

Many thanks to Andy Hall and others for their help earlier this year on
the
design of my new plumbing system (http://tinyurl.com/3pwck etc.).
Unfortunately, thanks to a few months illness, I'm still at it - with a
few
issues to resolve.

A question regarding an unvented domestic hot water system: Cambridge
Water
tested my incoming main supply and informed me that the pressure = 2.8
bar
and the flow rate = 25 litres per minute. Is this sufficiently high for
an
unvented supply feeding a family home containing two power showers, a
bath,
five sinks, three toilets, a washing machine and a dishwasher?

Hi,

Before going for an expensive solution it may be worth checking that
they don't mean 25lpm _at_ 2.8 bar.

Also a garden tap is quite restrictive, a better measure would be to
open all direct mains taps at a time when the mains water pressure is
likely to be lowest.

[For information, the hot water will be heated indirectly in a 250 litre
tank by a condensing boiler. The mains supply pressure will be reduced
(to 2
bar?) by a water softener, although a "high flow" softener, such as the
Kinetico 2020c HF, will be fitted to minimise this problem.]

Presumably the central heating could operate as a sealed system, even if
the
hot water tank is vented?

One way round could be to use the hot tank as a heat bank with a
tanked supply for the bath and direct mains for the showers, both
heated via separate plate exchangers.

The initial problem is that he "thinks" his mains water supply can't handle
demand. A "full" mains pressure system appears out of the question. He
needs to assess the proper flowrate and pressure of his supply. He may be
OK.

If the mains is fine then a heat bank would be fine. One plate should be
fine with one bath and 2 showers.

If the mains is not that good then an element of stored water is required.
He could use a cold tank/cylinder setup heated to 70-80C with the draw-off
blended down, via a blending valve, to 50-55C. A bronze pump could pump
water out of the cylinder into a plate heat exchanger which will provide
mains fed showers.

I did that for a neighbour, who wanted a power shower. It was about the same
price to buy the plate and pump, etc, at the time.

In this case, if the mains can't cope, I would most likely go for a:

- combination cylinder for all low pressure hot water
- high flow combi for mains showers.

Divide and rule. A combination cylinder is about the same size as an
unvented cylinder but on low pressure, so no BBA approved fitter or large
overflows, pressure safety devices, etc.