"VisionSet" wrote in message
...
I'm about to start the last major upheaval in my house.
Replaceing the ancient back-boiler.
The whole project may involve most of these:

Move or remove Water tank. I don't want it where it is it is in the way of
a
subsequent minor loft conversion/over stairs mezzanine.

Move or remove water cylinder from landing to a cupboard in 3rd bedroom or
loft. Mainly for the same reason as above.

Install new boiler in 1/ under stairs, 2/ in loft 3/ somewhere else.
Though
exhaust of 1/ may interfere with subsequent garage updgrade at future
date.

Finally new gas fire in lounge to replace back boiler.

These are the major considerations, how else do I choose Combi boiler or
stored water?

Current shower is balanced gravity fed which is newish in a new bathroom
that I don't want to upset - though I would like to improve shower
performance possibly in conjunction with raising water tank since I need
to
move/replace it. Alternatively a combi boiler would give the needed extra
pressure or I could add a pump?

As you can see I have so many variables it is hard to pin any of them
down?
Where do I start?

Your requirements point to a mains water fed system with no tanks. Check
the mains pressure/flow. The Powermax may be suitable, with all being in
one box.

Some info:

A combi explanation for you....

Firstly, a combi is a "combination" of the heating and water system in one
case, eliminating external tanks and cylinders, and generally supply hot
water at high main pressure. To confuse a little, some can run at very low
pressures and even off tanks. Generally most are fed from the mains. It is
generally a matter of mounting the boiler and connect up the pipes. The
expert designers have done the hard work for you and put all in one case.

There are three types of combi:

1) The Infinitely Continuous Combi -

Heats cold mains water instantly as it runs through the combi. It never runs
out of hot water. This is the most common type of combi, generally having
lower flowrates than Nos 2 & 3 below. The largest flow rate instant combi is
a two bathroom model, 22 litres/min ECO-Hometec. Being a condenser it is
very economical too. http://www.eco-hometec.co.uk

2) Unvented Cylinder Combi -

An unvented cylinder is a similar to a conventional cylinder but run off the
high-pressure cold mains. A combi with an integral unvented cylinder has
approx a 60 litre cylinder heated to approx 80C, with a quick recovery coil
that takes all the boilers output. A fast acting cylinder thermostat ensures
the boiler pumps heat into the cylinder ASAP with a recovery rate from cold
around 5-8 mins (Ariston claim 8 mins). The 80C water is blended down to
about 45-50C. e.g's, Ariston Genus 27 Plus, Glow Worm, Powermax, Alpha
CB50.

3) Invinately Continuous/Unvented cylinder combi -

The Alpha CB50 is a combination of both having atwo stage flowrate, of high
flowrate when using the stored water with an automatic flow regulator
switching in to reduce flow to an invinately continuous flowrate of approx
11 litres/min.
http://www.alpha-boilers.com/products/CB50.html

4) Heat Bank Combi -

Incoming water is instantly heated running through a plate heat exchanger
(as is most instantaneous combi's) that takes its heat from a "domestic hot
water only" store of water at approx 80C (instantaneous combi's take the
heat from a heat-exchanger heater via the burner). A fast acting thermostat
ensures the boiler pumps all of its heat into the store ASAP with a recovery
rate about 5-8 mins from cold. The 80C water is blended down to about
45-50C. They are generally two stage flow rates, in that when the thermal
store is exhausted it reverts to what the bunrer can produce, which is
approx 11-12 litre/minute. e.g. Vokera & Worcester floor standing models
(standard washing machine sizes).

N.B. The heat bank is a variation of a thermal store, but is "not" a thermal
store in the conventional sense in that a coil carrying cold mains water
runs though a store of hot water kept at about 80C. Heat-banks are far more
efficient and give higher flowrates than conventional coiled thermal stores.
The stainless steel plate heat-exchangers do not scale up so easily.

5) Combined Primary Storage Unit
(Not classed as a combi, but a derivative of a combi, but still a one box
solution, so still in the same family)

These are a combination of a large thermal store, or heat bank, and boiler
in one casing. The units are large (larger than standard washing machine
size) and floor mounted. The heating is taken off the thermal store, which
in many cases the DHW taken off the store using a plate heat-exchanger
(heat-bank). Unlike the Heat-bank in 3) above the thermal store supplies
heating "and" DHW, giving the "combined" to the title. They are available
from 1 to 2.5 bathroom models. Gledhill do an excellent condensing
version, the Gulfsream 2000. http://www.gledhill.net

Nos. 2) 3) 4) & 5) have high flowrates. No. 1 "generally" has low flowrates
but there are always exceptions and some can be high - e.g. the ECO-Hometec
infinitely continuous combi, actually has a very high flowrate. Nos 2), 3)
4) & 5) use stored water, but in different ways. Unlike No. 1 "some"
versions will eventually run cold, but that takes quite a time, hence some
are referred to as "two bathroom" models, having the ability to fill two
baths with very fast recovery rates. As hot water is being drawn off the
high rating burner is also reheating. Very rare do these combi's run out of
hot water in average use. When taking one shower the burner may be
re-heating faster than what can be drawn-off. No. 3) above uses stored
water but will not run out of hot water (high and low flowrates). Most
versions of N. 4) above are two stage flowrate models (high and low
flowrates) and will also not run out of hot water.

There are combi models that give hot water and heating simultaneously as
Combined Primary Storage Units do. Most don't as they are hot water
priority.