"Phil Addison" wrote in message
...

Mr Drivel...

That's me.

Whilst I might agree that Ed's FAQ does
not cover the whole range of
options it IS a very good introduction.

It is 20 years out of date, which I have told him. He asked for me input, I
have given it. He has basically rejected the lot. The FAQ as it stands it
pretty useless for a novice to grasp in parts as he is using catch all
phrases which are meaningless to a novice.

Things have moved on in 20 years - the introduction of the plate heat
exchanger to domestic appliances as led to great leaps in instant hot water
in combis and heat banks. Most boilers sold now are combis - a FACT. About
1 million boiers are sold each year with combis being 60 to 70% of sales.
The "traditional" boiler now is the combi. Mains pressure hot water is the
prime way ahead, if the mains pressure and flow is good enough a mains
pressure system is the first choice, whether combi (of various types), heat
bank, thermal store or unvented cylinder. Combis (the floor mounted jobs)
can even supply up to 3 baths. Instant hot water doesn't cover all as it is
mains pressure and flow dependent, so the older methods of stored water have
to be covered too. That is the way it is. That is way it is going. An up
to date FAQ must cover the choices merely stating what is available. It
doesn't it whitters on about yesterdays technology and methods and
approaches.

I have highlighted some relevant innovations that are highly useful. Ed, no
doubt has never seen an Rinnai or Andrews multi-point, so he dismisses them
out of hand - not down the local merchants guv so can't be right. These
high flow Japanese appliances are a God send to some homes being fitted
outside if you want and doing two bathroom too - superb for increasingly
popular multi-jet showers (even B&Q sell them). Stay in a multi-jet for not
too long and you will run out of hot water on a normal cylinder, a Rinnai
will supply the needs, and cheaper than massive space taking unvented
cylinders with their £100 annual service charge - which Ed thinks is not
relevant, yes. That is just two innovations that have come to the UK
recently, which the FAQ does not even mention.

For dismissing the combi myths and giving the levels available look at
below. I have periodically posted a comprehensive post on combis on the
group, that could go in the FAQ. Here it is:
______________
Here is a run down on combi's:

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. The case could be small and wall mounted or
large and floor mounted, but a "one-box" solution. 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.

Combis are rated by DHW "flowrate". The CH side is generally large enough
to heat a 5 bedroomed house.

Types of combi:

1) The Infinitely Continuous Combi -

Generally gives low flowrates with exceptions giving high flows. The low
flowrate models fill bath slowly but excellent for showers. 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. Typical flow rates are 11 to 12 litres per
minute, with some 40kW versions giving around 15 litres per minute. The
largest flow rate instant combi generally abailable in the UK is a two
bathroom model, 22 litres/min ECO-Hometec. Being a condenser it is very
economical too. http://www.eco-hometec.co.uk The largest flow rate model
is the special order wall mounted MAN Micromat at 76kW which requires a
commercial sized gas meter.

Some models may have a small 10 to 15 litres store of water to prevent the
"combi lag" at the taps. Water is delivered to the taps at the same rate as
an cyldiner system. Other models keep the internal plate heat exchanger hot
to eliminate the "combi lag".

2) Unvented Cylinder Combi -

Gives High flowrates. 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, wall mounted: Ariston
Genus 27 Plus, Glow Worm, Alpha CD50. Floor mounted: Powermax, Ideal Istor,
ACV HeatMaster (unvented cylinder/thermal store hybrid), Geminox.

3) Infinitely Continuous/Unvented cylinder combi -

Gives High flowrates. A wall mounted example being the Alpha CD50, a
combination of both having a two stage flowrate, of a high flowrate of 18 to
19 litres per minute when using the stored water with an automatic flow
regulator switching in to reduce flow to an infinitely continuous flowrate
of approx 13-14 litres/min when the stored hot water is exhausted.
http://www.alpha-boilers.com/products/CD50.html

Most models on sale now tend to have this two stage function.

4) Heat Bank Combi -

Gives High flowrates. Incoming water is instantly heated running through a
plate heat exchanger that takes its heat from a "domestic hot water only"
store of water at approx 80C. This store of water is "primary" water, water
that stays inside the system at all times - the water is used to store heat.
A fast acting thermostat ensures the boiler pumps all of its heat into the
heat 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 burner can
produce, which is approx 11-12 litre/minute. e.g. All floor standing
models: Vokera & Worcester-Bosch (both standard washing machine sizes),
Viessmann 333,

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 as the
plates flex.

5) Combined Primary Storage Unit

Gives High flowrates. 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 3 bathroom models. Gledhill do an excellent condensing version,
the Gulfstream 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. Most models have two stage
flowrates so hot water never runs out. As hot water is being drawn off the
high rated 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 No. 4) above are two stage flowrate models (high and low
flowrates) and will also not run out of hot water.

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