"Capitol" wrote in message
...
The smallest of comi's will do the CH
on a "very" large house. The
boiler sizing is not a problem if it is not a big house.
Sadly not true, as our friend has found out the hard way!
Please read again. The smallest combi is
approx 80,000 Btu/hr, which is big
enough for a largish 4/5 bedroomed house.
Most houses in the Uk are a lot smaller than that.
The average sized combi is approx 95,000 btu/hr.
They must have a large house or one with no
insulation.
It's taken me some time to check the figures quoted
above. The range of common condensing combi
boilers is 75Kbtu/hr to 156Kbtu/hr. The central
heating output of these units however varies from
53Kbtu/hr to 156Kbtu/hr.
The CH output is generally lower than the hot water output to reduce boiler
cycling as in 90% plus of cases the boiler, for CH purpose, is too large.
In most cases the CHG output can be raised to hot water output. Very few
combi's fitted are 53,000 btu/hr. The average is way above that. Go to B&Q
and you will see a 80-85,000 btu/hr off-the-shelf BIASI combi, for £400.
Small units cannot be relied upon
to cope with a 4 bedroomed house in
winter.
The super small ones of 53,000 btus, no. But I have never actually come
across one of these and they are not the normal units you will see on sale.
An average run of the mill combi that is fitted in most homes? It will cope
with power to spare.
The figures are from the architectural digest,
so I have no reason to doubt them.
Neither have I, but they are so vague as to be useless giving only the highs
and the lows The average combi fitted in UK homes will cope with a 4 bed
house.
The house is modern with insulation to
the relevant standard.
How modern? Insulation regs have risen recently and progressively risen
since the regs first insisted on insulation in 1974. Many people would
consider an insulationless 1973 house modern. What is the CH rating of the
house in btu/hr?
Looking at the heat exchanger materials,
with some manufacturers using
copper and aluminium, I'd be very surprised
if the average life expectancy
would ever exceed 10 years for these units.
On what experience/figurers do you base this assumption? A copper/alum'
heat exchanger will last 20 years plus if the installation is fitted right
and serviced right too. A boiler with the burner pressure far too high may
not last that long (poor servicing). Modulating burners have helped improve
longevity. Electronic controls on some boilers (combi and system) will lock
out if the burner pressures are way out. BTW, in most cases the same heat
exchangers are in the system boilers. Makers use the same exchangers and add
a water section to make a combi. Many condensing boilers have the same
exchangers as combi's and system boilers and add a smaller condensing
exchanger. They keep their production lines and component counts down.
I guess that is why better
manufacturers use stainless steel.
Stainless is becoming more common and 5 year guarantees are given, even with
the Hepworth cheapie from Travis Perkins. Yet, the reason why many combi's
are scrapped is not that the heat exchanger has burnt out. It is because of
other factors, the biggest is ignorance of the combi operation by
"plumbers". In many cases it is that the repair cost is not far off a new
combi as they have tumbled in price, with many being below the price of many
system boilers. Also a new combi will invariably have a better flow rate, be
physically smaller and probably a faster response rate with pre-heat
exchangers or built-in water store vessels.
The figures for water delivery at 35C
rise are in the region of 10L/min
for most of the units. 17 minutes to
fill the bath!
And your point? 10l/min is the rate for the cheap end of combi's. Flowrates
far in excess of this are available, even up to two bathroom jobs (well 1.5
and 2 with a push). A Worcester HighFlow will fill bah in 5 mins, as will
many other high flow combi's. 17 minutes to fill a bath? Even if the temp
coming out is at bath temp of 45C that is 10 minutes fillup, as 100 litres
is the average bath capacity. After all you are on about "most" here, and
most baths will be 100 litres. If the combi is delivering hot water at
above 45C then 7 to 8 mins fill up time. Then get a 18-19 litres/min combi
and the fill up time is 4-5 mins. Wickes even sell a 13 L/min
off-the-shelf combi for £525. At these prices you can afford to buy two
combi's and have one do upstairs and one down for natural zoning and split
the bathrooms. Highly cost effective.
And the pumpless high pressure showers in all combi's are for free.
Don't go on old wives tales and ignorant "plumbers", who are very good at
drains and gutters. Go to heating engineers.
Many combi's are designed for ease of design and installation, true DIY.
For e.g., the Ariston Microgenus even has a built-in filing loop and
pressure differential valve, among other failsafe goodies. These are
combi's are designed for people with a limited knowledge of heating and are
the models DIYers should aim for.
I still think that an ion exchange water
softener is essential in hard
water areas.
Not so. A phosphor descaler will do. They are very effective in eliminating
scale, or at least 98% of it. Any scale left around is easily removed by
normal washing.
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