"Andy Hall" wrote in message
...
On Tue, 5 Aug 2003 22:10:48 +0100, "IMM" wrote:
"Andy Hall" wrote in message
.. .
Unvented cylinders are pressure
reduced to approx 3.5 bar for steel and
2 bar copper. A combi is not reduced.
A heat bank is a far better solution
than an unvented cylinder.
Thanks for the advice, but what's a heat bank ?
See:
http://www.heatweb.com
They will explain.
DPS make a lot of claims about the performance of their systems but
don't quote all the operating conditions. For any system which
instantly heats mains cold water, be it this or a combi, you need to
look very carefully at the small print. Combis are usually quoted
with a flow rate that they can produce based on a certain temperature
rise - normally 30 or 35 degrees. This means that if the water
temperature from the mains drops to 5-7 degrees as it can in winter,
the output temperature will only be at around 40 - i.e. about shower
temperature. So if you consider a 15 litre/minute combi, it may be
enough to run one decent shower (just), but it won't run two.
You can pre-heat mains water if running an UFH system. Run the mains
pipe
through the concrete slab. This will pre-heat water to approx 25C, so a
combi in winter will deliver 60C at say 15 litres/min. In summer the
cold
water mains pipe at approx 10C temp will cool the floor. Simple and easy
to
do and gains all around.
This is a nice theory, but I am not convinced that it would be as good
as you describe. You are talking about a 20 degree rise at 15
litres per minute. That needs just over half the energy that a combi
would require at similar volume (about 21kW, as I calculate it).
Getting that through a heat exchanger made of MDPE pipe when the heat
source (i.e. the floor) is not that much warmer is not very plausible.
It would need a very long length of pipe and that would add problems
with flow resistance.
MDPE? You can use two way santoprene pipe. This is 2 or 3 way counter feed
pipe. Mains water in one way hot water in the other. Running PEX side by
side is good enough. Many in the USA have done this and reported good
results.
The USA has different water regs to use. They use a hot water cylinder and
take off fresh water and run it through a non-ferrous UFH system. Many
companies run cold mains water through the same pipe loop by turning a
manual 3-way valve to cool the floor in summer.
I have not seen similar figures quoted for heat banks. That is not
to say that the technology is bad, but remember that the amount of
heat that you store is directly proportional to the volume of the
cylinder and the temperature. In a heatbank you normally run it at
76-80 degrees as compared with a conventional HW cylinder which is
normally run at 60 degrees - so you store about 25% more energy in the
same space. However, when that energy is used up, the performance
degenerates to that of a combi of equivalent power.
So you never run out of hot water.
Provided that the cylinder is large enough, which takes us back to the
point that it is only as good as the energy stored plus the energy
that can be delivered into the heatbank from the boiler while the
former is delivering energy to the heat exchanger.
No. The system acts like a combi as a fall back.
Therefore in considering this, do your
sums in terms of whether there
really is much of a space saving
There is.
It's easy enough to calculate that when comparing a heatbank at 76
degrees with a conventional cylinder at 60 degrees.
For the same amount of stored energy, the height of the cylinder could
be reduced by 60/76. For a 1500mm high cylinder that would be to
1185mm approx. - a saving of 315mm - about a foot. Enough space to
put a couple of pairs of shoes. Hmmmm. Not a decision making
criterion that seems that big an issue unless you are in a small flat.
And millions are.
- the claims made are optimistic
IMHO. Unless you are really tight
on space for a cylinder, you can
achieve an equivalent or better
result by using a pressurised cylinder
and storing the hot water to be used.
Behave yourself. A heat banks outperform an unvented cylinder an day.
Unvented cylinders required pressure to be reduced to, Unvented cylinders
are pressure reduced to approx 3.5 bar for steel and 2 bar copper. A
combi
or heat bank need not be reduced.
A heat bank is a far better solution
than an unvented cylinder as it does not run at high pressures, have
large
bore overflow pipes, tundishes, pressure control equipment, etc.
This isn't really relevant because the flow rate through a combi is
the key issue, not the pressure that it will withstand. Pressure
and flow control is required on most mains fed systems anyway,
regardless of technology, in order to balance the flows of hot and
cold to a reasonable extent.
Combi??
I therefore installed a cylinder capable
of running pressurised but am
currently running it from a roof tank. It
is entirely satisfactory for two people
and two teenagers with two baths, two
showers or one of
each being run simultaneously.
And very large.
200 litres is not particularly large and could not be substantially
reduced by the use of heatbank technology as already illustrated.
It could. Store water at 80C and have a large output boiler to assist when
drawing hot water.
The cylinder is a fast recovery type
of 200 litres with a boiler able to deliver 28kW.
Unvented cylinders are minimally quick recovery. They can't match an open
vented cylinder.
That depends on the cylinder design and
specifically the heating coil.
Yes, and unvented cylinders do not have very large coils.
You should have fitted a heat bank and then it would be easy to convert
it
when a decent mains pipe comes along.
In fact I could since the cylinder has plenty of connection points.
I am also quite impressed with the Swedish made GEA Ecobraze heat
exchangers - the one I have for the workshop circuit performs very
well.
Those sold by DPS are about the best. I forget right now who makes them,
but they are about the most efficient and not expensive.
However, the match of a direct heatbank to a more sophisticated
condensing boiler is not a good one, and this does not leave any
significant advantages for the heat bank, unless one is really tight
on space.
A smart condenser will work well with a heat bank. A heat bank with the top
half at 76-80C for DHW and the bottom at 45C for UFH works very well with
condensers. A diverter valve sends the boilers output to either section.
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