"Andy Hall" wrote in message
...
On Wed, 6 Aug 2003 00:54:56 +0100, "IMM" wrote:



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.

Fine, but probably not applicable here. SInce most houses in the
U.S. have timber floors, I wonder where this is used.

It was used in a concrete floor.

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.

Sure, but the performance level will have dropped off.

The point is you "never" run out of hot water. Your hot water is 2 stage of
high flow and then low flow.

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.

.. and millions are not. Either way, I don't see this as a major
selling point. Minor one maybe.

Few British house have space to spare, hence combi boilers account for
60-70% of the one million boilers sold each year.

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??

To balance at the points of delivery.

It was about unvented cylinders, heat banks.

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 net equation is still the same. It doesn't matter whether the
boiler is providing the energy via a conventional cylinder or a
heatbank. Once the stored energy is used up, both arrangements fall
back to combi behaviour.

No. With a beat bank the boioers output goes straiught to the DHW draw off.
This si noit the case with a vented or unvented cylinder.

The only real difference is that in the
heatbank case you have stored 25%
more energy in a given volume or a
small reduction in space.

You don't understand.

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.

That depends on what you order and from whom.

Again.. 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.

Looking at the pictures, it looks as though these are the ones that
they carry.

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.

We've already had this discussion and I illustrated why this would not
be the case. I'm not going to go over the same ground again.

You don't understand.


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