On Thu, 4 Nov 2004 17:18:44 -0000, "IMM" wrote:


"Andy Hall" wrote in message
.. .
On Thu, 4 Nov 2004 16:41:30 -0000, "IMM" wrote:




Then there is the matter of effectively making your
cylinder bigger by adding some simple controls.

But you haven't - you made it "smaller"

No bigger as it gives a larger volume of hot water.

From where?

The cylinder.

Well I didn't imagine that it would be from the Yellowstone Geyser.

Please answer the question. How does having an arrangement which
causes a large proporion of the cylinder contents to fall in
temperature to practically cold water temperature increase the volume
of available hot water?



, except for adding a flow
switch to start the boiler earlier
than the thermostat would.
Everything else made it worse
from the capacity perspective.

Nonesense.

How does adding a thermostat which has the effect of allowing most of
the cylinder contents to fall to cold water temperature increase the
volume of hot water?

You should read properly. That was to make the re-heat more efficient.
Other methods were given to increase the hot water volume a cylinder will
produce. Duh!

OK, so let's summarise before you attempt to draw attention away from
your confused solution any further.

1) You suggested a scheme involving two thermostats and a relay as an
attempt to improve the efficiency from the boiler because it would
burn for longer and hence be more efficient than running it for
shorter periods.

2) I said that from tests that I did with a condensing boiler and fast
recovery cylinder that I found no evidence that a scheme like this
would make any notable difference to efficiency.
I also said that I could see no reason why any boiler of condensing
type of recent design and in the 90-91% SEDBUK category would be
substantially different.
I did say that I could see a reason why this approach *might* help an
old fashioned cast iron boiler with natural draft flue, especially
with a non fast-recovery cylinder.

3) I pointed out that implementing such a scheme with two thermostats
will result in times where there is as little as a quarter of the
total volume of the cylinder available as hot water. If that
happens to be at a time when there is a sudden large demand for water
to run a bath or shower, then that volume will exhaust quickly and the
user will be left with whatever the boiler can do on an instant basis.
Given the mixing effect of incoming cold water rushing into the
cylinder, the results will be fairly poor.
This was my point about reducing the available volume of hot water

4) You raised the point about having a flow switch to fire up the
boiler the moment that hot water is used and therefore bringing the
boiler on before the thermostat otherwise would.

5) You suggested that both methods together were a good idea, both
improving efficiency and increasing available hot water.

6) I said that although having a flow switch does have the effect of
bringing the boiler into operation earlier and was worth doing, it
won't compensate for the loss of 75% of the storage capacity from the
other scheme with two thermostats

In fact the effect of the flow switch, if just on the cylinder output
would be to fire up the boiler each time you start drawing even small
amounts of hot water, thus defeating the object of the thermostats
scheme.

One way that this could be circumvented would be to only allow the
flow switch to activate the boiler if both thermostats were
indicating that a recovery and boiler run should happen. At this
point, there is less than 25% of hot water left anyway. It is likely
that the boiler won't have done enough in time to prevent the stored
water being exhausted.

For most of the time, the lower thermostat will be calling demand and
the upper one not and there is no way to know with a simple thermostat
arrangement like this whether there is 25% or 75% of HW left and
therefore whether the boiler should be fired up when water is drawn.
This could be handled by having separate plumbing runs for the bath
and shower and having the flow to those monitored.
Otherwise the arrangement will be worse overall than just having a
single thermostat with its hysteresis. You will fire up the boiler,
every time the hot tap runs at all, whcih is precisely what you didn't
want to happen because it's less efficient (or so you say).

I don't think you thought through the implications of your solution
before suggesting it.



--

..andy

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