"Andy Hall" wrote in message
...
On Thu, 4 Nov 2004 11:32:40 -0000, "IMM" wrote:

"Andy Hall" wrote in message
.. .

I found it to be poor on the old system
because it was a long time
before the boiler would come on, by
which time a lot of water had been
used. Then the temperature would
overshoot by a long way and the
water too hot during later use.

The simplest method to re-heat a cylinder.

This isn't the simplest,

It is.

and I am not sure that it has value in terms
of usability either.

You are not sure. I am sure. It has.

Have two cylinder stats, top and
bottom of the cylinder either strap-on or immersed. Immersed is better,
but
not that crucial. Have the bottom stat set to 60C for Legionella
problems,
the top, about 1/4 of the way down and set to just below the desired DHW
temp you want. Have a DHW blending valve on hot draw-off pipe, which are
to
become mandatory. Set this to your desired DHW temperature, say 50C.
DHW
is then user adjustable.

When the cylinder is up to temp at 60C the bottom stat cut out the
boiler.
When the temp get below say 48C, the top stat will bring the boiler in
when
the temp is below say 47-48C. The boiler heats the cylinder with one
long
burn and no inefficient boiler cycling.

This would need at least a latching relay arrangement to work.

yep.

The top stat, as I read the way you describe its use, would delay the
commencement of reheating until most of the contents had been used.
If you happen to be at the point when the cylinder is down to 25% of
capacity and then run a bath or shower, you are back into the same old
game of the water running out quickly and relying on the instantaneous
delivery rate of the boiler. Having the flow switch idea would
help the timing a bit at least but doesn't replace having a full
cylinder of water.

The stats can be set to give a half full cylinder if you like.

Boiler cycling would occur only if the heat production rate exceeds
the capacity of the cylinder to absorb it. That can be avoided if
the cylinder is a fast recovery type or if the boiler modulates down.

Fast recovery is highly desirable, modulation down? No.

If the cylinder is running out of hot water too quickly, then there are
three options:

1. Raise the bottom stat temperature to give more stored hot water, say
up
to 65 to 70C. The DHW blending valve will ensure the draw-off
temperature
is what the user wants.

I think you mean water stored at a
higher temperature at the top,
which when mixed gives more at
the wanted temperature.

No. water stored at a higher temp all through the cylinder.

Even this
is not going to compensate for letting the
stored amount of heat fall
to 25% of the full capacity.

Wrong.

2. Insert a flow switch into the pipework just before the high flow taps
(bath and shower), and install a third high limit cylinder stat, set to
80C.
The flow switch will bring in the boiler immediately heating the DHW
cylinder preventing the bottom of the cylinder being very cold with
entering
cold water before the boiler cuts in.

This could be worth doing to deal with the poor response of bimetal
thermostats.


3. Do both 1 and 3 above.

Doing the above effectively increases the size of your cylinder.

Actually it does the opposite

Wrong again.

Quite
simple and easy to do. Many people when installing full lengthy body
jets
have a larger cylinder installed. This in many cases is unnecessary. The
above could be done and money saved in boiler efficiency.

The boiler efficiency argument
here is marginal and is at the expense
of performance.

Wrong again.

You are sacrificing the stored capacity for a
presumed improvement in efficiency.

Whether there is an efficiency gain to be had really depends on the
type of boiler and cylinder.

If it's an older type cylinder, incapable of taking all the heat from
the boiler, then the cycling will happen anyway if the mismatch is
great even if the coil is effectively immersed in cold water because
the cylinder was allowed to drop to 25% of capacity.

If we assume that the cylinder is fast recovery, so will absorb all
the heat and therefore no cycling during recovery,

equals more efficient.

then the question
becomes one of how long did the inefficient parts of the boiler cycle
vs. the efficient parts last as a proportion. The inefficient parts
relate mainly to energy used to heat up the heat exchanger before
useful heat is delivered to the load, .and at the end of running, heat
that is not usefully delivered to the load. If the heat
exchanger is not a high thermal inertia type such as cast iron, then
unless the recovery time is very small, the inefficient periods will
be a small proportion of the total.

With a condensing boiler, having a cooler load means that the run time
at lower temperatures is longer and hence the boiler is operating at a
more efficient point - however when it does run, it runs for longer.

I did some experiments on my system where the cylinder was recovered
totally from cold. I recorded the temperatures of flow and return
water and rate of rise of the cylinder temperature and also measured
the amount of gas used to do the reheat. The temperatures can all be
logged to a PC from the boiler. Modulation down of the output only
comes in the last 30 seconds or so of the run and is there to prevent
overshoot of temperature.

When using a DHW blending valve overshoot doesn't matter.

I then repeated the experiment in two different ways. The first was
to run off the same volume of hot water (basically a cylinder full)
continuously and letting the boiler do its thing of starting to
replenish when determined by the sensor.
The second was to run off small amounts of water more slowly so that
the boiler would come on as the cylinder temperature at the sensor had
reached 55 degrees. This is indicative of smaller use during the day.
I stopped running water as soon as the boiler came on and waited for
recovery. In total I drew the same amount of water in each of the
three cases.

In terms of total gas used for each, the amount was the same to within
about 2%. So in this context at least, I don't buy the long burn
argument as making a huge practical difference with recent equipment.

As boiler cycles heat from the boioer is lost through the flue and primary
pipes when the boioer is in an off cycle period.