"Andy Hall" wrote in message
...

The simplest is to have one thermostat
or sensor and done with it.

You've added an additional thermostat,
a relay and a flow switch.

To improve efficiency, reduce boiler wear and in effect extend the cylinder
size, without installing a larger cylinder.

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

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

So how does reducing the available
volume of hot water in a storage
system improve usability?

When you require that volume of hot water.

Or just have one at the bottom,
maybe plus a flow switch and done with
it.

That creates inefficient boiler cycling and excess wear on boiler controls.

Fast recovery is highly desirable, modulation down? No.

This would only happen if the cylinder
is incapable of absorbing all
the heat or as part of a control system
to avoid overshoot.

99% of boilers do not have sophisticated DHW controls.

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

Wrong.

Please explain how this scheme
improves usability

Usability? Stop making things up.

and makes more
stored water available.

Read again.

Doing the above effectively increases the size of your cylinder.

Actually it does the opposite

Wrong again.

Please explain how this scheme improves usability

Usability? Stop making things up.

and makes more
stored water available.

Read 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.

I demonstrated, with figures how
it makes little diference with recent
equipment. It might make more difference
with older boilers.

Figures from one sophisticated boiler which few people have heard of.

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.

Obviously. The question is whether that is significant.

yes.

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

More complexity.

They are to be mandatory soon, so get used to them. They are simple
mechanical devices.

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

If the boiler is of a fanned flue type,
especially with small gauge
flue pipes, the flue loss is negligible.

It is lost and adds up over the year, and over millions of houses too.

It might make more
difference with an old cast iron or
natural air flow model.

Heat loss through primary pipes is
in the building envelope so is
largely irrelevant.

In the summer it is not. There is also the wear of the boiler controls with
excessive unnecessary cycling.