wrote in message
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John Rumm wrote:
Dave Plowman (News) wrote:

relatively "slow" cylinder (reducing
the cycling *does* reduce the fuel
used a little).

I've always wondered by how much
though. The only difference I can see
is the boiler water output temp will be
hgher during a brief full power
burst than it will be in modulation.
Modulation will improve efficiency,
but not by much afaics. A difference
of perhaps 30C out of a flame temp
of 10s of times that doesnt look like
it would amount to
much, a few percent maybe.

A set rate burner designed for maximum efficiency is more efficient that a
modulation burner. A compromise is reached with modulation, although the
more expensive burners are more efficient when modulating.

A small cylinder coil ensures a high return temperature reducing condensing
efficiency, and promoting inefficient boiler cycling. A large cylinder coil
does the reverse. A large cylinder coil takes more kW reducing the size of
the cylinder, as rapid re-heats are ensured. Less DHW stored buffer space is
needed.

To get the best efficiency:
- The boiler needs to be 20kW and above, more like 25-30kW,
- The cylinder coil must take all the boilers output
- The boiler has a set rate burner
- The cylinder has two cylinder stats to prevent boiler cycling.

Would 1960s be about the right guess for a cylinder that manages all of
2kW throughput?

Yep. Some cowboy makers had 1" coils with few turns.

You could make your own quick recovery cylinder quite cheaply. Buy a
standard direct cylinder, without a coil. That is cheap. Get a bronze pump,
a plate heat exchanger and two cylinder stats, a relay and a plastic
electrical wall box.

The boiler pumps into the plate heat exchanger as if it is pumping into a
coil. The bronze pump pumps water out of the bottom of the cylinder and
through the plate. The two cylinder stats prevent boiler cycling (the relay
is needed for this to work). The total cost will be less than an off the
shelf similar sized Albion with a coil. With the plate the efficiency will
be much higher as the plate is far more efficient than the coil. The
cylinder size can be vastly reduced. The heat extracted from the boiler is
far more than a coil giving a very low return temperature promoting
efficiency.

If there is excellent mains flow and pressure it is quite easy to do similar
and make a directly heated heat bank. No bronze pump needed then.