"Andy Hall" wrote in message
...
On Sun, 05 Sep 2004 22:49:02 +0100, Pete C
wrote:

On Sat, 4 Sep 2004 21:53:07 +0100, "IMM" wrote:

It is. Even a modulating boiler cycles when demand is below the minimum
kW.
With a heat bank, no cycling.
Hi,

How much heat is lost when a modulating condensing boiler cycles? Not
much I would have thought, as the heat exchanger probably has low
thermal mass and operates at a lower temperature.

Precisely - and it will be running at very low power anyway.



Another way of doing it would be to connect the lower half of the heat
bank/thermal store in line with a differential bypass valve and 2 port
valve in parallel.

Then as the system gets up to temperature and the TRVs close, the
thermal store would take the excess heat. When that reaches
temperature and the burner switches off, the 2 port valve would open
to release the heat back into the system.

The problem is that for DHW, the heatbank needs to have very different
characteristics.

If you segment the heatbank into two and deliberately run the lower
half at a lower operating temperature, you are cutting into the
storage for the DHW application. This implies either a larger or a
separate heatbank to do the job.

A taller heat bank that's all.

This is why it is far better to treat the CH and the DHW as the two
totally different systems that they are in terms of thermal
characteristics and time constants and to simply drive the radiators
from the modulating boiler.

Not cheap to do this. and the boiler will cycle anyway. Better ways of
doing it.

This is the intent of the manufacturer's
design

The boiler manufacturers design you mean.

and without a more sophisticated set of sensors and controller
than is inside the boiler, and a boiler that can be analogue
controlled, if the gas boiler is the only source of energy, you won't
improve on it.

What tripe. Stop making things up.