On 15/06/2014 02:28, John Rumm wrote:


One consideration to keep in mind is that modern part L compliant
cylinders will have a much higher power heat exchanger than old
cylinders. On many old ones the maximum transfer rate was often limited
to less than 5kW, so you needed some load from the rads to prevent the
boiler cycling endlessly during a cylinder reheat. A modern one may be
able to sink 15kW or more, and hence the need to provide additional load
is no longer there, and you can get a much faster reheat (in fact, you
can reheat fast enough to match a smallish demand (e.g. a shower) in
real time, giving combi style "unlimited" hot water.



That's an interesting perspective which, of course, makes a lot of
sense. Being on the verge of dealing with my DHW (I have been for a
while but it's really going to happen soon), it's a current area of
interest and the heat transfer rate could be a significant factor in
choosing one cylinder over another and perhaps a factor in deciding on
what size to get (although I suspect that 'enough to fill the bath' is a
good rule of thumb) .
Very few suppliers seem to quote a figure for heat exchange, however,
and instead give reheat times or just say "it's good, 'onest guv", if
anything. Similarly, I couldn't find a specific minimum performance in
part L, though I'm sure there should be and I could easily be looking at
the wrong document.