On 09/11/2016 17:44, John Rumm wrote:

You have a conflict of interests here really... Thermal stores work
well at high temperatures, and condensing boilers get bigger efficiency
gains at lower temperatures.

Even at return temps above 54 degrees (the dew point of the flue gasses)
the modern boiler will be more efficient than an older style one as a
result of its larger heat exchanger. The efficiency graph just gets a
bit steeper at that point.

Like this (reposted):
http://www.maxwell.myzen.co.uk/uk.d-i-y/AM3_fig2.6.gif

The design seems mad to me. Surely for optimum efficiency you want the
UFH running straight off the boiler primary, at the lowest feasible
temperature. Weather compensation would help here, by reducing the
temperature further in mild weather. Then use the store for DHW only
with appropriate controls to heat it to ~65 deg. a couple of times a
day, or as required, by running the boiler flat out into it for short
periods (i.e. HW priority control).

--
Andy