heating system advice

[jackg]

I have been building an extension and so I need to consider the heating design and layout.
Single storey, existing was 60m sq new is 110 m sq.
The existing has a bathroom with a shower and bath. The new has a shower so two showers.
The existing heating was oil WB 12-14 combi heatslave. 12 years old
A water softener is fitted, which does reduce the flow rate.
Mains water pressure is quite good, three ish bar.
Existing rads were four 800 and two 500 all doubles. Two of the 800s have been removed.
It is very easy to heat and the boiler is never on more than a third of maximum.
Additional rads will be doubles: 1000 + 800 x 2 plus four small ones 4-500.

Not sure if a larger combi or a HWC in an unvented system is the most suitable?
Any advice would be welcome. Thanks

 

[dilalio]

Flow rate with two mains taps open?

Picture the scenario... decent combi with, say, 12lpm at 35 degrees rise. 2 showers running gives roughly 6lpm hot to each outlet, given that it is mixed with cold at about 60:40; is this enough for you, out of the showers?

Then, a toilet is flushed, kitchen hot or cold is opened, washing machine or dishwasher kicks in... they all have a detrimental effect on the flow... it’s down to what you expect from the system.