Unvented and boiler heat loss calcs

[gingalig]

I have a house with 14kw heat loss using the whole house method, ( Navien oil system boiler ) I aimed to use heat engineer software on acceptance of the quote but have been struck down with covid, the unvented cylinder is 300ltr due too a family from Ukraine living there ( 8 people living there,) the manufacturer kw on the coil is 16kw would this mean a sizing of 30kw to be adequate or could I get away with less, there,s very little scope to alter existing pipework.
Many thanks

 

[Knappers]

I don't actually know but I'd have thought you could half the HW coil output for calcs and still have an acceptable design recovery time. Have a look at pipework and flowrate too, if you've got a restricted flow then there's no point oversizing boiler. I'd prob look at 18Kw for HW priority or 24kw for standard system.
If someone could weigh in to save me with a more substantiated response, I'm sure there's a more technical answer than my thumb.
(I'm assuming that with heatloss your talking rad outputs with space heating accounted for?)
Are you aware of new 55deg heating temps (I don't even know if they're mean or flow temps?) and TRV requirements, or are you working with existing rads?