16kw ashp advice

[strideredc]

Got a few niggling questions for the plumbers out there.... I have run 28mm copper pipe from where the monobloc will sit outside to the plant room upstairs in the spare room. the total run one way I.e flow is about 9m ish (return almost identical) and it then hits the 3 port valve (28mm connections and then about .5m into a 250l buffer tank and 300l hp dhw cylinder with a low resistance 3m2 coil. Its going up prob about 5m from heatpump to plant room. not sure what make of heatpump to buy yet but it needs to be 16kw (heat loss is about 13kw ish at -3 deg) most seem to state a max flow rate of 3m3 per hour and min of about 1.8m3 per hour. 

 

should I have gone 32mm copper for the flow and return to be on the safe side or will 28mm copper be ok???