The manual for the combi condensing boiler we have bought does not mention anything about fitting additional pressure vessels if specific circumstances apply.
There are 14 rads in total, 7 double panels with double convectors (type
22), 5 towel type radiators, one double panel single convector (Type 21) and a single panel single convector (type 20)7 of the rads are on the upstairs zone, the other 7 on the downstairs zone.
Clearly the flow pipework has been duplicated twice in 22mm to permit the upstairs and the downstairs zoning arrangement and to allow for additional rads to be added should the house be extended so there is a lot more copper tube than usual.
Regards,
Stephen