Do I need a bigger boiler/accumulator ?

[Daddy Boots]

Son's bought a standard 3 bed terrace with a loft conversion which has 4th bedroom and ensuite shower running off the boiler. Fully double glazed so heating is not an issue.
Currently has a Baxi duo-tec 24Kw which spec says will deliver 9.8l/min.. confirmed by filling a 9.5L bowl downstairs in 55secs with hot water. The same bowl fills with cold water in 46 secs but fills in 39 secs in the upstairs bathroom so the max flow is somewhere between 12.5 and 14.5. The 1st floor main bathroom has a Triton 8.5Kw electric and the temp varied a lot even with no other flow in the house so looking to run off the boiler when the bathroom is refurbished. There's currently only 1 occupant but potentially 3. Assuming the 2 showers are not in simultaneous use does the boiler need an upgrade to deliver more Kw or do we need an accumulator to deliver more l/min to have the 2nd shower running off the boiler ?

 

[muggles]

If the showers are not going to be in simultaneous use and you're happy with the existing flow rate then the existing boiler will be fine. If you're looking for an increased flow rate, you'll need a new boiler

 

[dilalio]

and/or new main/unvented cylinder.
Depends on what you have at the carrier supply/space/expectations and budget.