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Sounds cheap on both labour and materials

Boilers alone and bits would come to 3-4K

Unless you have all the bathroom stuff ?

Yeah it doesn't include fitting any of the showers or bathroom units. Literally just getting the stated equipment (boilers, cylinders) installed and commissioned.

Fitting the rads, basins, toilets and showers / tiling etc will be extra.
 
We've done quite a few commercial type installations with Rinnai equipment.

Why don't you get them to size the equipment for you.
We've done them with banks of Rinnai's or Rinnai's bolted onto their cylinders.

What you want is recovery rate so I would go for something like 1 cylinder with 2 Rinnai units connected to it. The recovery rate of something like that would almost be instant. The Rinnai's only heat the cylinder water via pump(s) operated by the tank stat.
Never ever had a issue with anything they have sized or designed.

Give them a try
 

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