Combi for a sealed system ???

[Collectors]

I was just thinking of upgrading my gravity hot water system to a sealed system on tank side of things. Sods law the boiler just packed up & we think it might be the heat exchanger @ around £600 + fitting. Now i cant afford to update everything & was thinking of going down the combi route feeding 2 bathrooms, 13 rads & a towel rail `with a Worcester Greenstar 4000 30kW Combi. The heating engineer is not a fan of two bathrooms on a combi. So my question is::
Can i fit a combi & if it was a problem with 2 bathrooms , then fit a sealed system 200lt tank to be heated by the combi.

 

[Johntheo5]

Thermally, the calcs are quite simple.

A 30kw combi will give 12.65LPM, 6.33LPM/shower in the debths of winter with mains of 6C and showering temp of 40C,
At a more realistic winter mains temp of 10C will give 14.33LPM/7.17LPM/shower. Other numbers
15C mains, 17.2LPM/8.6LPM/shower & in the height of summer with 18C mains, 19.55LPM/9.77LPM.
So you could restrict the showers permanently to say 7.0LPM which is a reasonable showering temperature or restrict them based on the above/season.

You can certainly install a 200L unvented cylinder as well but whether mixing them with one bathroom on the combi and one on the UV cylinder is debatable IMO, or retrofitting the UV cylinder.