Balancing system/ pump not working or boiler too small

Hi, I have a pressurised Glowworm space saver complheat 60 (60000 btu\\\'s) As I have been renovating and re-decorating the house, I have been replacing the old rads with new ones (Kermi) I now have the following : Downstairs 1x 1800 x 600 DC; 2x 1800 x 600 SC; 1 x 1400 x 600 SC; 1 x 1400 x 500 SC; 1 x 1100 x 500 SC; 1 x 900 x 600 SC; 1 x 600 x 700 SC AND 1 x 600 x 600 SC.
Upstairs I have 2 x 1400 x 600 SC; 1 x 800 x 600 DC; and 1 x 600 x600 SC.
I presume that the boiler will never heat all these rads efficiently and plan to replace it this summer. What size of boiler will I need to operate efficiently and why even when I turn most of the rads off do I have 2 that either won\\\'t get warm or only get warm at the top and are cooler at the bottom.
Would balancing the system help and what exactly does balancing achieve. Many thanks
Ron

I presume that the boiler will never heat all these rads efficiently

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Why presume? You can add up the outputs of all your rads, add 10% for pipework losses and 2 kW for hot water to see what output would be required. Balancing is covered in the FAQ topic.