Is there a formula or online calculator available whereby I can measure all my current radiators and find out their outputs? I can find outputs for similar sized radiators from manufacturers online catalogues, but is there an exact method?

I doubt if you can work these out exactly as the heat output will be based on surface are of radiator and fins which would be difficult to measure and model.

The safest way is to compare to known ones and then to make an allowance for error such that you oversize the rads.
Providing you have trv's you are better of oversizing the rads as they will assist in the operation of your condensing boiler by working at a lower temperature.
Also be careful as the rates output of a radiator is stated at a defined water temperature these are often higher than you would choose to run a condensing boiler at in order to optimise condensing operation. A condensing boiler needs a return temp of 55 or less to condense whereas non condensing boilers had to have a higher return temp to prevent condenstation and corrosion in the cast iron heat exchanger.

Tony

You dont seem to understand that radiator systems for non condensing systems require lower output radiators because they are designed to run at a higher temperature. With a condensing boiler you need higher output radiators and run them at a lower temperature. Hence my suggestion is unlikely to be belt and braces just good design and far less less wasteful than running a condensing boiler that will never condense.

Posted By: tonythe system can be adjusted so that it will run in good condensing mode for the majority of the time without changing the radiators for bigger ones.

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How?

That wont work Tony.

Quoted from the technical director of ideal standard

"The fact remains that a high-efficiency boiler will only condense when the system allows it to do so, crucially, when the return temperature is maintained below 50°C. However, commercial and industrial heating systems in UK buildings can be as much as 20 to 30 years old, and many cannot easily adopt condensing technology — either through design or suitability. With the replacement boiler sector accounting for 70% to 80% of all commercial boilers installed, careful consideration must therefore be given to overall system design before choosing either a high-efficiency or standard-efficiency boiler for a replacement project.

So what factors do installers need to take into account before making the right selection for their end-user customers? The main consideration is the flow and return temperatures that commercial buildings are designed on. To maximise the benefit of a condensing boiler, for example, the system must be designed for 60°C flow and 40°C return. However, most of systems are designed on 82°C flow and 71°C return, and if this is overlooked, the rated output of the appliance can significantly reduce by as much as 8%."


So following your logic and assuming a system design temperature drop of 11c the max flow temperature you could have would be 61c at an average system temp of 56c the heat available would be reduced by 60% which wont be enough to keep a building warm so even in mild weather your idea would not work.