By single panel I assume you mean the type without fins on the back. In which case you have about 6.25kW of rads. If they do have fins then you have about 10.2kW of rads.
You can find out what you should have by using the Boiler Size Calculator. If you deduct 2kW from the result you will get the heating requirement, which is the same as the required rad output.
Which version of the Vitodens 100 do you have? Is it the 100-W or an earlier model?
I have a veissman Vitodens 100 boiler that was fitted about two years ago. I've felt that recently it wasn't really getting the water hot enough in the bathroom and it's taking an age to heat the house.
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