Thanks for all the responses which are helpful and reassuring.
1) I didn't know about the priority for HW but I think our system is not smart enough for that. Only the controller knows what circuits are on and the boiler just shoves out hot water.
2) 30-40 radiators sounds good, we have 24 plus...
Thank you Flameport, I thought that would be the way to go as the radiators vary from 30 years old to new and add up to a lot of kWs. I note your comment about HW - is the modern assumption that it will never have to heat the HW at the same time as the radiators and therefore will have plenty of...
Hopefully simple answer - I want to check the size of my system boiler. Do I do this against the output value of all my radiators or against the heat requirement of my rooms?
I know I have to add some for the HW tank.
Surprisingly I can't find the answer on the web.
Thanks