boiler sizing

[lilleville]

I'm having to have an unvented cylinder made specifically to fit in my loft, it will be a 350litre twin coil. I asked them to tell me how much heat would be required for the coil (from the boiler) so I could add it into the boiler calc for sizing the boiler. They came back with its 17kw rated. What does this mean? Using the whole house boiler sizing method it comes to 18.5KW (my house is hard to do accurately though coz its not a box shape), so wanted to back it up with adding the rads etc together.

So I've also worked out the rad sizes + underfloor heating and added them up to make 10.9KW. I thought that the cylinder coil may need 5KW so was making a total of 16kw, not far off the 18.5KW. It just depends really on what they mean by 17kw rating for the cylinder.

Any suggestions?

Thanks