B
BigBurner
But if you said "I need a boiler which will be capable of heating all rooms to 21°C from cold within half an hour", and the boiler fails to meet your requirements, then the fault lies with the installer.
That is what a responsible installer does. If the customer specifies something which the installer has reservations on, he puts in in writing to them. Like, "the boiler is inadequate and will not heat the house in the worst case scenario".