If its a heat only boiler and the existing is 18 kW then its most likely that all that is needed is about 15 kW.
But a whole house heal loss calculation should be done to check.
There is no point in over sizing a boiler as that only reduces efficiency.
Tony
But a whole house heal loss calculation should be done to check.
There is no point in over sizing a boiler as that only reduces efficiency.
Tony