Every installer of heat pumps and biomass boiler, under their MCS accreditation,  must use calculation software that complies with EN12831

In simple terms, this means that the property has to be carefully surveyed and calculations prepared to model the thermal efficiency in order that precise boiler sizing can be carried out as well as for the emmiters i.e. raidators or underfloor heating.  As the calculations are carried out on a romm-by-room basis and not as an overall volume (such as the EPC survey), this means that any areas not reaching optimum thermal efficiency are identified and modifications can be implemented

All applications for the Renewable Heat Incentive are required to have the application backed up by Heat Loss Calculations that comply with current software regulations.  

It is clear that in many installations this does not happen and sizing is based on a 'guesstimate' by the installer, leading to possible very expensive running costs, cold areas in the property or both