The survey behind our answer is slow work. It records the dimensions of each room, the window and door areas, the construction of walls, roof and floor, and whether each ground floor is solid or suspended, then works out the heat loss of each room against an external design temperature. I still carry out those surveys myself, and for householders I have written a plainer account of what a room-by-room heat loss survey covers. The tools are a tape measure and a few hours in the house.

The cost of each wrong answer
An oversized unit costs more to buy and takes more space outside. The installer has to account for its sound in the Microgeneration Certification Scheme (MCS) 020 calculation if the installation relies on permitted development in England and the distribution network operator (DNO) notification has to cover a larger electrical load. In service, a unit sized well above the building's load spends mild weather at the bottom of its modulation range; below that floor it cycles on and off, which wears components and costs efficiency.

A wrong refusal costs more in carbon terms. The Street house ran on heating oil. A householder told the property is not suitable, then quoted 16kW by the next company, could conclude that low-carbon heat is a gamble and order another oil boiler. That outcome does not appear in any installation statistics, because no installation takes place.

Funding that pays the same for any size
The UK’s Boiler Upgrade Scheme grant does not vary with the size of the heat pump. The 16kW proposal and the smaller unit the house needed would have drawn the same grant, and the installer applying for it must hold MCS certification. A flat grant keeps the scheme simple to administer and leaves sizing to the installer's design. That leaves the public money resting on a survey the householder has no means to check.

As a freelance National Association of Professional Inspectors and Testers (NAPIT) MCS assessor, I audit other installers' heat pump work against the MCS standard. MIS 3005-D asks for a room-by-room heat load calculation to BS EN 12831-1, and unless the system is a hybrid, the heat pump must meet the full calculated load at the design flow temperature without help from a supplementary electric heater. The requirement exists; but the householder loses sight of it in the sequencing: the customer receives a performance estimate before the contract is awarded, and the detailed room-by-room design before installation starts. Householders comparing three quotes compare unit sizes and prices, and the design behind each size can arrive after they have chosen.

Proportionate changes, in my view
My proposals sit inside the existing standard. First, the room-by-room summary should travel with the quote: heat loss per room in watts, the design flow temperature and the unit's output at that temperature set against the total load. That fits on one page. A householder holding three of those pages can compare designs as well as prices, and a relative or adviser with an engineering background can read them.

Second, a refusal should carry the same evidence. If an installer judges a property unsuitable, the householder should receive the calculation and the reason in writing, naming the rooms that fall short, the flow temperature tested and the emitters assumed. Another installer can check a refusal with numbers attached. A bare verdict leaves the oil boiler in place.

Neither change asks for new software. The MCS heat load calculator already produces room-level figures, and an installer quoting a unit size should hold them before the quote goes out. The work lies in handing them over earlier, and in accepting that a competitor may end up reading them.

The views and opinions expressed in this article are those of the author and do not necessarily reflect those of the Energy Institute.

• Further reading: ‘Living with a heat pump: why behaviour matters as much as technology’. In this personal account, Dr Lin Gao, a researcher at Energy Bridge Research, tells how installing a heat pump in a domestic property became a two-year journey of research, renovation, frustration, learning and, ultimately, surprise.
• ‘Smart heat pumps: the potential for demand flexibility in the home’. We are partly through a journey into widespread acceptance of the efficacy and suitability of heat pumps to serve UK homes. But a new study has demonstrated another valuable set of advantages to operators of electricity networks that should tip the balance decisively, argues Professor Patrick James from the University of Southampton.