The Certificate and the City
Exhibit 02, from our own modelling run of 2026-07-06. The full exhibit is live on our home page, with every number drawn from the committed data.
A certificate records how one machine performed in one laboratory test. The conditions are fixed and the result is repeatable, which is the point of it. What it cannot tell you is how the same machine will behave in a particular home, because a home is not a fixed condition: the weather moves, buildings get extended and re-glazed, and plenty of radiators were sized for a boiler that left years ago.
So we asked a plain question. What does one certified machine do across a whole city of homes, over a whole year?
What we did
We took one certified 8.5 kW air source heat pump and modelled it in 24,931 homes across Greater London, each home reconstructed from its public EPC record. Every home ran a full year, hour by hour, through the Home Energy Model, against a standard London weather year.
Modelled is the operative word. No meter was read and no home was visited; this is a simulation of the housing stock as the public record describes it. The measure is seasonal efficiency in the home: the space heating a home received over the year, divided by the electricity the heat pump used to provide it.
What came out
The certificate figure is 5.0. That is the SCOP at 35 °C flow, flow being the temperature of the water the heat pump sends to the radiators. The hotter that water has to be, the harder the machine works, and 35 °C is the kindest case a test rig can be given.
The median home delivers 3.6.
Nine homes in ten land below the certificate figure. A tenth of homes sit below 3.24 and a tenth clear 4.92, so the same machine with the same certificate produces a citywide spread of outcomes, and the spread is decided by the homes.
The same product carries a second certified figure in the same public record, taken at 55 °C flow, which is the water most existing radiator circuits actually need: 3.6. The median home lands on it exactly. The certificate was right all along; the mistake is reading the headline figure rather than the relevant one.
If you make heat pumps
In this study the product performs to its certificate at both flow temperatures, so nothing here reflects on the machine. What varies is everything around it: fabric, emitters, demand, hours of use.
The problem it leaves you with is routing. You answer for a base of homes, and one figure for the whole range cannot tell you which of them are ready now, which need evidence first, and which are not yet worth a visit. Sorting that takes a number for each home, before anyone is sent to the door.
If you run housing
A budget built on the certificate buys 5.0 on paper and 3.6 in the median home. The difference is not slack to be clawed back later; it is a promise already made to a board, a funder and a resident before any evidence existed.
The spread matters more than the median. In the same city, with the same measure and the same machine, one street spends well and the next does not. What makes a budget defensible, when the funder eventually asks what it bought, is evidence for each home before the money moves.
About the method
Every number above is simulated: a full year at hourly steps, a London test reference year for weather, a design flow temperature of 55 °C. Nothing was measured. There is no monitoring data, no field trial and no installed system behind these figures.
The homes are reconstructions from public EPC records, and those records age. Any single reconstructed home may be wrong in ways its record cannot show, which is why the claim here is the population rather than any one address.
The product is anonymised deliberately. It performs to its own certificate, and naming it would turn a finding about housing into a story about a brand.
None of this makes the certificate the problem. A rig test is repeatable and comparable, and that is its job. It only goes wrong when a single point is read as a prediction for a particular home. A certificate is one point; a city is a distribution.
Where this leaves you
The full spread is drawn as Exhibit 02 on our home page. If you answer for a base of heat pumps, start at the OEM door. If you answer for homes and the budgets that improve them, start at Housing & Place. The question in both cases is where your homes land, not what the certificate says.
Source notes
- Exhibit 02, generated 2026-07-06: one certified 8.5 kW air source heat pump modelled in 24,931 Greater London homes reconstructed from public EPC records, full year at hourly resolution, London CIBSE test reference year, 55 °C design flow temperature
- Certificate figures from the public record for the product: SCOP 5.0 at 35 °C flow, 3.6 at 55 °C flow
- Home Energy Model simulation; metric is annual space heating delivered divided by heat pump space heating electricity input