Heat loss survey

How a room-by-room survey establishes the building fabric, ventilation and design heat load needed to size a heat-pump system.

A heat loss survey establishes how much heat each room and the whole home lose at the chosen winter design condition. It provides the design basis for the heat pump, emitters, flow temperature and water flow rates.

This is a physical and analytical survey, not an estimate based only on floor area, the existing boiler or an Energy Performance Certificate. A valid EPC may help with an annual energy estimate, but it is not a substitute for the design heat-load calculation.

The current MCS basis

As at 22 July 2026, MCS heat-pump design is governed by MIS 3005-D Issue 3.0. For a hydronic system that is not a hybrid, it requires the design heat load to be calculated to BS EN 12831-1:2017 using the MCS internal and external design temperatures. The selected heat pump must then provide at least the calculated load at the design external temperature and the proposed design flow temperature, without counting output from a supplementary electric heater.

The MCS Heat Load Calculator implements the room-by-room method. Its April 2026 update changed important reference inputs, including design-temperature choices, airtightness defaults, ventilation rates and the sources used for U-values. A report should therefore identify the calculation method and software version or reference data used, rather than simply saying that it is an MCS calculation.

What the survey records

Every heated space should be represented separately. The surveyor records enough information to describe both the external envelope and the heat transfer between rooms, including:

room length, width and height
external walls, roofs or ceilings, floors, windows and doors
net areas, with openings deducted from their parent elements
construction type, age and known insulation for each element
adjoining heated rooms, unheated spaces, neighbouring properties and the ground
thermal bridges and exposed junctions where the method requires them
intended room temperature and the property’s external design condition
purpose-provided ventilation, chimneys, flues and relevant extract systems
airtightness evidence or the permitted default assumption
mechanical ventilation type, airflow and heat-recovery data where present
existing emitters, main pipe routes and any planned fabric changes

Photographs, drawings, measurements and product or installation records help support facts that cannot be checked later. An assumed construction should be labelled as an assumption, not quietly presented as measured fact.

Fabric heat loss

Transmission through a building element is based on its area, thermal transmittance and temperature difference:

heat loss in W = area in m² × U-value in W/m²K × temperature difference in K

A low U-value means less heat passes through the element. The value should come from reliable evidence for the actual construction where that exists. Otherwise, the calculation uses the appropriate recognised default for the element, age band and location.

The MCS calculator’s external default U-values are drawn from RdSAP 10, while its internal-element values use the CIBSE Domestic Heating Design Guide 2026. It also applies the relevant thermal-bridging treatment. These reference sources matter because two walls that look similar can have very different calculated losses if their construction or insulation assumption changes.

Ground-contact elements, party elements and unheated adjoining spaces need the method intended for them. Treating every surface as though it faces outdoor air can materially overstate the load.

Ventilation and air leakage

Ventilation heat loss covers the energy carried away when warm indoor air is replaced by colder air. The calculation distinguishes infiltration through the building fabric from intentional ventilation and air transferred between spaces.

Measured airtightness evidence is preferable when it is suitable for the method. Without it, the permitted calculation default must be used and identified. A visual impression that a house is “draughty” is not a measurement.

Mechanical ventilation with heat recovery can reduce the loss associated with planned ventilation, but it does not make infiltration disappear. Its effect should be calculated from the designed airflow, heat-recovery performance and system arrangement. The MCS calculator warns when MVHR is selected without measured air-permeability data because the result is particularly sensitive to that combination of assumptions.

Design conditions

The external design temperature is a statistical winter design condition for the location, not the lowest temperature ever recorded. The current MCS calculator offers the applicable CIBSE 99% and 99.6% design-temperature choices and applies the method’s location and altitude treatment.

Each room also has an internal design temperature. The resulting temperature difference drives its fabric and ventilation losses. A calculation can therefore be internally consistent yet wrong for the customer’s needs if the room temperatures were never agreed.

MIS 3005-D treats intermittent-heating uplift and winter heat gains within the formal method. They should not be added informally on top of a hidden safety margin. Where the 99.6% external design condition is selected, the standard says that no intermittent-heating uplift is to be applied.

Existing and proposed fabric

Only improvements that will be completed should reduce the design load. If insulation, glazing or airtightness work is proposed but not yet certain, the report should show the relevant cases separately:

the home as surveyed
the home after clearly specified work

This prevents a smaller heat pump being selected on the assumption that an unrelated fabric project will happen later. It also makes clear which insulation depth, wall treatment or window specification the revised result depends on.

Design heat load is not annual energy use

The survey answers a peak-power question: what heat output in W must be supplied at the design condition? Annual energy use answers a different question about heat delivered over a season.

Historic fuel use can be a useful sense-check when meter data, boiler efficiency, hot water and occupancy are handled properly. It cannot by itself replace the room calculation, and annual kWh must not be treated as a heat-pump capacity in kW.

What a useful report contains

The homeowner and system designer should be able to follow the result back to its inputs. A useful report contains:

the calculation standard, tool or method and relevant version
the chosen internal and external design temperatures
measured dimensions and the construction or U-value assigned to each element
ventilation, airtightness and MVHR assumptions
room-by-room design heat losses
the whole-building design heat load
warnings, uncertain inputs and any sensitivity checks
separate results for any proposed fabric-improvement case
an emitter schedule linked to the selected design flow and return temperatures

The whole-building figure selects the heat source. The room figures select the emitters. Those results need not be a simple arithmetic match because the BS EN 12831 method accounts for heat transfer and ventilation at room and building level.

Signs that the design basis is incomplete

The design basis is incomplete if it relies on:

floor area multiplied by a generic W/m² figure
the rated output of the old boiler
an EPC heat-demand number used as peak load
a whole-house total with no room data
unexplained U-values or airtightness assumptions
proposed insulation that is absent from the scope of work
a heat-pump model selected before the design temperature and flow temperature are known

The right response to uncertainty is to record it, inspect further or test a credible range. Adding several untraceable allowances at different stages can oversize the heat pump and the emitters without making the design more reliable.