A heat loss survey under MCS standard MIS 3005-D is a room-by-room calculation of how many watts each room loses at design outdoor conditions, summed to a total system size in kW. It must precede any compliant heat pump quote in the UK, because the Boiler Upgrade Scheme grant is gated on MCS certification, and MCS certification is gated on the survey. Any installer who quotes from floor area, or from the kW rating of the gas boiler being replaced, has not met the standard.
The catch is that this still happens on roughly half the heat pump quotes UK homeowners receive.
Last reviewed: May 2026
TL;DR
- MCS MIS 3005-D requires a room-by-room heat loss calculation under BS EN 12831 methodology before any compliant heat pump quote.
- The survey records U-values for fabric elements, air change rates, design indoor and outdoor temperatures, and total watts of heat loss.
- UK design outdoor temperatures range from -1.7°C in central London to -4°C and below in Aberdeen and the Highlands.
- A heat loss survey cannot be skipped: BUS grant payment is contingent on MCS lodgement, which audits the calculation.
- "Quick quote from floor area" is the most common red flag and the most common cause of oversized, inefficient installs.
What MIS 3005-D actually requires
MIS 3005-D is the Microgeneration Certification Scheme installation standard for domestic heat pumps. It is the document that governs how an MCS-certified installer must design, install and commission a heat pump in a UK home. The standard requires a heat loss calculation in line with BS EN 12831:2017, the European standard for design heat load calculation that succeeded the older CIBSE TM30 methodology.
The calculation is room by room. Every habitable space must be assessed individually, not aggregated. The installer needs to produce:
- Fabric U-values for external walls, internal walls (where they border unheated space), roof, floor, windows and external doors.
- Areas of every fabric element, measured in square metres.
- An air change rate per hour, based on construction era and ventilation strategy.
- Design indoor temperatures: typically 21°C in living rooms, 18°C in bedrooms, 20°C in bathrooms (CIBSE Guide A convention).
- The design outdoor temperature for the postcode.
- The resulting heat loss in watts per room at design conditions.
- The total system heat load in kW, summing all rooms.
Anything less is non-compliant. MCS auditors check this document first when complaints are escalated.
Design outdoor temperatures by UK region
The design outdoor temperature is the 99th percentile cold figure for the postcode: the temperature that is statistically exceeded for only 1% of the heating season. It varies materially across the UK and drives the kW sizing of the heat pump.
| Region | Design outdoor temperature (approx) | Indicative postcode area |
|---|---|---|
| Central London | -1.7°C | EC, WC, SW, NW |
| South coast (Brighton, Bournemouth) | -1.5°C to -2.0°C | BN, BH |
| Birmingham, West Midlands | -3.0°C | B, WV, DY |
| Manchester, Greater Manchester | -3.5°C | M, BL, OL, SK |
| Newcastle and the North East | -3.5°C to -4.0°C | NE, SR, DH |
| Edinburgh, central Scotland | -4.0°C | EH, FK, KY |
| Aberdeen and Highlands | -4.0°C to -6.0°C | AB, IV, KW, PH |
| Northern Ireland (Belfast) | -2.5°C | BT |
A 1.5°C difference in design temperature changes the calculated heat loss by roughly 5 to 8% in a typical fabric. Across the UK that is a meaningful sizing difference between, say, an 8 kW unit in Bournemouth and a 10 kW unit in Aberdeen for the same dwelling.
What gets measured on the survey visit
The on-site element of an MIS 3005-D survey is usually a half-day visit by an experienced installer or heating engineer. The work splits across three phases.
Fabric and dimensions. Every external wall is measured. Windows and doors are sized and the glazing type recorded (single, double, triple, low-e coating presence). Roof construction is checked: pitched with loft insulation, room-in-roof, flat. Floor construction is recorded: solid, suspended timber, suspended concrete, ground-bearing slab. Insulation status. Cavity insulation presence is checked by visual inspection at the wall ties or by reference to the EPC's RdSAP assumption. Loft insulation depth is measured. Underfloor insulation, if present, is recorded.
Ventilation and air permeability. An assumed air change rate per hour is applied based on age of dwelling and ventilation strategy. A 1900s solid wall terrace with natural ventilation might be assigned 0.8 to 1.0 ACH; a 2010s new build with mechanical ventilation might be assigned 0.4 to 0.5 ACH.
The flow temperature design link
The output of the heat loss survey is not just a heat pump size. It is also the design flow temperature, which determines the emitter sizing for every room. A heat pump operating at 35°C flow delivers a high SCOP and a low running cost. At 55°C flow it delivers a much lower SCOP, often around 2.5, because the refrigerant cycle has to compress harder.
The room-by-room watts figure is divided by the chosen design flow temperature delta to size the radiator or underfloor circuit. A living room losing 1,200 W at design conditions needs roughly 1,800 W of emitter at 50°C flow temperature, or about 2,500 W of emitter at 40°C flow.
This is why heat loss surveys and radiator schedules are inseparable. An installer who completes the heat loss calculation but does not produce a corresponding emitter schedule has done half the job. In practice, this is the most common cause of underperforming installs.
Red flags: what a non-compliant quote looks like
Several patterns appear repeatedly in non-compliant heat pump quotes UK homeowners receive. Each one is a signal that the installer has not run MIS 3005-D.
Quote from floor area. "Your house is 110 m², so you need a 12 kW heat pump." This is a rule-of-thumb shortcut that ignores fabric, ventilation and regional design temperature. It typically oversizes by 20 to 40%.
Quote from boiler kW rating. "Your gas boiler is 24 kW, so let's put in a 12 kW heat pump." Gas boilers are routinely oversized for the home's actual heat loss, often by 100% or more, because gas was cheap and oversized boilers gave fast hot water recovery. Sizing a heat pump from the boiler propagates the over-sizing.
No room-by-room schedule. If the customer never sees a spreadsheet or printout showing watts per room, it has not been done.
No mention of design outdoor temperature. A survey that does not state the design outdoor temperature for the postcode is incomplete.
No emitter schedule. No radiator schedule means no link from heat loss to flow temperature. The system will likely be commissioned at 55°C flow and underperform.
The Ofgem and DESNZ link to BUS grant payment
The MIS 3005-D heat loss calculation is the document MCS auditors check when reviewing a Boiler Upgrade Scheme claim. The £7,500 BUS grant is paid by Ofgem to the installer only after the installation is lodged on the MCS database, and lodgement is only valid if the calculation is on file. DESNZ quarterly BUS statistics through 2025 noted MCS audit failure rates on a small but persistent proportion of claims, with calculation quality the most common citation.
On the ground, this is the leverage homeowners actually hold. Ask for the MIS 3005-D calculation in PDF before signing the deposit. An installer who refuses or stalls is signalling that the calculation does not exist yet, which means the quote is provisional and the price almost certainly inaccurate.
What a complete survey deliverable looks like
A complete MIS 3005-D heat loss survey deliverable is typically 8 to 16 pages of PDF including:
- Address, EPC reference, date of survey.
- Design outdoor temperature for the postcode.
- Room-by-room schedule: dimensions, fabric U-values, ventilation rate, design indoor temperature, calculated watts of heat loss.
- Total system heat loss in kW.
- Proposed flow temperature.
- Radiator or underfloor schedule per room.
- Hot water demand calculation.
- Proposed heat pump model, with MCS approved product list reference.
This package is what gets uploaded to the MCS portal at lodgement and is the artefact Ofgem can audit.
Regional and devolved-nation notes
Scotland's Home Energy Scotland grant and loan scheme requires the same MCS standard for installer eligibility. The Energy Saving Trust, which administers Home Energy Scotland on behalf of the Scottish Government, audits against the same MIS 3005-D calculation. The £7,500 grant plus interest-free loan available in Scotland does not relax the survey requirement; it intensifies the audit because two pots of public money are committed.
Wales operates under the same MCS and BUS framework as England. The Nest scheme adds additional checks for households claiming on income grounds, but the heat loss methodology is identical.
Northern Ireland does not currently have a direct BUS equivalent at the £7,500 value level, and routes installs through Department for the Economy energy efficiency support. The MCS standard still applies where heat pumps are installed under the Northern Ireland Sustainable Energy Programme.
The honest sizing point
Heat pumps are not boilers. A boiler can be oversized at modest efficiency cost. A heat pump sized 30% too large will short-cycle, hit poor SCOP, and deliver a winter electricity bill the homeowner did not expect. The MIS 3005-D survey is not a bureaucratic hoop; it is the only thing standing between a well-designed install and a regrettable one. Where it breaks is where the installer is too thinly resourced to do the room-by-room work properly.
Frequently asked questions
Is a heat loss survey free?
Most MCS installers fold the survey cost into the install quote rather than charging separately. Some installers charge a standalone survey fee of £150 to £400, refundable against the eventual install order. Free surveys are common but the quality varies.
How long does a heat loss survey take?
The site visit is typically two to four hours for a three or four bedroom home. The calculation and deliverable take another half-day to a day of office work. A complete survey package is usually delivered within five to ten working days.
Can a heat loss survey be done remotely?
Some installers offer a remote pre-screen using floor plans, EPC data and a video walkthrough, but MIS 3005-D requires on-site fabric verification before MCS lodgement. A purely remote survey is not compliant.
Does an EPC replace a heat loss survey?
No. An EPC uses RdSAP methodology with default assumptions, not BS EN 12831 room-by-room calculation. The EPC is a screening tool. The MIS 3005-D survey is a design document.
What happens if the installer's calculation is wrong?
A poorly sized heat pump will short-cycle, deliver low SCOP and miss its performance specification. MCS has a complaints route, and the installer's MCS certification can be suspended. The Renewable Energy Consumer Code (RECC) also adjudicates.
Does the survey need redoing when an extension is added?
Yes. A new extension changes the heat loss profile and may push the system size beyond the original spec. An updated MIS 3005-D calculation is required if the heat pump is being upgraded.