MIS 3005 heat pump standard

MIS 3005 is split into two MCS standards for heat-pump systems:

• MIS 3005-D covers design
• MIS 3005-I covers installation, setting to work and commissioning

The split matters because a good heat pump is not simply a product attached to an existing heating system. Its output, emitters, controls and hydraulic arrangement have to be designed for the building, then the installation has to deliver and prove that design.

Current editions during the MCS transition

As at 22 July 2026, two MCS installer-scheme tracks are operating:

Scheme track	Design standard	Installation standard
Previous MCS installer scheme	MIS 3005-D Issue 3.0	MIS 3005-I Issue 1.0
MCS:2025	MIS 3005-D:2025 Issue 1.0	MIS 3005-I:2025 Issue 1.0

The documents are not identical. For example, the previous-scheme design standard was revised in December 2025 to include air-to-air heat pumps, while the MCS:2025 Issue 1.0 document predates that revision. During the transition, check the installer’s actual certification scope and the document listed for that scheme rather than assuming every heat-pump category is handled identically.

Scope and system size

The MCS:2025 design and installation standards cover heat pumps supplying permanent buildings with space heating, domestic hot water or both. They include ground, air and water heat sources, monobloc and split units, and heating-only or reversible systems, although a reversible system has to be designed and optimised for heating.

They exclude cooling-only systems, direct-expansion ground loops and heat pumps extracting heat from loft spaces. A domestic-hot-water heat pump within this scope is used for hot water only.

The scheme defines a microgeneration heat pump as having a thermal output no greater than 45 kW. Multiple units may serve one installation with a total design heat load no greater than 70 kW, provided no single unit exceeds 45 kW. These are stable scope thresholds in the named standards rather than current market figures.

MIS 3005-D: design around the building

The design starts with the building, not the nominal capacity on a sales brochure. MIS 3005-D requires the building and its curtilage to be assessed by a competent professional experienced in heat-pump systems.

For a hydronic space-heating system, the core evidence is a room-by-room heat-load calculation in accordance with BS EN 12831-1:2017 and the MCS method. This establishes the design conditions and the heat required in each room. The heat pump and emitters are then selected at the actual design flow temperature, not by comparing boiler and heat-pump nameplate figures.

For a heat-pump-only design, MCS:2025 requires the selected heat pump to provide at least 100% of the calculated heat load at the relevant flow and external design conditions, without relying on a supplementary electric heater for that capacity.

The design also needs to establish:

• the design flow temperature
• emitter output in each room at that temperature
• domestic-hot-water demand and cylinder suitability where applicable
• hydraulic volume, flow and control requirements
• electrical-supply adequacy
• ground- or water-collector design where relevant
• how defrost and other operating modes affect delivered heat
• the product performance data used to select the unit

The official MCS Heat Load Calculator implements the MCS approach and BS EN 12831-1:2017. It is a tool, not a substitute for a correct survey. Its reference data and calculation features can be updated, so the project record retains the completed calculation report and inputs.

Flow temperature and consumer choice

Flow temperature affects emitter size and heat-pump efficiency. Where a design proposes a flow temperature above 55°C, MIS 3005-D:2025 requires an alternative design at 55°C or below and an explanation of the differences in efficiency and energy consumption.

That does not make 55°C a universal maximum. It creates a comparison point so that a higher-temperature design is an informed choice rather than a hidden consequence of leaving undersized emitters unchanged.

The post-design information should preserve the calculated heat loss, emitter details, design emitter temperature, design flow temperature, hot-water specification and the seasonal performance figure used. Those values should agree across the calculation, quotation, final design and commissioning record.

Hybrid heat-pump systems

MIS 3005-D:2025 includes a specific route for a heat pump combined with a supplementary heat source. It requires:

• all heat sources together to cover at least 100% of the calculated heat load
• the heat pump to provide at least 55% of that load when its output is rated at 55°C flow temperature and the site design outdoor temperature
• a control philosophy capable of prioritising heat-pump use
• the heat sources to be operationally integrated under a single master control

These are design requirements for an MCS hybrid system. They do not establish that a hybrid will be cheaper to run or lower-carbon in every home. The controls, tariffs, fuel prices and temperatures at which the supplementary source operates remain important.

Electricity network and planning checks

The design contractor must confirm that the electricity supply is adequate and follow the current ENA/DNO process, seeking advance permission where required. A heat pump is a new electrical load, so this is a demand-connection question rather than G98/G99 generation approval unless the project also includes generation or storage.

For an air source heat pump intended to use permitted-development rights, the applicable planning conditions and MCS sound-assessment method also have to be met. Passing MCS 020 a) is one part of that planning route; it does not replace the other conditions or a planning application where permitted development is unavailable.

MIS 3005-I: install the approved design

MIS 3005-I covers the physical installation, setting to work and commissioning. The building must remain compliant with the applicable Building Regulations, and the equipment must be installed to the design and manufacturer instructions.

Practical requirements include suitable location and access, protection of pipework and equipment, safe condensate disposal, measures to avoid unreasonable noise, appropriate system controls and weather compensation where it is useful for the system.

Refrigerant work is controlled separately by the applicable fluorinated-gas rules. A split system with connections made on site can require qualified refrigerant work, while a factory-sealed monobloc presents a different task. MIS 3005 should not be used as a shortcut for deciding the current legal F-gas requirements; those are covered in the dedicated entry.

The electrical work remains subject to BS 7671 and the applicable Building Regulations. MCS certification is not an electrical installation certificate or a Competent Person Scheme notification.

Commissioning and evidence

MIS 3005-I requires a documented commissioning procedure showing that the system is safe, installed to the standard and manufacturer requirements, and operating in accordance with the design.

The useful record is not merely a tick saying “commissioned”. It should connect the installed system back to the design, including:

• installed heat pump, cylinder, pumps, controls and emitters
• design and commissioned flow temperatures
• flow rates and hydraulic settings required by the manufacturer
• weather-compensation and supplementary-heat settings
• electrical test and Building Regulations evidence
• sound-assessment assumptions and final location
• water treatment, pressure and safety controls where applicable
• ground-loop flushing, pressure, fluid and test evidence where applicable
• user instruction and maintenance requirements

If the installed unit, emitter sizes, cylinder, pipework or controls differ from the design, the designer should confirm the effect and update the calculation. A commissioning sheet cannot validate an undocumented substitution.

Pre-sale estimates and the Boiler Upgrade Scheme

Under MCS:2025, pre-sale information and annual performance estimates sit in a separate standard rather than in MIS 3005-D itself. The estimate is for consumer information; the system must still be sized from the building design load, not from an EPC annual-energy figure or a projected running cost.

The Boiler Upgrade Scheme currently requires the installer business to hold MCS certification for the relevant technology. Ofgem checks the MCS installation certificate at voucher redemption, but BUS has additional product, property and process conditions. An MCS-compliant design alone does not guarantee grant eligibility.

Related entries

• MCS
• MCS installation standards
• Room-by-room heat loss
• Design flow temperature
• Hybrid heat pump systems
• SCOP performance estimate
• Acoustic assessment for ASHP
• MCS 020 heat pump noise calculation
• F-Gas qualification rules
• Boiler Upgrade Scheme voucher process