Heat and hot-water requirements
This guide covers an air-to-water ASHP serving a hydronic heating and hot-water system. GSHP, water-source and air-to-air projects have additional source-side or distribution work. The heat pumps and air conditioning section links to those separate routes.
The design inputs include the rooms to be heated, intended room temperatures, hot-water use and any part of the building that will change before installation. Extensions, insulation, window replacement, changes of occupancy and removal of the existing heat source all affect that boundary.
An old boiler can often deliver more heat than the building needs and may have been selected before later insulation work. Its output is not a reliable heat-load measurement. Annual gas use and an EPC can help sense-check a result, but neither replaces a room-by-room design calculation.
The survey of the existing system covers:
- heat source and controls
- radiator or underfloor-heating schedule
- cylinder and hot-water arrangement
- visible pipe sizes and distribution routes
- rooms that struggle to warm
- known faults, cold spots or persistent noise
- actual fuel and electricity use
This gives the designer evidence about the property and provides a useful comparison after commissioning.
Building survey and room-by-room heat loss
For an MCS hydronic design, the peak heat load is calculated room by room using BS EN 12831-1:2017 with the applicable MCS provisions. The survey should leave a traceable record of the geometry and assumptions used.
Important inputs include:
- external dimensions and room volumes
- walls, floors, roofs, windows and doors
- construction, insulation and U-values
- exposed, ground, unheated and neighbouring-room boundaries
- thermal bridging
- ventilation and infiltration
- intended room temperatures
- local external design condition
Existing and proposed fabric cases should be kept separate. Where the heat-loss case depends on insulation or glazing upgrades, those upgrades must be complete before the corresponding design case and commissioning results are valid.
A copied room, missing extension, assumed cavity wall or incorrect window area can materially change both the building result and individual emitter sizes. The heat-loss report therefore needs to describe the property actually surveyed.
Use each room heat load to size its emitter and the whole-building heat load calculated under the applicable MCS method to select the heat source. The room values may not add directly to a displayed building value because internal heat transfer and ventilation are handled within the calculation method.
The heat-loss survey and room-by-room heat loss entries explain the evidence behind the calculation.
Selecting the heat pump
The selected unit must provide the required output at the design outdoor temperature and proposed water temperature. A model described as 8 kW does not necessarily deliver 8 kW at every condition.
Selection is tied to the exact model and the relevant manufacturer data point. The design states:
- calculated building heat load
- design outdoor temperature
- flow and return temperatures
- declared heat-pump output at those conditions
- whether defrost has been accounted for
- any supplementary electric-heater contribution
- minimum output or modulation information
- the design approach for domestic hot water
A heat-pump-only design under the applicable MCS design standard is not made compliant by relying on an electric immersion or backup heater to cover an undersized unit at the design condition. Hybrid systems have a separate, explicitly defined design route.
Selecting the next model above the calculated requirement may create a visible allowance because product sizes are discrete. That is different from adding several hidden safety margins to the heat-loss inputs and then oversizing the unit again.
Minimum output matters during mild weather. A unit that cannot reduce its output sufficiently may cycle unless the water volume, controls and system demand are suitable. The heat-pump sizing margin entry covers capacity evidence and uncertainty in more detail.
Design flow temperature and emitters
The design flow temperature is the heat-pump leaving-water temperature required at the design condition. It connects three parts of the design:
- heat-pump output and efficiency
- radiator or underfloor-heating output
- hydraulic flow required to move the heat
Weather compensation should reduce the requested flow temperature when the weather is milder. The design figure is therefore a cold-weather endpoint rather than a temperature the system should use throughout the year.
Each room needs enough emitter output at the proposed mean water temperature and room temperature. Radiator catalogue outputs are normally declared at a test temperature difference that may be much higher than the heat-pump design. The installer should correct the output using recognised or manufacturer data.
Existing radiators can remain where their calculated output, condition and water flow are adequate. There is no requirement to replace every radiator or to install underfloor heating in every heat-pump property. A room may instead need a larger panel radiator, additional emitter, fan-assisted unit or fabric improvement.
The emitter schedule shows:
- room heat load
- existing and proposed emitter
- declared or corrected output at the design temperatures
- design water flow where relevant
- valve or control changes
- excluded decoration and making good
If the design proposes a high flow temperature, compare the alternative equipment and emitter changes needed to reduce it. The design flow temperature and radiator emitter upgrades entries provide the calculation context.
Pipework, pumps and system volume
The system must move the design heat at the required water-flow rate and available pump head. Pipe suitability follows from internal bore, length, fittings, valves and total circuit resistance. A statement that existing microbore is always suitable or always unsuitable is not a hydraulic design.
Hydraulic calculations and drawings set out:
- retained and replaced primary pipework
- design flow and return temperature difference
- required circuit flow
- calculated resistance and available pump head
- system pump or additional pump duties
- minimum-flow route
- active water-volume requirement
- flushing, cleaning, inhibitor and water treatment
- insulation for all new and disturbed pipework
A buffer vessel, volumiser, low-loss header or hydraulic separator should have a stated purpose. It may provide active volume, hydraulic separation, minimum flow or defrost support. The same component can also introduce mixing or extra heat loss if it is arranged or controlled poorly.
Zoning and TRVs (thermostatic radiator valves) cannot be considered independently of hydraulics. Closing circuits changes available flow and system volume. The hydraulic design has to keep the heat pump within its requirements when room controls operate.
DHW cylinder and hot-water design
DHW is a separate higher-temperature duty. The cylinder is selected from the household’s draw-off pattern, usable mixed-water volume, heat-exchanger performance, recovery and available space.
The word “heat-pump cylinder” is not a complete specification. Ask for:
- exact cylinder model and storage volume
- vented or unvented arrangement
- heat-exchanger performance at the proposed heat-pump conditions
- recovery assumptions
- cold-main flow and pressure checks for an unvented system
- standing loss
- immersion heater and control
- hygiene strategy
- safety devices and discharge route
- access for service and eventual replacement
An existing indirect cylinder can stay only where the designer verifies that its heat exchanger, volume, condition and controls suit the proposed system. Coil area by itself does not establish heat-transfer performance.
The DHW strategy determines whether space heating pauses during cylinder recovery and how that affects the property. It also names the controller governing DHW temperature, schedule, priority and immersion use.
Outdoor unit position
The ASHP location affects airflow, sound, appearance, service access and the pipe run.
The survey should consider:
- manufacturer clearances and airflow
- risk of discharged air recirculating
- access for maintenance and replacement
- base, brackets and vibration isolation
- condensate and defrost-water drainage
- snow, leaves and other obstruction
- exposure to impact or deliberate damage
- refrigerant and split-system restrictions where applicable
- distance to neighbouring habitable-room windows and doors
- appearance and planning conditions
MCS 020 a) provides the noise calculation used by the current English permitted-development route where all relevant conditions are met. Passing that calculation does not by itself establish permitted development. Other limits on location, unit numbers, size and property type still apply.
Planning law differs across the UK. Obtain the appropriate nation and local-authority position for the actual site. A planning calculation, decision or lawful-development evidence should use the final unit and position, not an earlier option.
Electrical supply and DNO process
The electrical design covers the heat pump, immersion heater, supplementary heater, pumps, controls and any trace heating or accessories. The electrical assessment covers the service, main fuse, earthing, bonding, consumer unit, maximum demand and circuit route.
Heat pumps must be registered with the DNO in Great Britain. The installer uses the current ENA process to establish whether prior approval is required or whether the installation can be notified after connection.
The heat pump’s electrical input, starting behaviour, power-electronic characteristics and flexible-connection capability may affect that route. The unit’s thermal kW rating is not the figure used for every electrical assessment.
The electrical design and handover records include:
- proposed circuit and protective devices
- isolation arrangements
- maximum electrical demand for all heat-pump system loads
- DNO submission
- any agreed connection limit or required network work
- DNO acknowledgement or authorisation
- Electrical Installation Certificate and Building Regulations evidence
Northern Ireland has a separate network operator and connection process.
MCS, BUS and consumer protection
MCS certification concerns the installer, product and installation standard within the scheme scope. The certificate identifies the responsible legal entity and applicable technology. Consumer-code membership, payment protection, workmanship cover and product warranties sit outside that technical certificate.
BUS (Boiler Upgrade Scheme) is an installer-led grant scheme in England and Wales. Eligibility, technology categories and grant values are governed by the live Ofgem and GOV.UK scheme material.
Where BUS applies, the installer submits the voucher application and the property owner confirms consent directly with Ofgem. The Ofgem process, voucher and scheme records establish the grant position. The contract and invoice record how the grant affects the payment schedule, but those commercial documents do not themselves prove BUS compliance.
MCS certification, consumer-code membership, payment protection, workmanship cover, product warranty, BUS administration, planning and the DNO process have separate scopes and may be evidenced by different organisations.
Installation work and disruption
The physical scope may include removal of the former heat source, cylinder replacement, radiator changes, primary pipework, outdoor-unit base, electrical circuits, controls, wall penetrations, condensate drainage and lifting access.
The programme should state:
- when the home will be without heating or hot water
- who drains, flushes and refills the system
- treatment of asbestos or other hazardous materials
- floor, cupboard, wall and garden access
- responsibility for decoration and making good
- removal and disposal of old equipment
- protection of retained equipment
- contingency for hidden pipework or an inadequate electrical supply
Photograph concealed routes and final valve positions where it will help later maintenance. Changes to the design should be recorded before they are covered or commissioned.
Commissioning and controls
Commissioning should record that the installed system operates safely and matches the design, including the hydraulic, control, DHW and electrical functions that apply.
The records should cover as applicable:
- water cleanliness, pressure and inhibitor
- purge and air removal
- pump and valve operation
- design or commissioned water flow
- flow and return temperatures
- emitter balancing
- heat-pump and supplementary-heater settings
- weather-compensation curve and limits
- room-control and zone behaviour
- DHW setpoint, schedule, sensor and immersion control
- defrost and condensate path
- electrical test results
- DNO and MCS details
Controls should be explained using the installed screens and apps. The owner needs to know which device sets space-heating temperature, weather compensation, room influence, DHW, schedules and backup heat.
The heat-pump controls and settings guide explains how to document and adjust these functions after handover.
Handover records
The complete digital and paper record includes:
- final room-by-room heat-load report
- emitter schedule and design flow temperature
- exact heat-pump and cylinder models
- system schematic and hydraulic calculations
- MCS 031 a):2025 pre-sale performance estimate
- MCS certificate
- BUS application and voucher records where applicable
- DNO submission and response
- planning evidence and MCS 020 a) calculation where applicable
- Electrical Installation Certificate and Building Regulations evidence
- commissioning results and final settings
- product manuals, warranties and serial numbers
- consumer-code and insurance-backed protection documents
- maintenance requirements and fault contacts
Owner or administrator access to each app and portal forms part of handover. Installer-only access can make later troubleshooting and service transfer unnecessarily difficult.
Evidence from normal operation
The building and weather change slowly, so a single warm commissioning day cannot prove winter performance. During normal operation, room comfort, outdoor temperature, flow temperature, electricity use, DHW behaviour and supplementary-heater operation provide the relevant evidence.
Relevant symptoms include rooms consistently below their design temperature, persistent cycling, repeated loss of flow, excessive noise, uncontrolled immersion use, frequent high flow temperatures or repeated manual overrides needed to keep the home comfortable. They need interpretation against the design and commissioning record.
Change one non-safety control at a time and record the old value. Do not alter installer limits, protection settings or hydraulic balancing without understanding their purpose.