The short answer
An air source heat pump installation follows a clear sequence. First an installer carries out an MCS heat loss survey, measuring how much heat the home loses room by room. From that they produce a system design: pump size, radiator changes, flow temperature and the hot water cylinder. On installation, the team fits the outdoor unit on a base or wall bracket, connects it to the heating system, installs or connects a hot water cylinder, upgrades any radiators that are too small, and wires the controls and electrical supply. They then commission the system — filling, pressurising, configuring the weather compensation and balancing the heating. Finally the installer issues the MCS certificate and paperwork needed for the Boiler Upgrade Scheme grant. For a typical home the on-site work usually takes a few days, though preparation and design come first.
A heat pump installation is more design-led than a simple boiler swap. Understanding the stages helps you know what a good installer should do and what to expect on the days they are on site.
Installation at a glance
- Stage 1MCS heat loss survey
- Stage 2System design and quote
- Stage 3Fit outdoor unit and cylinder
- Stage 4Upgrade radiators if needed
- Stage 5Commission and balance
Stage 1 and 2: survey and design
The work begins before anyone lifts a tool. An installer carries out an MCS heat loss survey — a room-by-room calculation of how much heat the property loses at a cold design temperature. This determines the correct pump size, which radiators need upsizing, the design flow temperature and the right hot water cylinder.
From the survey the installer produces a system design and quote. A good design is shared with you and explains the assumptions: the heat loss figure, the chosen flow temperature, the expected efficiency and any emitter changes. This design stage is what separates a properly engineered installation from a guessed one, and it underpins the MCS certification and grant eligibility.
Stage 3 and 4: fitting the system
On installation day(s), the team works through the physical fit:
- Outdoor unit: positioned on a level base or wall bracket, sited to meet noise and permitted-development rules, with space around it for airflow.
- Pipework: insulated flow and return pipes connect the outdoor unit to the heating system, usually entering the house at a discreet point.
- Hot water cylinder: a heat-pump-suitable cylinder is fitted (or an existing suitable one connected), as the pump heats and stores domestic hot water rather than producing it instantly.
- Radiator upgrades: any radiators too small to deliver enough heat at the low flow temperature are replaced, as set out in the design.
- Electrics and controls: the pump's electrical supply, the controller and the weather compensation sensor are wired in.
The old boiler is removed where it is being fully replaced, and the system is reconfigured around the heat pump.
The order of work on these days is planned to keep the household as comfortable as possible. Installers usually fit and connect the new components — outdoor unit, cylinder, any new radiators and the controls — as far as they can before disconnecting the old heating, so the period without heat and hot water is kept short. Pipework runs are chosen to be as discreet and direct as possible, and the electrical work may involve a dedicated circuit and, in some homes, liaison with the network operator if the supply needs checking. Where radiators are being changed, the team drains the relevant part of the system, swaps the emitters and refills, which is why the heat loss design listing exactly which radiators change is so useful on the day. None of this is dramatic, but it is more involved than lifting out one boiler and hanging another, and a tidy installer will protect floors, plan cable and pipe routes in advance, and reinstate any disturbed surfaces before leaving.
| Component | What happens |
|---|---|
| Outdoor unit | Mounted, sited for noise and airflow |
| Hot water cylinder | Installed or connected |
| Radiators | Upsized where the design requires |
| Controls | Wired with weather compensation sensor |
Main components handled during installation. Sources: Energy Saving Trust; MCS.
Stage 5: commissioning and handover
With everything connected, the installer commissions the system. This means filling and pressurising the circuit, removing air, setting the weather compensation curve, configuring the controls and balancing the radiators so heat is distributed evenly. Commissioning is where a system is tuned to run at the low flow temperature it was designed for — skipping or rushing it is a common cause of poor performance.
Finally, the installer hands over the documentation. For an MCS-certified installation this includes the MCS certificate, which is required to claim the Boiler Upgrade Scheme grant. They should also explain how the controls work and how to run the system efficiently. A good handover leaves you understanding how to operate the heat pump, which is different from operating a boiler.
How to prepare and what a good installer looks like
Because a heat pump installation is design-led, the choice of installer matters more than for a simple boiler swap. The standards to look for are practical:
- MCS certification: essential for the Boiler Upgrade Scheme grant, and a baseline assurance that the installer works to recognised standards.
- A documented heat loss survey: a quote that arrives without a room-by-room heat loss calculation should be treated with caution, because the sizing may be guessed.
- A shared system design: a good installer explains the heat loss figure, the design flow temperature, the expected efficiency and which radiators need changing — and answers questions about all of it.
- References and track record: heat pumps reward installers who design for low flow temperatures, so previous work and reviews are worth checking.
On your side, a little preparation helps. Decide where you are happy for the outdoor unit to go, bearing in mind airflow and noise to neighbours; identify a location for the hot water cylinder, which takes more space than a combi boiler; and address any obvious insulation gaps, since better insulation lowers the heat demand and can reduce both the pump size and the radiator work. Planning the timing also matters — because the full process from survey to commissioning spans several weeks, it is sensible to start well before winter rather than during a cold snap. Good preparation and a careful installer together are what turn a heat pump from a source of worry into a quietly reliable heating system.
Frequently asked questions
Do I need to leave my home during installation?
Usually not for the whole job, but you will be without heating and hot water for parts of it while the old system is removed and the new one connected. Installers typically work to minimise disruption and restore heat as quickly as possible. Discuss timing with your installer, especially in cold weather or if anyone in the home is vulnerable.
Will the installer remove my old boiler?
When the heat pump fully replaces your existing heating, yes — the old boiler is normally removed as part of the work. The heating system is then reconfigured around the heat pump and hot water cylinder. If you are fitting a hybrid system, the boiler is retained, but that is a different installation and generally not grant-eligible.
Does the installation have to be MCS certified for the grant?
Yes. To claim the Boiler Upgrade Scheme grant, both the installer and the installation must be MCS certified, and the installer applies for the grant on your behalf as part of the process. The MCS certificate issued at the end of the job is the document that confirms eligibility.
Sources & further reading
- Energy Saving Trust — Air source heat pumps
- MCS — Heat pumps for consumers
- gov.uk — Boiler Upgrade Scheme
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.