Air to air heat pump installation: what actually happens

In short

What gets installed, and where

Every air-to-air system has three parts.

The outdoor unit is a condenser containing the compressor and a fan, mounted on an external wall, a flat roof or a ground-level bracket. It needs clear airflow around it and it needs to sit somewhere the noise will not become a neighbour dispute. Siting is the decision most likely to cause trouble later.

The indoor units are usually wall-mounted, high on the wall, positioned to circulate air across the room rather than blow directly at a bed or sofa. Floor-standing and ceiling cassette options exist at higher cost.

The pipework is two insulated copper refrigerant lines, a condensate drain and a control cable running between the two. Shorter and straighter is cheaper and more efficient. Long runs through lofts or across floors typically add £200 to £300.

The installation process, step by step

Survey. A competent installer visits, measures the rooms, calculates the cooling and heating load, agrees unit positions and plans the pipework route. If anyone quotes without visiting or without asking room dimensions, treat the quote as unreliable.

Sizing and design. Capacity is calculated at roughly 0.1 kW per square metre of floor area, more for kitchens and sun-facing rooms. An undersized unit runs flat out and never satisfies the room; an oversized one short-cycles, leaving the space cold and humid.

Planning and permissions check. Many domestic installations are permitted development, but conservation areas, listed buildings, flats and front elevations facing a highway frequently are not. Leaseholders will usually need freeholder consent regardless of the planning position.

Mounting. Brackets are fixed for the outdoor unit and the indoor backplates are mounted. A 60 to 80 mm core hole is drilled through the external wall for each indoor unit.

Pipework and electrics. Refrigerant lines, condensate drain and control cable are run and insulated. A dedicated electrical circuit is installed if one is needed, which is common.

Pressure test and vacuum. The system is pressure tested for leaks, then evacuated with a vacuum pump to remove air and moisture. Skipping or rushing this is the single most common cause of premature failure, and it is the step a cheap installer cuts.

Charging and commissioning. Refrigerant is released or added, the system is run in both heating and cooling modes, and performance is verified.

Handover and certification. You should receive the F-Gas certificate, the manufacturer warranty registration, the commissioning record and, for a grant installation, the MCS certificate.

How long it takes

Add time for scaffolding, difficult access, or where floors need lifting.

The two certifications, and why the difference matters

This trips up almost everyone, so it is worth being precise.

F-Gas certification is a legal requirement. Under the F-Gas regulations, anyone working on the refrigerant circuit of a split system must hold it. This applies to every installation, grant or no grant. Ask for the certificate number before you book. An F-Gas certified installation typically costs £200 to £400 more than an unregulated one, and the cheaper option is not legal.

MCS certification is a grant requirement. The Microgeneration Certification Scheme is the quality mark that unlocks the Boiler Upgrade Scheme. Since 28 April 2026 it sits in the primary regulations rather than in guidance, so only an MCS certified installer can claim BUS. MCS certification is technology-specific: an installer certified for air-to-water is not automatically certified for air-to-air, and adding it requires a fresh assessment by their certification body.

These are separate schemes with separate assessments. An installer can be F-Gas certified and perfectly competent while being unable to claim your grant.

And a current constraint worth knowing about. As of mid-2026, air-to-air product certification through MCS was still being rolled out, and without a listed product an installer cannot raise a compliant grant voucher. See where the grant actually stands right now.

Can you install it yourself?

For a split system, no. The refrigerant circuit is legally restricted to F-Gas certified engineers, and self-installation voids the manufacturer warranty and any insurance position.

There are two partial exceptions. Some manufacturers sell pre-charged systems where a competent DIYer can mount the units and run the pipework, but a certified engineer must still connect and commission the refrigerant side. And portable units require no installation at all, only a window to vent the exhaust hose, which is the genuine self-install option if the alternative is not viable.

If you live in a flat

Flats are the most common source of installation problems, and the constraints are usually contractual rather than technical. Freeholder consent is nearly always required for anything fixed to the external fabric of the building. Lease terms frequently prohibit alterations to the exterior without written permission. Permitted development rights are more restricted for flats and maisonettes than for houses. Siting on a balcony or light well needs checking for airflow and noise. And managing agents can take weeks to respond, so start early.

None of this makes it impossible. It does mean starting the consent conversation before you get quotes. See our guide to planning permission for air conditioning.

How to choose an installer

Ask for the F-Gas certificate number, which is non-negotiable and verifiable. Ask for MCS certification and scope if the grant matters to you, specifically whether air-to-air is on their scope. Ask for a written specification listing model numbers, kW output and SCOP for each unit. Ask what is excluded, because electrical works, making good, scaffolding and pipework routing are the usual gaps. And get three quotes, because variance on identical specifications is routinely 30 per cent.

For what the whole job costs, see our air to air heat pump cost guide.

Frequently asked questions