Air Source Heat Pumps and Air-to-Air Systems in UK Retrofit

A Familiar Technology Wearing an Unfamiliar Badge

A recent installation video followed a homeowner fitting a five-room mini split system into an ordinary three-bedroom semi-detached house in England. Nothing was torn apart. No ductwork snaked through the loft. Each room received a wall-mounted indoor unit, connected by a slim pair of copper refrigerant pipes and a control cable running through a cored hole to a compressor outside. The whole job took five days.

For most homeowners, that is a story about cooling. For anyone working under PAS 2035, it is a story about heating — because a reversible mini split is an air-to-air heat pump, and the same hardware that dumps heat outside in July pulls heat indoors in January.

That distinction matters in exams, and it matters more on site. Candidates who only associate “heat pump” with a large monobloc unit feeding a radiator circuit tend to lose marks when a question describes a room-by-room air-to-air arrangement and asks how it should be recorded, ventilated, or sequenced.

Tip: When a question describes refrigerant pipework and an indoor fan coil rather than a flow and return to radiators, you are being asked about air-to-air, not air-to-water. The efficiency logic is similar; the assessment, controls, and emitter questions are not.

Air-to-Air Versus Air-to-Water

Both are heat pumps. Both extract low-grade heat from outdoor air and upgrade it using a vapour compression cycle. The difference is what carries the heat into the dwelling.

Feature	Air-to-air (mini split)	Air-to-water
Heat carrier	Refrigerant to indoor fan coil	Water to radiators or underfloor
Domestic hot water	Not provided	Usually provided via cylinder
Cooling capability	Yes, if reversible	Rarely
Emitter upgrade needed	No wet emitters involved	Often, to suit lower flow temperatures
Control	Per room, often remote or app	Whole-house, usually weather compensated
Fabric-first dependency	Still applies	Strongly applies

The installation in the video drew a maximum of around 1500 watts per outdoor unit and was wired into an existing ring final circuit through the back of a socket. Five outdoor units together consumed roughly 0.8 kWh a day in standby alone — about 30 watts each, continuously. Heating the kitchen for a couple of hours on a chilly November morning averaged around 250 watts, against a gas boiler rated at 6000 watts on full output.

Those figures are a useful reality check for anyone revising services. They show two things at once: the running efficiency of a heat pump serving one occupied room can be genuinely low, and the parasitic standby load of a multi-unit system is not zero. A coordinator comparing options for a work-from-home household needs both halves of that picture, not just the headline coefficient of performance.

Did you know? Standby consumption across a multi-split installation can quietly add up to more than 290 kWh a year. That is a real cost line that never appears in a manufacturer’s seasonal performance figure.

Where This Sits in PAS 2035

PAS 2035 sets out the whole-dwelling approach: assess, plan, design, install, monitor and evaluate. A heating technology choice is never made in isolation from that sequence.

The fabric-first principle still governs. Fitting an efficient heat generator into a leaky, poorly insulated dwelling shifts the running cost problem without solving it. The medium-term improvement plan exists precisely to record the order in which measures should be delivered, and heating plant almost always follows fabric and ventilation work rather than preceding it. Our Retrofit Coordinator Level 5 Topic Map walks through how the MTIP fits alongside the other coordination duties.

Three points that regularly appear in coordinator assessment questions:

1. Risk assessment drives the path. The dwelling’s risk category determines the level of assessment and design input required before any measure is specified.
2. Ventilation must be reassessed whenever fabric changes. An air-to-air unit filters and recirculates internal air. It does not introduce fresh air. Background ventilation, extract in wet rooms and purge provision remain separate obligations.
3. Interactions between measures must be identified. Adding a heating measure to a dwelling that has recently received insulation changes the heat demand profile, the internal surface temperatures and the moisture risk balance.

That last point links directly to condensation control. If a dwelling is being cooled in summer, internal surfaces can fall below the dew point of humid incoming air, and moisture can appear in places where nobody expected it. Our guide to damp, mould and condensation covers the underlying physics that examiners keep returning to.

Remember: A measure that reduces energy use and a measure that reduces risk are not automatically the same measure. PAS 2035 asks you to demonstrate both.

What the DEA Needs to Capture

For a Domestic Energy Assessor, the practical question is narrower and more precise: how is this system recorded?

RdSAP conventions require the main space heating system, its fuel, its efficiency and its controls to be identified and evidenced. Where a room-by-room air-to-air system is genuinely the main heating for the dwelling, it is recorded as such, with appropriate evidence of the equipment installed. Where it supplements an existing gas boiler — which is the more common British arrangement, and the one described in the video — the boiler usually remains the main system and the units are treated as secondary heating.

Getting that call right requires evidence, not assumption. Useful items to collect on site include:

• Manufacturer and model of both indoor and outdoor units
• Whether the units are reversible or cooling only
• Which rooms are actually served, and which are not
• The presence and type of controls, including app or programmable control
• Photographic evidence of the outdoor unit and data plate
• Whether any existing wet system remains in place and operable

Consistency here matters more than speed. Two assessors visiting the same property should reach the same recorded system, and the way to achieve that is a repeatable site routine. The DEA Exam Workflow Revision Guide sets out a structure for that sequence, from preparation through to EPC production.

Tip: In DEA questions, watch for the word “main”. Examiners often describe a supplementary system in detail to see whether you will incorrectly promote it to main heating.

Installation Detail Worth Recognising

The video showed a competent installation, and several details in it map onto knowledge that assessors and coordinators are expected to recognise:

• Brazed connections at the indoor unit, made before commissioning, to reduce the chance of a refrigerant leak inside the dwelling
• Pressure testing of the pipework before charging
• Evacuation of the pipework to remove moisture and non-condensable gases prior to filling with refrigerant
• Penetrations through the external wall for pipes and cables, sleeved and made good
• Trunking used to conceal and protect runs both internally and externally

Two of those matter for fabric integrity. Every core drilled through an external wall is a potential thermal bridge and a potential air leakage path, and every penetration through an insulated wall must be sealed correctly. Where external or internal wall insulation is already present or planned, the sequencing of a pipe penetration becomes a genuine coordination issue rather than a detail left to the installer on the day. If you work on the installation side, our post on external wall insulation covers the same interface from the other direction.

Refrigerant handling itself is governed by F-Gas requirements and sits outside the PAS 2035 assessment role. You are not expected to charge a system. You are expected to know that the person doing it must be suitably qualified, and to record that the work was carried out by a competent installer.

Revision Angles to Take From This

If you are preparing for a coordinator or DEA assessment, this topic is worth revisiting through four specific lenses rather than as a single block:

Lens	Question to ask yourself
Technology	Can I describe the vapour compression cycle in two sentences?
Assessment	How would I record this system on site, and what evidence proves it?
Coordination	Where does this measure sit in the improvement plan, and why?
Risk	What moisture, ventilation or overheating risk does it introduce?

Working through the same technology from four angles is far more effective than reading a single description four times. It is the same principle behind using short practice sessions — retrieval from different directions builds a more usable memory than repetition from one.

How RetrofitReady Fits Into This

Heating systems, controls and their interaction with fabric and ventilation appear right across the retrofit qualifications, and they are exactly the kind of topic where confident-sounding guesswork gets punished in an exam.

RetrofitReady: Coordinator provides Level 5 Retrofit Coordinator preparation with 600 PAS 2035 questions across domestic retrofit, risk, fabric, services, ventilation, MTIP, and monitoring. The services and MTIP sections are where heat pump strategy, measure sequencing and interaction risk are tested most directly.

RetrofitReady: DEA offers Domestic Energy Assessor revision with 360 DEA questions covering safe practice, preparation, assessment, EPC production, and focused supplement units. If your uncertainty is about what to record rather than what to specify, that is where to start.

Both apps sit within the wider RetrofitReady suite of 1900+ questions across 4 apps, alongside Retrofit Assessor and Retrofit Installer, so you can move between routes as your role develops without changing platform.

A sensible approach with this topic:

1. Practise the services and heating topic on its own until your score stabilises.
2. Move to ventilation and moisture questions, watching for the interactions between the two.
3. Read the explanations for every question you answer correctly by elimination, not just the ones you get wrong.
4. Return to the same topic four or five days later before moving into mixed practice.

Alongside app practice, the recommended books and tools page lists building services, ventilation and site-inspection references chosen for RetrofitReady learners.

Start with the free quizzes or browse the full RetrofitReady app suite.