10 air source heat pump installation mistakes and how to avoid them

What to know first

These points cover the design decisions that most affect an air-to-water heat pump’s comfort and performance.

Poor performance is rarely explained by the outdoor unit alone. A successful installation needs the heat pump, emitters, pipework, controls, hot-water system and building to work together. Current MCS standards cover the design, installation, commissioning and handover of the complete system.

This article mainly covers air-to-water heat pumps connected to wet central heating. Air-to-air systems distribute heat through indoor fan units, so some mistakes do not apply in the same way.

Why whole-system design matters

Issue 3.0 of the MCS design standard became mandatory for relevant MCS contractors on 5 December 2025. It can be used with the installation standard to cover supply, design, installation, commissioning and handover.

A proposal should therefore show the calculated heat losses, design temperatures, emitter sizes, cylinder specification where relevant, expected performance and work included—not only a model name. MCS requires much of this information to be supplied in writing.

Expert insight

“The outdoor unit is one part of the heating system. A strong proposal makes the design logic visible, showing how the calculations, equipment and settings fit together.”

Giles Crosse

Energy Editor

Experienced editor, journalist and communications consultant specialising in consumer energy and low carbon technologies.

• Editor and campaign author for Shell, EDF Energy and Good Energy.
• Consultant to the United Nations and contributor to the World Economic Forum.
• Journalist for Reuters, the BBC, The Economist and The Guardian.

More about Giles Crosse

The 10 mistakes—and how good design prevents them

1. Sizing from floor area or an EPC instead of heat loss

Sizing should use a heat-load calculation based on the building, ventilation, room temperatures and local winter design temperature. MCS says the annual heat demand on an Energy Performance Certificate is not suitable for sizing.

For a whole-home, non-hybrid system, the selected pump must provide at least 100% of the calculated load at the stated flow and outdoor temperatures. Good design uses room-by-room figures and manufacturer data, not a rough rule.

2. Oversizing the heat pump “just in case”

Some model-selection headroom may be unavoidable because equipment comes in fixed ranges. The problem is an unexplained jump to a much larger unit.

Every model has a minimum output. If this exceeds the home’s demand for much of the season, the unit may repeatedly switch on and off. Government research found that rapid cycling can reduce performance, so selection should consider minimum as well as maximum output.

3. Leaving radiators or other emitters undersized

A radiator releases less heat when the water passing through it is cooler. One that worked with a boiler may therefore be too small at the heat pump’s proposed temperature.

Not every radiator must be replaced. The installer should compare each room’s heat loss with its emitter output. MCS requires the customer to receive the emitter type, dimensions and design emitter temperature before work starts.

4. Designing around an unnecessarily high flow temperature

Flow temperature is the temperature of water leaving the heat pump. Raising it can help smaller radiators, but normally makes the heat pump work harder. Manufacturer performance changes with both the outdoor temperature and the required flow temperature.

MCS says high-temperature heat pumps should be avoided unless more than 55°C is needed. A design above 55°C should be accompanied by an alternative at 55°C or below.

A higher-temperature design may still be reasonable where larger emitters are impractical. The trade-off should be calculated and explained rather than hidden behind a single headline performance figure.

5. Ignoring pipework, water flow and system volume

Heat pumps commonly need higher water flow than boiler systems. Narrow pipework can work in some homes, but flow velocity and pressure loss must be calculated rather than assumed. Longer runs of narrow pipe are more likely to need alteration.

An air source heat pump also needs enough water available for stable operation and defrosting. The layout may need an always-open circuit, volumiser or buffer vessel. Each addition should solve a defined design need and reflect the manufacturer’s requirements.

6. Using controls that make the system stop and start

Heat pumps generally work best when controls support steady operation at the lowest temperature that maintains comfort. Weather compensation, or a heating curve, changes flow temperature with outdoor conditions. MCS says it should be enabled where it can optimise efficiency.

Poorly coordinated thermostats, valves, schedules and heating curves can cause cycling or unnecessarily hot operation. Commissioning should confirm how heating and hot-water controls interact and which settings the householder can adjust. Rapid cycling can adversely affect heat pump performance.

Expert insight

“A familiar boiler-style schedule is not automatically right for a heat pump. Controls should follow the building’s response and system design, then be explained at handover.”

Giles Crosse

Energy Editor

Experienced editor, journalist and communications consultant specialising in consumer energy and low carbon technologies.

• Editor and campaign author for Shell, EDF Energy and Good Energy.
• Consultant to the United Nations and contributor to the World Economic Forum.
• Journalist for Reuters, the BBC, The Economist and The Guardian.

More about Giles Crosse

7. Putting the outdoor unit wherever it is easiest to fit

The unit needs clear airflow. MCS says siting should avoid chilled-air recirculation, vibration through the building and nuisance to neighbours. Condensate must drain safely.

A position close to a bedroom, in a tight enclosure or between reflective surfaces may create avoidable problems. The installer should follow manufacturer clearances, assess vibration and keep the unit accessible for servicing.

Good to know

Permitted development is conditional. MCS 020a uses a 37 dB(A) sound limit at defined assessment positions, but meeting that figure alone is not enough. England made MCS 020a the sole permitted certification scheme from 28 May 2026, while Wales updated its permitted development rights on 1 June 2026. Scotland and Northern Ireland have separate rules, so check the relevant planning authority.

8. Specifying an unsuitable hot-water cylinder

This applies to air-to-water systems that heat domestic water. The cylinder must suit expected demand and heat-pump output, and the desired reheat time should be agreed.

Its internal heat exchanger matters too. MCS notes that heat pump cylinders generally need a larger heat-exchanger surface than boiler cylinders because the water temperature is usually lower. An existing cylinder must be assessed for its condition, insulation and suitability.

9. Treating commissioning as a quick switch-on

Commissioning checks and sets the finished system so it operates safely and as designed. It is more than confirming that the unit turns on.

The procedure should follow manufacturer requirements and record the checks completed. Controls, design settings and whole-system operation should be confirmed before handover.

10. Finishing without a complete handover

MCS requires a handover pack with commissioning forms, maintenance information, manuals, warranty details and relevant scheme paperwork. The contractor must provide and explain it.

The MCS certificate should be generated only after full installation and commissioning. The contractor must register the installation within 10 working days of commissioning and send the certificate to the customer.

What a sound proposal should show before work starts

A useful proposal should show:

• room-by-room heat losses;

• model output at the stated outdoor and flow temperatures;

• each emitter and proposed replacement;

• design flow temperature and expected seasonal performance;

• cylinder size and recovery time, where relevant;

• included pipework, hydraulic, electrical and building work;

• outdoor location, sound calculation and condensate route;

• responsibility for planning, building-control and network notifications.

MCS requires much of this information in writing before installation begins.

If the agreed design or estimated performance changes after detailed design, the customer should receive an updated estimate, a contract variation and an opportunity to cancel without further cost, obligation or liability.

The installer must assess building suitability and deal with the distribution network operator—the local electricity-network company—under Energy Networks Association procedures. Advance permission is required in some cases. Planning, heritage, leasehold or freeholder requirements should also be settled where relevant.

Good to know

In England and Wales, Boiler Upgrade Scheme grants checked on 13 August 2026 were £7,500 for air-to-water heat pumps and £2,500 for residential air-to-air systems. A temporary £9,000 grant applies to eligible off-gas-grid properties replacing oil or liquefied petroleum gas until 31 March 2027. The installer applies on the property owner’s behalf. Eligibility does not prove that a particular design is suitable.

Key takeaways

• Start with room-by-room heat loss.

• Select enough capacity without unexplained oversizing.

• Design emitters, pipework, controls and the cylinder around the same temperatures and flows.

• Explain any high-temperature design and its performance trade-off.

• Site the outdoor unit for airflow, drainage, noise, vibration and planning.

• Treat commissioning records and handover as part of the installation.

• Grants and permitted development remain conditional; the relevant rules were checked on 13 August 2026.

Frequently asked questions