If you’ve been in construction for any length of time, you’ll have noticed the change on site. Where a combi boiler used to be the default on every domestic job, there’s now an outdoor unit bolted to a concrete plinth and a hot water cylinder taking up cupboard space inside. Heat pumps are becoming a fixture on residential builds and retrofits, and that trend is only going one way.
The UK passed 250,000 cumulative MCS-certified heat pump installations in 2024, and the pace has only picked up since, with 2025 the strongest year on record for installs. The government’s Warm Homes Plan, published in January 2026, sets a target of 450,000 installations a year by 2030. For general builders and contractors, that means more jobs where a heat pump is part of the spec, even if you’re not the one fitting it. So how does the work actually differ from a standard boiler swap, and where does your responsibility end?
Why a Heat Pump Isn’t a Boiler Swap
A gas boiler heats water by burning fuel. An air source heat pump extracts heat from the outside air and compresses it to a higher temperature using a refrigerant cycle. That difference is important because it changes nearly everything about how the heating system is designed.
Boilers produce flow temperatures of around 60 to 80°C. Heat pumps work most efficiently at lower flow temperatures, typically 35 to 45°C. That means the radiators, pipework and hot water storage all need to be sized differently. Oversized radiators or underfloor heating are common on heat pump installs because they can deliver enough warmth at those lower temperatures. A standard panel radiator that worked fine with a boiler might not cut it with a pump.
Air source heat pumps now account for the vast majority of low-carbon heating installs in the UK, and MCS-certified specialist firms handle the design, heat loss calculations and compliance paperwork that sit behind every installation. As a general builder, you won’t be making those design decisions, but you will need to understand why the system looks different to what you’re used to.
Heat Loss Calculations Drive Everything
On a boiler job, the plumber often sizes the system based on experience and rough rules of thumb. Heat pumps don’t leave that margin. Every installation starts with a room-by-room heat loss calculation, usually to MIS 3005 standards, which is the MCS installation standard for heat pumps.
The calculation accounts for wall insulation, glazing, floor area, ceiling height, ventilation rates and local design temperatures. The result determines the pump’s output, the cylinder size, the emitter type in each room and the flow temperature. Get the heat loss wrong and the system will either underperform in cold weather or run inefficiently year-round.
For builders, this matters because the building fabric has to be right before the pump goes in. Insulation levels, draught-proofing and glazing all feed directly into the heat loss numbers. A well-insulated property can run a smaller pump at lower cost, while a draughty one will need a bigger unit and higher running costs.
DNO Notification and Electrical Work
Heat pumps run on electricity, and adding one to a property changes the electrical load. The installer is required to notify the local Distribution Network Operator (DNO) before or after installation, depending on the property’s maximum demand.
There are two routes. If the new maximum demand stays at or below 60 amps, the installer can connect the pump and notify the DNO within 28 days. If it falls between 60 and 100 amps, they’ll need to apply to the DNO for a connection assessment before installation, which takes up to 10 working days.
The electrical work itself, including a dedicated circuit from the consumer unit and sometimes a consumer unit upgrade, must be done by a Part P competent electrician. That’s usually handled by the heat pump installer’s team, but if you’re managing the wider project, make sure this is coordinated early. Delays on DNO approvals or consumer unit upgrades can push back the whole programme.
Where Your Work Ends and the MCS Installer Begins
This is the bit that catches some contractors out. Heat pump installations must be carried out by an MCS-certified installer to qualify for the Boiler Upgrade Scheme grant, which currently covers £7,500 towards the cost of an air source heat pump. That figure rises to £9,000 for off-gas-grid properties replacing oil or LPG heating, under a temporary uplift running until March 2027. The installer also handles the MCS certification, SCOP documentation and warranty registration.
As a general builder, your involvement will typically cover the groundwork (the concrete plinth or wall brackets for the outdoor unit), any internal building work to accommodate a hot water cylinder, pipework routes through the building fabric, and making sure the property is properly insulated before handover to the heat pump team.
You won’t be commissioning the refrigerant circuit, running the heat loss software or filing the DNO paperwork. But if you’re project-managing the build, you’ll need to know the install sequence. The outdoor unit needs clear airflow around it, typically 300mm at the sides and rear with no obstructions above. It also needs to sit on a level base that can handle the weight and vibration. Getting the plinth wrong or boxing the unit into a tight corner are common mistakes that create noise issues and reduce efficiency.
What’s Coming Next for Builders
The Warm Homes Plan and the Future Homes Standard mean heat pumps will increasingly feature on new-build specs over the next few years, and the retrofit market is accelerating fast. The Boiler Upgrade Scheme has already processed over 100,000 applications, and every one of those installs needed a builder somewhere in the chain.
You don’t need to retrain as a heat pump engineer, but you do need to know enough to work alongside one. The builders who treat heat pumps as someone else’s problem will keep losing days to rework and miscommunication. The ones who learn the basics will find themselves on more projects, more often.
