Replacing an old boiler

How to plan the change from a boiler to a heat pump across heat loss, emitters, hot water, services, removal and handover.

Replacing a boiler with a heat pump is a heating-system project, not a like-for-like appliance swap. The heat source, radiators, pipework, hot water, controls, electrical supply and outdoor site all have to work at the same design conditions.

Planning before the old boiler fails gives time to survey those parts properly. An emergency winter replacement creates pressure to accept a design that has not been tested against the building.

Define what the old system does

The first survey should record more than the boiler make and output. It needs to establish:

rooms and areas currently heated
how well each room reaches temperature in cold weather
radiator, underfloor or fan-convector types and sizes
pipe layout, bore, condition and known one-pipe or microbore sections
hot-water type, outlets, occupancy and usage pattern
boiler controls, zones and any secondary circulation
the incoming electrical supply and consumer unit
possible outdoor-unit, cylinder and internal-equipment positions
flue, gas, oil, LPG or solid-fuel equipment that will be removed or retained

Existing fuel use can be a useful cross-check, but it does not replace a room-by-room design. Bills reflect past weather, occupancy, boiler efficiency and thermostat behaviour.

Calculate heat loss before selecting the heat pump

MIS 3005-D requires a heat-load calculation using the applicable design method. The designer uses it to select a heat pump that can provide the required output at the design outdoor and flow temperatures, not merely at a mild catalogue test point.

The calculation should use the building that will exist when the system is commissioned. If insulation, windows or an extension are part of the project, their final specification and timing need to be clear. Aspirational future work should not be used to under-size today’s system.

For a heat-pump-only design under the current MCS:2025 standard, the selected unit must provide at least the calculated heat load at the relevant design conditions without counting a supplementary electric heater toward that capacity. Defrost performance also needs consideration for an air-source unit.

An oversized unit is not a safe shortcut. It can cycle more, need greater electrical capacity and sit outside its efficient modulation range in mild weather.

Check emitters and pipework room by room

Boilers are often run at higher water temperatures than an efficient heat-pump design. Each existing radiator therefore needs its output recalculated at the proposed flow, return and room temperatures.

Some radiators may already be large enough. Others may need greater surface area, additional panels, a fan-assisted emitter or a fabric improvement. One difficult room should be investigated before it drives the entire system to a higher flow temperature.

Pipework suitability depends on required flow and pressure loss. Microbore is not automatically unusable, but long shared runs, one-pipe systems, restrictive valves and hidden sludge can require changes. The hydraulic design should show the required circuit flows and how the heat pump’s minimum flow is maintained as zones close.

The final design includes a room heat-loss and emitter schedule, not a vague allowance for “radiator upgrades if needed”.

Plan domestic hot water

A combi boiler heats water as it is drawn. Most air-to-water heat-pump systems instead use stored hot water, so the project may need a cylinder and the space, pipework and safety controls that go with it.

MIS 3005-D sizes domestic hot-water demand and requires a cylinder and heat exchanger suitable for the selected heat pump. A heat-pump cylinder commonly needs a larger heat-exchanger surface than a boiler cylinder because the primary water is cooler.

An existing cylinder can be retained only after checking:

capacity against household demand
coil or heat-exchanger suitability
insulation and standing loss
mechanical condition, corrosion and scale
compatibility with the heat pump and required water temperature
safety devices and current installation requirements

Hot-water reheat, space heating and pasteurisation controls should be designed together. An immersion heater may provide high-temperature pasteurisation or backup, but it should not conceal an undersized cylinder coil or poor control setup.

Where no suitable cylinder space exists, the designer should discuss real alternatives and compromises before contract. A brochure promise that a heat pump “replaces any combi” is not a hot-water design.

Outdoor unit, services and approvals

The outdoor position must pass airflow, noise, condensate, access, structure and refrigerant-safety checks. Pipe length alone should not decide it.

The electrical designer assesses the compressor and every auxiliary load, the consumer unit, maximum demand and DNO connection route. Planning status, building regulations and any listed-building or landlord consent are separate checks.

The project should also identify routes for:

insulated flow and return pipes or split-system refrigerant lines
power, controls and communications
defrost water to a suitable drain or soakaway
cylinder discharge pipework where applicable
removal or safe termination of the old fuel supply

Coordinating these on one drawing avoids discovering during installation that the planned cylinder blocks access or the condensate route enters an R290 protection area.

Decommission the old boiler safely

The old appliance should normally remain available until the new design, equipment and approvals are ready. The changeover plan should state how long the home will be without heating or hot water and what happens if commissioning cannot be completed.

Gas work, including disconnection and making safe, is for an appropriately registered Gas Safe engineer. Oil, LPG, solid-fuel and electrical systems have their own competence and environmental requirements. Old tanks, flues, asbestos-containing materials and contaminated pipework may need specialist handling.

Decide whether redundant pipework, flues and tanks are removed, capped or left for another phase. A capped fuel pipe should be identifiable and safe; an abandoned external opening should be properly weathered.

The heating circuit itself normally needs cleaning, flushing and water treatment before connection to new equipment. Reusing dirty water because the radiators stay in place risks transferring debris into the heat pump and small waterways.

Controls change with the heat source

A boiler is often operated in shorter high-temperature bursts. A heat pump usually performs better with longer operation, weather compensation and the lowest water temperature that meets demand.

The control design should coordinate space-heating zones, hot-water priority, backup heat and any retained supplementary appliance. In a hybrid system, MCS requires one master control strategy rather than two heat sources competing through separate thermostats.

The owner needs practical guidance on normal schedules, weather-curve adjustment, hot-water settings and what not to change. Recreating the old boiler’s on-off pattern can undermine an otherwise good installation.

Funding and contract timing

As at 22 July 2026, the Boiler Upgrade Scheme is an installer-led grant scheme for eligible properties in England and Wales, and its categories and values have recently changed. The live Ofgem and GOV.UK rules govern the current amounts, application process and technology categories.

Grant eligibility is not a substitute for system suitability. The contract should separate the full project price, grant assumption, customer contribution, enabling work and what happens if eligibility or project scope changes.

Do not remove a serviceable boiler solely because an application has been started. Equipment availability, DNO or planning approval and the grant process should be sufficiently advanced for the chosen project sequence.

Commissioning and handover

Before the project is complete, the installer should prove:

the system matches the final design and schematic
heat-pump flow and hydraulic balance are correct
emitters heat and controls operate as intended
domestic hot water, safety devices and pasteurisation work
backup heat is controlled and not masking a fault
electrical tests and DNO requirements are complete
condensate and outdoor clearances are satisfactory
weather compensation is enabled and set for the design

The handover includes the heat-loss and emitter calculations, product and commissioning records, electrical and building certificates, planning and DNO evidence, warranties, applicable MCS certificate, control settings and maintenance instructions.

A heat pump is not properly handed over when it merely makes the radiators warm on installation day. The records should show why it can heat the building at the design condition and how it is intended to operate through the year.