Hybrid & Boiler-Replacement Retrofit: Heat pumps for landlords

Why a hybrid retrofit is the pragmatic route for most let buildings

The hybrid, or bivalent, heat pump is the route a landlord reaches for when the building needs decarbonising but a full heat-pump-only design is either too expensive or impractical given the existing emitters. A hybrid pairs a heat pump that covers the bulk of the annual heat demand with a retained or new peaking boiler that tops up only on the coldest days. For a landlord this is often the easiest project to get over the line: the capital is lower than a heat-pump-only design, the board approval is simpler, and the worst-case cold spell is de-risked because the boiler is still there for the handful of days a year it is genuinely needed.

It is also the sensible answer for the very common situation where a building's radiators and pipework were sized for a 70 to 80C gas boiler flow, because the hybrid keeps the project affordable while still cutting gas use and carbon by 70 to 90%. For a landlord under MEES and EPC pressure who cannot face the disruption or the cost of re-emittering an occupied building, the hybrid is the stepping stone that makes decarbonisation start now rather than in five years. It delivers most of the carbon benefit and the running-cost protection of a full heat pump, while leaving the worst-case heating security in place and spreading the capital commitment over a more comfortable path. For a multi-let portfolio, it is frequently the route that lets a landlord begin decarbonising across many buildings at once rather than committing the whole budget to one flagship full-heat-pump scheme. It is the design that turns net-zero from an intention into a programme a landlord can actually start this year.

What a typical install looks like and how we size it

A hybrid system usually pairs a 60 to 400 kW heat pump cascade with a retained or new bivalent gas boiler, in a plant area of around 20 to 120 square metres. The heat pump is deliberately sized for roughly 70 to 90% of the annual heat load, which covers the vast majority of operating hours, while the boiler handles only the rare coldest days. A hybrid of this kind delivers in the region of 90,000 to 800,000 kWh of heat a year, with the heat-pump share at 70 to 90% of that, and saves between 12 and 140 tonnes of CO2 annually.

The critical design decision is the changeover point: we set the bivalent control strategy to maximise heat-pump run hours, because the further you push the heat pump before the boiler takes over, the more carbon and gas you save. Set the changeover too cautiously and you leave savings on the table; set it without surveying the emitters and you risk poor comfort on cold days. As always, we keep the flow temperature low where the surveyed emitters allow it, since that protects the SCOP across the hours the heat pump is carrying the load. Because the heat pump is sized for the bulk of the load rather than the absolute peak, it is smaller, cheaper and easier to site than the unit a heat-pump-only design would need, which is a large part of why the hybrid economics work for a landlord.

Sizing the hybrid correctly is also what keeps the smaller electrical load manageable. A heat-pump-only design for the same building would draw considerably more power and is far more likely to trigger a DNO supply upgrade, whereas the hybrid's right-sized unit can frequently work within the building's existing supply, which removes one of the longest-lead and least predictable cost items from the project. For a landlord weighing several buildings, that difference in electrical impact, alongside the smaller plant footprint, is often what makes a portfolio-wide hybrid programme deliverable where a full heat-pump rollout would not be.

Costs, payback and tax relief

A hybrid retrofit typically runs £70,000 to £500,000, with the shortest payback in the heat-pump family at around 7 years, because the smaller heat pump and the retained boiler keep the capital down while still capturing most of the saving. For a landlord, the tax position is the same backbone as any heat-pump project: full expensing for companies, a 100% first-year deduction with no cap that is permanent from April 2026, worth up to 25p of tax saved per pound at the 25% rate, or the Annual Investment Allowance for unincorporated landlords, with the heat pump qualifying as plant and machinery.

Where the project retains or replaces a gas boiler, the boiler element and ancillary works should be checked with your accountant for the right capital-allowance treatment. On running cost, the hybrid captures most of the SCOP benefit of a full heat pump across the majority of operating hours, while the boiler covers only the small number of hours when an all-electric design would be at its least efficient and most expensive, which is part of what makes the hybrid running-cost case so robust. The domestic Boiler Upgrade Scheme does not apply to commercial premises. Our cost guide sets out worked numbers for the hybrid route specifically.

Funding routes in detail

Hybrid schemes can draw on the same commercial funding routes as any heat-pump project, matched to the building. A public-sector landlord or occupier can access the Public Sector Decarbonisation Scheme through Salix for buildings such as schools and local-authority property, which funds the cost over a like-for-like fossil replacement. An eligible industrial site can look at the Industrial Energy Transformation Fund for fuel-switching to heat pumps and waste-heat recovery. A multi-building scheme can consider the Green Heat Network Fund at up to 50% of eligible costs.

And every UK-taxed private landlord has full expensing or the Annual Investment Allowance as a reliable route on every qualifying project. We assess which apply at feasibility, because even where a grant is modest, stacking it with capital allowances meaningfully improves a hybrid business case. The hybrid's lower capital can also make a grant go further across more buildings in a portfolio, so we look at the funding question across the whole estate rather than building by building where a landlord is decarbonising at scale.

It is worth being clear about what the hybrid funding case is not. The £7,500 Boiler Upgrade Scheme is domestic-only and does not apply to a commercial hybrid, so a landlord should not plan around it. The genuine commercial routes above are what a hybrid project draws on, and because a hybrid is frequently the first decarbonisation step a landlord takes, getting the funding established on that first building also sets the template, and often the evidence base, for the funding applications on the rest of the portfolio that follow.

Compliance and sector considerations

A hybrid plant room puts both F-Gas and Gas Safe in scope at once, because you have refrigerant plant and a combustion appliance side by side, so we coordinate both certifications on the same project rather than leaving a landlord to manage two trades. The existing emitters and pipework are surveyed for flow-temperature compatibility before design, since the whole hybrid logic depends on understanding what the building can run at and at what temperature.

The general commercial compliance set still applies: MCS certification or a recognised equivalent for systems up to 45 kW thermal where grants require it, BS EN 14511 and BS EN 14825 performance ratings, the BS EN 378 safety standard, a BS 4142 acoustic assessment for the external heat-pump plant, and an early DNO supply check, since even a part-load heat pump adds electrical load that a constrained supply may not accommodate without an upgrade. Because the boiler stays in service, the hybrid also keeps the building within familiar gas-safety territory during and after the changeover, which can ease the transition for a landlord and tenants used to a conventional plant room.

How we approach this kind of project

The hybrid route lives or dies on the survey and the control strategy, so that is where we concentrate. We model running cost and carbon from your real 12-month consumption data, survey your existing emitters and pipework first so we know exactly how hard the heat pump can run, and set the bivalent changeover point to maximise heat-pump hours rather than letting the boiler cut in early and quietly erode the savings.

We size for self-consumption, keep the flow temperature as low as the emitters allow, submit any G99 grid application and open the DNO conversation early, and we can keep the existing boiler live through commissioning so the building is never without heat. You receive a fixed-price proposal specified to BS EN 14825 with an insurance-backed warranty. Because the hybrid de-risks the cold spell and keeps the capital manageable, it is often the design a cautious landlord approves first, and once it is proven across one building it becomes the template for the rest of the portfolio.

An illustrative example

As an illustrative composite, and not a real named client or project, consider a landlord with a mixed-use building whose radiators were originally sized for a high-temperature gas boiler and where a full strip-out was neither affordable nor desirable in an occupied demise. The design paired a heat pump cascade carrying around 80% of the annual heat load with the retained gas boiler held for the coldest days, cutting gas use and carbon by roughly 70 to 90% while keeping worst-case heating security in place.

Modelled heat delivered fell within the 90,000 to 800,000 kWh band for this technology, the qualifying spend was written off under full expensing, and the payback came in near the 7-year mark typical of hybrid retrofits. The changeover was planned for a shoulder season so tenants were not disrupted in peak heating weeks, and the retained boiler meant heating continuity was never at risk through commissioning. Every figure here is illustrative and depends on your building, emitters, changeover strategy and tariff.

If your building can in fact run a low flow temperature, a full air-source heat pump may pay better, and for a long-hold year-round asset see commercial ground-source heat pumps. When you are ready, read the cost guide and the funding routes, then request a feasibility or browse the heat pump FAQs.