Heating and Cooling Systems

Our Approach

1: Understand the project

Start by understanding what the homeowner wants to match an approach, then work out the detail. Do not lead with a product.

2: Review the current system

Carry out a light-touch assessment before choosing a path. This determines what's feasible.

3: Choose the high-level approach

[IMAGE PLACEHOLDER — upload separately: "Choosing the high-level approach flowchart"]

Choosing between A2A and A2W

• If there's no wet system, and homeowner wants heating (and maybe cooling):

  • Default to A2A, plus a hot water solution.

  • This avoids installing a whole water system. Fitting a wet system for A2W is the alternative if they want radiant heat, UFH or efficient built-in hot water.

• If there's a wet system, and homeowner wants to get off gas with no cooling:

  • Default to A2W.

• If there's a wet system, and homeowner wants heating and cooling:

  • Default to reversible A2W with fan coils (one system for heating, cooling and hot water).

  • If they want strong, air-con-style cooling, read the note below first.

• If homeowner wants to keep the boiler but add cooling:

  • Specify A2A in the rooms they want cooled.

  • The boiler stays for hot water and space heating.

Cooling coverage

Cooling is homeowner-led. Homeowners usually know the rooms they want cooled (e.g. the ones they use most, or all the bedrooms). This drives the number of indoor units, which in turn drives the number of outdoor units.

We validate this against any constraints: how many indoor units one outdoor unit can carry (e.g. up to 6 for Mitsubishi, up to 5 for Daikin), where the outdoor unit(s) can go within permitted development, and the hot water solution. Some customers will want cooling throughout and the wet system gone, which points to all A2A rather than a few rooms treated. Our value is confirming what is feasible and sizing it correctly, and it is greatest when cooling sits alongside fabric work.

4: Size the system and identify emitter upgrades

Heating is sized from the heat loss calc (BS EN 12831, room by room). Heat loss does not depend on emitter type, so the same figures apply for A2W and A2A. Whole-house loss drives the outdoor unit size, and room-by-room loss drives the required emitters or indoor heads.

Cooling is sized from a heat gain (cooling load) assessment, not heat loss. Factors for this assessment include solar gain through glazing, occupants, equipment and ventilation. A reversible or A2A system should be checked against both duties. In UK homes heating is usually the larger load, so units sized for heating normally cover the cooling, but the cooling figure is not the heat loss figure.

A definitive room-by-room heat loss calc is later carried out by the MCS-certified installer in approved software, and is required for the BUS grant. Our sizing is indicative, to set scope and expectations. The installer confirms the final unit and emitter sizes.

A2W

Size the outdoor unit to the whole-house heat loss, then work out which emitters are required. Low flow temperature is what makes a heat pump efficient, so radiators are often upsized, or UFH is used (section 8). High flow temperature R290 units (Vaillant 75°C, Viessmann 70°C) can drive existing radiators harder and cut the number of changes, at some efficiency cost. Fabric work cuts demand and shrinks the emitters and the unit.

A2A

Size room by room, and put an indoor head only in the habitable rooms (living room, main kitchen and bedrooms). Bathrooms, WCs and the utility can take a small electric heater (a heated towel rail or a panel heater). Halls and landings do not require indoor heads. Indoor heads are usually the smallest class (about 1.5 to 2 kW) and are often oversized against the room's demand, which is fine. The system self-balances, and total indoor capacity can exceed the outdoor unit.

5: Analyse costs and impacts

We run a cost and impact analysis so the homeowner can decide the scope. This includes the following factors:

• Running cost: driven by efficiency (SCOP), tariff and fabric, not by insulation's effect on the pump (section 14). A well-designed heat pump is typically cost-neutral to cheaper than gas, and definitively cheaper on a heat pump tariff or paired with solar.

• Hot water: a heat pump cylinder (e.g. Mixergy iHP) runs roughly three times cheaper than a direct electric cylinder.

• Grants: these change the net cost sharply (section 12). A2A is self-funded today.

• Disruption: A2A is faster and lighter to install. A2W and reversible A2W need a water system and a cylinder.

• Fabric: alongside the system it cuts running cost and emitter size, and is where A2A earns its place.

Indicative installation costs

We use ballpark cost ranges (as of Q3 2026) to set expectations. No grants are confirmed at this stage. Install costs vary depending on the property, access constraints, scope and grant availability, so confirm these figures per project.

6: Specify the detail

Once the route and scope are set, specify:

• Products from the Products reference (section 10).

• Hot water for A2A: a Mixergy iHP (ducted), a Daikin Multi+, or keep the boiler (sections 10, 13).

• Siting: outdoor unit position and clearances, refrigerant zone, indoor unit positions, condensate drains and power runs (section 13).

• Emitters: the radiator changes or UFH, or the hydronic fan coils for a cooling wet system (section 8).

Reference Information

7: The systems

Fan coil (DX) = refrigerant in the coil; the A2A indoor unit in each room. Fan coil (hydronic) = hot or chilled water in the coil, fed by an A2W. Same forms (wall, cassette, ducted, floor); DX coils run colder, so A2A cools harder.

How the two heat, in plain terms

A2W heats water that flows through radiators or underfloor heating, warming the home the way a boiler system does but at a lower, steadier temperature. It is gentle, even, background warmth: radiators feel less hot to the touch but stay on longer.

A2A heats air directly and blows warm air into the room, like an air conditioner running in reverse. It warms a room quickly and gives room-by-room control, but it is blown warm air rather than radiant warmth off a radiator or floor. Either way a heat pump moves roughly three to four units of heat per unit of electricity; a gas boiler gives back under one. Radiators cannot cool (section 9), and A2A on its own makes no hot water.

What an air-to-air install looks like

[IMAGE PLACEHOLDER — upload separately: "Anatomy of an air-to-air system"]

Outside there is one box: the outdoor unit, a fan and condenser on a wall bracket or ground stand. Inside, small insulated refrigerant pipes in white trunking run from it to a wall-mounted head in each room. Each head needs a power supply, and hot water is handled separately, by a heat pump cylinder or the existing boiler.

8: Emitters

We cover emitters both alongside a heat pump and on their own.

• Radiators: a heat pump runs at low flow temperature, so existing radiators are often undersized and need upgrading. High flow temperature R290 units reduce the number of changes. Fabric work shrinks what is needed.

• Underfloor heating: the ideal low-temperature emitter, and it heats evenly. Wet system only (A2W or reversible A2W).

• Fan coils (hydronic): blow air across a water coil; heat and, on a reversible system, cool. Every cooling fan coil needs a condensate drain (section 9). Forms: ceiling cassette, concealed / ducted, wall, low-level floor (Daikin FWx, Myson iVector, Jaga Briza, Smith's Ecovair).

9: Cooling

How A2A and reversible A2W compare on cooling. A2A is a proper air conditioner: the coil runs cold with refrigerant, so it cools hard and fast and removes humidity well. Reversible A2W cools through chilled water in fan coils, and the water only goes down to about 7°C, so its cooling is gentler and removes less humidity. For most UK homes, where the aim is comfort on hot days, that is enough. Where a customer wants strong, fast cooling, A2A is the better cooler.

How it feels. A2A feels like air conditioning: a noticeable flow of cool, drier air that drops the room temperature quickly. Reversible A2W feels subtler, it takes the edge off and stops a room overheating, more “not hot” than actively cold, with little airflow. Set that expectation early: a customer expecting air-con chill from fan coils will be disappointed.

Dew point, and why radiators cannot cool. The dew point is the temperature at which moisture in the air condenses into water. A cold radiator drips, with no tray or drain, so it cannot cool. Fan coils have a tray and a condensate drain, so they can. We do not offer underfloor cooling: a cold floor risks surface condensation and gives little useful cooling.

Rooms and units. Customer-led room selection, validated against the units-per-outdoor-unit caps and siting (section 3).

10: Products

A2W heat pump

Ranked on real-world efficiency, network and aftercare: 1. Vaillant aroTHERM plus (R290, 75°C, biggest network). 2. Viessmann Vitocal 150-A (R290, very quiet, strong monitoring). 3. Mitsubishi Ecodan (the workhorse, biggest spares network). 4. Panasonic Aquarea (7-year standard, strong aftercare, good cold-weather). 5. Daikin Altherma 3 (widest range, the choice where cooling is wanted).

A2A

Mitsubishi Electric is the benchmark for splits and its MXZ-F carries up to 6 heads on one outdoor unit, which often keeps a whole house on a single unit. Then Daikin (strong cooling, up to 5 heads), Panasonic (nanoe X, value) and LG (compact, smart controls). Avoid portables. F-Gas certified install.

Reversible A2W (cooling mode)

The mainstream ranges all offer a cooling mode: Daikin Altherma 3, Viessmann Vitocal 150-A, Panasonic Aquarea, Samsung EHS and Vaillant aroTHERM plus. The established fan coil cooling system is the Daikin Altherma 3 with Daikin heat pump convectors.

Hot water cylinders (for A2A, or all-electric)

A heat pump cylinder runs roughly three times cheaper than direct electric for hot water. Two siting types:

• Ducted (the default): draws and exhausts air through two ducts to outside, so it keeps its efficiency wherever it sits, even a cold loft. It needs a route for the ducts (two 150 to 160 mm ducts to an external wall, ideally out a gable). Examples: Mixergy iHP / iHP X (R290, CoP ~3.4, smart, solar-optimised) and Dimplex Edel (R290, 60°C, CoP ~3.2).

• Non-ducted / ambient: draws air from and exhausts into the room it sits in, so it needs no ducts, but it must sit in a large, reasonably warm space (a garage or plant room, not a small cupboard or a cold loft) and it cools that space as it runs. Use it where duct routes are not possible. Some units, e.g. the Vaillant aroSTOR, can be set up either ducted or on room air.

Mixergy is hot water only: it cannot serve UFH or a towel rail.

11: Planning

• How many units (England, since 29 May 2025): permitted development caps the total number of heat pumps on a property, and A2A and A2W both count. A detached house may have two; a semi, terrace, end-terrace or flat may have one.

• When a second unit needs planning: on a detached house a second unit is fine under permitted development (a third would need an application). On a semi, terrace or flat only the first is permitted, so a second unit of any kind (an A2A added to an existing A2W, or a second A2A) needs planning. This is why keeping an attached home to one outdoor unit matters.

• Size and boundary: the 1 metre boundary rule is gone. The outdoor unit may be up to 1.5 m³ on a house, 0.6 m³ on a flat.

• Cooling: A2A is permitted development only if it also heats. A unit used solely for cooling is not, so always specify cooling units as heating and cooling.

• Noise: the MCS 020 condition (37 dB(A) at the neighbour's window) must pass.

• Check the rights have not been removed: listed buildings, conservation areas and Article 4 areas always need an application. Permitted development can also be removed by a condition on a previous planning permission, which is common on new-build estates, so check the property's planning history before assuming permitted development.

• Nations: Scotland, Wales and Northern Ireland have not adopted the 2025 changes (Wales still has a 3 metre boundary rule).

12: Grants

• Air to water, £7,500: for an A2W (or ground source) that fully replaces a fossil-fuel system. Keep the boiler and there is no grant.

• Off-gas uplift, £9,000: a temporary rate (21 July 2026 to 31 March 2027) for homes off the mains gas grid replacing oil or LPG with an A2W or ground source heat pump.

• Air to air, £2,500: in the scheme since 28 April 2026 (England and Wales), residential properties only. Note the common “direct electric only” claim is not in the Ofgem guidance. The general retrofit rule (para 3.37) is that the system replaced was fossil fuel or electric; the fossil-fuel or electric system must be fully replaced (a retained gas boiler is an ineligible fossil-fuel hybrid, para 3.89); and the installed system must meet the full space heating and hot water demand (para 3.75). Because A2A makes no hot water, that means pairing it with a non-fossil-fuel hot water solution (an immersion or a hot water heat pump, para 3.81). So a gas, oil or LPG home is not categorically barred, but it must remove the fossil-fuel system entirely and cover hot water another way. In practice most such homes take the larger, simpler £7,500 A2W grant, which is why the shorthand exists. Also not claimable yet in practice: no A2A products are on the MCS product directory. Confirm any specific case with Ofgem.

• EPC condition removed: outstanding insulation recommendations on the EPC are no longer a precondition for the grant (SI 2026/390, 28 April 2026).

• Rates and rules change. Confirm the live position against Ofgem's BUS guidance before quoting.

13: Siting

• Outdoor unit: position within permitted development and the noise limit, with the manufacturer's clearances.

• Refrigerant: R290 (propane, GWP 3) reaches higher flow temperatures but is flammable and carries a protective zone, so batteries, inverters and ignition sources stay outside it (roughly 1 m to the sides, 2.1 m up for a wall-sited aroTHERM plus; use the unit's manual). R32 (GWP 675) sites more easily.

• Indoor units (A2A): each needs a power run and, if it cools, its own condensate drain to a waste point (or a small condensate pump).

• Hot water cylinder: cylinder space, plus for a Mixergy iHP two 150 mm insulated ducts (intake and exhaust, at least 1.5 m apart, ideally out a gable end). A ducted unit keeps its efficiency; an unducted one in a loft draws cold loft air and underperforms in winter.

14: Efficiency evidence

[IMAGE PLACEHOLDER — upload separately: "SPF by model chart"]

Source: the heatpumpmonitor.org community dataset, 295 monitored systems, mostly at the H4 boundary, so conservative. It is enthusiast-skewed, so read it as best-practice potential. Vaillant aroTHERM plus and Viessmann Vitocal 150-A are the standouts on strong averages and the largest samples.

Benchmarks: MCS minimum SCOP for grant eligibility is 2.8; at or above 3.5 is solid, 4.0 is excellent. Break-even against a good gas boiler is roughly SCOP 3.7 at current prices.