Shhh… air-to-water
heat pumps are dead

Say it quietly in a heat-pump forum and you’ll start a fight. But somebody needs to: for a large and growing slice of British homes, the air-to-water heat pump — the big white box, the buffer tank, the new cylinder, the radiator upgrades, the room-by-room heat-loss survey, the flow-temperature wizardry — is a solution looking for a problem it no longer uniquely solves.

It is a beautifully engineered, frequently magnificent piece of kit. It is also expensive, fiddly to get right, and, for half the year, an asset that does almost nothing but heat your hot water. Strip away the £7,500 Boiler Upgrade Scheme grant and an MCS ecosystem that exists almost entirely to install it, and a simpler, cheaper, harder-working technology has already overtaken it on the metrics most households actually care about.

The £12,000 radiator that sleeps through summer

Here is the uncomfortable truth about a typical air-to-water install. You spend £10,000–£14,000 (before grant) on a heat source, a hot-water cylinder, often a buffer tank, and frequently a set of larger radiators or new pipework so the system can run at the low flow temperatures that make it efficient. Then, from roughly May to September, the heating side switches off. The expensive outdoor unit spends the entire British summer doing one job: nudging a hot-water tank up to temperature for a few minutes a day.

It cannot cool your house in a heatwave — the vast majority of UK air-to-water systems are heating-only. So you’ve bought a premium, capital-intensive asset and you use perhaps half of its capability for half of the year.

Air-to-air never clocks off

An air-to-air heat pump — what most people still call “air conditioning” — does the opposite. In winter it heats. In summer it cools, which in an increasingly warm UK is no longer a luxury. And the latest generation has quietly closed the one gap that used to count against it: hot water.

Modern units are genuinely efficient. Daikin’s current Perfera range is rated A+++ in both heating and cooling, with a seasonal coefficient of performance (SCOP) of up to 5.1–5.2 on the datasheet. At Rose Cottage, the real-world, metered figure is a space-heating SCOP of 4.44 — 444% efficiency, in an actual home, through an actual British winter (Daikin Onecta data, January to June 2026). That is squarely in the same league as a well-commissioned air-to-water system, for a fraction of the cost and disruption.

Daikin Multi+ quietly changed the maths

The objection used to be simple: “air-to-air can’t make hot water.” Not any more. The Daikin Multi+ connects a single outdoor unit to up to three or four indoor units and a domestic hot-water tank (90 to 230 litres), all from one compact box. It produces hot water at an ECO set point of 50°C or a COMFORT set point of 60°C, with an electric element for backup, and the whole thing is controlled from the Onecta app.

Read that again. One outdoor unit. Whole-home heating in winter. Whole-home cooling in summer. And your hot water. The same asset, working every single day of the year — not half of them.

Here is the part nobody in the trade wants to say out loud. The single biggest reason air-to-water still wins on the showroom sticker is the subsidy. From 28 April 2026, the Boiler Upgrade Scheme finally extended to air-to-air — but at £2,500, against £7,500 for air-to-water and ground source. That is a £5,000 thumb on the scale.

Think about what that means. The government is handing air-to-water an extra £5,000 of public money per install — and it’s still the more expensive, more disruptive, more complex option for many homes. Take the subsidy gap away and the comparison isn’t close. The grant isn’t evidence that air-to-water is better; it’s the thing disguising that it often isn’t.

Rose Cottage did the experiment

We don’t deal in showroom theory. At Rose Cottage, a Daikin air-to-air system went in for £5,300, fully installed — roughly half the typical pre-grant cost of an air-to-water retrofit, and that’s before you factor in the radiator upgrades and cylinder works air-to-water so often demands. No buffer tank. No two-day heat-loss survey. No re-piping. Over the 2026 heating season it delivered 6,130 kWh of heat from just 1,381 kWh of electricity — a measured space-heating SCOP of 4.44 (444%) — and it cools the house in summer for nothing extra. Hot water is handled separately by a Sunamp heat battery charged from solar, so that 4.44 is a pure heating figure, with no electric-element top-up dragging it down.

Simple beats clever

Anyone who has followed our analysis of the HeatpumpMonitor.org dataset knows our central finding: with air-to-water, the hardware barely matters — commissioning dominates. Get the flow temperature, sizing, weather compensation and emitter design wrong and a brilliant unit limps along at a SCOP of 2.8, costing the owner hundreds of pounds a year. The performance gap between a well- and badly-commissioned system is enormous, and it depends entirely on finding a genuinely good installer.

Air-to-air largely sidesteps that minefield. There is no low-temperature radiator circuit to design, no buffer tank to misconfigure, no weather-compensation curve to tune. It is closer to plug-and-play. You do not need a heat-geek guru to wring acceptable numbers out of it — the commissioning gap that quietly wrecks so many air-to-water installs barely exists.

What’s coming: the Altherma 4 X

This is where it stops being speculation. Daikin already ships the Altherma 4, its first residential heat pump on R-290 (propane), good for leaving-water temperatures up to 70°C with hot water and slick, software-driven commissioning. But the unit that really matters is still a prototype: the Altherma 4 X, previewed at the ISH exhibition in Frankfurt in 2025.

The 4 X is the first Daikin system to combine air-to-air and air-to-water in a single box — exactly the “Multi+ on steroids” the market has been waiting for. One compact outdoor unit drives warm-air indoor heads and a hydronic circuit — radiators or underfloor, low- or high-temperature — while also making your hot water. The previewed system connects up to four air-to-air indoor units (white, silver or matt black) plus an optional DHW tank, and runs on R454C, a lower-GWP A2L refrigerant (GWP 146).

Think about what that does to the debate. The last genuine advantage of dedicated air-to-water — that it drives the radiators you already own — disappears the day a hybrid like the 4 X reaches homes. You get the wet circuit where you want it, warm-air heads and cooling where you don’t, and one year-round asset doing all of it. It is still a prototype, and the refrigerant, specifications and timing may yet shift before it lands in the UK — but the direction of travel could not be clearer.

And air-to-water is fighting back: the Samsung EHS Quint

Daikin isn’t the only one reading the room. At InstallerSHOW 2026 (NEC Birmingham, 23 June 2026), Samsung Climate Solutions launched the EHS Quint — and it is the mirror image of the convergence story. Where the Altherma 4 X is air-to-air learning to drive radiators, the EHS Quint is air-to-water learning to cool. It is a hybrid air-to-water system designed to deliver simultaneous heating, cooling and domestic hot water from a single unit, cleverly separating the refrigerant line from the hydraulic line to combine easy installation with direct-expansion comfort.

The headline specs are serious: high-temperature heating up to 65°C, performance maintained down to −15°C, R32 refrigerant, a quiet Night Mode, and compatibility with multiple indoor unit types — all managed through Samsung’s new EHS Remote Care and Cascade Control software. In other words, the single biggest stick we’ve been beating air-to-water with on this page — “it can’t cool, so it sleeps through summer” — is exactly the gap Samsung has just closed.

So read the two launches together. Air-to-air is sprouting a wet circuit; air-to-water is sprouting cooling and smarter controls. Both point at the same destination: one box, all year, heating and cooling and hot water. Which badge gets there first matters far less than the thing they agree on — the heating-only, summer-idle outdoor unit is the design that’s actually dying. The EHS Quint doesn’t undermine our argument; it confirms it from the other side of the fence.

We’d be poor analysts if we pretended this was one-sided. Air-to-water is not dead for everyone, and there are homes where it remains the right answer:

• You already have a good wet system. If your home has decent radiators or underfloor heating and you want even, whole-house warmth through them, air-to-water uses what you’ve got. Air-to-air means visible indoor heads in multiple rooms — and the cost climbs with every head.
• High hot-water demand. A Multi+ tank tops out around 230 litres and can lean on its electric element in tough conditions. A big household with heavy, simultaneous hot-water use may still be better served by a dedicated cylinder and a powerful air-to-water unit.
• The EPC and valuation catch. Because air-to-air heats air rather than a water circuit, it is generally not recorded as the primary heating system on a standard EPC — your old boiler can remain the “official” heat source. That has real consequences for EPC ratings, some mortgages and resale, and it’s the strongest practical argument against going air-to-air today.
• Deep cold and gentle, constant heat. Some people simply prefer the steady radiant warmth of a wet system over moving warm air, and very cold rural sites may favour a system designed around radiators.
• The bigger grant. That £7,500 is real money. For a household that qualifies and wants wet heat, it genuinely narrows the cost gap.