Daikin Altherma 3 H HT high-temperature heat pump: models and specifications

The Daikin Altherma 3 H HT is a high-temperature air-to-water heat pump, and the point of it is simple: it can hold a 70°C flow temperature even when the outdoor air drops to -15°C¹. That is hot enough to feed the radiators and hot water tank an older house already has, which is why Daikin describes it as a heating system replacement for older homes rather than a whole-house renovation project³.

It comes in two indoor formats. The W is wall-mounted and compact enough to sit in a cupboard, connecting to a separate cylinder⁴. The F is floor-standing with the hot water tank built in⁵. Both are rated up to A+++ for heating, and both are certified Quiet Mark⁵.

What the Altherma 3 H HT is: a high-temperature air-to-water heat pump

Daikin classifies the Altherma 3 H HT as a high-temperature air-to-water heat pump, sold as a set with an outdoor unit and an indoor unit⁵. It provides heating, cooling and domestic hot water, and the independent product listing describes it as delivering "heating, cooling and instant hot water" while using less energy to do it⁶. The range is aimed squarely at the renovation market, and Daikin's own framing is that it is designed to be the perfect heating system replacement in older homes⁶.

The distinction that matters is temperature category. Daikin sells mid-temperature Altherma models alongside this one, and the mid-temperature units are described as designed to upgrade an existing heating system or suit large new-build homes⁷. The high-temperature version exists for the houses where a mid-temperature unit would mean changing the emitters: older, leakier, less insulated properties where the radiators were sized for a boiler running hot.

The practical consequence is that the H HT can connect directly to underfloor heating, heat pump convectors and high-temperature radiators, and it can connect to older radiators and hot water tanks⁵. Daikin states plainly that it has no gas dependency⁵. For a household, that is the whole proposition: the fuel changes, the pipework and the radiators largely do not.

Flow temperature: 70°C at -15°C outdoors

The headline figure is a consistent 70°C flow temperature, maintained even when outdoor temperatures fall to -15°C¹. The independent listing puts it the same way: a leaving water temperature of 70°C down to -15°C with heat pump operation only, meaning no backup heater is doing the work at that point⁶. Daikin's guidance repeats the -15°C figure in its rural homes material¹.

The operating envelope is wider than that. Both the W and the F are listed as operating down to -28°C⁵, and the independent listing gives an operation range down to -28°C⁶. The distinction is between the temperature at which the unit will run at all and the temperature at which it will still deliver 70°C. The -15°C figure is the one that matters for design, because it is the point at which the unit is still producing boiler-level flow temperatures from the heat pump alone.

For context on how unusual that is, a weather-compensated heat pump system in ordinary UK conditions might run a flow temperature of around 43°C when the ambient temperature is 7°C⁸. The H HT is built to run far hotter than that when it has to. That does not mean it should: a heat pump is most efficient when flow temperatures are low, and the high-temperature capability is a fallback for cold snaps and for undersized radiators, not a target to run at year round. More on how flow temperature drives running costs is at heat pump flow temperature.

Two indoor formats: wall-mounted W and floor-standing F

The W is a wall-mounted indoor unit, and Daikin notes that this compact unit can even be installed in cupboards⁴. It connects to a separate domestic hot water tank, available from 150 litres to 300 litres⁴. It also supports an optional thermal store, which Daikin says maximises energy usage and connects to additional heat sources⁴.

The F is a floor-standing indoor unit with an integrated hot water tank⁵. The specific product listing for the ETVH16E6V7 describes a combined stainless steel domestic hot water tank of 180 or 230 litres with the heat pump, for easier installation⁹. A separate listing for the ETVZ16E6V describes a floor-standing unit integrated with different temperature zone management¹⁰.

The choice between them is mostly about space and hot water. A household with a cylinder already in place, or with a cupboard that can take the wall unit, has the W option. A household with nowhere to put a cylinder, or replacing a floor-standing boiler, has the F. Both connect to the same outdoor units, and a documented UK installation used a 16-class indoor unit, which Daikin notes is compatible with outdoor units of the 14, 16 and 18 classes¹¹.

Efficiency: A+++ for heating, and what drives it

Both the W and the F are rated up to A+++ for heating on the A+++ to D scale⁵. That is the top of the domestic heating label, and it is the same rating Daikin gives its mid-temperature and lower-temperature Altherma ranges, so the label alone does not separate the models¹².

What separates them is the conditions under which that efficiency holds. Daikin states that models such as the Altherma 3 H HT operate at superior high efficiencies across high, medium and low temperatures². That is the claim worth testing in a real house: a high-temperature heat pump that only hit its best efficiency at low flow temperatures would be no different from a standard unit. The design intent is that the efficiency holds up when the unit is asked to run hot.

The independent listing credits the single fan outdoor unit with achieving some of the highest efficiencies and lowest sound levels in the market⁶. The refrigerant is R32, with a global warming potential of 675⁶. R32 has a lower GWP than the R410A it largely replaced, and it is a factor in reaching high flow temperatures without an oversized charge. The rules around refrigerants, and what the F-Gas regulations require, are set out at heat pump refrigerants.

Efficiency in a fitted system still depends on the house. A heat pump's seasonal performance is driven by flow temperature, emitter sizing and how the system is controlled, not by the badge on the outdoor unit. The A+++ label describes the appliance under test conditions; the household's actual figure will be lower and will vary with the weather and the radiators. The difference between the two is explained at heat pump efficiency: COP, SCOP and SPF.

Noise: 43 to 48 dBA outdoors, Quiet Mark certified

The outdoor unit is listed at a sound pressure of 43 to 48 dBA on both the W and the F⁵. Daikin describes the range as having Quiet Mark certification and a sound power level of 54 dB(A)². The independent listing confirms Quiet Mark certified models in both three phase and single phase variations⁶, and Daikin's own installer brochure describes it as Quiet Mark certified and operating at whisper-quiet levels¹⁵.

The indoor side is quieter still. Daikin states that the indoor units of its heat pumps generally produce a sound pressure level of about 30 dBA, which is comparable to a quiet room⁵. That matters for the W in particular, since it may be going into a cupboard or a utility space rather than a garage.

The range has a Low sound mode¹⁶. Quiet Mark certification is an independent scheme, and the certification is a useful check because it is not the manufacturer marking its own homework. For comparison within the wider market, other high-temperature units are listed at different figures, and the general subject of how noise is measured and assessed is covered at how noisy is a heat pump and MCS 020: the heat pump noise assessment.

Works with existing radiators and hot water tanks

This is the section that decides whether the H HT suits a particular house. Daikin states that the W is compatible with older radiators and hot water tanks⁴, and that it connects directly to different heating and cooling emitters including underfloor heating, heat pump convectors and high-temperature radiators⁴. The F connects to underfloor heating, high-temperature radiators and heat pump convectors⁵. The F also has no need for additional building works, according to Daikin, because it has no gas dependency and can connect to older emitters⁵.

Domestic hot water runs up to 65°C on both formats⁵. That is hot enough for a stored cylinder to meet normal household demand, though the exact setting is a matter for the installer and the cylinder specification. Cylinder sizing and coil requirements for heat pumps are covered at hot water cylinders for heat pumps.

The evidence from real installations is worth weighing. A case study of a Denbighshire cottage records the replacement of an oil heating system with a 16kW Daikin Altherma High Temperature heat pump, with the system delivering up to 65°C to satisfy peak demands for heating and domestic hot water¹⁷. A separate Cornish cottage case study records a switch from oil heating to a Daikin Altherma high temperature heat pump with a Sunamp heat battery, alongside a 250-litre Daikin hot water cylinder serving a kitchen, a bathroom and two other shower rooms¹¹. Both are documented conversions of older, off-gas properties.

What this means for independence is straightforward. The H HT removes the gas or oil supply from the heating and hot water side of the house, and it does so without demanding a fabric-first retrofit or a full emitter replacement. What remains is electricity: the heat pump runs on mains power from a supplier, so the household is still on the grid, and the running cost depends on the electricity price and the tariff. The unit does not generate anything. Households wanting to cut that remaining dependence further would be looking at solar and a battery, covered at solar panels and a battery with a heat pump, and the wider picture is at heat pumps and household energy independence.

Smart control, smart grid and the Onecta app

Both formats offer smart control through the Onecta app, Google Assistant and Amazon Alexa⁵. The app allows a household to create schedules and track usage from a phone or tablet¹⁸. The same control set appears across the Altherma range, including the mid-temperature and lower-temperature models¹².

The dependence this creates is worth stating plainly. Remote scheduling, usage tracking and voice control all run through Daikin's app and servers, and through a home broadband connection. Local control at the unit continues without either, but the app-based features do not. A household that wants the heating to work regardless of the internet should expect the basic controls to keep working and the smart features to stop.

The range is also described as smart grid ready, which is the capability to respond to signals from the electricity network or a time-of-use tariff. That is a capability rather than a guarantee of savings: what a household actually gets depends on the tariff and the supplier. The subject of controls and weather compensation more generally is at heat pump controls, weather compensation and smart operation, and the rules on when new heat pumps must be smart are at when do new domestic heat pumps have to be smart.

Refrigerant, F-Gas and who can install it

The system uses R32 refrigerant, with a global warming potential of 675⁶. No F-gas licence is needed for the installation, because only a water connection runs between the indoor and outdoor unit⁶. The refrigerant circuit is factory-sealed, which is what makes the monobloc-style installation possible without a refrigerant-qualified engineer on site.

That does not make it a DIY job. Daikin states that only an authorised technician should carry out the installation and in a suitable location¹⁹. The distinction is between the refrigerant work, which is not required, and the installation itself, which is. The difference between monobloc and split arrangements, and what each means for who does what, is at monobloc and split heat pumps.

Outdoor unit model numbers, single and three phase

The E-Series outdoor units are listed in both phase variants⁶. Single phase outdoor units are EPRA08EAV3, EPRA10EAV3, EPRA12EAV3, EPRA14DAV37, EPRA16DAV37 and EPRA18DAV37. Three phase outdoor units are EPRA08EAW1, EPRA10EAW1, EPRA12EAW1, EPRA14DAW17, EPRA16DAW17 and EPRA18DAW17⁶.

The matching indoor units follow the same split. Single phase indoor units are ETBH12EF6V, ETBX12EF6V, ETBH16EF6V7 and ETBX16EF6V7. Three phase indoor units are ETBH12EF9W, ETBX12EF9W, ETBH16EF9W7 and ETBX16EF9W7⁶.

Three phase matters for larger properties and for homes where the single phase supply would need upgrading. Whether a property needs a supply upgrade, and who pays for it, is covered at who pays if my electricity supply needs upgrading for a heat pump.

Where the Altherma 3 H HT sits in the range

Daikin sells a wide Altherma line-up in the UK, and the H HT is the high-temperature end of it. The mid-temperature H MT comes in the same W and F formats, with the W connecting to a domestic hot water tank and the F carrying an integrated tank¹². The lower-temperature R and M ranges cover the rest of the market, with the Altherma 3 M described as a single outdoor unit for heating and cooling²⁰, and the Altherma 4 H models as the newer high-temperature additions that work with existing hot water tanks²¹.

The H HT's position is defined by the 70°C flow temperature and the -15°C figure. A household with a well-insulated property and modern radiators may find a mid-temperature unit cheaper and equally capable. A household in an older, less insulated property, or one replacing an oil boiler off the gas grid, is the case the H HT was built for. Which homes suit which approach is covered at which homes suit a heat pump and replacing a gas, oil or LPG boiler with a heat pump.