An air-to-water heat pump heats water for underfloor heating or radiators and usually the hot-water cylinder; it is the most common choice in new Irish homes. An air-to-air heat pump heats and cools the room air directly through indoor units, and hot water comes from a separate system. In a home with very low heating demand, the small heat load suits the simpler air-to-air route with MVHR. Ask your BER assessor early how your choice is modelled.
Both systems take heat from the outdoor air and move it indoors using electricity, at a fraction of the energy a direct electric heater needs. They differ in what they deliver the heat to: water or air. That choice decides how heat reaches your rooms, whether the system can cool, how hot water is made, how much the installation costs, and how the home is assessed for its Building Energy Rating (BER). This guide compares the two for a new, well-insulated home in Ireland.
How does an air-to-water heat pump work?
An air-to-water heat pump extracts heat from the outdoor air and transfers it to a water circuit. The warm water flows through underfloor heating or radiators and, in most installations, also heats a hot-water cylinder. The lower the water temperature the heating needs, the more efficiently the heat pump runs, which is why underfloor heating at low flow temperatures suits it well.
Some air-to-water heat pumps can cool through the floor or fan coils. The floor must stay above the dew point; suitable fan coils with condensate drainage can also dehumidify the air.
How does an air-to-air heat pump work?
An air-to-air heat pump moves heat from the outdoor air directly into the indoor air through one or more indoor units, the wall or ceiling units of a split system. The same unit heats in winter and cools in summer, and it warms a room quickly because it heats the air, not a floor screed.
It does not make hot water. A home with air-to-air heating needs a separate hot-water system, such as an electric cylinder or a heat pump water heater.
Air-to-air or air-to-water: how do they compare?
| Air-to-air | Air-to-water | |
|---|---|---|
| Heat delivery | Indoor units warm the room air | Water through underfloor heating or radiators |
| Cooling | Yes, with the same indoor units | Possible with a reversible heat pump and suitable emitters |
| Hot water | Separate system needed | Usually from the same heat pump via a cylinder |
| Response | Fast: warms the air directly | Slow with underfloor heating, steady temperature |
| Installation | Outdoor unit, indoor units and refrigerant lines | Outdoor unit, hydraulic circuit, emitters and cylinder |
| In Irish new builds | Less common; works best with very low heat demand | The usual choice under NZEB |
| Best fit | Very low heating demand, wish for cooling | Higher heating demand, wish for underfloor heating |
The lower the heating demand, the less the heat distribution system matters. In a very well-insulated home, a few kilowatts are enough, and air-to-air units with MVHR deliver them without pipes in every floor.
How does the heat pump count for Part L and the BER?
Part L requires new dwellings to meet the nearly zero energy building standard, including a renewable energy ratio of 20%. Heat pumps count as a renewable source, alone or together with solar panels. The BER is calculated in SEAI's DEAP software from the design, so the heating system, hot-water system and controls all affect the result. Ask your BER assessor at design stage how your chosen system is modelled.
New homes are not eligible for the SEAI heat pump grant, which is for homes built and occupied before 2021, so the choice comes down to running costs, comfort and installation.
What heating does a BioBuilds home use?
BioBuilds homes are built on a Passivhaus-certified building system with a heating demand target of around 15 kWh/m² per year. In the Turnkey finish, Zehnder MVHR works with a Samsung WindFree or Panasonic heat pump for all-electric heating and cooling. In Serenity, Sanctuary and Bloom, the Zehnder ComfoClime Q module adds active heating or cooling through the ventilation air and regulates indoor humidity. Hot water comes from an Ariston smart two-tank system of 100 to 150 litres.
Underfloor heating can be added by preference or for very cold sites; it is an option, not a requirement of the system. In Ireland, BioBuilds Wanderlust in Turnkey starts from €163,000, including 23% Irish VAT, with transport and installation on prepared ground screws through BioBuilds' local partners included. Crane hire and other site works are separate. To see the price with your options, configure your home.
