ATMO Europe: Unic Air Set to Launch AHU with CO₂ Heat Pump Delivering Space Heating and Cooling

In spring 2026 Danish HVAC manufacturer Unic Air will launch COOLHEAT, a bespoke air handling unit (AHU) that delivers space heating and cooling via a CO₂ (R744) heat pump. The self-contained unit was developed in collaboration with the Danish Technological Institute (DTI).

Christian Heerup, Business Manager of Refrigeration at DTI, along with Allan Kjær, Head of Research and Development at Unic Air, outlined the unit’s development and testing in a presentation at the ATMOsphere (ATMO) Europe Summit 2025. The event, organized by ATMOsphere, was held November 24–25 in Padua, Italy. ATMOsphere is the publisher of NaturalRefrigerants.com.

“We have seven years of experience cooling with CO₂,” Kjær said, adding that Unic Air has more than 250 CO₂ HVAC units operating in Denmark. “However, we see a bigger and bigger need for adding heating.”

Unic Air and DTI initiated their collaboration in 2021 with the goal of producing an “HFC-free alternative for conditioning fresh air,” according to Heerup. By 2023, the partnership had a prototype in development. In summer 2024 DTI conducted laboratory testing, followed by Unic Air’s installing the unit in its office building for demonstration.

Details/performance: The design incorporates two inverter-driven CO₂ Dorin compressors, with a rotating heat exchanger covering the majority of the load, according to Heerup.

• The unit shares components across its circuits while featuring separate heating and cooling coils, integrated into a single block handling both supply and exhaust air. U.K.–based 3T supplies the coils, while Italian-based Carel provides the controls.
• Testing in the Unic Air office building gave a COP of 3.7 in ambient conditions ranging from 19 to 23°C (66 to 73°F) and a COP of 2.2 in temperatures over 27°C (81°F). In temperatures below −5°C (23°F), it provided a COP of 3.2, with defrosting taking 5‒10% of the COP.
• “It’s market–ready and suitable for northern climates,” Heerup said, adding that it costs roughly 20% more than a comparable HFC-based AHU.

Overcoming challenges: Conditions in Denmark require seven times more airflow in warm months compared to cold ones, making it difficult to maintain stable operation and high COPs in the operating extremes.

• DTI optimized one of the compressors for summer loads and one for low–frequency/part–load winter operation.
• Increasing the superheat setpoint from 7 to 10K (7 to 10°C/12.6 to 18°F) resolved issues with cold compressor operation and foaming oil, which increased the COP from 2.2 to 3.1. The partnership also replaced the evaporator coil in the exhaust air to support even frosting.

Potential upgrade: The next generation design by DTI and Unic Air for an AHU using a CO₂ heat pump includes coils in both parallel and counter flow, a four-way changeover valve, an integrated ejector and an internal heat exchanger/oil rectifier. Heerup also suggested evaluating permanent magnetic motor compressors.

• The team completed a high ambient temperature design for a potential unit in May 2025, but ran out of funds to build it with the project terminating in February 2026, according to Heerup.

Potential options: A couple of other European manufacturers have AHUs using CO₂ refrigerant in various stages of development.

• German HVAC designer and manufacturer Menerga showcased a prototype of an AHU with a CO₂ heat pump for public-swimming-pool dehumidification at ISH 2025, with 20kW (5.7TR) of cooling capacity and 25kW (7.1TR) of heating. It produces outlet water temperatures of up to 65°C (149°F).
• U.K.-based heat pump manufacturer Clade Engineering Systems offers a combined AHU and heat pump using a direct expansion CO₂ heat transfer coil. The modular unit comes in 50, 100, 125, and 150kW (14.2, 28.4, 35.5 and 42.7TR) capacities.