This article examines where the heat pump integrated heat recovery (HPHR) approach becomes rational, which field problems must be anticipated, and by what technical logic climate and layout decisions should be made. In most applications, the main purpose of HPHR is not to "increase comfort" but to recover energy from exhaust air more aggressively, and so reduce ventilation-driven heating and cooling loads. For this reason, HPHR should not be judged on catalogue efficiencies alone, but on operating realities such as defrost behaviour at low outdoor temperatures, supply temperature stability and the need for auxiliary heating.
We will also show that, in warm-climate scenarios, the layout of evaporator, condenser and heat recovery exchanger (rotor/plate) is critical for holding the target supply temperature. In some architectures, a wrong layout can reduce the rotor's contribution and leave most of the load to the heat pump circuit. The final section summarises the selection logic of crossflow, counterflow and rotary exchangers, together with their frosting, leakage and ΔP effects, and shows how the HPHR decision is completed at system level.
What is a heat pump integrated heat recovery unit?
Heat pump integrated heat recovery (HPHR) is an integrated solution that extracts energy from exhaust (waste) air more effectively through a refrigerant cycle (heat pump) and transfers it to the fresh air side. In practice it usually works in series with a passive heat recovery exchanger (rotary or plate): the passive exchanger recovers first, then the heat pump cycle uses the "remaining potential". Manufacturer documents justify this approach with integration of "heating, ventilation and (where suitable) cooling in one unit" and with reduced site installation and piping [3], [4], [7].
Primary objective: maximum heat recovery, not comfort
In most applications, the main goal of HPHR is maximum heat recovery, that is, to reduce the ventilation heating load and, when needed, bring the supply air to its target temperature with less additional energy. For this reason, reading HPHR only as cooling, like "DX cooling inside an air handling unit", is incomplete for most projects. In real-world operation, the main scenario is usually the following [1], [7]:
- High-efficiency recovery with a passive exchanger (rotor/plate),
- "Top-up" heating or cooling with the heat pump where passive recovery falls short,
- Safe operation with an auxiliary heater when required under extreme conditions (such as very low outdoor temperatures).
Spaces and process requirements
HPHR solutions become more rational under the following conditions:
- Systems with high outdoor air ratios (100% outdoor air or very high fresh air fractions): Schools, offices, assembly areas, sports facilities, some industrial spaces and process ventilation applications fall into this class.
- Buildings dominated by ventilation load: Even with low internal heat gains, high fresh air flow rates are a major energy component, especially in the heating season.
- Projects targeting a serial recovery concept: Within the Ecodesign/Eurovent framework, basic recovery comes from the passive exchanger, and further recovery is targeted with the heat pump cycle [1], [2].
- Projects with installation or space constraints: The integrated heat pump approach reduces outdoor units and site piping, so it can be preferred in retrofits and in projects where roof space is valuable [3], [4], [7].
Application-driven motivation (heat recovery focus)
- Approaching a high supply temperature in the heating season: The passive exchanger + heat pump sequence brings the supply air to target while reducing the need for auxiliary heaters; this is especially emphasised [7].
- Reducing defrost impact in cold climates: If the frost/defrost strategy is set up correctly, energy and comfort deviations caused by defrost can be limited [6], [7].
- "Cooling energy recovery" and integrated cooling in the cooling season: In some manufacturer concepts, the heat pump coil placement is said to enable recovery of both heating and cooling energy [7].
Defrost and low-temperature scenarios
Defrost is a critical operating condition for heat pump-based solutions, both for energy use and for supply temperature stability. The Swegon RX/HC function guide defines several defrost methods against frosting on the exhaust (extract) coil side: reversing the refrigerant circuit, support with an electric heater and support with recirculation [6]. The GOLD RX/HC documentation also states that the defrost approach should be chosen according to the climatic design temperature; for example, recirculation-supported defrost can be used down to lower outdoor temperatures [7].
In a similar way, some manufacturers treat "defrost management" not only as a control function but as a layout and product-architecture goal: reducing or avoiding the need for defrost. IV Produkt states that in its ThermoCooler HP solution, the heat pump coil is placed on the warmer side of the rotor to reduce the frosting risk on the coil and to minimise defrost demand under certain conditions. For very low outdoor design temperatures, auxiliary heating (trim heater) is treated as part of the design for safe operation [4], [5].
On the Systemair side, the Geniox HP DF variant is positioned as a product that runs at low outdoor temperatures without needing a defrost cycle. For the standard Geniox HP variant, the operating limit at lower temperatures is secured with complementary measures such as auxiliary heating or a boost heater [3].
Practical design notes:
- Defrost strategy is not only a "works / does not work" issue; defrost duration, frequency and the supply temperature deviation during defrost are design criteria [6], [7].
- In cold climates, coil placement and the temperature levels along the air path have a clear effect on frosting risk; some manufacturers place the coil in a "warmer zone" to reduce unnecessary defrost [5].
Component layout and climate zones
Here, "layout" means the sequence, along the air paths, of the heat recovery exchanger (rotor, crossflow and so on) and the two heat pump coils (the exhaust-side coil and the supply-side coil). Even with the same compressor and the same exchanger type, the side of the rotor on which the coils sit clearly changes the following [1], [7]:
- Overall heat recovery ratio,
- The temperature lift the compressor has to work against,
- Defrost frequency,
- The "cooling recovery" potential in summer.
The discussion below follows two climate conditions, warm and cold. Table 1 gives the reference temperatures of the two climate classes; the list below it is a short overview of European and Turkish climate zones.
| Region | Summer | Winter |
|---|---|---|
| Warm climate | >30 °C | >−5 °C |
| Cold climate | <30 °C | <−5 °C |
Table 1 — Climate zones, reference temperatures: summer and winter thresholds of the warm and cold climate classes.
- Northwestern Europe – maritime / mild-humid (frequent but relatively mild defrost): Netherlands (Randstad), Belgium (Brussels region), northern France (Lille corridor), western Germany (NRW).
- Central Europe – continental / mild-cold (defrost becomes critical at design hours): southern and central Germany (Bavaria), eastern Austria (Vienna), inland/eastern France (Strasbourg–Lyon corridor), northern Italy (Milan).
- Alpine climate – severe winter (defrost + preheating/recirculation as part of the design): Austrian Tyrol, the German, French and Italian Alps.
- Mediterranean coast – warm and often humid (strong motivation for cooling recovery): Aegean and Mediterranean coasts of Türkiye, southern Italy/Sicily, the southern coast of France.
- Inland regions – hot-dry / hot-continental (high dry-bulb, relatively low latent load): central and southeastern Türkiye (Ankara–Konya; Gaziantep/Şanlıurfa) and similar inland zones.
Layout for cold climates
Design objective: In cold climates, the aim is to sustain the highest possible net recovery (including defrost and auxiliary heating) with the lowest possible defrost loss.
Frost behaviour depends on the exchanger type: Eurovent stresses that condensation and frosting risk can occur at low outdoor temperatures, especially in plate exchangers without moisture transfer (sensible only). Solutions with moisture transfer (enthalpy/sorption) reduce the frosting risk. This framework shows that the frosting limit of rotary/sorption wheels can shift to lower temperatures, depending on the application [1].
The air temperature the coil "sees" is a design parameter: The placement of the integrated heat pump coils sets the evaporator inlet temperature and humidity ratio, so it directly affects defrost demand. IV Produkt states that placing the heat pump's extract air coil on the warmer side of the thermal wheel can reduce the frosting risk on the coil below 0 °C, with the aim of avoiding unnecessary defrost [5].
Three factors that define the net benefit:
- Rotor effectiveness (temperature + moisture transfer),
- Evaporator inlet condition (especially T and ω),
- Defrost strategy (reverse cycle, electric preheating, recirculation and so on).
If these three are set up wrongly, a "high heat recovery" claim can come back from site as low capacity and high energy use.
The weak point of this layout type shows up in cooling mode. When the compressor system runs, the condenser outlet air becomes the rotor inlet, so the rotor's exhaust-side inlet temperature is higher than on the fresh air side. For this reason, in cooling mode only one of the two, the rotor or the HP module, should be used.
Layout for warm climates
In warm climates, HPHR is usually justified by two drivers:
- Cooling recovery: Recovering "cooling energy" from exhaust air to lower the inlet enthalpy of the fresh air and reduce compressor load.
- Integration: Reducing site complexity of outdoor units, piping and refrigerant circuits (especially in projects with limited roof and façade space), and lowering commissioning risk with a packaged solution.
Unlike the cold-climate layout, this layout must not be treated under a "defrost-free" assumption. The coil on the exhaust path after the rotor (usually the evaporator in heating mode) can be fed with sub-zero air, and the frosting risk rises. Swegon states in its RX/HC documentation that frost can form on the exhaust coil in heating mode, and it sets the defrost approach by outdoor temperature band [7]:
- Down to about −5 °C: Reverse-cycle defrost alone may be sufficient.
- Around −10 °C: An electric preheater in front of the rotor comes into play.
- At lower design temperatures: Recirculation-supported defrost is described.
Manufacturer examples and temperature ranges: "which approach for which climate?"
The summary below is an interpretive framework showing how manufacturer documents justify their layout and operating logic. Project realities (airflow, indoor conditions, filter fouling, control sequence, the unit's bypass/recirculation equipment) directly affect the result.
| Manufacturer / series | Layout concept |
|---|---|
| Swegon – GOLD RX/HC | Rotor + integrated reversible heat pump; supply and exhaust coils sit on the two air streams of the rotor. Compared with "module solutions", Swegon argues that this layout gives more effective recovery in heating and cooling [7]. |
| IV Produkt – ThermoCooler HP | Integrated reversible heat pump module inside the AHU; the coil is placed on the "thermal wheel warm side" to reduce frosting risk [5]. |
| Systemair – Geniox HP DF / Geniox HP | Rotary exchanger + integrated reversible heat pump; the DF variant is positioned around "Defrost Free" and "high heat recovery" [3]. |
Table 2a — Manufacturer examples: layout concept.
| Manufacturer / series | Stated operating / design temperature notes |
|---|---|
| Swegon – GOLD RX/HC | Defrost selection by design outdoor temperature: reverse cycle alone down to a −5 °C design temperature; electric heater in front of the rotor recommended for −5…−10 °C; recirculation defrost down to a −25 °C design temperature [7]. |
| IV Produkt – ThermoCooler HP | Example operation: at −12 °C outdoor air and 22 °C return air, fresh air can be heated to 20 °C with the compressor at ~60%; a dry temperature efficiency of ~88% is given [5]. |
| Systemair – Geniox HP DF / Geniox HP | Geniox HP DF: "Heating mode down to −20 °C without preheater" and "No defrosting cycle required". Geniox HP: "Heating mode down to −12 °C without preheater", with a re-heater need noted. A boost heater is stated as standard for design temperatures colder than −17 °C [3]. |
Table 2b — Manufacturer examples: stated operating and design temperature notes.
| Manufacturer / series | Defrost approach |
|---|---|
| Swegon – GOLD RX/HC | Frost can form on the extract air coil in heating mode; the sorption rotor's moisture recovery makes the air reaching the coil drier, which is stated as a factor that reduces frosting [7]. |
| IV Produkt – ThermoCooler HP | "Warm side" placement aims to reduce frosting on the coil below 0 °C and avoid unnecessary defrost. The manufacturer documentation also states "No defrost cycle required" (to be verified under project conditions) [5]. |
| Systemair – Geniox HP DF / Geniox HP | For the DF variant, "no defrosting cycle required" is highlighted. This claim depends on the unit's internal control sequence and coil/air conditions; check it with the selection software and the project operating sequence [3]. |
Table 2c — Manufacturer examples: defrost approach.
Heat recovery exchanger types and selection logic
The Ecodesign framework and Eurovent assessments make it clear that choosing the exchanger type is not only about efficiency; frosting, leakage, pressure drop and hygiene must be considered together [1], [2].
Crossflow
According to Eurovent, crossflow plate exchangers are a common solution, especially in larger units; but within the same class they tend to show lower temperature efficiency than counterflow. Efficiency can be increased by connecting two crossflow exchangers in series [1]. The main advantage of crossflow is its suitability for large airflows and its flexibility in manufacturing and configuration. However, when very high efficiency is targeted, optimising exchanger size and pressure drop (ΔP) becomes difficult.
In the plate exchanger family, the condensation and frosting risk also grows as the efficiency target rises, especially at low outdoor temperatures; Eurovent notes that this risk can appear earlier than with rotary solutions [1]. For this reason, crossflow plate solutions are rational in projects where efficiency is kept at a "reasonable" level and a compact, economical solution is needed for large airflows. In very cold climates, frost management (bypass, preheating, control sequence) must be an integral part of the design [1].
Counterflow
Eurovent notes that very high temperature efficiencies can be reached with counterflow plate exchangers, especially in small AHU sizes, but that a higher efficiency target can also increase frosting risk [1]. The clear advantage of counterflow is its high temperature efficiency potential with proper sizing. On the other hand, at high effectiveness targets the condensation–frosting dynamics inside the exchanger become more critical. Frost management (bypass strategy, preheating need, operating/control approach) therefore becomes a key design driver [1].
Counterflow plate solutions are a strong option where efficiency is the "main goal" and the unit size must stay small. To keep operation continuous at low outdoor temperatures, however, the frost/defrost strategy should be verified, where possible, with the manufacturer data used for selection [1].
Rotary
According to Eurovent, rotary exchangers offer high temperature efficiency, compactness and (with a suitable rotor) moisture recovery; they also start to frost at lower temperatures than other types, which can be an advantage. However, a wrong fan configuration or poor commissioning can increase EATR/OACF-related carryover and leakage risks, so they are not recommended for applications with very high hygiene requirements [1].
Conclusion (recommendation)
In the right scenario, heat pump heat recovery solutions can significantly reduce ventilation-driven heating and cooling loads. However, "success" usually depends less on single catalogue efficiency values and more on the net operating performance achieved over the whole year.
The key parameters that set net performance are [3], [5], [6], [7]:
- The project's climatic design conditions,
- The layout between evaporator, condenser and heat recovery element (rotor/plate),
- The choice of defrost strategy (reverse cycle, preheating, recirculation and so on),
- Management of the auxiliary heating share that may be needed at very low outdoor temperatures.
References
- Eurovent AISBL/IVZW/INPA, Eurovent 6/18 – Quality criteria for Air Handling Units (First edition), 2022, https://www.eurovent.eu/wp-content/uploads/eurovent-rec-6-18-quality-criteria-for-air-handling-units-2022-en-2.pdf
- European Commission, Commission Regulation (EU) No 1253/2014 – Ecodesign requirements for ventilation units, 2014, https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32014R1253
- Systemair, Geniox HP – Integrated reversible heat pump and rotary heat exchanger (product brochure), 2024, https://a.storyblok.com/f/108972/x/aa7e39befa/systemair-geniox-hp-brochure_20240508_.pdf
- IV Produkt, ThermoCooler HP – Integrated reversible heat pump (product page), n.d., https://www.ivprodukt.com/products/integrated-reversible-heat-pump/thermocooler-hp
- IV Produkt, ThermoCooler HP – Integrated reversible DX heat pump for the Envistar range (brochure), 2024, https://www.enawent.pl/files/39/2024_1_ThermoCooler_HP_en.pdf
- Swegon, Guide to the GOLD version F/G functions – Reversible heat pump, RX/HC (function guide), n.d., https://www.swegon.com/globalassets/digizuite/9295-en-funkrxhc_functionguide_en.pdf
- Swegon, GOLD RX/HC – Packaged ventilation, heating and cooling (brochure), n.d., https://www.swegon.com/globalassets/digizuite/8450-en-goldrxhc_brochure_en.pdf
- Systemair, Geniox HP (product page), n.d., https://www.systemair.com/en-gb/products/air-handling-units/geniox/geniox-hp
- IV Produkt, Envistar – A flexible range of air handling units with integrated control equipment (brochure), n.d., https://www.byggematerialer.dk/envistar-serien-1227674/fil-files/Brochure_Envistar_191017_02_EN.pdf