Defrost-Free Heat Recovery in Cold Climates: Evaporator–Rotor Order

Picture two heat-pump heat recovery units: same compressor, same rotor, same fans, same capacity class. The only difference is whether the evaporator sits before or after the rotor on the return (exhaust) path. Nearly identical on the datasheet, these two units behave in opposite ways at low outdoor temperatures.

One keeps frosting its evaporator. The compressor cuts out and re-engages once the ice melts, so it falls into a continuous "stop-start" cycle. The other keeps its evaporator above freezing even at −20 °C outdoor and runs its compressor without interruption.

In this article we look at a design decision that is often overlooked in heat-pump heat recovery (HPHR) units: the return-path layout. The data comes from a one-to-one comparison on the same unit in HVACinOne [9]. We name the two layouts as follows:

• Temperate-climate design (HP-HR-R) (return path: rotor → evaporator): the most common HPHR architecture in temperate, cooling-dominated climates, including Türkiye.
• Cold-climate design (HP-HR-R-C) (return path: evaporator → rotor): aimed at heating-dominated regions with long, harsh winters; in demand especially in the Nordic countries.

HP-HR-R and HP-HR-R-C are the function names of these two layouts in HVACinOne; "HPHR" denotes the general device class. Both architectures are sold as an "integrated reversible heat pump + rotary heat exchanger" concept. Manufacturers position these product families by climate coverage and defrost strategy [1], [2].

Why it matters

In a cold climate, an HPHR unit's success is measured less by a single catalogue efficiency figure than by its ability to get through winter without interruption. At low outdoor temperatures, evaporator frosting is not just an efficiency loss. As ice covers the coil surface, the compressor stops on low pressure/protection, then restarts once the ice melts. The system therefore enters a continuous stop-start regime, which makes stable heating impossible and shortens compressor life.

The answer to "down to what temperature does it run safely", i.e. the minimum operating temperature, is a direct selection criterion. We will show that this is not a single catalogue number but largely a consequence of the layout.

Two layouts: is the evaporator before or after the rotor?

The two configurations differ in the order the return air follows:

• Temperate-climate design (HP-HR-R): Return-path order Filter → Rotor → Evaporator → Fan. The rotor recovers heat from the return air first; the evaporator then sees this pre-cooled air.
• Cold-climate design (HP-HR-R-C): Return-path order Filter → Evaporator → Rotor → Fan. The evaporator sees the raw, mild return air from the building; the rotor then captures the energy left after the evaporator.

Manufacturers also address the fact that the coil's position relative to the rotor changes frosting behaviour. One manufacturer's documentation places the heat-pump coil in the warmer zone of the rotor. The goal is to reduce the risk of coil frosting below 0 °C and to avoid unnecessary defrost [3].

This ordering difference looks like a small detail. But it sets the source temperature the heat-pump cycle sees, and therefore the evaporation temperature (TEvap).

What temperature is the air passing over the evaporator?

In HVACinOne we solved the same unit in two configurations, in heating mode from +10 °C down to −20 °C outdoor [9]. The unit handles 8000 m³/h and has a single-circuit reversible heat pump; its rotor is the type the manufacturer calls "sorption hybrid". Return air is 20 °C / 50% RH in both cases. The moisture/heat classes of rotary exchangers (condensing, enthalpy and sorption wheels) are defined in the Eurovent Certita AARE programme [4].

The results clearly separate the air passing over the evaporator and the evaporation temperature:

Table 1a — Temperate-climate design (HP-HR-R): evaporator inlet, evaporation temperature and outlet condition versus outdoor air.

Table 1b — Cold-climate design (HP-HR-R-C): evaporator inlet, evaporation temperature and outlet condition versus outdoor air.

Method: what governs frosting?

Frost depends not on the evaporation temperature TEvap itself but on the coil surface temperature. When the surface falls below both 0 °C and the dew point of the incoming air, moisture condenses and turns to frost [5]. Since the surface temperature is not reported directly, it is estimated from the cycle value as Tsurf ≈ TEvap + 2 K. On this basis the surface freezing threshold corresponds to TEvap ≈ −2 °C (for the dew-point calculation, see [6]).

Indeed, the evaporator outlet in Table 1a being below 0 °C and at 100% RH shows directly that both conditions (wet + freezing surface) are met. The same data can be read as the change of evaporation temperature with outdoor air (Graph 1). The temperate-climate design's line dives below the freezing threshold, while the cold-climate design stays flat in the +2.2…+2.7 °C band.

In the temperate-climate design, the evaporator sees air that the rotor has already cooled. That is why the evaporation temperature collapses as outdoor air drops. At +10 °C, TEvap (−1.0 °C) is still just above the freezing threshold (≈ −2 °C). At 0 °C outdoor, however, the evaporator outlet is already below 0 °C and at 100% saturation, so frosting has begun.

In the cold-climate design, the evaporator always sees the raw 20 °C return air. So TEvap stays in the +2.2…+2.7 °C band even at −20 °C outdoor. The surface temperature is around +4 °C: it is below the dew point, so the coil runs wet and dehumidifies, but it stays above 0 °C and does not frost.

What this means: Same unit, same compressor, same evaporator and condenser. But the layout keeps the evaporator surface out of the freezing zone in cold climates.

At low outdoor temperatures: the compressor's stop-start cycle

The real consequence of frosting shows up in the compressor in heating mode. In the temperate-climate design, the evaporator keeps collecting ice as outdoor air drops. As ice blocks the coil, heat transfer degrades, suction pressure falls and the compressor cuts out for protection. The unit runs a defrost step; once the ice is gone, the compressor re-engages and the evaporator starts frosting again.

Defrost methods for reversible heat pumps, such as reverse cycle, electric pre-heating and recirculation, are defined in manufacturer function guides; all of them exist to manage exactly this load [2]. The result is a continuous stop-start regime at low outdoor temperatures. The compressor cannot carry a stable load, supply temperature fluctuates, and heating continuity breaks down.

In the cold-climate design this cycle never starts. Because the evaporator sees raw 20 °C return air, the evaporation temperature stays positive and the compressor runs continuously even at the lowest outdoor temperatures. The extreme cold is handled by the rotor at the very end of the return path. Since the rotor turns continuously, it carries this load without pushing the compressor into cycling.

This matches a known difference between exchanger types. In high-efficiency plate exchangers, frosting starts at higher outdoor temperatures. Rotary exchangers, by contrast, run without frosting down to lower temperatures and are structurally more tolerant of build-up [7].

What this means: In a cold climate the real question is not "how many points does efficiency drop" but whether the compressor stays operable. The right layout does not manage defrost with controls; it removes the need in the first place. (This study does not measure defrost frequency or duration; it shows that the frosting mechanism does not occur in the cold-climate design.)

Minimum operating temperature: what the compressor envelope says

We overlaid the operating points of both layouts on the operating envelope the manufacturer publishes for the PSH023-4 scroll compressor [8]. This chart makes it visible why the minimum operating temperature is a consequence of the layout.

Temperate-climate design (winter)

As outdoor air drops, the operating points move toward the cold (lower-left) edge of the envelope. At −20 °C outdoor the point (TEvap −24.4 °C / TCond 19.4 °C) falls below the envelope's 20 °C lower condensing limit, so it lies outside the envelope. TEvap alone is still inside the −30 °C lower limit, but the condensing temperature is below the lowest permitted value. Combined with early evaporator frosting, this is the region that feeds the stop-start regime described above.

Cold-climate design (winter)

The operating points cluster around TEvap +2.5 °C, in the centre of the envelope with plenty of margin from the cold edge. In this layout, the minimum operating temperature is no longer limited by defrost or the compressor envelope but by the supply temperature. The cold-climate design's supply temperature is 20.5 °C at 0 °C outdoor and drops to 14.2 °C at −20 °C.

At very low outdoor design conditions, what comes into play is not defrost but a small trim heater. This is an electric or hydronic coil that tops up the last few degrees the heat pump falls short of.

What this means: The answer to "down to what temperature does this unit run?" comes not from the catalogue but from the layout's trace on the compressor envelope.

Performance: on-paper COP versus field reality

Let us read the heating performance of the two layouts together with evaporation/condensation temperatures and supply temperature:

Table 2a — Temperate-climate design (HP-HR-R): heating operating point, supply temperature and system COP.

Table 2b — Cold-climate design (HP-HR-R-C): heating operating point, supply temperature and system COP.

On paper, the temperate-climate design also gives a reasonable COP. But these values come from a steady-state solution. In reality the temperate-climate design cannot sustain these COPs at low outdoor temperatures, because it keeps frosting its evaporator and stop-starting its compressor.

The cold-climate design's values, by contrast, are actually achievable. Because TEvap stays fixed at +2.5 °C, the temperature lift the compressor has to cover stays small, and the rotor takes on a growing share of the heating load. The COP values are system COP, including the rotor's passive heat recovery; they are not compressor-only COP.

What this means: COP must be split into "on paper" and "in the field". The cold-climate design already gives a higher COP and can sustain it. The temperate-climate design's reasonable on-paper COP cannot be reached at low outdoor temperatures because of stop-start, so the real gap is even larger than the table shows.

The summer side: two operating modes of the cold-climate design

To read the cold-climate design's summer behaviour, its two operating modes must be separated. In this layout the rotor and the compressor cannot run efficiently together in summer. When the compressor runs, the rotor on the return path sees the hot air leaving the condenser (≈ 43.5 °C). With no temperature difference left against the fresh air, the rotor becomes ineffective.

For 30–45 °C outdoor air, the three operating cases are:

Table 3 — Supply temperature / total electrical power in the three summer operating modes (30–45 °C outdoor).

Rotor recovery is the same in the temperate and rotor-only modes: 45 °C → 108.7 kW · 40 °C → 74.8 kW · 35 °C → 44.9 kW · 30 °C → 19.0 kW. In compressor-on mode the rotor contribution is zero.

The table and the graph show that, in summer, the operating mode and the outdoor air decide the result together:

• At high outdoor temperature (≈ 45 °C): In the cold-climate design, rotor-only mode gives both a lower supply temperature (27.1 °C) and much lower power (2.71 kW). Running the compressor disables the rotor, so power rises and the supply gets warmer (31.26 °C). On the hottest days the smart mode is rotor-only, i.e. free-cooling behaviour.
• At moderate outdoor temperature (≈ 30–35 °C): Compressor-on mode gives deeper cooling (22.67 °C supply at 35 °C, 18.54 °C at 30 °C). Rotor-only only reaches ~25 °C in this band.
• The crossover is around ≈ 40 °C: The two modes give a similar supply temperature, so the choice shifts to rotor-only to save power.

In every case the temperate-climate design gives the lowest supply temperature, because it uses rotor pre-cooling and the compressor at the same time. In short, the cold-climate design works like a free-cooling unit on hot days. In hot climates that need deep mechanical cooling down to setpoint, the temperate-climate design is the better fit.

What this means: There is no "single correct mode" for summer. The unit should be managed between rotor-only (economical/free cooling) and compressor-on (deep cooling) according to outdoor air and the comfort target.

Practical takeaways

• Heating-dominated / cold climate (Nordic countries, etc.): Prefer the cold-climate design. The compressor runs without stop-start even at the lowest outdoor temperatures, and in summer the rotor provides free cooling. Depending on equipment selection, the supply temperature drops below the comfort band at roughly −10 … −15 °C outdoor; plan a trim heater for conditions below that band.
• Cooling-dominated / temperate climate (most of Türkiye): Prefer the temperate-climate design. The rotor and compressor work together in summer for deep cooling, and the winter defrost load in these regions stays limited.
• Minimum operating temperature should be treated not as a catalogue number but as a result of the layout. When selecting a unit, ask for the TEvap trend.

One-sentence lesson: In cold climates, the limit of heat recovery is set not by the compressor's power but by the evaporator surface temperature; when the layout feeds the evaporator with raw return air, the compressor runs without stop-start even at the lowest outdoor temperatures.

References

1. 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
2. Swegon, GOLD RX/HC – Function Guide, n.d., https://www.swegon.com/globalassets/digizuite/9295-en-funkrxhc_functionguide_en.pdf
3. IV Produkt, ThermoCooler HP – Integrated reversible heat pump, 2024, https://www.enawent.pl/files/39/2024_1_ThermoCooler_HP_en.pdf
4. Eurovent Certita Certification, AARE – Air-to-Air Regenerative (Rotary) Heat Exchangers (certification programme), n.d., https://www.eurovent-certification.com/en/third-party-certification/certification-programmes/aare
5. ASHRAE, ASHRAE Handbook – HVAC Systems and Equipment, chapter "Air-to-Air Energy Recovery Equipment", n.d., https://www.ashrae.org/technical-resources/ashrae-handbook
6. Wood, L.A., The use of dew-point temperature in humidity calculations, Journal of Research of the National Bureau of Standards – C, 74C(3–4), 117, 1970, https://nvlpubs.nist.gov/nistpubs/jres/74C/jresv74Cn3-4p117_A1b.pdf
7. Eurovent AISBL/IVZW, 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
8. Danfoss, PSH heating-optimized scroll compressors – Application Guide (PSH019–PSH039), n.d., https://assets.danfoss.com/documents/latest/525181/AB492637854347en-000301.pdf
9. HVACinOne, HVAC selection and calculation software – device technical outputs (8000 m³/h reversible HPHR), 2026

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