Hot-Climate Rotor Selection: Summer Latent Deviation, Winter Defrost

In warm and temperate climates, equipment arrangement in "heat recovery + integrated heat pump" (HP-HR-R) units determines more than the nominal COP/EER. It also directly determines supply-air temperature stability, compressor power demand and winter defrost continuity. Using calculation outputs from HVACinOne, this article interprets how rotor types (sensible/condensation, enthalpy/hygroscopic, sorption) and the summer–winter air-path arrangement affect cooling/heating performance. We also take potential field issues into account.

The HVACinOne air flow for summer mode is shown in the diagram below. In winter mode the cycle is reversed via a four-way valve, and the evaporator and condenser swap roles. In this arrangement, the results are interpreted using rotor types that Eurovent Certita Certification programmes separate into clear classes such as "condensing / enthalpy / sorption" [1].

Rotor types and application areas

This study considers four rotor families. Each family behaves differently in terms of the nature of heat transfer (sensible/latent) and the moisture transfer mechanism (condensation, hygroscopic surface, adsorption). In an HP-HR-R unit these differences clearly affect the evaporator load in summer and the defrost risk in winter [1], [2].

Condensation rotor (ST)

This class of rotors is optimised mainly for sensible heat recovery; moisture transfer is not a target under normal operating conditions. However, if the local surface temperature drops below the dew point while the exhaust (extract) air cools over the rotor, condensation can occur inside the rotor matrix. A limited latent effect may then appear under "wet" operation. This effect is not a controlled/hygroscopic transfer but a condition-dependent condensation phenomenon.

Therefore, in hot-humid climates its ability to reduce absolute humidity on the supply side is more limited than that of hygroscopic/sorption classes, and the evaporator may have to do more dehumidification work. In Eurovent terminology this group is treated as a separate product type, the "condensing wheel" [1].

Enthalpy / hygroscopic rotor (SE)

These rotors target not only sensible but also latent (moisture) transfer, using a coating/treatment that makes the surface hygroscopic (able to hold moisture). The mechanism works by holding water vapour on the rotor surface and carrying it to the other air stream (an adsorption/desorption-like process). As a result, the dew point / humidity ratio on the supply side can be reduced more effectively.

However, this class may not move moisture as aggressively as sorption rotors; its performance is bounded by "moisture recovery" targets. Especially under extremely hot-humid conditions, moisture-driven performance deviation may not disappear completely. This is how the enthalpy/hygroscopic class is positioned within Eurovent's moisture-recovery approach [2].

Sorption rotor (HM)

Sorption rotors use materials/coatings with a higher adsorption capacity to increase moisture transfer, and "sorption" is treated as a separate category in certification and terminology [2]. The practical advantage of this class is that, in hot-humid climates, it can lower the post-rotor absolute humidity of the supply air further, so less water condenses on the evaporator. This improves compressor power draw and/or supply-temperature deviation. In short, in an HP-HR-R configuration the sorption rotor acts in summer like a "pre-conditioning element that reduces the evaporator's latent load" [2], [3].

Sorption hybrid rotor (SH)

Hybrid sorption rotors sit at an "intermediate" point between hygroscopic (enthalpy) and sorption behaviour. The goal is to strengthen latent transfer noticeably while keeping some application constraints (cost, pressure drop, carryover targets, control flexibility) in balance. For this reason, in hot-climate designs they are often chosen as a compromise that clearly reduces latent-driven performance deviation without going to the maximum sorption effect [3].

Rotor classes and manufacturer equivalents

What a rotor type is called is not only a marketing question; it also sets the certification and comparison methodology [1], [2], [3]. Below is a comparison of products from Hoval, Klingenburg and Heatex [4], [5], [6].

Table 1 — Rotor classes and their equivalents in Hoval, Klingenburg and Heatex products.

To summarise briefly:

• Condensation / sensible rotor: Rather than saying "no moisture transfer", the right statement is this: if condensation occurs, a limited latent effect appears, but it is not controlled moisture transfer (it is a condition-dependent phenomenon). Therefore, in hot-humid outdoor air the expectation of "reducing absolute humidity" is limited [1].
• Enthalpy / hygroscopic rotor: Moisture transfer works in both directions: in winter, humidity recovery so the indoor air is not "over-dried"; in summer, partial "drying" of the supply air to reduce the coil's latent load. However, it is not as aggressive as sorption [2].
• Sorption (3Å molecular sieve): The molecular-sieve coating stands out with the argument of reducing summer cooling demand thanks to high moisture effectiveness. Some documents also state that it helps reduce odour transfer (selectivity) [7].
• Hybrid: In practice the "middle solution" is usually a partial-coating approach; it becomes the most commonly used class in the cost/ΔP/performance balance. Heatex's definition "Hybrid … (Enthalpy)" describes this approach clearly [6].

Calculations in HVACinOne for dry and humid summer conditions and winter conditions

In the context of this article, the calculation workflow in HVACinOne follows this sequence:

1. Outdoor condition: The outdoor air condition (DB/RH; for example a constant-DP scenario) is defined.
2. Rotor step: From the rotor inlet conditions on the supply and extract streams, the rotor outlet temperature and relative humidity are calculated (in summer on the supply stream before the evaporator; in winter on the extract stream before the evaporator). Latent transfer behaviour differs by rotor class [2].
3. Heat-pump cycle: The cycle is solved iteratively from the air conditions on the evaporator/condenser sides and the target supply setpoint. The results are TEvap/TCond, compressor power draw and supply temperature.
4. Winter defrost evaluation logic: The risk of condensation and icing in the evaporator inlet air after the rotor on the extract stream is based on comparing the evaporator inlet dew point (Tdp,evap,in) with the coil surface temperature (Tsurf). Tdp,evap,in is calculated from the rotor outlet temperature and relative humidity; the condition Tsurf < Tdp,evap,in marks the threshold where condensation starts. When the surface temperature drops below 0 °C, the condensate turns into ice and an icing/defrost band forms. The risk trend can be read from the difference ΔT = Tdp,evap,in − Tsurf; as ΔT grows, the risk trend grows [14].

Note: The HVACinOne coil library outputs do not report the coil surface temperature directly. For comparison, Tsurf was therefore estimated representatively from the cycle's TEvap.

Outdoor air conditions used in the calculations

Table 2 — Outdoor air conditions used in the calculations.

In four unit designs, all equipment except the rotor was kept the same, and the effect of the rotor type on the system in summer and winter was examined. The equipment used is summarised below.

Table 3 — Equipment kept the same in all four designs.

Table 4 — The four rotors compared.

Effect of rotor type on supply temperature and compressor power in humid and dry outdoor air

The numerical comparisons below are taken from two HVACinOne summer cases at 35 °C.

Humid air: 35 °C, RH 41.6% (DP ≈ 20 °C)

Table 5 — Humid air (35 °C, RH 41.6%): rotor outlet, supply temperature, compressor power and TEvap.

In humid outdoor air, the post-rotor relative humidity differs strongly by rotor type. This increases the evaporator's latent load and therefore the compressor power. Rotor type definitions and moisture transfer behaviour are explained in the Eurovent framework through the "sorption / enthalpy / condensing" distinction [1], [2].

In the configuration that carries the most moisture to the evaporator (ST in this dataset), the evaporator's latent load rises, so compressor power increases even though TEvap goes up. In practice this pushes the supply temperature upward as a latent-driven performance deviation in summer operation.

Dry air: 35 °C, RH 30%

Table 6 — Dry air (35 °C, RH 30%): rotor outlet, supply temperature, compressor power and TEvap.

Latent impact map (humid − dry, 35 °C)

Table 7 — Latent impact map: difference in supply temperature and compressor power between humid and dry air at the same 35 °C.

At the same DB = 35 °C point, when absolute humidity (DP) rises (dry → humid), an adverse deviation of +0.5 °C to +2.7 °C appears in supply temperature, depending on rotor type. In parallel, compressor power increases by +0.02 kW to +0.28 kW.

The physical meaning of this difference is as follows: in summer operation the rotor sits upstream of the evaporator on the supply stream. So the rotor's moisture (latent) transfer changes the dew point and humidity ratio of the air entering the evaporator. The evaporator duty can then shift from purely sensible cooling toward a load dominated by moisture removal (latent cooling/condensation). As a result, the cycle works harder to reach the same supply temperature target and compressor power tends to rise.

Eurovent's "moisture recovery" framework defines exactly this point by classifying rotors as condensing / enthalpy / sorption: under which conditions and by which mechanism the rotor transfers moisture, and how this reflects on system loads. That is why separating a hot-dry profile from a constant-DP profile makes the differences between rotor types visible in a "latent-sensitive" way [2].

Compressor continuity under winter inlet conditions for four rotor types

In winter mode the extract-path order is Filter → Rotor → Evaporator → Fan, so the rotor's heat recovery cools the extract air before it reaches the evaporator. In this arrangement, two axes that decide continuity must be read together:

• Whether icing will start (risk): If the evaporator surface drops below 0 °C and at the same time stays below the dew point of the inlet air (Tsurf < 0 °C and Tsurf < Tdp,evap,in), the system enters the icing band [9], [14].
• How fast ice builds up (rate): Once in the icing band, the build-up rate tends to rise as the mass-transfer "driving difference" between the moisture level in the air stream and the saturation level at the frost surface grows. Frost growth models define this as the driving potential [15], [16].

Assumption linking evaporation temperature and surface temperature

The quantity read directly from the cycle solution in HVACinOne output is TEvap (the refrigerant evaporation temperature). A practical approach is therefore used for the coil surface temperature:

Tsurf,est ≈ TEvap + 2 K

This assumption suits a relative continuity comparison between rotor types rather than a prediction of absolute defrost time.

Post-rotor humidity ratio, dew point and surface temperature approach

The air condition at the evaporator inlet is read after the rotor on the extract stream. At this point:

• Dew point (Tdp,evap,in) lets us read the condensation/icing threshold with a single number. Comparing Tdp,evap,in with Tsurf gives the "dry coil / wet coil" distinction, and the icing band is defined below 0 °C [14].
• Humidity ratio (w_in) is useful for separating the build-up rate. At the same surface temperature, a higher w_in tends to increase the water-vapour flux to the frost surface (the mass-transfer driving difference grows). The w_in values come from the humidity-ratio column reported for the post-rotor evaporator inlet air in the HVACinOne winter technical output [15], [16].

Table 8 — Post-rotor evaporator inlet in winter: TEvap, estimated surface temperature, dew point and humidity ratio.

Table 9 — Frosting risk / ice build-up rate by rotor type and outdoor temperature.

Note: The "frosting rate" classification is consistent with the driving-difference approach that governs mass transfer to a frost surface. As w_in decreases (drier air), the build-up rate also tends to weaken [15], [16].

Which system is more prone to defrost at which temperature?

This dataset gives a very clear continuity message driven by the arrangement:

• DB_out = 10 °C: For three rotor families (ST, SE, SH), Tsurf,est stays at or above 0 °C, so the system does not enter the icing band. In the HM rotor, by contrast, Tsurf,est drops just below 0 °C, which creates a possibility of icing in the borderline zone.
• DB_out = 5 °C: For all four rotor families, Tsurf,est drops clearly below 0 °C and the condition Tsurf,est < Tdp,evap,in is met, so all of them enter the high-risk band. Here the question shifts from "does it start?" to "how fast does it progress?". On the ST/SE/SH side the higher w_in makes the fast class dominant; HM stays at a medium rate at the same outdoor point thanks to a relatively lower w_in.
• DB_out ≤ 0 °C: The icing band is strongly met for all types. As TEvap converges across rotor types, the continuity difference shrinks and defrost management becomes decisive.
• DB_out = −10…−15 °C: Risk is still high; however, because the air is drier, the build-up rate shifts toward the slow class.

Once icing starts, two critical field effects come into play. The air path narrows, pressure drop rises and fan power increases; at the same time, heat recovery/heat transfer performance deteriorates [8].

As a result, in this arrangement continuity at low outdoor temperatures usually requires an active defrost strategy, regardless of rotor type. In integrated reversible heat-pump applications, defrost options such as cycle reversal, auxiliary heating and recirculation are clearly defined in manufacturers' function guides [10].

Conclusion

• Summer performance (example DB = 35 °C): In humid air, rotor types change the evaporator's latent load. In this dataset they create a latent-driven performance deviation of +0.5…+2.7 °C in supply temperature and a compressor power increase of +0.02…+0.28 kW.
• The decisive effect of the arrangement: In summer, placing the rotor upstream of the evaporator offers a potential to pre-condition the supply air (especially on the moisture/enthalpy side). In winter the same arrangement carries a colder source to the evaporator after the rotor on the extract stream; this tends to pull TEvap down and can widen the defrost window.
• Winter continuity: At DB = 5 °C and below, risk rises for all rotor types. Below this band, continuity often makes defrost management (cycle reversal, electric backup, recirculation/bypass, etc.) and, if needed, a preheater part of the design, regardless of rotor type [8], [9], [10].
• Market approach: In the integrated-solutions market, manufacturers such as Systemair (Geniox HP) and Swegon position the "integrated reversible heat pump + rotary heat exchanger" concept as a product family. Climate coverage and defrost strategy are treated as core decisions of the product architecture [11], [12].
• Component/section variety: IV Produkt offers an integrated cooling section with a rotary heat exchanger (e.g. EcoCooler) or a modular section approach. This shows that different heating/cooling architectures can be built on the same "air path" [13].

References

1. Eurovent Certita Certification, AARE – Rotary Heat Exchangers (certification programme), n.d., https://www.eurovent-certification.com/en/third-party-certification/certification-programmes/aare
2. Eurovent AISBL, Eurovent 17/14 – Moisture recovery in ventilation and air-conditioning systems (First Edition), 2025, https://www.eurovent.eu/wp-content/uploads/publications-files/eurovent-rec-17-14-moisture-recovery-in-ventilation-and-air-conditioning-systems-first-edition-2025-en.pdf
3. Systemair, All you need to know about: Energy Recovery (blog post), 2022, https://www.systemair.com/en/blog/efficiency-performance/all-you-need-to-know-about-energy-recovery
4. Hoval, Sorption rotary heat exchangers – Hoval Energy Recovery Portal, n.d., https://www.hoval-energyrecovery.com/en/Sorption-rotary-heat-exchangers/sorption-rotary-heat-exchanger/
5. Klingenburg, HUgo Sorption Rotor (DekaTru® coating), n.d., http://www.klingenburg.co.uk/en/products/rotary-heat-exchangers/hugo-sorption-rotor/
6. Heatex AB, Heatex Rotary Heat Exchangers – Technical Information (Version 2), 2025, https://www.heatex.com/wp-content/uploads/2023/03/Rotary-Heat-Exchangers-Technical-Information-English.pdf
7. Östberg, 3Å molecular sieves, 2017, https://na.ostberg.com/3a-molecular-sieves/
8. Lawrance, W. (Swegon), Condensation and frosting in rotary heat exchangers (blog post), 2024, https://blog.swegon.com/en/condensation-and-frosting-in-rotary-heat-exchangers
9. ASHRAE, ASHRAE Handbook – HVAC Systems and Equipment, chapter "Air-to-Air Energy Recovery Equipment", n.d., https://www.ashrae.org/file%20library/technical%20resources/covid-19/si_s20_ch26.pdf
10. 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
11. Systemair, Geniox HP range with integrated reversible heat pump (product brochure), n.d., https://a.storyblok.com/f/108972/x/aa7e39befa/systemair-geniox-hp-brochure_20240508_.pdf
12. Swegon, GOLD RX/HC – all-in-one AHU and reversible heat pump (product page), n.d., https://www.swegon.com/products/air-handling/air-handling-units/gold/gold-rxhc/
13. IV Produkt, EcoCooler – Integrated speed-controlled DX cooling (product page), n.d., https://www.ivprodukt.com/products/integrated-cooling-unit/ecocooler
14. 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
15. Na, B., Webb, R.L., Mass transfer on and within a frost layer, International Journal of Heat and Mass Transfer, 47(5), 899–911, 2004, https://doi.org/10.1016/j.ijheatmasstransfer.2003.08.023
16. Kandula, M., Frost growth and densification in laminar flow over flat surfaces, NASA technical report KSC-2011-043, 2011, https://ntrs.nasa.gov/citations/20110009969

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