Analysis of Window Trickle Vents at Various Pressure Differences

Abstract

Air pollution remains a major global health concern, contributing to millions of premature deaths annually. Poor indoor air quality (IAQ) is strongly associated with sick building syndrome (SBS), which can lead to various health problems and reduced workplace productivity. This study examines the role of trickle vents as a passive component in natural and hybrid ventilation systems aimed at improving IAQ and occupant comfort. Two types of factory-produced trickle vents were tested in a controlled climatic chamber under systematically varied indoor–outdoor pressure differentials, generated using a blower system. Airflow measurements revealed a strong relationship between pressure difference and vent performance. Differences between the two vent types were largely due to variations in cross-sectional areas, influencing airflow resistance and pressure drop. Although neither vent achieved the required ventilation rates for standard conditions, their integration into hybrid systems, particularly in combination with mechanical exhaust fans, was found to significantly enhance potential airflow. The findings underline both the challenges and opportunities in achieving effective ventilation, especially in upper building floors where natural driving forces are reduced. This work contributes to the understanding of passive ventilation components and their potential to support healthier, more sustainable indoor environments.

1. Introduction

1.1. Public-Health Context

Urbanization has led to a significant increase in population density in the cities around the world. With an average person spending approximately 90% of their time indoors, the significance of maintaining good indoor air quality (IAQ) is of particular importance [1]. According to World Health Organization data, the direct and indirect impact of air pollution caused approximately 3.2 million deaths in 2020 [2]. Deterioration of indoor air quality and related health issues may lead to phenomena called sick building syndrome (SBS) [3]. It reduces work efficiency and can cause symptoms such as headache, dizziness, as well as difficulty in concentration, fatigue and even increased incidence of asthma attacks [4,5,6,7,8]. A properly designed and managed mechanical ventilation system is an effective measure to dilute and remove excessive pollutants and moisture build up from premises, and thus preventing the risk of SBS or other factors related to poor IAQ [8,9]. To accurately calculate the required amount of air which is necessary to maintain comfort and health in the indoor environment, various parameters are to be considered: occupant activity level, clothing level, indoor pollution level and floor area [9,10,11].

1.2. Regulatory Framework and Ventilation Targets

In accordance with the European standard EN 16798-1:2021 [12], upon which the Latvian local standard LVS EN 16798-1:2021 is based, the recommended ventilation rate comprises two components: the airflow per person and the airflow per unit area. These airflows depend on factors like building classification, contamination level, space size, and occupant count [11,13]. For the first building category, characterized by a 15% dissatisfaction threshold among occupants, the prescribed ventilation rate is set at 10 L/s per person. Conversely, for the fourth building category, where the level of occupant dissatisfaction is at 40%, the specified ventilation rate stands at 2.5 L/s per person. In addition, the airflow per unit area is influenced by the contamination level within the building environment. Buildings are categorized based on contamination rates as very low polluting, low polluting, or non-low polluting [14].

1.3. Natural and Hybrid Ventilation: Physical Drivers and Constraints

Natural ventilation is a key passive cooling strategy used to achieve low-carbon building design. It reduces energy consumption, and improves occupants’ health, comfort, and productivity [15]. Various studies have shown the importance of natural ventilation solutions for maintaining good indoor air quality and improving building energy efficiency [16,17,18]. Natural ventilation concepts using vents can be used for various purposes, such as ventilation, cooling and heating [19]. Some of the examples include residential buildings [20], industrial [21,22,23], commercial and public premises such as schools [17,24,25,26], sports halls [27].

Urban microclimate and morphology strongly influence the natural ventilation potential (NVP): by combining the urban surface model with EnergyPlus, Xie et al. show that NVP efficiency and cooling energy savings vary with building density and season, and that in dense structures, one-way ventilation can be comparable to cross-ventilation efficiency. This finding supports the use of facade-integrated passive air inlets, such as cross-ventilators, when cross-ventilation is geometrically impossible. They also demonstrate that relying on rural weather data biases NVP efficiency estimates in urban areas, highlighting our focus on pressure-dependent performance, which is sensitive to the real-world urban wind and smoke conditions our devices will encounter [15]. At the room scale, the hydromechanical basis determining “how” inlet openings remove pollutants matters: Kay and Hunt quantify the transient development and washdown efficiency of naturally ventilated rooms as a function of heat source distribution and the effective area of the inlet openings; for large openings, a uniformly distributed airflow provides faster washdown, while for smaller effective areas, localized jets are more effective. This model explains why the effective area and placement of small inlet openings significantly influence pollutant removal and supports our decision to consider flow through drip openings as a function of pressure drop and resistance [17]. Building-level retrofits interact with these airflow mechanisms through building envelope selection and management: thermal renovation studies emphasize cost-optimal thermal resistance and portfolio-level selection of energy-saving measures under budget constraints using dynamic programming and life cycle indicators—a context that motivates the combination of low-leakage building envelopes and calibrated background inlets to match indoor air quality without compromising the economic viability of the retrofit [19]. In schools, where occupancy density is high and windows are intermittently used, field campaigns report mean airtightness of n⁵⁰ ≈ 7 h⁻¹ and average CO₂ ≈ 1878 ppm, with 42% of classrooms exceeding 2000 ppm when windows are closed; such evidence has led authorities to promote natural-ventilation strategies even where mechanical ventilation is nominally required, highlighting the need for reliable, passive air pathways that can temper CO₂ without continuous window opening [25]. Additional classroom research conducted during the COVID-19 period uses CO₂ as a practical indicator of indoor air quality and develops opening schedules that balance infection risk reduction with thermal comfort, again pointing to the role of controlled background air inlets to maintain baseline air exchange with doors and windows closed [18]. In addition to indoor air quality, thermal comfort/energy studies show that building envelope upgrades and heating control changes lead to changes in radiant temperature and PMV, while periodic temperature reduction can reduce heating energy consumption by approximately 13% over the course of a season, provided transient comfort levels remain acceptable. These results support hybrid solutions that combine small passive air intakes with temporary exhaust ventilation or heating controls to stabilize both comfort and indoor air quality in renovated schools [26,27]. Taken together, these applications justify our methodological focus on characterizing airflow through vents as a function of indoor-outdoor pressure differences, and on discussing their integration into hybrid systems, particularly for upper-story spaces where stack and wind pressures are low but reliable base airflow is needed.

However, there is also a wide use of vents in hybrid/passive novel heating systems such as Trombe wall applications (particularly in warmer climate regions) [28]. While there are numerous studies aimed at exploring efficient approaches to implement natural ventilation systems, very few explore air vent configuration and characteristics for effective air exchange and distribution. Some research dedicated to examining various vent patterns and configurations in natural ventilation systems included experimental and simulation studies in different settings.

Lyu et al. conducted a thorough investigation into the impacts of factors such as vent opening, wind speed, and crop height on the microenvironment within a three-span arched greenhouse using natural ventilation. The simulation outcomes revealed that under thermal pressure ventilation conditions, the effectiveness of ventilation and cooling, along with the temperature difference between the interior and exterior of the greenhouse, diminishes as the thermal pressure driving effect decreases [29].

Higton et al. investigated airflow patterns through a doorway and an upper vent in a room, revealing that accurate predictions of airflow regimes were possible based on simple physical principles. This study found that interface height depended on door aspect ratio, door height relative to room height, high-level vent area, and plume entrainment, with unbalanced exchange flows leading to increased ventilation rates compared to conventional models [30].

Castillo et al. examined how the arrangement of vent openings affects coefficients related to plume entrainment and virtual origin position, noting that despite variations in opening configurations, the theory accurately predicts interface height with minor discrepancies. Introducing vents, particularly three top vents, in a room with a standard window and a heat source was found to improve the indoor comfort conditions by lowering temperatures, providing valuable insights for natural ventilation design in low wind speed areas with floor-level heat sources [31].

Zhang et al. (2014) [32] focused on the influence of vent location in naturally ventilated building compartment fires through numerical simulations. The findings indicated that, for fires with equivalent heat release rates, smoke filling showed minimal variation across different vent locations, but fires with center vents had higher oxygen concentrations and lower gas temperatures compared to those with corner vents, which exhibited higher temperatures and distinct pressure differences, underscoring the importance of cautious modeling for fires situated directly under ceiling vents [32].

Fitzerald et al. investigated the influence of middle vent placement in naturally ventilated rooms based on heat source types and vent arrangements. The research revealed that for distributed heating at the base, upper and lower vents operated as outlets and inlets, respectively, with flow direction through the middle vent influenced by its height relative to neutral buoyancy, highlighting the potential for predicting ventilation and temperature control outcomes based on vent configurations and heating methods [33].

A novel adaptable ventilation approach, multi-vent module-based adaptive ventilation (MAV), employing multi-vent modules to alter inlet and outlet configurations for diverse indoor conditions while maintaining constant grille positions was proposed and evaluated by Zhang et al. (2022) [34]. Investigating its effectiveness in managing contaminant dispersion, the research established that different MAV modes exhibit varying performance levels in reducing contaminant diffusion and concentration around occupants’ oronasal areas. Specifically, the vertical MAV mode proved most effective in safeguarding indoor occupants from contaminants, emphasizing its potential benefits for infection risk mitigation [34].

Molkov et al. developed a model for passive ventilation in the case of sustained gaseous leaks within an enclosure with a single vent, based on perfect mixing assumptions. It demonstrated that the traditional assumptions about the location of the neutral plane in natural ventilation equations do not apply in the context of passive ventilation due to accidental gas releases, potentially leading to significant safety consequences as the exact analytical solution for passive ventilation diverges from the approximate solution for natural ventilation by a factor of ±2 [35].

In general, natural ventilation can be performed using two methods: trickle ventilation with a constantly slightly opened window (pane) and periodic short-term ventilation performed by a briefly widely opened window [36].

1.4. Trickle Vents: Definition, Typologies, and Prior Evidence

Creating efficient natural ventilation requires supplying the right amount of air as per local standards and regulations [37,38]. A commonly used natural-ventilation solution in the market is the “window trickle vent”. A window trickle vent is a small ventilation opening installed in windows to allow for controlled and continuous airflow into a building, maintaining some level of security and protection from weather, as well as ensuring protection from condensation, mold, and stale air. Trickle vents are especially useful in well-insulated and tightly sealed buildings where natural ventilation (infiltration) might be limited. Vents typically consist of a narrow opening with adjustable louvers or slats that can be opened or closed to regulate the airflow. Some research explores how window trickle vents perform in indoor spaces, looking at their construction, effectiveness, and effects on airflow and occupant comfort [39,40]. A literature review of existing research reveals that trickle vents are increasingly being adopted as prevalent natural ventilation solutions due to their ease of installation and their contribution to improved building energy efficiency. A comprehensive literature review on trickle vent application was conducted by Biler et al., focusing on their performance parameters, control strategies, positioning, and energy implications. The study underscored the need for optimizing trickle vent design in natural ventilation setting to balance their performance criteria and addresses the influence of factors such as window positioning, climate, and occupant behavior on their effectiveness in ensuring indoor air quality and energy efficiency [39]. Hoffmann et al. explored the integration of passive window ventilation openings (PWVO) and trickle vents with additional exhaust fans in residential buildings to achieve consistent air change rates. Findings indicated that while outdoor air flow rates often fall short of recommended standards, factors like heating systems impact air draft and CO₂ concentrations, highlighting considerations for future natural ventilation concepts [40,41,42]. Karava et al. conducted an experimental study investigating the performance of two types of trickle ventilators, slot and pressure-controlled, in low-rise buildings under wind influence. Pressure-controlled ventilators demonstrated better performance, maintaining stable airflow even under low pressure differences, thereby enhancing indoor air quality and occupant comfort. Empirical equations were derived from experimental data to predict trickle ventilator performance, and integration into hybrid ventilation systems proved effective in providing fresh air, offering insights such as a recommended minimum opening area of 5 cm²/m² of floor area for small office spaces in Montreal with occupancy under 12 m²/person [43].

Choi et al. investigated the utilization of a trickle ventilator for natural ventilation in a residential building during spring and summer, evaluating four quantitative assessment methods for natural ventilation rate. It was concluded that wind turbulence near the trickle ventilator plays a significant role in influencing the rate, with outdoor wind speed and direction having less impact. These findings emphasized the importance of local turbulence in shaping hybrid ventilation system design and suggest potential avenues for future research involving IoT technologies to predict and manage combined natural and mechanical ventilation [44].

1.5. Research Gap, Objectives, and Contributions

The objective of this study is to provide practical empirical data for two factory-assembled window trickle vents units tested in a controlled climate chamber with indoor-outdoor pressure differentials ranging from 5 to 100 Pa. This data is presented in the form of simple regression relationships suitable for preliminary sizing in accordance with EN 16798-1:2021 in a model room. This study does not propose new theory but rather provides calibrated measurements and transparent fits that can assist in early design decisions and assess when hybrid support may be required.

This study presents unit curves with linear regressions for two representative geometries, illustrating how cross-sectional design affects airflow resistance at stack pressures dependent on floor and season. The data is incremental and complements previous work on ventilation devices and hybrid concepts by providing product-specific measurements under standardized conditions along with design-ready regressions.

This study is motivated by the need to ensure healthy indoor environments in increasingly airtight and energy-efficient buildings, where infiltration and natural ventilation are insufficient to maintain adequate air exchange. While mechanical ventilation systems can effectively mitigate these risks, they are often expensive, energy-intensive, and impractical in some buildings. Trickle vents, as passive components of natural or hybrid ventilation, represent a potentially low-energy and easily integrated solution for maintaining air exchange in modern airtight buildings. However, despite their increasing prevalence, empirical data on their performance under various pressure conditions and their ability to meet established ventilation standards remains limited. This gap highlights the need for experimental evaluation to elucidate their effectiveness, limitations, and role in natural and hybrid ventilation systems.

We present the results of a controlled evaluation of two factory-built jet-air window vents, documenting airflow over a wide range of indoor and outdoor pressure differentials and summarizing the results as fitted pressure-flow relationships. Within the experimental setup, these calibrated regressions provide practical data for preliminary sizing and consideration of integration into a hybrid ventilation system.

2. Materials and Methods

Within the framework of this study, two window trickle vents were tested for their application in a real operational setting. The experimental study was conducted in a closed environment of climatic chamber, simulating the variable pressure drop conditions that can be formed between indoor space and the outdoor facade of the building. The window trickle vents (Figure 1) were installed in a window frame, which was installed in the climate chamber’s wall (Figure 2). Climate chamber was tightly sealed to ensure variable air conditions simulation. The airtightness inside the chamber allowed for an experimental simulation of any environment. The chamber dimensions were as follows: Chamber is installed in laboratory, and its dimensions are W: 3 m, L: 4 m, H: 2.3 m., the total volume of chamber was 27.6 m³. To conduct the measurements, tilt and turn double glazed PVC window of 0.5 × 0.5 m was installed in one of the chamber walls.

Outdoor air entered the chamber through the gap between window frame and closed sash. Air flow rate measurements in the vents were obtained at various pressure drops—from 5 to 100 Pa with increments of 5 Pa. To generate the pressure, drop during the test in climate chamber Retrotec 300 Series DucTester blower ((Retrotec, Ottawa, ON, Canada) was installed (

Table 1. Retrotec 300 technical data.

Flow	Range
Maximum Flow	1365.8 m3/h
Minimum Flow (102 mm flow range rings)	279.1 m3/h
Minimum Flow (74 mm flow range rings)	47.4 m3/h
Minimum Flow (47 mm flow range rings)	21.2 m3/h
Minimum Flow (29 mm flow range rings)	8.8 m3/h
Flow Accuracy	±3%
Digital gauge model	DM32

) with digital manometer DM32 (Retrotec, Ottawa, ON, Canada;

Table 2. Technical data.

Parameter	Range
Pressure measuring	−2488 Pa to +2488 Pa
Accuracy	±0.4% of pressure reading or ±0.07 Pa at 22 °C
	±0.6% of pressure reading or ±15 Pa at 0–44 °C
	±0.9% of pressure reading or ±1 Pa at 70 °C
Resolution	0.1 Pa

). Blower unit was created under pressure inside the chamber, and it was connected to the in the chamber wall. The air flow ranged from 4.5 to 1365 m³/h. With the use of manometer, blower can adjust the constant rotation speed which maintains needed pressure difference between external environment and climate chamber. Flow range rings of various connection sizes were used (47 mm, 74 mm and 102 mm) to ensure accurate blower operation at different minimum flowrates.

Testo 417 vane anemometer was used to measure volumetric air flow rate flowing through the window trickle vent. Anemometer was connected to a funnel, which was sealed to the window and adjacent surfaces. The volume flow measurements ranged from 0 to 440 m³/h (±0.1 m³/h (0 to +99.9 m³/h), ±1 m³/h (100 to 440 m³/h)) in combination with funnels. The instrument technical data is shown in

Table 3. Testo 417 technical data.

Parameter	Volume Flow
Volume flow measuring range	0 to 99,999 m3/h
Volume flow resolution	0.1 m3/h (0 to 99.9 m3/h)
	1 m3/h (100 to 99,999 m3/h)
Flow speed measuring range	0.3 to 20 m/s
Flow speed accuracy ±1 digit	±(0.1 m/s + 1.5% of m.v.)
Flow speed resolution	0.01 m/s

.

For an airtight fit of the funnel with anemometer to the window and adjacent surfaces, the surface around the window and the glass was levelled by construction materials, such as pressed cardboard and EPS sheets. Also, funnel was attached using foiled tape to adjacent surfaces for better sealing. This solution allows us to accurately measure the amount of air entering the chamber through trickle vent. Full experimental setup is shown in Figure 3.

To ensure the reliability of the experimental results, a series of calibration, environmental control, and measurement repeatability measures were implemented. Throughout the tests, stable boundary conditions were maintained in the climate chamber: the indoor temperature was 21 °C and the relative humidity was 45–50%. External laboratory conditions were maintained by closing windows and doors to minimize uncontrolled air movement. This protocol isolates pressure and flow characteristics from thermal transients, allowing device curves to be matched to building-scale pressure models during post-processing.

All measuring instruments were factory calibrated before the experiment, and the Retrotec DM32 pressure gauge and Testo 417 vane anemometer (Testo SE & Co. KGaA, Titisee-Neustadt, Germany) was further calibrated against reference instruments to confirm accuracy within the specified tolerances. Each pressure step was maintained for at least one minute to stabilize the fan and anemometer readings, after which three consecutive measurements were taken for each pressure level. The average value was used in the analysis, and the deviations between repetitions remained within the accuracy of the instruments (±3% for air flow, ±0.4% for pressure). These procedures ensured data reproducibility and minimized the influence of random variations.

3. Results

The results of the measurements conducted are plotted in Figure 4. The result illustrates the throughput (flowrate, m³/h) performance of two trickle vents under varying pressure conditions between indoor and outdoor environments. The x-axis represents the pressure difference, ranging from the 5 Pa to 100 Pa, while the y-axis showcases the corresponding throughput values for each vent. The data points on the plot represent a clear correlation between pressure and throughput for both vents. As the pressure increases, so does the throughput, indicating a positive relationship between these variables. However, slightly different throughput trends developed between the two vents, with vent B having a large capacity from the initial stage and featuring a steeper increase compared to vent A. The difference between vents “A” and “B” suggests potential variations in their respective design configuration. Vent “B” has a larger cross-sectional area compared to vent “A”, stipulating that the larger cross-sectional area affects the airflow through the vent. With a larger cross-sectional area, vent “B” allows for more space and thus less resistance for air to flow through it. Consequently, a higher volume of material or fluid can pass through the ventilation system at a constant pressure compared to vent “A”. Furthermore, the throughput of the system is influenced by the cross-sectional area of the space between the window frame and the closed sash, which is where the vent is located. This area may vary depending on the manufacturer and can be considered a potential “bottleneck” of the ventilation system. The larger this space is, the more air and materials the vent can handle, effectively increasing its throughput capacity. On the other hand, if this area is too small, it can limit the overall efficiency of the ventilation system by restricting the airflow.

The main outcome of this study is a pair of design-ready regression models that re-late airflow Q to the indoor–outdoor pressure difference Δp for two factory-built trickle vents (Figure 4). For each device we fitted equation Q = a∙Δp + b, obtaining strong correlations (R² ranging from 0.91 to 0.98, p < 0.001). For vent “A,” slopes ranged from 0.140 to 0.202 m³/h Pa, with intercepts between 7.28 and 10.28 m³/h, indicating moderate sensitivity to pressure difference and some variability among repeated measurements. Vent “B” demonstrated significantly higher slopes of approximately 0.315–0.316 m³/h Pa and intercepts near 11.5 m³/h, reflecting both its larger airflow capacity and more consistent performance over the course of testing. These results confirm the descriptive observation that Vent “B” outperforms Vent “A” under all test conditions. The fitted regression equations and their 95% confidence intervals, presented in Figure 4, demonstrate that experimental variability remained within narrow limits, thereby providing statistically reliable relationships suitable for predictive purposes when designing natural or hybrid ventilation systems.

For design use, the least-squares relationships are:

$$Vent A:Q_A=0.171∆p+8.131,{ R}^2=0.91,$$

(1)

$$Vent B:Q_B=0.316∆p+11.648,{ R}^2=0.98,$$

(2)

These regression models reproduce measured curves within the experimental scatter and are intended for preliminary sizing at known pressure drops.

The difference in air pressure between inside and outside a building can also depend on the height of the building’s floor due to a phenomenon known as the draft effect. The draft effect occurs in buildings due to the difference in temperature and air pressure between the inside and outside of the building [41,45]. It results in the movement of air in and out of the building through openings like windows, doors, and vents. During cold weather, the warm air inside the building rises to the upper levels, generating a positive pressure at the top and a negative pressure at the bottom. This pressure difference drives air infiltration from the outside at the lower levels and exfiltration at the upper levels [42,46]. As a result, natural ventilation units, including trickle vents, will allow more air to pass through on the first floor, with progressively decreasing airflow on each subsequent floor [33,39,47]. Typically, in Baltic states, in cold weather periods, the pressure difference is around 20 Pa in the first floor, 15 Pa in the third floor and 10 Pa for the fifth floor [48].

The experimental study was conducted in a 12 m² climatic chamber. According to EN 16798-1:2021, the required ventilation rate for this climatic chamber is 15.4 L/s or 55.44 m³/h, which corresponds to the second-class category of low-pollution buildings with a single occupant. However, conventional vents are unable to achieve this ventilation rate under typical conditions. With a pressure difference between the indoor and outdoor environment of 20 Pa, vent “A” provides a ventilation rate of ≈12 m³/h (with filter), and vent “B” provides a ventilation rate of ≈18 m³/h.

From a practical perspective, integrated vents can potentially complement hybrid ventilation systems when integrated with exhaust fans in ventilation shafts, creating negative pressure to facilitate the influx of fresh outdoor air. While experimental measurements show that vent “B” is better suited for this purpose due to its higher airflow performance at various pressure drops, it is important to recognize that hybrid ventilation systems have drawbacks such as air quality uncertainty, the risk of drafts, lack of protection from outdoor noise, and associated system costs. As a result, hybrid ventilation is more effective as a solution for retrofitting buildings where natural ventilation is insufficient to achieve the desired indoor air quality, and the implementation of mechanical ventilation is considered less feasible.

4. Discussion

Measurements demonstrated a clear relationship between indoor and outdoor pressure differences and the airflow capacity of both tested vents. This trend is consistent with the results of earlier studies, which identified pressure as the primary factor influencing the effectiveness of passive ventilation [33,39,47]. Across the test range, vent “B” demonstrated superior performance, particularly at low pressures, due to its larger cross-sectional area, which facilitates improved airflow by reducing drag. This result is consistent with previous studies highlighting the importance of vent geometry in determining ventilation performance [41,42].

Despite these differences, neither product delivered the EN 16798-1:2021 target ventilation rate of 55.44 m³/h for the climate chamber under realistic pressure conditions. This limitation supports previous findings that standard trickle vents are often insufficient in colder climates, particularly at higher building levels where stack effect pressures are reduced [42,46,48].

In practical terms, higher-performance drip vents can be useful when incorporated into hybrid ventilation systems alongside mechanical exhaust units, which can create the negative pressure needed to draw in outside air. However, this approach must consider issues such as indoor air quality control, draft prevention, and noise reduction.

4.1. Temperature and Real-World Variability

Our measurements provide airflow as a function of the pressure differential under controlled conditions. The relation (Equation (3)) is given by the orifice equation [49]:

$$Q=C_{d}A\sqrt{{\displaystyle \frac{2∆p}{\rho }}},$$

(3)

which we use to map device pressure–flow characteristics to operating conditions. To extrapolate from the reference temperature 21 °C any T at fixed Δp, we apply the density correction:

$$Q\left(T\right)=C_{d}A\sqrt{{\displaystyle \frac{{\rho }_{ref}}{\rho \left(T\right)}}},$$

(4)

with air density evaluated as follows:

$$\rho \left(T\right)={\displaystyle \frac{p}{RT}},$$

(5)

Over a representative range from −15 °C to +30 °C, in Equations (4) and (5), there is simply a change of approximately −6–−7% and 1–2%, respectively, in Q at fixed Δp. On the contrary, the stack component of pressure varies with the indoor–outdoor temperature difference and height:

$$∆p_{stack}=gH\left({\rho }_{out}- {\rho }_{in}\right).$$

Typically, in Latvian cold climate pressure difference ∆p produced by the floor-dependent and wind is ≈22.3 Pa for first floor and ≈−1 Pa for 9 floor in multiapartment building in winter months (

Table 4. The air pressure difference for nine-storey apartment building calculated using average Latvian climatic conditions, ΔP, Pa.

	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII
1st floor	23	21	19	15	11	9	7	8	12	16	21	23
9th floor	−1	−3	−1	1	2	4	4	4	4	4	3	1

) [50].

Consequently, the pressure–flow curves (Figure 4) should be read against floor- and season-specific Δp values to estimate seasonal performance. We explicitly note that chamber temperature was held constant; this controlled approach isolates the intrinsic pressure sensitivity of the vents and enables subsequent coupling with building-scale models and field data.

4.2. Limitations and Future Work

The results presented here are limited to two factory-made window vents tested under controlled laboratory conditions in a sealed climate chamber with constant boundary conditions. Therefore, the fitted regression models and resulting sizing recommendations are specific to the specific device and reflect its performance under a given pressure drop Δp, rather than under the transient wind and stack pressures observed on real facades. Generalization of the results beyond the two vent types studied requires broader validation in residential buildings at different floors, orientations, and seasons, as well as replication with other vent typologies and manufacturers.

Future studies will repeat selected pressure measurements at different indoor-outdoor temperature differentials to quantify any residual temperature dependence of the flow coefficients; conduct multi-season field monitoring of identical vents in residential spaces, recording wind, outdoor/indoor temperature, CO₂, and indoor pressure on both lower and upper floors; and integrate these data with analytical models of the chimney and wind pressure to generate recommendations for the design of hybrid systems, taking into account floor and season. Given the typical floor-dependent Δp in Baltic winters and our measured device curves, this program will allow for the validation of predicted flows throughout the year and the evaluation of comfort/indoor air quality tradeoffs under realistic transient conditions. Future studies should also evaluate integration with demand-controlled systems and refine vent geometry to improve both airflow and acoustic properties.

5. Conclusions

This work provides design-ready practice-oriented regression models linking trickle-vent airflow to pressure difference and translates them into sizing guidance under EN 16798-1:2021 requirements. These models, validated across Δp = 5–100 Pa, enable rapid estimation of device counts and the assessment of when passive fresh outdoor air supply alone is insufficient.

Comparative testing of two models under controlled pressure conditions showed that increasing the pressure drop leads to increased airflow in both designs, with vent “B” outperforming vent “A” due to its larger cross-sectional area and lower airflow resistance.

Real-world performance is influenced by many factors, particularly building height, where the draft effect can significantly impact natural ventilation rates; so, the applicability of the present regressions is bounded by the tested products and conditions. The test results showed that none of the vents met the performance requirements set by EN 16798-1:2021 for the test conditions. Vent “B” delivered a flow rate of 18 m³/h at a pressure drop of 20 Pa, significantly lower than the required 55.44 m³/h for the test room.

From a design perspective, using the fitted regression models, at ∆p ≈ 20 Pa, a single “B” vent delivers ≈ 18 m³/h, which corresponds to the EN recommended rate for the chamber. Therefore, approximately four units are required per room, increasing to five on the upper floors, where ∆p drops to ≲3 Pa. Therefore, meeting the requirements with trickle-vent alone is unlikely on the lower floors without mechanical assistance; hybrid operation with moderate exhaust settings can reduce the required number of supply openings and improve reliability across seasons.

While the findings suggest that trickle vents can contribute to IAQ improvement when combined with mechanical assistance, achieving compliance with ventilation standards remains challenging, particularly in upper floors. Continued development in vent design, integration with hybrid systems, and application-specific adaptations will be essential to overcome the current limitations. Future research should also consider occupant behavior, aesthetic integration into building facades, and smart control strategies to maximize performance while minimizing energy use.