Comparison of air quality in rooms with balanced mechanical ventilation and ventilation with windows

“Go outside and breathe in the fresh air” - this is the advice we received from our grandparents on a daily basis when we were children. And as usual, they turned out to be right and it was a very valuable piece of advice that we often forget today.

We inhale and exhale about 11,000 litres of air every day - more than 50 air-filled baths. The World Health Organization is also constantly emphasising the importance of fresh air for our health.

Various studies have shown the effects of poor quality and stale air on human and building health. Apart from dust and other pollutants, the main indicator of indoor air quality is the level of CO₂ in the room. When the level of CO₂ in a room exceeds 1,000 ppm (ppm - the number of CO₂ particles per million air particles), it can cause headaches, fatigue and an inability to concentrate. Concentrations of contaminants over a long period of time can lead to serious respiratory diseases such as asthma.

Given that we spend 90% of our time indoors and 65% of our time directly in our own homes, it is definitely important to have fresh air available indoors. Today it is not so easy to accomplish. We often live in very densely populated cities, as well as close to industrial areas, so even if we regularly ventilate rooms with windows, there is a high risk of pollution in our lungs. In addition, both new and renovated buildings today are very well insulated and dense, which ensures high energy efficiency, but at the same time reduces the natural air exchange in the premises. In order to maintain healthy air quality in the premises, the air in them must be changed regularly.

The public is of the opinion that opening a window a few times a day and ventilating the premises is enough to maintain a healthy and comfortable climate in the premises. The Zehnder Group, which deals with indoor climate issues on a daily basis around the world, has decided to check if this is really enough.

The survey was conducted for one calendar year from 8 July 2019 until 30 June 2020. The apartment building was chosen as the object of the study, in which four apartments were constantly monitored in order to obtain data on indoor air quality, outdoor conditions and energy consumption. Two of these apartments were equipped with balanced mechanical ventilation with heat and moisture recovery, while two apartments were ventilated with windows.

Object of research

 

The building of the research is an apartment building built in 2017 in Buren, Switzerland (Figure 1). The building is built to modern standards - with effective thermal insulation and a high degree of air density. The building has four floors: two living floors, a roof floor and a basement. There are two apartments on the first and second floor, with an area of 80 m² for each apartment on the left side of the building and 113 m² on the right side of the building. The penthouse was not monitored. In the basement there is a children's playroom in one part, which is heated and cooled, and in the other part there is an unheated room, which proved to have a significant impact on the heating consumption of the apartment on the right side of the 1st floor.

Households with two people live in three apartments and one person in one apartment (Figure 2). Each of the larger apartments also has pets - one with a dog and the other with two cats.

The apartments on the 2nd floor (hereinafter referred to as the upper floor) are ventilated by windows and doors, while on the first floor (hereinafter referred to as the lower floor) apartments are equipped with balanced mechanical ventilation systems with heat and moisture recovery with Zehnder ComfoAir Q 350 ERV equipment. All windows and doors are openable.

Measurements taken

The aim of the measurements was to compare the types of ventilation in terms of indoor air quality and energy consumption for heating and cooling.

Indoor CO₂ concentration (ppm) was used as an indicator of air quality, taking into account the recommendations of health organisations, assuming a concentration above 1 000 ppm for unhealthy air, between 800 and 1 000 ppm as medium quality, and below 800 ppm as healthy.

Energy consumption was also measured to ensure a comfortable temperature in both the heating and cooling seasons in the apartments. The building is heated and cooled by a heat pump.

Figure 2. Apartment plans with number of occupants, type of ventilation and possible places for ventilation through windows and doors marked in blue

The data obtained were collected from several sources:

• ComfoAir Q equipment - registers air exchange data recorded every 5 minutes;
• Air quality sensors - measurements every minute;
• The opening of the windows was recorded by contact sensors, indicating separately whether the window is fully or only partially open;
• Distance sensors measured how far the sliding windows and doors were open;
• The heat pump provided hourly energy consumption data for each apartment.

A ventilation factor was adopted, which includes how many windows are open and for how long. The factor ranges from 0% when all windows and doors are closed to 100% when all are fully open.

Achieved results

• Indoor air quality

Figure 3 shows that in apartments with balanced mechanical ventilation, the average CO₂ level in a room is between 400 and 600 ppm. During the night, CO₂ levels increase to 600 ppm. During the day when the inhabitants may not be at home, it decreases as mechanical ventilation continues to operate, reducing CO₂ levels to 400 ppm - the same as in a pine forest. In the evening, the level increases, but when looking at the measurements in a daily section, the average air quality is healthy.

Figure 3. Ventilation factor and CO2 concentration in rooms with mechanical ventilation (lower apartments)

In apartments with window ventilation (Figure 4), several windows are left open at night. When leaving in the morning, residents leave one window half open, but this does not ensure sufficient air exchange in the premises. CO₂ levels remain at 800 ppm throughout the day, and when people return home and their windows are closed, CO₂ levels rise to 1,200 ppm. It can be seen that the CO₂ level does not fall below 600 ppm throughout the day. Despite the ventilation with windows, the air quality in the rooms is average and even unhealthy.

Figure 4. Ventilation factor and CO2 concentration in rooms with window ventilation only (upper apartments)

• Air quality during the year

Figure 5 shows the air quality in different rooms throughout the year. This chart gives a better idea of which hours of the day and which months the air quality is better or worse. Colours indicate indoor air quality: green - quality air with CO₂ concentration < 800 ppm, yellow - medium air quality with CO₂ 800–1000 ppm, red - unhealthy air with CO₂ > 1000 ppm.

Each rectangle shows the air quality in one room (living room, bedroom) during the year from July 2019 to June 2020 (horizontally from left to right), every day from 0:00 to 24:00 (vertically from top to bottom).

Figure 5. Indoor air quality throughout the year in each room

Figure 5 shows the difference between air quality in rooms with mechanical ventilation and in rooms with window ventilation. Most of the time, balanced mechanical ventilation ensures sufficient air exchange in the premises. Of course, there are times when, for example, due to visitors, the air quality deteriorates for a while, but it is improved in a short period of time. It should be noted that during the study, residents were asked not to change the capacity of the ventilation equipment based on air quality, but to maintain a constant flow at all times.

• Unhealthy indoor climate hours

Hours during the year (a total of 8,760 hours per year) when indoor air quality with a CO₂ concentration above 1000 ppm was also counted. As shown in Figure 6, there is a significant difference between apartments with mechanical ventilation and apartments with window ventilation.

Figure 6. Unhealthy indoor climate hours and ventilation factor during the year

In mechanically ventilated apartments, the number of such hours in living rooms typically does not exceed 30, and in bedrooms around 100, while in apartments with window ventilation, the number of unhealthy hours is between 3,000 and 5,000. It can be concluded that indoor air quality with window ventilation alone is on average 30-50 times worse. In the case of mechanical ventilation, the unhealthy indoor climate is 1% of the year, with window ventilation 30-60% of the year.

• Energy consumption for heating and cooling

Given that the air quality in apartments with balanced mechanical ventilation was so high, it was decided to measure the impact of a constant supply of fresh air on heating and cooling energy consumption compared to the window ventilation.

Figure 7. Heating energy consumption (kWh/m2) in each apartment during the year. The ventilation factor of the premises is indicated in the period from October to March. Heated rooms are green, unheated blue. The heat loss directions are marked with red arrows.

Figure 8. Cooling energy consumption (kWh/m2) per year. The ventilation factor of the premises is indicated in the period from April to September. Cooled rooms are green, non-cooled blue. The directions for the effects of heat are marked with red arrows.

The heating energy consumption of both apartments on the left is equal, although the ventilation factor with windows in the lower left apartment is slightly higher.

The lower right-side apartment needs almost twice as much heating energy, as it has large heat loss to the underfloor basement below, and a higher indoor temperature was maintained in this apartment. In other apartments, the so-called “neighbourhood effect” can be observed, when there is a heated area above and below them and the amount of heating energy consumed is significantly affected by the ambient temperature.

The ventilation factor was chosen as the basis for comparing the apartments, because the air introduced through the window must be heated or cooled to the selected room temperature. Comparing the lower left and upper right apartments, where the ventilation factor was 5%, the heating consumption in the apartment with mechanical ventilation is 24% lower (Figure 7). Comparing the two apartments on the left, where the ventilation factor was 7%, the cooling consumption in the apartment with mechanical ventilation is 35% lower (Figure 8).

Executive summary

A comparison of air quality clearly shows that it is healthier to reside in a room with balanced mechanical ventilation than in a room that is only ventilated with windows.

Balanced mechanical ventilation system ensures a constant exchange of fresh air in the rooms. Manual window ventilation only provides fresh air, when several windows are open. Comparing the two principles, mechanical ventilation provides much better air quality, but more importantly, window ventilation does not guarantee a healthy indoor climate, given that 30-60% of the year's indoor CO₂ concentration exceeds 1000 ppm.

Heating energy consumption depends on how much and how often the room is ventilated with windows, as well as on the selected heating temperature and heat loss and benefits.

Mechanical ventilation with heat and moisture recovery does not increase energy consumption and significantly improves indoor air quality.

This article was written in collaboration with Zehnder Baltics