Radon protection in apartments using a ventilation system wireless-controlled by radon activity concentration

The new German Radiation Protection Act (StrlSchG) of 31 December 2018 established a reference value of 300 Bq m−3 for the annual average radon activity concentration in buildings with recreation and living rooms, as well as in workplaces. It is expected that the reference value will be exceeded in a vast number of buildings throughout Germany and that radon protection measures will become indispensable. A simple and inexpensive radon protection measure for existing buildings is ventilation. In the scope of a joint project, ventilation systems with zone control and heat recovery are to be extended by the control parameter radon activity concentration. A highly sensitive, miniaturized radon monitor will be developed for this purpose, which can be integrated wirelessly into ventilation systems. Radon measurements were carried out in 13 apartments of an unoccupied heated apartment block in Germany over a period of three weeks in the wintertime. High radon activity concentrations were found on all three floors. The maximum values were 14000 Bq m−3 on the first floor, 6000 Bq m−3 on the second floor, and 2000 Bq m−3 on the third floor. Ventilation experiments were carried out in an apartment with high radon activity concentration. Two decentralized ventilation systems with heat recovery were installed in each of the two opposite outside walls. The controlling device of the system was activated wirelessly depending on the radon activity concentration. The radon activity concentration was reduced from 8000 Bq m−3 to 800 Bq m−3 in a first experiment in the living room.


Introduction
Every type of soil produces a lot of radon, and that is why there is a radon problem in buildings around the world. A review can be found in [1]. In addition, meteorological parameters influence the variation of indoor radon activity concentration [2]. The new German Radiation Protection Act (StrlSchG) of 31 December 2018 established a reference value of 300 Bq m −3 for the annual average radon activity concentration in buildings with recreation and living rooms as well as workplaces [3]. It is expected that the reference value will be exceeded in a vast number of buildings throughout Germany and that radon protection measures will become indispensable.
There are a large number of radon protection measures for buildings [4]. The measures can be divided into constructional measures and ventilation measures [5]. Constructional measures include The ventilation of rooms is a simple and inexpensive radon protection measure that everybody can carry out by hand. Therefore it is the scope of the research work to automate the ventilation in apartments to give people an inexpensive tool to reduce the radon activity concentration in their homes. Existing inVENTer ventilations work with zone control and heat recovery. Each zone has two ventilation units. A zone may include one large room or several small interconnected rooms. Four zones can be set up in one apartment. Each zone is controlled separately. The ventilation units of a zone work through the counter-current principle and change the flow direction every 70 s. The units have heat exchangers so they can recover up to 93% of the heat present in the air that passes through them.
The project 'RadonVent' has two goals. The first goal is the extension of ventilation systems with zone control and heat recovery by the control parameter radon activity concentration [6]. The existing inVENTer ventilation system works well and is controlled by the optional parameters humidity and carbon dioxide. The system is to be augmented with a new radon monitor and new software to remove radon, to minimize radon intrusions, and to minimize heat losses in apartments. The new system should work automatically. In the future, the system will also use machine learning and artificial intelligence to adapt the parameters of the ventilation units.
The second goal is the development of a highly sensitive, miniaturized radon monitor for wireless integration into ventilation systems. The radon monitor will be based on the silicon photo multiplier technology and the height will be 30 mm. Figure 1 shows the test site in Bad Schlema in the Ore Mountains (Erzgebirge) 220 km south of Berlin, Germany. The Ore Mountains have a particularly high radon potential [7,8]. The block has 19 apartments on three floors, two entrances with staircases, and a semi-basement. All apartments are unoccupied but were heated for the field test. Two ventilation units on the wall of the block are shown forming a ventilation zone.

Field test at an unoccupied apartment block
The apartment has five rooms: a hallway, kitchen, bathroom, living room, and bedroom (figure 2). In each room, a radon monitor measures the radon activity concentration at a time interval of 1 h. The radon monitor in the living room has a radio transmitter and switches the ventilation controller in the hallway on or off depending on the measured radon activity concentration. The ventilation controller controls four ventilation units in the two exterior walls, forming two ventilation zones. Both zones are controlled by only one radon monitor in

Results
At first, the radon activity concentration was measured in the basement and in 13 apartments of the block in the wintertime. Very high radon activity concentrations were found, of up to 14000 Bq m −3 on the first floor, 6000 Bq m −3 on the second floor and 2000 Bq m −3 on the third floor ( figure 3). This is a typical situation for the town Bad Schlema in the Ore Mountains. Because of such very high concentrations, it is important to improve the living conditions for the people in Bad Schlema soon using this project's results.
Based on the radon measurements in the block, apartment 3 on the first floor was selected as the test site. Two basic measurements without ventilation are first shown in the winter and in the summer. Figure 4 displays the course of the radon activity concentration in the five rooms of the heated apartment with mainly opened doors in the wintertime. The radon activity concentration is very high and reached up to 7000 Bq m −3 . Figure 5 shows the same in the summertime. The radon activity concentration is also very high and reached up to 4000 Bq m −3 . Figure 5 displays the course of the radon activity concentration in the five rooms of the apartment with opened and closed doors. During the time with closed doors a specific radon activity concentration was found for every room, with wide variations between the rooms in the unoccupied apartment. This was surprising because radon is really mobile and it was expected that radon would move from room to room through keyholes and gaps in closed inner doors.   But this was not the case. The phenomenon was detected in all measurements in the apartment and can be seen in further figures.
It is common knowledge that heating buildings in wintertime can draw radon from the ground into buildings. The heating unexpectedly failed for 12 d in the unoccupied apartment. The heating failure resulted in a strong decrease in the radon activity concentration caused by stopping the draw of radon into the building and the mixing indoors of air from outside ( figure 6). This is a good example of the well-known process.  The first ventilation experiments were successful. Figure 7 shows the course of the radon activity concentration in the five rooms of the apartment during a ventilation experiment with opened doors and wireless activation of the ventilation controller by a radon monitor. Four ventilation units worked in the operating mode 'heat recovery' at a performance level of 100%. S115 Figure 8. Course of the radon activity concentration in the five rooms of the apartment during a ventilation experiment with closed doors and wireless activation of the controller 'MZ-Home' by a radon monitor 'Radon Scout Plus' as well as four ventilation units 'iV14-Zero' in the operating mode 'heat recovery' at the performance level 100%. Figure 9. Course of the radon activity concentration in the five rooms of the apartment during a ventilation experiment with opened doors and wireless activation of the controller 'MZ-Home' by a radon monitor 'Radon Scout Plus' as well as four ventilation units 'iV14-Zero' in the operating mode 'cross ventilation' in the direction from kitchen/bedroom to living room at the performance level 100%. The automatic ventilation system controlled by the radon activity concentration worked well. The radon activity concentration was reduced from 8000 Bq m −3 to 800 Bq m −3 in a first experiment in the living room. Figure 8 displays the result of the same experiment once more in heat recovery mode, but with closed doors. The automatic ventilation system worked well again. The reduction of the radon activity concentration depended on the rooms and resulted in concentrations that ranged from 300 Bq m −3 in the bedroom to 800 Bq m −3 in the living room.
Finally, a result of an experiment with cross ventilation mode is presented. Figure 9 shows the course of the radon activity concentration in the five rooms of the apartment during a ventilation experiment with opened doors and wireless activation of the ventilation controller. The four ventilation units worked in the operating mode 'cross ventilation' in the direction from kitchen and bedroom to living room at a performance level of 100%. The radon activity concentration in the rooms decreased to about 300 Bq m −3 . This is good news for the people of Bad Schlema. As can be seen, the automatic ventilation system controlled by the radon activity concentration worked well with the cross ventilation mode as well. The modes 'heat recovery' and 'cross ventilation' are to be mixed in further experiments on radon protection in homes.

Conclusion
The first results of the project 'RadonVent' are the following: Next steps include an investigation into the ventilation with excess pressure in the ventilation zones. New radon protection software for the ventilation controller to mix automatic 'heat recovery' mode, 'cross ventilation' mode, and ventilation with excess pressure is to be developed. Also, a new small radon monitor to use as a sensor for the ventilation system is in development.
It is the vision of the research group that radon protection becomes the new standard for ventilation systems to protect people in their homes.