For example, people ask whether a supply and exhaust ventilation with heat recovery, which has more expensive installation costs, is a cost-effective solution. Alo Mikola, who defended his doctoral thesis ‘Ventilation Performance in Deep Renovation of Multifamily Apartment Buildings’ in TalTech this year, says that the aim of the thesis was to provide answers to those kinds of questions and to find the best possible solutions to problems.

‘When I started writing the thesis, problems related to ventilation systems were the biggest obstacle in regard to renovating Estonian apartment buildings. This problem had to be solved and as I had already addressed it in my master’s thesis, I continued the research in-depth in my doctoral thesis. Thus, my doctoral thesis is quite thorough,’ Alo Mikola emphasises. Although he could have defended a smaller-scale thesis, it would not have been able to cover all questions related to ventilation renovation that Estonians are currently facing.

What did the research of ventilation systems show?

The performance of ventilation and air change in Estonian apartment buildings with different ventilation systems is analysed by field measurements. The average air change rate in old apartment buildings was 0.24 h⁻¹ and the technical inspection of studied ventilation systems pointed out that natural ventilation systems are in poor condition. ‘Laboratory and on-site studies of the CO2 based tracer gas method proved that the daily variation of CO2 level can be used to estimate the air change rate in apartments and CO2 based tracer gas can be used to measure air change efficiency. In summary, a simplified conclusion can be made that the air change rate in the studied old apartment buildings was too low and those systems definitely need comprehensive renovation.’

Advantages and disadvantages were revealed

According to Mikola, one renovation solution for ventilation that was used in apartment buildings were single room heat recovery ventilation units. The initial analysis of air change rates of these units showed that the solution did not work as designed.

Therefore, the performance of two types of single room ventilation units was measured and their suitability as ventilation renovation solutions was assessed with simulations. As a result of the study, Mikola suggested suitable pressure difference values for the correct design of single room ventilation units. Additionally, according to the author, studies were performed in laboratory conditions to analyse the performance of air change in various single room ventilation systems.

The measurements of air change performance showed that a perfect mixed ventilation was achieved for both ventilation radiators and two types of single room units!

However, there were some problems as well. ‘The ventilators of some single room ventilation units were not dimensioned, taking into account the underpressure created in apartments. Additionally, the single room units did not have the ability to recover heat from the exhaust air of the toilet, bathroom, and the kitchen,’ Mikola says. In conclusion, the problems mentioned meant that the yearly efficiency of units used in renovation was significantly lower than allowed. ‘In addition, the single room units were prone to freezing during the cold period, which meant that they had to be switched off for that time,’ states Mikola, author of the doctoral thesis.

What are the best ventilation systems?

Coming back to the question asked earlier, Mikola says yes – centralised mechanical supply and exhaust ventilation with heat recovery resulted in the best performance. ‘However, an alternative system with an exhaust heat pump and ventilation radiators may also be suggested,’ added Mikola. According to him, the results of the doctoral thesis can successfully be used when renovating similar European apartment buildings.

‘Currently, the most commonly used solution in the renovation of apartment buildings is the previously mentioned supply and exhaust ventilation with heat recovery, where ventilation ducts are installed inside the additional insulation layer and the ventilation unit is installed on the roof. A mechanical exhaust ventilation with an exhaust heat pump is also used. With this system, new ventilation ducts installed in old ventilation shafts are used to extract air from apartments and outdoor air enters the apartments via ventilation radiators, where the outdoor air preheats.’

The doctoral thesis of Alo Mikola is available in the Digital Collection of TalTech.