# Performance Analysis of the Demand-Based Ventilation in a Nordic Apartment Building
## 3 Results
### Performance of Standard Demand-Based Ventilation System  
The simulation assesses the airflow rates and airflow balance at room, apartment, branch and AHU levels as a function of the number of apartments with kitchen boosting active. Figure 4 represents the target and simulated exhaust and supply airflow rates for apartments and zones in different boosting scenarios. Figure 5 illustrates the airflow rates at the AHU level concerning each apartment type. The scenarios are selected by the number of boosting apartments in each type. These scenarios represent 25, 50, 75 and 100% boosting modes of the ventilation system with one, two, three and four floors boosting simultaneously. The results are assessed concerning two main criteria: balance of the apartment supply and exhaust airflow rates and the ability to follow room airflow demand in the boosting mode. For one-, two- and three-room apartments, it is required to reduce the bathroom exhaust airflow rate in the boosting mode. For the studio, the supply airflow rate in the living room is increased instead. The target airflow rate in two-, three- and four-room apartments should be constant in normal and boosting modes. However, the simulation shows that an imbalance exists between the apartment supply and exhaust. The worst-case scenario of apartment airflow rate imbalance occurs with the one floor boosting case. The bathroom exhaust airflow rate is 8 L/s higher, which leads to a 7 L/s difference in the apartment level supply and exhaust  
Figure 4: Airflow rates of the different sizes of apartments when a different number of apartments are boosting. Figure 5: Target and simulated airflow rates at the AHU level when a different number of apartments are boosting. airflow rates. The worst-case scenario for airflow demand redistribution happens when all the floors are boosting. The kitchen airflow rate, in this case, is 7 L/s lower than the target in each apartment. The trend is also the same in the other boosting cases as bathroom exhaust airflow rates are higher and the kitchen hood airflow rates are lower than target values. In the studios, the apartment total airflow rate should be increased in the boosting mode. However, the simulation shows that neither supply nor exhaust airflow rates reach the target values. The worst-case scenarios are the same as in the larger apartments. Maximum apartment disbalance is around 10 L/s, and the kitchen hood airflow rate is 7 L/s lower than the target in boosting mode. The supply airflow is unable to reach the design values and is lower than the exhaust. The difference between the target and simulated kitchen hood airflow rates becomes higher when more floors are boosting. Figure 5 shows that the overall airflow rate at the AHU level is within 10% of the target value. The two-, three- and four-room apartment airflow rate is within 5% with higher exhaust airflow due to the mentioned issues. However, studio supply and exhaust airflow rates are lower by 56 L/s or 32% and 32 L/s or 20% than the target values in all boosting cases, respectively. Imbalance in a mechanical ventilation system may lead to pressure differences over the building envelope. The more airtight the building is, the higher this pressure difference may become. Three levels of airtightness were chosen to assess and compare the possible resulting negative to atmospheric pressure: 2.5 L/h representing older building stock, 1 1/h representing new buildings, and 0.5 L/h representing future energy-efficient buildings in Finland. To evaluate the meaning of pressure difference over the building envelope, the airflow rate imbalances of the worst scenarios were chosen for each type of apartment. The exponent for Equation 2 was assumed to be 0.74 [34]. The volume was calculated based on the building documentation and the height of the rooms was assumed to be 2.6 m.  
Table 4 presents the resulting pressure difference in the apartments with different airtightness level.