Infiltrationens inverkan på värmeeffektbehovet för en kontorsbyggnad

Sammanfattning: The purpose of this research was to investigate, in collaboration with the HVAC-engineering company Incoord, how air infiltration affected the heating demand for a building when selected environmental and structural parameters were changed and how the results varied between three different calculation methods. The building was a fictive office building located in Stockholm. The calculation methods analyzed were Incoord’s method, a method based on the Crack Method and finally by using the software IDA ICE 4.6.2. The building was assumed to have five floors with a centralized stairwell to which the door on every floor could be either opened or closed. All floors were identical with either an open floor plan without internal walls other than the stairwell or divided into 16 offices per floor. The ventilation system was assumed to be balanced. The results were mainly presented as graphs where the heating demand, as a result of air infiltration, was a function of the current varied parameter. The physical parameters that could be varied for all calculation models and thusly could be used for comparisons between the models’ results were the outside air temperature and the wind speed. Calculations were also carried out using parameters that were not explicitly part of all the models. The conclusions from this research were that, under reference conditions, almost all building models that included an open stairwell suffered from considerably higher infiltration rates than models without open stairwells. The exception was the partitioned model with an open staircase because of the internal walls hindered air flow from the building envelope to the stairwell. Incoord’s model showed similar infiltration characteristics as the other calculation methods when open building models with open stairwells were used. Of the models that incorporated open stairwells the Crack Method resulted in lowest infiltration rates over the entire temperature interval and for a majority of the wind speed interval. The infiltration rates for the open building models with stairwells were affected most by the change in outside air temperature. When the outside air temperature varied two clear groups within the methods emerged where the internal order considering infiltration rates did not change. This fact indicated that all the methods accounted for how the temperature affected the infiltration rate in a similar way. The exception was Incoord’s method which, compared to the other open building models, resulted in the lowest infiltration rate at low temperatures and the highest infiltration rate at high temperatures. When the wind speed was varied, the infiltration characteristics for the different methods were different which indicated that they accounted for the wind driven infiltration in different ways. When the infiltration rates for all models were compared two groups emerged, as it did when the outside air temperature was varied. However, the grouping here was not as clear. The sensitivity analysis was carried out for Incoord’s method and the Crack Method and it showed that they were similarly sensitive. When the values for the grand majority of the parameters were increased by 10 % the Crack Method and Incoord’s method resulted in a sensitivity of –19.3 and –20.4 % respectively, which means that the infiltration rate increased for both methods. The most sensitive parameters for Incoord’s method were the air tightness, envelope area per floor and the inside- and outside air temperatures. For the Crack Method, the most influential parameters were the inside- and outside air temperatures, stack effect coefficient and window circumference. The choice of reference conditions affected the results of the sensitivity analysis. For Incoord’s method and the Crack Method, within the given temperature- and wind speed interval, the outside air temperature appeared to be the dominant factor for air infiltration. This means that, for relatively open building layouts during reference conditions, there probably is no tipping point at which the increase in wind speed, because of the increase in outside temperature, leads to an increase in infiltration. This means that, for a building with a relatively open layout, the increase in outside air temperature and the resulting decrease in stack effect is more dominant than the infiltration increase on account of the increase in wind speed. IDA ICE can be a viable alternative when calculating infiltration rates for complex buildings with multiple zone-layers, unbalanced ventilation or if very high wind speeds at the building site are expected.