Summer season temperature control in Finnish apartment buildings

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1 Summer season temperature control in Finnish apartment buildings Jarek Kurnitski, Pasi Tauru and Jari Palonen Helsinki University of Technology, Finland Corresponding SUMMARY In modern apartment buildings with good insulation and heat recovery, often having large windows and high internal heat gains, overheating may be a serious problem not only in summer season, but also during heating season. This study determined the design curves for the assessment of room temperatures and cooling demand in apartments. The design curves are based on many simulations and they show the effects of window airing, solar protection glasses, window size and orientation, cooled supply air and room conditioning. Determined graphs allow to check quickly the thermal comfort in apartments and this will be a step forward from today s practice not to do such check. They suit well for preliminary design and are useful for architects and HVAC-consultants not using indoor climate and energy simulation software. INTRODUCTION Modern apartment buildings are well insulated, heat recovery is used in ventilation and they often have large windows and high internal heat gains especially in well-equipped apartments. These factors may cause overheating not only in summer season, but also during heating season. In the building process, it is common that the architect decides quite independently important choices as regards temperature control. Such issues are the shape and solar protection of building, window size, orientation and glass properties and the selection of structures. After that, the task of HVAC-consultant is to design heating and ventilation. This means in practice the sizing of heating and ventilation system according to architect s choices, not the indoor climate design. Occupant questionnaires have shown that the building process described gives good results for heating season, but in the summer, apartments are often overheated. This problem is seldom noticed when apartments are for sale, but usually afterwards, when it is not easy to fix the problem. Temperature control has also a concern of public health, which will be stressed with increasing frequency of heat waves. In the summer excess mortalities caused by high temperatures are between 1 cases per year [1] in Finland. Questionnaire survey conducted as a part of a wide European HOPE project [2] studied thermal comfort in apartments in summer and winter. Thermal responses in 7-point scale are shown in Figure 1. The responses of 3, 4 and 5 show acceptable conditions. Good thermal comfort was reported by 73% of occupants in the winter and by 6% in the summer. In the summer about 4% of occupants report that temperature is too high and even in the winter 11% reported too high temperature. There was high variation in the responses depending on the orientation of the apartment and presence of balcony glazing.

2 5 4 Winter Summer % 1 1 (Too hot) (Too cold) Figure 1. Thermal comfort reported by occupants in 7-point scale (n=6) in winter and summer. The main problem in the design of apartment buildings is that the temperature simulations are usually not conducted for typical flats. However, the results of the simulations should be taken into account in the choices of architect and in the design and sizing of ventilation and cooling. In this study we tried to overcome this problem by conducting a large number of simulations for typical cases and presenting the results in the form of design curves which can be used for the design of thermal comfort in all typical apartment buildings. These design curves give a simple design tool for architects and HVAC-consultants. METHODS We simulated two apartments from relatively new apartment building, construction year 1, representing typical construction solutions and size of apartments. The building consisted of 1 apartments with two bedrooms (7,5 m 2 ) and 33 apartments with one bedroom (6,1 m 2 ). On apartment of both types was modeled with IDA-ICE indoor climate and energy simulation software, Figure 2 and 3. All apartments were equipped with mechanical supply and exhaust ventilation with heat recovery. Total design supply airflow rate was 5 L/s and exhaust 6 L/s in all of apartments. Figure 2. A photo of the simulated apartment building.

3 Figure 3. Simulated apartments with 2 and 3 bedrooms. Summer time temperature measurement data from one flat was used for the verification of the IDA-ICE model, Figure 4,. This was not the attempt of detailed validation as the occupancy data and the use of window airing was not known. However, such comparison indicates how realistic temperatures are predicted by the model Temperature, C 25 Measured Simulated Room temperature, C Ch excess Date, dd.mm Relative time, % Figure 4. Typical measured temperature from the summer of 3. The simulated temperature shows some discrepancy as window airing is not taken into account. Accepted temperature exceeding for good indoor climate category S2 during three months in the summer. 1 In the parametric simulations the effect of 22 building and HVAC-factor was studied. Based on these simulations, the most important factors were chosen for sensitivity analyses. Three basic cases were studied: 1. the reference case corresponding to measured apartment building without cooling 2. cooling of the supply air 3. room conditioning unit in the apartment For these basic cases sensitivity analyses were conducted for the following parameters: orientation of the building (four main directions) window area equal to the reference case, and increased by 1,5 and 2 times

4 with and without window airing with solar protection glasses and with common clear panes 5 W/m 2 and 1 W/m 2 internal heat gains The results are presented as simple graphs, from which the effect of each factor can be seen for two defined indoor climate classes. Indoor climate classes were defined because exact key figures are needed for the comparison of design alternatives. For that purpose the target values of Finnish indoor climate classification [3] and the weather data of 3 was used. This approach follows the principles given in new indoor climate standard pren [4] and leads to categories: Basic category S3, where room temperature is usually below 27 C Good indoor climate category S2, where room temperature is usually below 25 C For both indoor climate categories the exceeding of the limit temperature by 1 Ch degree hours is accepted during the summer period from June 1 st to Aug 31 st, Figure 4,. RESULTS Design curves for room temperature and cooling demand Design curves are intended for the assessment of room temperatures and cooling demand in apartments and can be used by architects and HVAC-consultants. They show the effects of window airing, solar protection glasses, window size and orientation, cooled supply air and room conditioning. All design curves are given for the four main directions and as a function of the ratio of window area to floor area. Design curves show, would the indoor climate category achieved in the studied case or what should be changed to achieve the category. Room conditioning curves are for the good indoor climate category S2 and all other curves for the basic category S3. Design curves are not sensitive to the apartment size and may be used for sizing of full range of apartments. However, they are sensitive to internal gains. 5 W/m 2 applies for common apartments (including occupants, lighting and equipment). In well equipped apartments with relatively small floor area the gains may be higher. In that case 5 W/m 2 curves may still be used if the part of gains exceeding 5 W/m 2 will be removed by increasing the airflows in the case of cooled supply air or cooling power in the room conditioning case respectively. All cases are calculated with solar protection by blinds between the outer panes in three pane window. Possible shading by neighboring buildings, etc. is not taken into account, i.e. all curves apply for fully exposed building. Solar shading may have significant effect especially in apartments on lower floors. This can be roughly estimated, if in the case of clear panes the curves of solar protection glass are used. In the graphs, the degree-hours exceeding the room temperature of 25 or 27 ºC are shown. For the estimation of maximum room temperature, P95% values of the room temperature frequency distribution curves are given (this is considered as reasonable estimate of maximum room temperature during hot days). Room temperature assessment should be done according to the room showing the highest temperature (= highest window are to south or west). If the room has windows facing to two directions, the window areas may be summed and the higher result can be used, or the result may be interpolated by weighting with window area.

5 The effect of window airing and solar protection glass (no cooling) The overheating corresponding to current building practice is shown in Figure 5. The results apply for the building with clear window panes and if window airing is not used. In that case the apartments are hot independently of window area; room temperatures in the hottest days are over C. Solar protection will decrease the temperatures, but this is not sufficient measure as alone, because the rooms are still hot, about 29 C. Effective cross ventilation by window airing will reduce room temperatures to acceptable level, Figure 6. With reasonable window areas the basic indoor climate category S3 is achievable. In the case of solar protection glasses, about 1.5 times higher window areas in south and west facades may be used compared to ones in Figure 6. No window airing No window airing, P95% max temperature Degree hours over 27 C, Ch Figure 5. The degree hours exceeding the limit temperature of 27 C, and the estimate of max room temperature during the hottest days, for the building with clear window panes and without window airing. Temperature, C Window airing Window airing, P95% max temperature 33 Degree hours over 27 C, Ch Figure 6. The effect of cross ventilation. Clear window panes and open windows (with size of.6 m 2 ) on two facades. Temperature, C

6 Cooling of supply air Cooled supply can satisfy a relatively low cooling need of 1 15 W/m 2. It is important to insulate supply air ducts. The following results apply for the supply airflow rate of.7 L/s per m 2 and temperature control so that the minimum supply air temperature to the room is 16 C (should be lower from the air handling unit). Supply air temperature is controlled according to exhaust air temperature. Results show that cooled supply air is not sufficient in the case of windows with clear panes. The basic category S3 is achievable, if solar protection glasses are used in south and west facades together with cooled supply air, Figure 7 (window airing is not used). Cooled supply air Cooled supply air, P95% max temperature Degree hours over 27 C, Ch 1 Temperature, C Figure 7. Cooled supply air (supply air temperature to the room of 16 C and airflow rate.7.7 L/s per m 2 ) and solar protection glasses with solar heat gain coefficient of.33. Room conditioning Room conditioning devices give all possibilities for temperature control. In common apartments, good results are often achieved with one centrally located device (in the living room or in the entrance-hall in open plan apartments) if bedrooms doors are kept open during the daytime. Due to higher investment cost, better indoor climate category S2 is recommendable to guarantee sufficient temperature control in all cases. For the reduction of cooling demand and the surface temperature of windows, the use of solar protection glasses is recommended on south and west facades. An example of the effect of solar protection glass on the surface temperature of the window is shown in Figure 8. Cooling demands are calculated room by room with the curves given in Figure 9.

7 .5 m T air= 34,8 C.5 m T air= 26,7 C 42,3 C 42,3 C 34,9 C 31,7 C 31,7 C 27, C Figure 8. An example of the effect of solar protection glass on the surface temperature of window. W/m 2 cooling and clear window panes, W/m 2 cooling and solar protection glasses. Room conditioning, T=25 C Room conditioning and solar protection glass, T= 25 C Cooling demand, W/m 2 Cooling demand, W/m Figure 9. Cooling demand for rooms with clear panes, and with solar protection glasses. DISCUSSION The results show that the current buildings tradition leads to strong overheating in apartments. The basic indoor climate category S3 (room temperature of the 27 ºC with 1 ºCh excess) is achievable with effective cross ventilation and relatively small window size. Cooling of the supply air used together with solar protection glasses is enough for S3 category at internal gains level not exceeding 5 W/m 2. This solution is sensitive to internal gains, and for higher gains level the temperatures were out of the comfort range. For the room conditioning case, the graphs show the cooling load corresponding to the good indoor climate category S2. Determined graphs allow to check quickly the thermal comfort in apartments and this will be a step forward from today s practice not to do such check. They suit well for preliminary design and are useful for architects and HVAC-consultants not using indoor climate and energy simulation software. ACKNOWLEDGEMENT This study was a part of national ASTAT project. This research was supported by the Finnish National Technology Agency Tekes and many companies participating the project.

8 REFERENCES 1. Keatinge WR, Donaldson GC, Cordioli E, Martinelli M, Kunst AE, Mackenbach JP, Nayha S, Vuori I. Heat related mortality in warm and cold regions of Europe: observational study. BMJ. Sep 16;321(7262): Palonen J, Kurnitski J. (7) Performance of ventilation systems in apartment buildings. Submitted to CLIMA Classification of Indoor Climate. Target values, design guidance and product requirements. FiSIAQ publication 5 E, Espoo, Finland pren15251:7. Indoor environmental input parameters for design and assessment of energy performance of buildings- addressing indoor air quality, thermal environment, lighting and acoustics, CEN 7.