Energy consumption and ventilation performance of a naturally ventilated ecological house in a cold climate

Energy and Buildings 37 (2005) 23–35

Energy consumption and ventilation performance of a naturally ventilated ecological house in a cold climate Carey Simonson Mechanical Engineering, University of Saskatchewan, Saskatoon, Canada Sk S7N 5A9 Received 17 February 2004; received in revised form 11 March 2004; accepted 2 April 2004

Abstract In this paper, the thermal and ventilation performance of an ecological house in Helsinki, Finland are presented. The single-family dwelling has a well-insulated, wooden frame construction with no plastic vapour retarder. The measured and simulated results show that the energy consumption of the house is low and that the outdoor ventilation rate is generally satisfactory based on the measured CO2 concentrations. Extrapolating the measured ventilation data shows that, when the operable windows are closed, the ventilation rate is expected to be about 0.45 air-changes-per-hour (ach) in the winter and about 0.25 ach in the summer. The consumption of total primary energy and space heating energy were measured to be 30% less (162 kWh/(m2 a)) and 36% less (76 kWh/(m2 a)) than in typical Finnish houses, respectively. The paper also uses a numerical model to investigate the sensitivity of energy consumption to the insulation level, household electricity and domestic hot water consumption, window area, ventilation rate and heat recovery effectiveness. © 2004 Elsevier B.V. All rights reserved. Keywords: Energy consumption; Ventilation performance; Ecological house; Natural ventilation; Space heating; Airtightness

1. Introduction There is a rising demand for sustainable, low-energy housing that has a limited impact on the environment and provides occupants with excellent IAQ and climate. The house studied in this paper fulfills these criteria, but there are several others in Finland as well [9,10]. Since a large fraction of the energy consumed in buildings is used to condition outdoor ventilation air, which is delivered to provide adequate IAQ, efficient ventilation is an essential component of modern, low-energy housing. The purpose of outdoor ventilation is to dilute the concentration of indoor contaminants and since occupants represent a key source of contaminants, the concentration of CO2 is often used to estimate IAQ [1,12,14,21]. Typically outdoor ventilation rates are set with the assumption that the building envelope and furniture are sources of contaminants. However, for carefully selected components, it is possible that the building envelope and furniture can act as contaminant sinks and actually improve the IAQ. If a building envelope is made from porous materials, the diffusion of pollutant gases through the envelope can reduce the indoor concentration of pollutants. For the house investigated in this paper, the diffusion of CO2

E-mail address: [email protected] (C. Simonson). 0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.04.006

and SF6 through the envelope increase the effective ventilation rate by about 10 and 5%, respectively when the ventilation rate is 0.5 air-changes-per-hour (ach) [24,28]. In addition, the permeable and hygroscopic envelope can reduce the peak indoor humidity by 20% RH at 0.5 ach [25]. Since the perception of air quality is closely linked to humidity [5], moisture transfer between indoor air and building structures, could possibly reduce the needed ventilation rate [29,30]. Reducing the ventilation rate would have a significant impact on energy consumption because the energy required to condition ventilation air typically constitutes 20–50% of the thermal load [2,19]. In most cases, adequate ventilation can provide good IAQ, but the indoor temperature and relative humidity have an important effect on comfort and IAQ as well [26,27,30]. Indoor temperatures are often specified assuming that the building is equipped with a mechanical HVAC system, but recent research has shown that occupants in naturally ventilated buildings adapt to their environment and can accept (and, in fact, prefer) a larger range of indoor temperature [4]. During cold weather, occupants of naturally ventilated buildings accept cooler indoor temperatures and during hot weather, they accept warmer indoor temperatures. This finding is based on the analysis of 21,000 sets of data from 160 different office buildings on four continents. Therefore, there is strong evidence that naturally ventilated buildings can provide ac-

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ceptable IAQ and comfort, while greatly reducing energy consumption. Energy consumption can be further reduced by applying energy recovery devices [3].

2. Test house “Tapanila ecological house” The house monitored in this study (Fig. 1) is located in the Tapanila district of Helsinki, Finland. Helsinki has 4100 heating degree days and an annual mean temperature of 4.5 ◦ C. The two-storey, single-family house has four bedrooms, a full basement and an unheated porch. All ventilation and infiltration results will be based on the internal volume of 470 m3 , while the energy consumption will be normalised with the gross heated floor area of 217 m2 . For the most part, natural and ecological materials have been used throughout the house. Wood and wood based materials have been used extensively and plastic materials have been limited. Tapanila ecological house was designed as a low-energy house and the wood fibre insulation is 250 mm thick in the walls (U = 0.16 W/(m2 K)) and 425 mm in the roof (U = 0.10 W/(m2 K)). The triple pane windows (U = 1.2 W/(m2 K)) are concentrated on the south facade to take advantage of solar heat gains in the winter and to provide natural lighting to the house. More detailed information on the thermal properties of the house are given in Simonson [24]. The house is connected to the Helsinki district heating system, which provides space heating through hot water radiators and pipes embedded in the floor and heat for domestic hot water. The unique aspect of Tapanila ecological house is that it employs several passive systems for controlling the indoor climate and IAQ. The house has no plastic vapour retarder to permit diffusion mass transfer between indoor air and the porous building envelope. A natural ventilation system provides outdoor ventilation, and a wood-burning fireplace supplements the space heating provided by the district heating system. The thick insulation is intended to keep energy consumption low, whereas the porous and hygroscopic envelope and natural ventilation

systems are examples of passive methods of controlling the indoor climate and IAQ. This paper presents the airtightness of the building envelope, the performance of the natural ventilation system and the energy performance. The performance of the other passive systems can be found in Simonson [24] and Simonson et al. [28].

3. Building airtightness A building with poor airtightness may have uncontrolled airflow through the building envelope, which can lead to problems related to: moisture, thermal comfort, energy consumption, ventilation performance and noise. To minimise air leakage and moisture transfer, wooden buildings in cold climates often use polyethylene on the warm side of the insulation. The house described in this paper does not employ a polyethylene moisture and air barrier, yet it has a good moisture performance [24,25,31]. The airtightness of the whole house and one bedroom (second floor west bedroom in Fig. 2) were measured. A variable-speed fan was ducted through a 5-mm thick, high-density wood fibre board that was sealed in a basement window during the tightness measurement of the whole house and in the door connecting the bedroom and the house during the tightness measurement of the bedroom. A calibrated orifice plate was used to measure the flow rate of air exhausted from the house and bedroom. In all tests, the natural ventilation supply vents (Fig. 3) were removed and sealed with tape. During the test of the whole house, the chimney and exhaust vents (Fig. 4) were sealed with polyethylene plastic and tape on the roof. All windows and external doors were closed during the pressure tests, but typically were not sealed with tape. Measurements were also performed in the bedroom with the balcony door and windows sealed to determine their leakage characteristics. The main leakage paths were found to be the front door of the house and the balcony door in the bedroom. During the tightness measurement of the whole house, the pressure difference across the building envelope was mea-

Fig. 1. Pictures of the south facade and dining area of the test house (Riikka Kostiainen). The triple pane windows are concentrated on the south facade to take advantage of solar heat gains in the winter and to provide natural lighting to the house.

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Fig. 2. The first and second storey floor plans have been designed with an open layout to improve the distribution of ventilation air and natural lighting.

Fig. 3. The house contains 13 supply air vents located in the upper window frame and 7 in the exterior wall above the hot water radiators. The occupants can control both types of supply vents. The supply vents above the radiators are seldom open, especially in the winter.

sured on both the first and second floors. The measured pressure differences on the first floor were only slightly higher (usually less than 3 Pa) than the pressure differences on the second floor, indicating good mixing. During the measurement in the bedroom, the main door of the house was kept open to minimise pressurisation of the house. The pressure difference between the house and outdoors was typically less than 1 Pa. The results of the airtightness test are summarised in Fig. 5 and show that the house is moderately airtight and

that the bedroom has less air leakage than the whole house, especially when the balcony door is sealed with tape. At an underpressure of 50 Pa, the air infiltration through the building envelope is estimated to be 3.1 ach, while the air infiltration into the bedroom is 2.2 and 1.5 ach when the balcony door and windows are unsealed and sealed, respectively. The airtightness is in the range of 1 to 3 × 10−5 m3 /(s m2 Pa) (0.5–1.5 L/(s m2 ) at 50 Pa) as recommended by Uvslokk [33] and Ojanen [17].

Fig. 4. Measuring the airflow rate through one of the exhaust vents. In total there are eight exhaust vents, of which four are fan assisted (bathrooms, sauna and range hood). Each exhaust vent is ducted separately to the roof to reduce cross contamination.

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Fig. 5. Airtightness of the test house and the second floor west bedroom.

4. Performance of the natural ventilation system The performance of the natural ventilation system was assessed by continuously monitoring the concentration of CO2 in the house, measuring the natural ventilation exhaust airflow rate during different weather conditions and predicting the ventilation rate throughout the year. 4.1. Concentration of CO2 The indoor concentration of CO2 (often used to estimate IAQ) was measured with a sensor based on infrared adsorption having an accuracy of ±5%. Fig. 6 shows that the ventilation rate is seldom below 4 L/s per person in the bedrooms and generally above 0.5 ach outside the bedrooms. This means that the general ventilation rate is adequate and that the IAQ is good, as the owners of the house have personally noticed. The peak concentrations of CO2 in the bed-

rooms are between 1400 and 1800 ppm when the bedroom doors are closed and between 800 and 1000 ppm when the bedroom doors are open. The position of bedroom doors has an impact on the ventilation rate of the bedrooms because the natural ventilation exhaust vents (Fig. 4) are located in the open areas of the house (Fig. 2). The CO2 concentration outside the bedrooms is usually less than 800 ppm. The measured concentrations of CO2 in the summer and winter are nearly the same, while the air flow rate through the ventilation ducts is clearly lower in the summer than in the winter (Figs. 7 and 9). This shows that the occupants open the windows to enhance the ventilation rate during warm weather. 4.2. Exhaust air flow rates The purpose of these measurements is to determine the airflow rate through the natural ventilation system under controlled conditions using a flow nozzle and an anemometer

Fig. 6. Measured concentration of CO2 in different rooms during normal occupation. The concentrations indicate that the ventilation rate is often above 4 L/s per person in the bedrooms and 0.5 ach in the rest of the house. A common threshold for CO2 is 1000 ppm.

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Fig. 7. Flow and pressure distribution on March 10, 2000 (a) and measured ventilation rates for each test showing the distribution between the three storeys (b). All windows are closed during the measurements.

as shown in Fig. 4. The airflow rate is calculated by: Q = kv = 5.8 v

(1)

where Q is the airflow rate in L/s, k is the constant (L/m) and v is the measured air speed (m/s). The uncertainty in the measured airflow rate is expected to be ±10%. The airflow rate through each exhaust vent was measured separately, except for one of the exhaust vents in the basement that was partially covered by a bookshelf. Since the bookshelf covered only part of the exhaust vent, it was assumed that the airflow rate through this vent was equal to the airflow rate through the other exhaust vent in the basement, which was less than 2 m away and in the same large room. The exhaust airflow rate through the chimneys (fireplace and sauna) was estimated with a velocity traverse of the chimneys near the roof. The fireplace and sauna dampers were closed during this measurement and therefore the exhaust airflow rate through the chimneys was quite small and the associated uncertainty is therefore quite high (approximately ±25%). During the measurements, all external windows and doors were closed, all bedroom doors were open and all bathroom doors were closed. Before and after the measurement of the exhaust flow rates, which took about 2 h, the pressure difference between indoors and outdoors was measured in several locations on both the first and second floors. Other data recorded during the tests were the outdoor temperature and the wind speed (measured on the roof of the house) and direction. The measured pressure differences between indoor and outdoor air on the windward and leeward sides of the house and the measured exhaust flow rates during the March 10 test are presented in Fig. 7a. Fig. 7b presents the outdoor ventilation rate for other measurement times. The outdoor temperature and wind speed on each test day were: −1 ◦ C and 2.5 m/s (March 10), 17 ◦ C and 2.5 m/s (March 31), 14 ◦ C and 2.9 m/s (May 11), 8 ◦ C and 1.7 m/s (June 2) and 27 ◦ C

and 1.3 m/s (June 21). The data covers a range of temperatures from −1 to 27 ◦ C and wind speeds from 1.3 to 2.9 m/s and the ventilation rate varies between 0.35 and 0.14 ach (10–4 L/(s person) for the five occupants). The distribution of outdoor ventilation air is also important and Fig. 7 shows that the second floor has a lower exhaust flow rate than the basement and first floor. This may be a problem because there are three bedrooms upstairs. When the bedroom doors are open, however, the CO2 concentrations (Fig. 6) indicate that there is adequate mixing in the house. 4.3. Extrapolation To predict the ventilation rate during other conditions, a simple model is developed in this section. The model is based on the basic equations for pressure differences due to wind and stack effects [2,18,35,36]: Pwind = Cp and Pstack

P = R

ρ v2 = Cwind v2 2


1 1 − Tout Tin



(2)


gh = Cstack

1 1 − Tout Tin

 (3)

where Cwind and Cstack are the overall wind and stack pressure coefficients, Cp the wind pressure coefficient, ρ the density of air (kg/m3 ), v the wind speed (m/s), P the absolute air pressure (Pa), R the specific gas constant of air (J/(kg K)), T the absolute temperature (K), g the acceleration due to gravity (m/s2 ), h the height between the inlet and outlet (m) and subscript out and in refer to the outdoor and indoor temperatures, respectively. The total pressure differences between indoors and outdoors is calculated by simple addition of the wind pressure and the stack pressure [34] rather than including the interaction of pressure effects given in Walker and Wilson [35], which was found necessary for

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Fig. 8. Expected ventilation rate at various outdoor temperatures and wind speeds compared to the measured data with error bars.

a wider range of leakage distributions. The total pressure difference is therefore: P = Pwind + Pstack

(4)

and the ventilation flow rate uses the standard exponential relation, with constants C and n: Q = CP n

(5)

Eqs. (2)–(5) describe the model used to calculate the outdoor ventilation rate for a given temperature difference and wind speed. The unknown constants in the model (Cwind , Cstack , C and n) are determined from the measured data. The calculated ventilation rate is:   0.602 1 Q = 0.125 0.07 v2 + 3650 Tout (6) − T1in Using Eq. (6), the ventilation rate at various wind speeds and outdoor temperatures can be predicted for the house by assuming a constant indoor temperature of 22 ◦ C (Fig. 8). The measured ventilation rates and associated wind speeds are also included in Fig. 8 and show that the model fits the experimental data quite well. The main discrepancy is that the model predicts a lower flow rate than that measured when the outdoor temperature is 27 ◦ C. This difference can be partly explained by the fact that the uncertainty in the measured ventilation rate is highest for the 27 ◦ C test because the ventilation through the chimneys was nearly half of the total ventilation, giving a total uncertainty of about ±15%. Another reason for the discrepancy is that the model uses a constant indoor temperature of 22 ◦ C, whereas the indoor temperature was nearly 25 ◦ C during the test when the outdoor temperature was 27 ◦ C. In-

creasing the indoor temperature reduces the difference between the measured and predicted ventilation rate but the predicted value is still outside the error bars. This indicates that the uncertainty in the measured data for the low airflow rates measured when the outdoor temperature is 27 ◦ C is greater than ±15%. Comparing the ventilation rates in Fig. 8 with those required in the National Building Code of Finland—D2 [14] (0.5 ach) and the National Building Code of Canada [12] (60 L/s for this house) shows that it is quite difficult to meet these requirements when the windows are closed, unless the wind velocity is high and the outdoor temperature is low. Additional ventilation, via open windows or exhaust fans (bathroom or sauna), is likely always needed to satisfy these standards when the outdoor temperature is above 0 ◦ C. On the other hand, the requirements of ANSI/ASHRAE Standard 62.2 [1] (30 L/s for this house) are often satisfied with the natural ventilation system alone. To estimate the performance of the natural ventilation system during the year, Eq. (6) is applied using the hourly weather data for Helsinki (1979) and an indoor temperature of 22 ◦ C (Fig. 9). The average wind speed measured at the weather station was 4 m/s for the whole year, which is slightly higher than the measured wind speeds at the test house. Therefore, the measured wind speeds were reduced by 25% to account for the local wind shielding. Fig. 9 includes the predicted daily, monthly and yearly average ventilation rate and shows that in the winter (November to March) the daily average ventilation rate varies between 0.3 and 0.7 ach (40–90 L/s), whereas the monthly average ventilation rate varies between 0.4 and 0.5 ach (50–65 L/s). In the summer (June to August), the average ventilation rate of-

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Fig. 9. Expected ventilation rate for the test house using the Helsinki (1979) weather data, reducing the wind speed by 25% to account for local wind shielding and assuming all windows are closed.

ten is about 0.25 ach (33 L/s), but additional ventilation by opening windows is possible at this time. Using the assumption of a constant wind speed of 3 m/s, the ventilation rate at the yearly average temperature of 4.3 ◦ C is calculated to be 0.35 ach (46 L/s), which is 70%, 77% and 150% of the required ventilation rate in the National Building Code of Finland—D2 [14], the National Building Code of Canada [12] and ANSI/ASHRAE Standard 62.2 [1].

5. Energy and water consumption 5.1. Measured data The energy consumption was monitored from August 1999 to August 2000. During this time, the consumption of district heating energy was recorded weekly and it was assumed that 95% of the delivered district heating energy provides useful heating. The total water consumption was recorded nearly every month and the consumption of hot water was recorded weekly starting in October 1999. The hot water consumption from October 1999 to August 2000 was extrapolated to determine the yearly consumption. The energy required to heat the hot water was calculated assuming constant cold (5 ◦ C) and hot (55 ◦ C) water temperatures. The occupants of the house began burning firewood in October 1999 and the mass of consumed firewood was recorded daily. The heating energy from firewood is calculated from the heat value of pine wood, which is 4.15 and 4.2 kWh/kg when the moisture content is 20% and 10% by mass, respectively [32]. The moisture content of the firewood was measured to be between 12% and 17%. Since the fireplace in the living room is massive and the two dampers in the chimney are nearly always closed after the burning of the

wood, it is assumed that a large portion (70%) of this energy provides space heating (3 kWh/kg). The sauna stove, on the other hand, is much less massive and the damper is nearly always open during use, so only 10% of the wood consumed in the sauna is assumed to provide useful space heating (0.4 kWh/kg). The wood that is burned during the months of June, July and August is assumed to provide no useful space heating. The consumption of electricity was determined from the electrical meter, which was read by the utility company on May 26, 1999 and May 29, 2000. In addition, the electricity consumption was recorded weekly from May to August 2000. These measurements indicated that the electricity consumption was quite constant throughout the year. Since little electricity was used outside the house, this electricity was not subtracted from the measured values. The yearly energy and water consumption are given in Fig. 10 together with the consumption of typical Finnish houses [11]. The values for typical Finnish houses are based on a house with four occupants and a gross floor area of 140 m2 . Fig. 10 shows that the normalised consumption of energy and water is significantly higher in typical Finnish houses than in Tapanila ecological house. Tapanila ecological house consumes 33% less total energy, 37% less heating energy, 20% less electricity, 37% less water and 30% less primary energy than a typical Finnish house. When calculating the primary energy consumption, 1 kWh of electricity is assumed to equal 2.5 kWh of primary energy, while 1 kWh of heat energy (wood, oil or district heat) is assumed to equal 1 kWh of primary energy. It should be noted that the total energy consumption in Tapanila ecological house is comparable to that measured by Laine and Saari [9] in two low-energy houses in Finland. Since heating energy is a large part (77%) of the total energy, the distribution of the heating energy is presented in

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Fig. 10. Normalised energy and water consumption in Tapanila ecological house and typical Finnish houses [11].

Fig. 11, where the heating energy is divided into space heating, water heating and heat losses. In Tapanila ecological house, space heating accounts for 82% of the heating energy, water heating accounts for 14% and heat losses account for the remaining 4%. The space heating was determined by subtracting the water heating from the district heating and adding the wood heating. The heat losses in Tapanila ecological house were assumed to be only 5% of the supplied district heat because the district heat exchanger is located in the basement and most of the heat dissipated will heat the house. Fig. 11 shows that the space heating in Tapanila ecological house is 36% lower than in typical Finnish houses. Fig. 12 shows that wood provides 19% of the total energy and 29% of the space heating energy (1550 and 570 kg of wood were burned in the fireplace and sauna, respectively providing 4880 kWh of heat). District heat provides 59% of the total energy and 71% of the space heating energy. Electricity accounts for 23% of the energy supplied.

5.2. Calculated energy consumption In this section, the energy consumption of Tapanila ecological house will be analysed using the WinEtana simulation program. The WinEtana simulation program has been developed at VTT Building Technology for estimating building energy consumption and is described in publications by Kosonen and Shemeikka [8] and Shemeikka [22]. The program calculates the energy consumption using a single-zone, steady-state thermal analysis, which is based on the National Building Code of Finland—D5 [13] and the prEN 832 [20]. Kalliomäki and Kohonen [7] have shown the validity of the steady-state method for estimating energy consumption in Finland. WinEtana calculates the heating, electrical and water consumption and peak demands in the building using monthly average outdoor temperatures and assuming a constant indoor temperature. Despite its simplicity, Kosonen and Shemeikka [8] have shown that, when simulating

Fig. 11. Normalised heating energy consumption in Tapanila and typical Finnish houses.

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Fig. 12. Distribution of total (a) and space heating (b) energy supplied to the house.

the IEA BESTEST validation case [6] with reasonable assumptions, the heating energy consumption calculated by WinEtana is similar to that calculated by several transient simulation models. 5.2.1. Input data The WinEtana program is particularly useful for Finnish buildings because it contains a database of typical input parameters (U-values, window types, water consumption, electricity consumption, internal gains, air change rates, etc.) for Finnish buildings and Finnish regulations [13–15]. These database values will be used together with the size of Tapanila ecological house when describing a reference case for Tapanila ecological house as shown in Table 1. The other investigated cases (Table 1) are arranged in such a way that each subsequent case uses increasing actual data from Tapanila ecological house. In this way, the impact of various parameters can be assessed. The difference between the code and reference cases shows the impact of

the actual electricity and water consumption on the energy consumption of Tapanila ecological house, while the difference between the window (15%) and reference cases shows the impact of the window area. The difference between the actual-MV case and the code case indicates the impact of the improved building envelope on the energy consumption. Finally, the difference between the actual-NV and actual-MV cases shows the impact of the natural ventilation system on energy consumption. In all cases, a district heating system is used (95% efficiency), the outdoor weather is from Helsinki (1979), the indoor temperature is 22 ◦ C, the infiltration rate is assumed to be 0.15 ach and the total outdoor ventilation rate is 0.5 ach. 5.2.2. Results The simulated energy consumption for the five different cases compared to the actual consumption measured in Tapanila ecological house are presented in Fig. 13. The input data used to simulate the actual-NV case most closely

Table 1 Source and values of input data for the energy simulation cases Case

Surface areas

Water consumption

Electricity consumption

U-values

Ventilation system

(UA) (W/K)

Reference Window (15%)

Database Database

Database Database

Database Database

Database Database

185 180

Code Actual-MV

Actual Window area 15% of floor area Actual Actual

Actual Actual

Actual Actual

Database Actual

Database Database

185 126

Actual-NV

Actual

Actual

Actual

Actual

Actual

126

Water consumption Database: total 152 m3 /a, hot water 53 m3 /a, water heating 3070 kWh/a Actual: total 127 m3 /a, hot water 50 m3 /a, water heating 2880 kWh/a Electricity consumption Database: household 4880 kWh/a, ventilation 990 kWh/a, other 1070 kWh/a Actual: household 5570 kWh/a, ventilation 0 kWh/a, other 400 kWh/a Envelope U-values (W/(m2 K)) Database: walls 0.28 (0.36 underground), roof 0.22, floor 0.36, windows 1.8 Actual: walls 0.16 (0.21 underground), roof 0.10, floor 0.25, windows 1.2 Ventilation system (infiltration of 0.15 ach, total outdoor ventilation of 0.5 ach) Database: mechanical supply and exhaust with 50% heat recovery Actual: natural ventilation system without heat recovery “Database” data are from the WinEtana database and “actual” data are measured from Tapanila ecological house.

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Fig. 13. Calculated energy consumption of Tapanila ecological house for the cases in Table 1.

represents the actual house and the only difference is the ventilation rate. The sensitivity of energy consumption to the ventilation rate will be presented later (Fig. 15). Fig. 13 shows that the reference case has a total, space heating and electrical energy consumption of 154, 102 and 32 kWh/(m2 a), respectively. This means that if the envelope and equipment in Tapanila ecological house were according to typical Finnish standards, the total, space heating and electrical energy consumption would be greater than that measured in Tapanila ecological house by 28%, 33% and 16%, respectively. The reference case has the highest total and space heating energy consumption. The actual-MV case (actual house, but with mechanical ventilation) has the lowest total and space heating energy consumption (118 and 69 kWh/(m2 a), respectively), but the actual-NV case (actual house with natural ventilation) has the lowest electricity consumption (28 kWh/(m2 a)). The total, space heating and electrical energy consumption in the actual-MV case are

2% lower, 10% lower and 17% higher than the measured values, respectively. The calculated total and space heating energy consumption for the actual-NV case are about 10% higher than the measured consumption because the actual ventilation rate is below the assumed value of 0.5 ach in the simulation. Assuming that 1 kWh of electrical energy requires 2.5 kWh of primary energy, the primary energy consumption of a natural and mechanical ventilation system are quite comparable as shown in Fig. 13. This emphasises the importance including total energy consumption (and possible primary energy consumption) when assessing the performance of ecological houses. Space heating energy consumption alone is not an adequate measure of performance. Fig. 14 presents the impact of various parameters on the calculated energy consumption. Reducing the window area from 16.4% to 15% of the gross heated floor area, decreases the energy consumption very little. Similarly, changing the water and electricity consumption from the database to the

Fig. 14. The change in calculated energy consumption when changing various parameters from the database value to the actual value.

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Fig. 15. Total (a) and space heating (b) energy consumption as a function of the total outdoor ventilation rate.

actual values has a small effect on the energy consumption because the consumption of water and electricity are very similar in both cases. However, if the changes in window area, electricity consumption and water consumption were greater between the database and actual values, the differences would be greater. For example, simulation results show that if the household electricity consumption increased by 10 kWh/(m2 a), the space heating would decrease by 4.4 kWh/(m2 a). The actual heating required by a house is independent of the electricity consumption, but the typically reported space heating consumption depends on the electricity consumption because the waste heat from electrical appliances is not included in the reported space heating. This is particularly important because electricity is a higher valued energy source than district heat [23] and the production of electrical energy is about 2.5 times more resource intensive than the production of heat energy [16]. Changing the U-value of the envelope from the database value (i.e., building code value) to the actual value (i.e., greater thermal insulation) decreases the total and space heating energy by 34 and 32 kWh/(m2 a), respectively. Changing the ventilation system from the database system (i.e., mechanical ventilation with heat recovery) to the actual system (i.e., natural ventilation) increases the total and space heating energy by 11 and 15 kWh/(m2 a), respectively, but decreases the electricity consumption by 5 kWh/(m2 a). As noted previously, the calculated energy consumption for Tapanila ecological house is slightly greater than the measured consumption. This may be a result of slightly warmer outdoor temperatures during the measurements than during the reference year of 1979 because simulation results show that if the average temperature increases by 2 ◦ C, the space heating will decrease by 13% (11 kWh/(m2 a)). Nevertheless, the main reason for the difference between measured and calculated energy consumption is that the ventilation rate in Tapanila ecological house is typically less than

0.5 ach. Fig. 15 contains the calculated total energy consumption and space heating energy consumption for the different cases as a function of the ventilation rate. Comparing the actual-NV case to the measured values indicates that the actual ventilation rate is likely about 0.4 ach, which is comparable to that expected during the heating season (Fig. 9). The effect of ventilation rate on energy consumption is also evident in Fig. 15. As expected, the energy consumption in the actual-NV case is the most sensitive to the ventilation rate because this case has no heat recovery. The increase in total energy consumption for every 0.1 ach increase in ventilation rate is 6.0 kWh/(m2 a) for the actual-MV case (mechanical ventilation) and 9.7 kWh/(m2 a) for the actual-NV case (natural ventilation). The increase in space heating energy consumption for every 0.1 ach increase in ventilation rate is 4.5 and 9.2 kWh/(m2 a) for the actual-MV and actual-NV cases, respectively, which means that the sensitivity to ventilation rate is double for the case without heat recovery. This means that a house with a 50% effective heat recovery system and a ventilation rate of 0.5 ach will have the same space heating energy consumption as a house with no heat recovery and a ventilation rate of 0.25 ach. Furthermore, a higher ventilation rate is possible with the same total energy consumption when a mechanically ventilated house includes heat recovery. If the heat recovery effectiveness is 50%, the ventilation rate can be 1.8 times higher and if the effectiveness is 70%, the ventilation rate can be 2.7 times higher. It is important to note that these results are based on the WinEtana simulation program, which neglects the fact that the fan energy consumption increases as the heat recovery effectiveness increases. The heat recovery effectiveness has a significant impact on the calculated energy consumption as shown in Fig. 16. Here a 10% change in heat recovery effectiveness changes the space heating energy consumption by 3.3 kWh/(m2 a) when the ventilation rate is 0.5 ach.

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Fig. 16. Space heating energy consumption as a function of heat recovery effectiveness for a ventilation rate of 0.5 ach.

6. Summary and conclusions The air flow rate through the envelope of the wood frame house studied in this paper was measured to be 3 ach at an underpressure of 50 Pa, which is quite airtight considering that the house has no plastic vapour retarder. The performance of the natural ventilation system was analysed by measuring the exhaust flow rates during different conditions, measuring the concentration of CO2 during occupation and developing an equation to predict the ventilation rate as a function of outdoor temperature and wind speed. The measured CO2 concentrations were generally satisfactory (i.e., below 1000 ppm) when the bedroom doors were open. The peak concentrations of CO2 in the bedrooms were between 1400 and 1800 ppm when the bedroom doors were closed and 800 and 1000 ppm when the bedroom doors were open. The CO2 concentration outside the bedrooms was usually less than 800 ppm. Since the occupants often keep the windows open in the summer, which increases the ventilation rate, the measured indoor concentration of CO2 in the summer and winter are quite similar, even though the average ventilation rate through the natural ventilation system is expected to be 0.25 ach (33 L/s) in the summer and 0.45 ach (60 L/s) in the winter. The results presented in this paper show that Tapanila ecological house uses significantly less energy than typical Finnish houses: 33% less total energy, 36% less space heating, 20% less electricity, and 30% less primary energy. The measured total, space heating, electricity and primary energy consumptions are 121, 76, 28 kWh/(m2 a) (5970 kWh) and 162 kWh/(m2 a), respectively. The calculated energy consumption using the WinEtana simulation program shows good agreement with the measured data when the ventilation rate is assumed to be 0.4 ach. The results presented in this paper show that Tapanila ecological house would have consumed 34 kWh/(m2 a) more energy (28%) if the building envelope was built according to the current building code. Similarly, the space heating

would increase by 32 kWh/(m2 a) (42%) without the highly insulated envelope. In other words, the space heating is expected to be about 40% higher (∼108 kWh/(m2 a)) if the envelope was insulated according to the building code. Since the well-insulated envelope decreases the heating energy consumption, but has no effect on the electricity consumption, the primary energy consumption is significantly lower with a well-insulated envelope. Unlike a well-insulated envelope, the natural ventilation system increases the space heating because there is no possibility for heat recovery from the ventilation air, but saves electrical energy because there are no ventilation fans. The natural ventilation system increases the space heating by 15 kWh/(m2 a) (22%), but decreases the electricity consumption by 5 kWh/(m2 a) (14%) compared to a mechanical ventilation system with a 50% effective heat recovery system. Therefore, if Tapanila ecological house had a mechanical ventilation system with heat recovery (50% effective) that provided the same ventilation rate as the natural ventilation system, the space heating consumption is expected to be about 60 kWh/(m2 a), which is significantly less than the measured value of 76 kWh/(m2 a). Even though a house with a mechanical supply and exhaust ventilation system has a lower total energy consumption than a similar house with a natural ventilation system, the primary energy consumption is nearly the same for both systems. Increasing the ventilation rate and decreasing the heat recovery effectiveness increases the energy consumption. The increase in space heating energy consumption for every 0.1 ach increase in ventilation rate is 2.7, 4.5 and 9.2 kWh/(m2 a) when the heat recovery effectiveness is 70%, 50% and 0%, respectively. At 0.5 ach, a 10% change in heat recovery effectiveness changes the space heating energy consumption by 3.3 kWh/(m2 a).

Acknowledgements Financial support from the Finnish Ministry of the Environment and the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated.

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