Problems with Using the Exhaust Air Heat Pump for ... · ventilation system the most efficient way of heat recovery is to use the water-to-water heat pump solution [18]. Typical COP

Danish Journal of Engineering and Applied Sciences, August, 2015, Pages: 44-55

44

Problems with Using the Exhaust Air Heat Pump for Renovation of Ventilation Systems in old Apartment Buildings

Alo Mikola, Kaspar Tennokese, Teet-Andrus Kõiv

Department of Environmental Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia http://www.ttu.ee

A r t i c l e I n f o r m a t i o n A b s t r a c t Article history: Received: 24 June Received in revised form: 18 July Accepted: 25 July Available online: August Keywords: Heat pump Exhaust air heat pump Heat recovery Heat requirement COP Coefficient of performance Corresponding Author: Alo Mikola [email protected] © 2015 Danish Journals All rights reserved

Different types of exhaust air heat pump systems were studied in Nordic climate conditions. Field measurements of indoor air climate and parameters of the exhaust air heat pump were made. The results of the measurements show that there can be several problems with using the exhaust air heat pump in old apartment buildings. The main problem is related to the high temperature graph of the heating system. It was also noticed that exhaust air heat pump systems depend on the usage and maintenance of the ventilation system. If exhaust airflows are reduced, the heat production of the heat pump will also decrease. The energy simulations show that the exhaust air heat pump enables to save about 21% of energy compared to the building with the mechanical exhaust ventilation system without heat recovery. An analysis of energy consumption shows that in winter conditions the COPHP is about 3.0 and in the transition period about 3.3.

To Cite This Article: Alo Mikola, Department of Environmental Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia , Danish Journal of Engineering and Applied Sciences, 44-55, 2015

Introduction The performance of residential ventilation systems plays an important role in good indoor air quality. At the same time the energy consumption of the ventilation systems in apartment buildings is a considerable part of the total energy consumption. The study discusses the issues of energy efficiency and indoor climate in renovating the ventilation in apartment buildings. In the review of ventilation in European dwellings, it was pointed out that the ventilation of residential spaces in Nordic countries is often poor [1]. The renovation of ventilation systems is important because as a result of refurbishing, the air tightness of the building envelope increases and as many buildings have natural ventilation, the air change rate is reduced [2, 3, 4, 5]. The quality of indoor environment mainly affects the well-being and comfort of the building users - tenants. Besides this, poor indoor environment may lead to various health problems. One of the main devices to ensure acceptable indoor air quality is certainly ventilation. Natural ventilation is characteristic of old apartment buildings. Replacement of windows in these buildings will significantly decrease the air change, at least twice. Ensuring normal air quality without mechanical ventilation in such a situation is not feasible. In the majority of apartments in such buildings the indoor climate does not correspond to the requirements of the European standards. With the continuous increase in energy prices and the growing need for energy economy, using renewable energy sources is becoming increasingly more important. One good option is the increasing use of heat pumps. The most widespread are air-to-air heat pumps, but in the cold climate regions, where outdoor temperatures fall below -15 degrees Celsius, it is more efficient to use heat pump systems with a stable heat source, for example the ground source heat pump (GSHP) or the exhaust air heat pump (EAHP) because at low outside air temperatures the COP of the air-to-air heat pump is relatively low. This article gives an overview of the use of heat pumps with special emphasis on the exhaust air heat pump. The study by Berntsson [6] concluded that the heat pump has the potential to reduce CO2 emissions, but achieving it depends on the COP and the working hours of the heat pump. In addition, it is stated that the heat factor of a typical EAHP is 3.1, and the return of heat is up to 60% of the heat consumption of the building. Heidt [7] considers it important for heat recovery that a building should be airtight and that the airflow could be changed in separate apartments. To achieve maximal profitability, the heat should be returned to the hot water and heating systems and the COP should be at least 4. Higher COP will be ensured by higher temperatures on the evaporator side and lower temperatures on the condenser side. In addition the heat recovery attention should be paid to the ratio of electrical and thermal energy [8]. Chauhan’s study [9] in the 1980s found that the EAHP system works as expected, but with slightly lower thermal efficiency. Depending on the climate, the heat pump system is capable of saving up to 48% of the


45

heating energy. Sakellari and Lundqvist [10, 11] compiled simulations studied the operation of the EAHP in private buildings. Particular attention was paid to the effects of free heat and the sun as well as thermal storage of envelope elements. It was concluded that for optimal performance complex heating control is needed. Johansson’s [12, 13] simulations show that the EAHP has a shorter payback period compared to the mechanical supply-exhaust ventilation system. At the same time the author warns that if electricity prices increase, the payback period is also extended. Karlsson et al (2003) and Fracastoto and Serraino [14, 15] found that it is economically feasible to install a heat pump system with heat recovery, which covers 55-60% of the needed heat capacity. This system provides 90% of the needed annual heat consumption and the installation costs are significantly lower compared to the mechanical supply-exhaust ventilation system. The work of the EAHP in apartment buildings and the bases of calculations are described in the Nordtest Method [16]. The studies by Fehrm et al (2002) in Sweden and Germany [17] showed that the EAHP gives an average of 20% savings on the building’s power consumption in older buildings compared to the traditional use of mechanical exhaust ventilation. Some studies have shown that when renovating the existing apartment buildings with a natural ventilation system the most efficient way of heat recovery is to use the water-to-water heat pump solution [18]. Typical COP values of water-to-water heat pumps at different temperature graphs of the condenser if the flow temperature on the evaporator side is 5° C [19]. It can be concluded from these data that the higher is the required temperature to be produced the lower is the COP of the heat pump. It is most useful to use the heat pump for the floor heating system. In the case of the radiator heating system the average COP is 2.5 at the temperature of 60/50°C and 3.5 at 43/35°C. In the case of floor heating (at 35/30°C) the COP is nearly 4.0. The heat pump can be economical compared to other heating systems when the heating requirement of the building is covered with water temperatures of 45-50°C [20]. This is due to the fact that the COP of the heat pump is relatively high. In this case domestic hot water (DHW) production needs a separate solution because the temperatures of 45-50°C are not always sufficient for producing DHW. Abel and Voll [21] consider that the heat pump should be large enough to produce 90% of the required annual thermal energy. They also emphasise that for economic purposes it is recommended that the building should also be insulated when a heat pump is installed in an existing building. As the temperature escaping from the heat pump is lower than from a fossil fuel boiler, already existing heaters can be used if the heat load decreases. The authors note that the low temperature of the heating system is an advantage if the heating system is based on heat pumps, because it is more efficient at low output temperatures. The heat pump system should be designed so that the annual input-output energy ratio SPF (Seasonal Performance factor) would be maximal. Low heat consumption temperatures are preferred. When the temperature on the condenser side is reduced by 1°C the COP increases about 2% [22]. Thus, for the efficient use of the heat pump it is necessary to find a heat source with as high temperature as possible and a heating system, which works with as low temperatures of the heat carrier as possible. Exhaust heat pump systems During the past few decades the need to renovate apartment buildings has strongly emerged. Since approximately 70% of the people in Eastern Europe live in apartment buildings, increasingly more attention is paid to the EAHP system used in renovating apartment buildings. The existing natural ventilation system in old apartment buildings works with different efficiency at different times of the year. In winter the natural ventilation works well or even too well whereas in summer when the outdoor temperature is high, in the absence of wind there is practically no air change. Using the exhaust air heat pump system it is possible to ensure the required indoor climate and at the same time reduce energy consumption. In the latter case it is especially important that would be as efficient as possible. The EAHP uses the heat of exhaust air as the source of energy, which is usually just lost if not used [22]. The heat energy collected by the mechanical exhaust ventilation system of an apartment building can be used with the heat pump for heating the building and domestic water heating. Figure 1 shows one of the possible solutions, where the recovery heat of the exhaust air heat pump is used for DHW heating and for increasing the temperature of the return water in the heating system.


46

Figure 1 Principle scheme of the EAHP of an apartment building

Methods Studied buildings To study the efficiency of the EAHP 2 different apartment buildings were observed. First studied building is the old five-storey apartment building with 75 apartments built in 1977 and with the renovated envelope. The building has a reinforced concrete bearing construction and a flat roof, the exterior wall panels are made of ash gas concrete, which have been additionally insulated on the outside with foam polystyrene. Most of the windows in the building have been replaced with modern plastic windows. There is an automatically controlled central heating system. Ventilation has been solved with mechanical exhaust from the sanitary rooms (toilets and bathrooms) and with natural ventilation in the kitchens. Supply air is obtained through the fresh air valve installed in each apartment. The warm air extracted from sanitary rooms passes through the cooling batteries on the roof, which with a water glycol solution transfers its heat to the heat pump (the nominal rating of the heat pump is 28.8 kW). The heat pump gives the heat energy to the returning water of the heating system. The second studied building is the new apartment building with 17 apartments and built in 2011. The building has a reinforced concrete bearing construction, the exterior wall panels are made of concrete and of ash gas concrete, insulated with 200mm of insulation. The windows are with wooden frame and double glazed.There is an automatically controlled central heating system. Ventilation has been solved with central mechanical exhaust system from the sanitary rooms (toilets and bathrooms) and from the kitchens. Supply air is obtained through the fresh air valve installed in the windows. The warm air extracted from sanitary rooms passes through the cooling batteries on the roof, which with a water glycol solution transfers its heat to the heat pump (the nominal rating of the heat pump is 17 kW). The heat pump gives the heat energy to the returning water of the heating system and to the domestic hot water. Indoor air quality In accordance with the requirements of regulation no. 38 [23] of the Government of the Republic of Estonia, the living area must have natural or technical ventilation, which guarantees the air exchange necessary for human activity. According to the requirements in Estonia EVS-EN 15251:2007 [24] the air velocity in living spaces, the volume of the room per person and the content of harmful substances in the indoor air must not exceed the values permitted. The values for assessing the room temperature are taken from the Estonian standard EVS-EN 15251:2007. This standard describes the indoor environmental input parameters for designing and assessing the energy performance of buildings. In Estonia the indoor air CO2 concentration is considered in the standard of the indoor environmental input parameters and designing criteria CR 1752 [25]. The parameters are described in. The EAHP has to ensure the air change in the building, which meets the limit values of indoor climate.


47

Table 1: Class of the indoor climate for rooms with human activity by CR 1752 [25]

Category Expected

percentage dissatisfied, %

CO2 concentration at outdoor air level 350

ppm, ppm

Indoor air CO2 concentration, ppm

I (A) 15 460 810 II (B) 20 660 1010 III (C) 30 1190 1540

Results The measurements of indoor air quality in old apartment building To study the indoor climate conditions before installing the EAHP, measurements of the CO2 concentration of the indoor air in the bedrooms were performed. The measurements were conducted in nine bedrooms, which is 12% of the total of bedrooms in the apartment building. An example of the measurement results is given in Figure 2. The CO2 level of indoor air varies to a relatively large extent within days, ranging from 361 to 3999 ppm. The cumulative CO2 distribution is shown in Figure 2.

Figure 2 CO2 concentration changes during a week (left) and the cumulative distribution of the measurement results of the CO2 content of indoor air (right).

Measurements of the CO2 concentration of the indoor air after installing the EAHP are shown in Figure 3. When the parameters of indoor climate were measured, it appeared that in the apartments studied the CO2 content of the indoor air corresponded to standard B 32% and to standard C 94% of the time during the measuring period. The results show that the CO2 concentration is lower after installing the EAHP.

Figure 3 The concentration cumulative of the measurement results of the CO2 content after installing the EAHP.


48

The measurements of indoor air quality in a new apartment building With mechanical exhaust ventilation in new apartment buildings the situation is different. They have a rather good indoor climate, but the costs of heating the air is excessively high and are often about half the cost of heating. The energy efficiency of such buildings does not meet contemporary requirements. At the same time in apartment buildings with mechanical exhaust ventilation the indoor climate is good. The duration curves of the indoor air temperature and the relative humidity in the apartment buildings are shown Figure 4. The carbon dioxide measurement results are shown in Figure 5. It can be seen that the indoor air temperature in apartments is acceptable, the relative humidity close to satisfactory and the carbon dioxide level in most apartments satisfies the requirements. Not only the indoor air quality, but also the energy efficiency in buildings depends on the successful organisation of ventilation in apartment buildings. The studied CO2 content of the indoor air corresponded to standard to standard B 82% and to standard C 97% of the time during the measuring period.

Figure 4. Indoor air temperature, °C and the relative humidity of the indoor air RH, %

Figure 5. The duration curve of the carbon dioxide concentration in apartment buildings.


49

Energy consumption after installing the exhaust air heat pump in old apartment building Energy consumption was analysed in 2014, since by that time the exhaust air heat pump had been installed and the external facades renovated. The results are shown in Table 2.

Table 2. Energy consumption of the building and proportions over the year 2014 Name of position Quantity Unit

Air flow rate of the heat pump 877 l/s Roof circle power of the heat pump 13.76 kW Total heat consumption of the building 568.12 MWh Heat energy from the district heating network 463.34 MWh Output of the heat pump 104.78 MWh Electrical energy consumed by the heat pump 42.57 MWh COP of the heat pump system 2.5 The share of the output of the heat pump of the total heat energy 18 % The returned heat energy with the help of the heat pump (without electricity) 62.21 MWh The percentage of heat energy returned by the heat pump (without electricity) 11 % Payable heat energy 463.34 MWh Payable electrical energy 42.57 MWh In 2014 the building consumed 568.12 MWh of heat energy, 463.34 MWh of which was obtained from the district heating network and 104.78 MWh was produced by the heat pump system. The percentage of heat energy produced by the heat pump is approximately 18%. If the percentage of electricity consumed by the heat pump is subtracted, the percentage of the heat energy produced by the heat pump is about 11%. The annual COP of the exhaust air heat pump system is 2.5 (taking into account the total heat produced and the total electrical energy consumed by the heat pump system). Energy consumption of the building in 2010 by month together with the COP numbers of the system and the proportion of heat energy produced by the heat pump is shown in Table 3.

Table 3 Energy consumption of the building for heating in 2014 by month.

Month Heat pump production, MWh

Electricity consumption of the heat pump, MWh

COP of the system

Heat energy from the district heating network , MWh

Percentage of the heat pump, %

January 14.93 6.23 2.4 91.85 14

February 13.53 5.73 2.4 66.91 17

March 14.58 5.70 2.6 55.20 21

April 14.65 5.01 2.9 27.59 35

May 6.21 2.41 2.6 18.08 26

June 4.08 1.96 2.1 10.94 27

6 months 67.98 27.04 2.5 270.57 20

Annual average 104.78 42.55 2.5 463.34 18

The table shows that the heat pump system produces a fairly stable amount of heat during the heating period. If the heating load is relatively small, the percentage of the heat produced by the heat pump is up to 35% of the total heat energy consumption. In January when the output of the heat pump system was biggest, its percentage of the total consumption of the building was 14%. Consequently, the exhaust air heat pump is in the current conditions (the air flow rate, indoor air temperature, heat transfer, efficiency of the device) able to produce only a certain amount of heat, irrespective of the outdoor air temperature and the actual heat load. In this building the efficiency of the heat pump during some months outside the heating period was low due to technical reasons. Thus, during the summer months the building consumed 11-15 MWh of heat energy from the district heating network for producing DHW, which, based on the winter data, would have been manageable for the heat pump system. Consequently, using the exhaust air heat pump is more effective if, in addition to heating,


50

it can also be used for producing DHW. This would considerably reduce the energy need of the building for the district heating system during the summer months. Simulated energy consumption in a new apartment building Energy consumption simulations in the investigated apartment building were performed with software IDA ICE in two cases: • With usual mechanical exhaust ventilation, • The same with the exhaust air heat pump and heat recovery. • Simulations were conducted in a 17-apartment building in Tallinn. The heated area of the building was 1482 m2. Table 4 presents the parameters of the envelope elements, the air change, DHW and energy consumption. In the simulations the heating system has temperature heating graph 70/50 and COP of the heat pump 3.0.

Table 4. Parameters of the envelope elements, air change and energy consumption. Parameter A B

U-value of envelope elements, W/(m2K)

External wall 0.17 0.17

Roof-ceiling 0.22 0.22

Floor 0.28 0.28

Window 1.4 1.4

Air flow rate l/s 795 795

DHW consumption l/d per person, 22 persons 40 40

Thermal energy consumption, kWh per m2 157.9 100.5

Electricity consumption, kWh per m2 4.7 27.6

Total energy consumption, kWh per m2 162.6 128.1

The simulation results show that a mechanical exhaust ventilation system with a Heat Pump heat recovery unit in the apartment building makes it possible to save energy consumption by about 21%. Processes of the heat pumps in old apartment building During the study the operating parameters of the heat pump were measured (power, quantity of heat, heating water temperature) and at the same time the heat transfer parameters of the roof circle were measured. The results are shown in Figure 6. The results show that the system works with short periodic pauses and the power from the heat pump system does not exceed 25 kW. The return water temperature of the heating system rises by 3-5°C in the heat pump. To assess the effect of outdoor temperature on the efficiency of the heat pump, measurements of energy consumption were performed during one week at the outdoor temperature of -10°C to -28°C degrees. This was due to the fact that with colder weather the temperature graph of the heating system of the building is higher and the heat requirement is maximal. Measurement results are shown in Figure 7. The figure shows that electricity consumption is not consistent and there are pauses in the work of the heat pump. Thus, at colder outside air temperatures, when the water temperature of the heating system is higher, the heat pump stops. When the temperature of the supply water of the heat pump rises to 50 °C degrees, the heat pump switches off automatically. Thus, in cold weather, when the heat energy requirement of the building is highest, the productivity of the heat pump decreases considerably. At the same time the electricity consumption of the system decreases from 160 kWh per day to the range of 45-130 kWh per day.


51

Figure 6 Operation of the heat pump system on 7 March 2014 and 9 March2014.

Figure 7. The electricity demand of the heat pump system on 14 February 2014

During the study of the daily productivity of the heat pump, the daily COP of the heat pump system was measured during the period of 5 March 2014 to 9 March 2014. The connections of the daily electricity consumption of the heat pump with its production are shown in Figure 8.

Figure 8. Heat pump production 9 March 2014.


52

The graph shows that electricity consumption is consistent; therefore, it can be assumed that the heat pump worked consistently and in a stable way during the period. The production, electricity consumption and COP values of the heat pump system by day are shown in Table 5. It can be concluded from the table that the daily COPs of the heat pump system are 16-24% higher compared to the annual average COP. Thus, if the heat pump worked consistently, the annual COP would be 2.9-3.1. Consequently, when designing a heat pump system it has to be considered that the heat pump should work in a stable way, irrespective of the outdoor air temperature and season.

Table 5. The daily energy consumption, productivity and COPs of the heat pump.

Date Heat energy, kWh Electrical energy, kWh COP The difference of COP from the annual average , %

6.03.2014 470 164 2.9 16

7.03.2014 420 147 2.9 16

8.03.2014 480 161 3.0 20

9.03.2014 490 160 3.1 24 Processes of the heat pumps in a new apartment building On the pipeline of the mechanical exhaust ventilation system a cooling coil is mounted on the roof of the building. The temperature of the exhaust air passing through the cooling coil falls by about 13°C and the glycol water solution warms up by 5°C and is directed to a heat pump, where it is transformed up to 50-55°C. Analysis of electrical energy consumption and thermal energy production data shows that the COPHP in winter conditions is about 3.0 and in the transition period about 3.3.

Figure 9. The temperatures of the exhaust air passing through the cooling coil and the glycol water solution


53

Figure 10. Units of air temperatures and the COP Inglise keelde!

Discussion When installing the exhaust air heat pump system in old building, pre-conditions have to be met, without which it is not possible to ensure effective work of the system and indoor climate that satisfies requirements. The heating system of the building has to be adjusted to a low temperature heating graph; the recommended graphs are 55/40 or up to 60/40, because the heat pump works more effectively at low temperature. There should be no towel dryers before water extraction devices in the domestic hot water system, so that the hot water temperature would not have to be higher than 55°C. The study revealed that if the air flow rate does not meet the parameters set for the device, the heat pump will never achieve its maximal nominal heating power. This is because the smaller the air change rate, the less energy the HP evaporator gets and the smaller is the production of the heat pump. It has to be noted that any increase in the air change will in turn raise the energy consumption for heating air and thus, increasing air change can actually raise the energy consumption of the building. The main problems why ventilation airflows were reduced in the studied buildings is related to low supply air temperatures. The measurements showed that the supply air temperature was too low to ensure thermal comfort. That is the main reason why people reduce the too fast air velocity. The following is a statement of the main problems with ventilation systems: • Reducing the airflows by switching the unit to low speed, • The old exhaust air devices are not replaced with modern exhaust valves or grilles, • The exhaust air devices are not connected to ventilation ducts, • Kitchens, toilets and bathrooms do not have transfer air grilles, • The ventilation systems lack maintenance service (the filters are not changed, the systems are not monitored), • Bad technical condition of the exhaust shafts, • Arbitrary alterations of the ventilation systems, • The ventilation system is not balanced. In conclusion there are many problems with the installation quality and also with the quality of maintenance the systems. During the measurement period some other impact factors, such the nightly temperature graph of the heating system, the reduction of the exhaust air flows in case of low temperatures, mistakes in designing and low building quality, have played a role.


54

Conclusions In this study the work of exhaust air heat pump systems were studied in Nordic climate conditions. Field measurements of indoor air climate and parameters of the exhaust air heat pump were made. The results of the measurements show that there can be several problems with using the exhaust air heat pump in old apartment buildings. The main problem is related to the high temperature graph of the heating system. It was also noticed that exhaust air heat pump systems depend on the usage and maintenance of the ventilation system. If exhaust airflows are reduced, the heat production of the heat pump will also decrease. During the measurement of different ventilation systems some important impact factors, such as the quality of maintenance, mistakes in designing and low building quality, also played a role. The energy simulations show that the exhaust air heat pump enables to save about 21% of energy compared to the building with the mechanical exhaust ventilation system without heat recovery. An analysis of energy consumption shows that in winter conditions the COPHP is about 3.0 and in the transition period about 3.3. In the old building the CO2 content of the indoor air corresponded to standard A 25%, to standard B 32% and to standard C 94% of the time during the measuring period. The studied CO2 content of the indoor air of new apartment building corresponded to standard to standard B 82% and to standard C 97% of the time during the measuring period. The results show that the CO2 concentration is lower after installing the EAHP. The main problems why ventilation airflows were reduced in studied buildings are related with low supply air temperatures. The measurements showed that the supply air temperature was too low to ensure the thermal comfort. Other problems were related with lack of maintenance service, old exhaust devices and unbalanced ventilation systems. Energy consumption did not significantly decrease after installing the exhaust air heat pump in an old apartment building and in 2014 the proportion of heat energy produced by the heat pump accounted for 18% of the total energy need of the building. If the proportion of the electrical energy consumed by the heat pump is subtracted, the percentage of the heat energy produced by the heat pump is only about 11%. The consumption analysis performed during a year showed that the exhaust air heat pump is able to produce heat according to the amount of exhaust air flow in these conditions (air flow rate, indoor air temperature, heat transfer, efficiency of the device) producing heat pursuant to the amount of exhaust air flow. The use of the exhaust air heat pump is more effective if, in addition to granting heat energy to the heating system, it can also be used for producing domestic hot water. Acknowledgements The research was supported by the Estonian Research Council, with Institutional research funding grant IUT1−15, and with the project “Development of efficient technologies for air change and ventilation necessary for the increase of energy efficiency of buildings, AR12045”, financed by SA Archimedes and by the project "Civil and Environmental Engineering PhD School, DAR9085". References 1- M. Frontczak, P. Wargocki. Literature survey on how different factors influence human comfort in indoor

environments. Building and Environment, 46, 2011, pp. 922-937. 2- A. Mikola, T.-A. Koiv. Indoor Air Quality in Apartment Buildings of Estonia. Computers and simulation in modern

science: selected papers from WSEAS conferences, 2011, pp. 257 – 261. 3- T.-A. Koiv, A. Mikola, K. Kuusk. Energy Efficiency and Indoor Climate of Apartment Buildings in Estonia.

International Journal of Energy Science, 2, 3, 2012, pp. 94 – 99. 4- T.-A. Koiv, H. Voll, A. Mikola, K. Kuusk, M. Maivel. Indoor Climate and Energy Consumption in Residential

Buildings in Estonian Climatic Condition. WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT, 6, 4, 2010, pp. 247 – 256.

5- T.-A. Koiv. Indoor climate and ventilation in Tallinn school buildings. Proceedings of the Estonian Academy of Sciences. Engineering, 13(1), 2007, pp. 17 - 25.

6- Berntsson, T., 2002. Heat sources - technology, economy and environment. International Journal of Refrigeration, Vol. 25, No. 4, pp. 428-438.

7- Heidt, F. D., 2006. Ventilation for Energy Efficient Buildings. ISES Summer Workshop. 8- Sakulpipantsin, P., Cauberg, J. J. M., van der Kooi, H. J., Itard, L. C. M., 2007. Application of the energy concept to

ventilation using heat recovery from exhaust air. Proceedings of Clima 2007 WellBeing Indoors. 9- Chauhan, R. B., 1985. Field test on an exhaust air heat recovery heat pump. Division of Building Reasearch, Ottawa. 10- Sakellari, D., Lundqvist, P., 2004. Energy analysis of a low-temperature heat pump heating system in a single-family

house. Inetrnational Journal of Energy Research, Vol. 28, No. 1, pp. 1-12. 11- Sakellari, D., Lundqvist, P., 2005. Modelling and simulation results for a domestic exhaust-air heat pump heating

system. International Journal of Refrigeration, Vol. 28, No. 7, pp. 1048-1056.


55

12- Johansson, D., 2007. The cost of indoor climate systems in dwellings taking into account airflow rate, health and productivity. Proceedings of Clima 2007 WellBeing Indoors.

13- Johansson, D., 2009. The life cycle costs of indoor climate systems in dwellings and offices taking into account system choice, airflow rate, health and productivity. Building and Environment, Vol. 44, No. 2, pp. 368-376.

14- Karlsson, F., Axell, M., Fahlen, P., 2003. Heat Pump Systems in Sweden - Country Report for IEA HPP Annex 28. Energy Technology, Bor´as. http://www.annex28.net/pdf/Annex28_N28.pdf

15- Fracastoto, G. V., Serraino, M., 2010. Energy analyses of buildings equipped with exhaust air heat pumps (EAHP). Energy and Buildings, Vol. 42, No. 8, pp. 1283-1289.

16- Nordtest Method, 1990. Exhaust air heat pumps: performance. Finland. 17- Fehrm, M., Reiners, W., Ungemach, M., 2002. Exhaust air heat recovery in buildings. International Journal of

Refrigeration, Vol. 25, No. 4, pp. 439-449. 18- Eriksson, L., Masimov, T., Westblom, S., 1986. Block of Flats with Controlled Natural Ventilation and Recovery of

Heat. Swedish Council for Building Research, Stockholm. 19- IEA Heat Pump Centre, 2013. Heat pump in residential and commercial buildings. Sweden.

http://www.heatpumpcentre.org/en/aboutheatpumps/heatpumpsinresidential/Sidor/default.aspx 20- ASHRAE, 2008. Handbook of HVAC Systems and Equipment. American Society of Refrigerating and Air-conditioning

Engineers. USA. 21- Abel, E., Voll, H., 2010. Building energy consumption and indoor climate (Estonian). Presshouse, Tallinn. 22- Kõiv, T.-A., Rant, A., 2013. Heating of buildings (Estonian), second ed. TUT publishing house, Tallinn. 23- VV määrus nr. 38. Requirements for dwellings (Estonian). 26.01.1999 (RT I 1999, 9, 38). 24- EVS-EN 15251:2007. Indoor environmental input parameters for design and assessment of energy performance of

buildings adressing indoor air quality, thermal environment, lighting and acoustics. Eesti Standardikeskus, 2010. 25- CR 1752. Ventilation for buildings: design criteria for the indoor environment / European Commitee for

Standardization. European Committe for Standardization. Brussels, 1998.

Documents

Problems with Using the Exhaust Air Heat Pump for ... · ventilation system the most efficient way of heat recovery is to use the water-to-water heat pump solution [18]. Typical COP