Krajcik, Michal

Olesen, Bjarne W.

Petráš, Dušan

Simone, Angela

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 19, 2017Experimental evaluation of thermal environment and ventilation effectiveness in aroom heated by warm airKrajcik, Michal; Simone, Angela; Olesen, Bjarne W.; Petráš, DušanPublished in:Proceedings of Indoor Air 2011Publication date:2011Document VersionEarly version, also known as pre-printLink back to DTU OrbitCitation (APA):Krajcik, M., Simone, A., Olesen, B. W., & Petráš, D. (2011). Experimental evaluation of thermal environment andventilation effectiveness in a room heated by warm air. In Proceedings of Indoor Air 2011Experimental evaluation of thermal environment and ventilationeffectiveness in a room heated by warm airMichal Kraj?ík1,2*, Angela Simone2, Bjarne W. Olesen2 and Dušan Petráš11Slovak University of Technology in Bratislava, Faculty of Civil Engineering, Slovakia2Technical University of Denmark, ICIEE, Denmark*Corresponding email: michal.krajcik@gmail.comSUMMARYThe study is focusing on experimental evaluation of the indoor climate in a residential roomheated by warm air supplied by a mechanical ventilation system. The experiments werecarried out in a full scale chamber, reproducing a typical residential room. The measurementswere done for various positions of supply/exhaust, at different outside conditions, internalgains and air flow rates. Additional setups with floor heating were performed and comparedwith the warm air heating system. Vertical air temperature and air velocity profiles weremeasured in order to describe the thermal environment; contaminant removal effectiveness,air change effectiveness and local air change index were measured to express ventilationeffectiveness in the room. Buoyancy forces play a key role in supply air distribution. Nosignificant risk of thermal discomfort was noted. The ventilation effectiveness may stronglydepend on the position of supply/exhaust; moreover, the three different ventilationeffectiveness methods may yield different results.IMPLICATIONSIn buildings with low energy consumption a mechanical ventilation system is often the onlyheating system used. The measurements investigate causes of thermal discomfort in roomswith such a heating system. It points out that the ventilation effectiveness in the occupied zonemay be less than 1, apart from ventilation effectiveness assumed in standards.KEYWORDSWarm air heating, radiant heating, thermal environment, ventilation effectivenessINTRODUCTIONIn low energy houses the heating demand for space heating is getting very low and the use ofthe ventilation system also for heating could become an interesting alternative instead ofinstalling a separate heating system. An acceptable indoor air quality in residential buildingsis mainly defined by specifying the required level of ventilation in air changes per hour(EN15251, 2007), which may differ according to national standards. The listed levels of airchange per hour in international or national standards are assuming full mixing i.e. ventilationeffectiveness equal to 1. However, the resulting ventilation in air changes per hour isdepending on the ventilation effectiveness. In existing standards the recommended values forventilation effectiveness depend on position of supply and exhaust device and on thedifference between supply and room air temperature (CR 1752, 1998). Especially by warm airheating supplying air at ceiling level the ventilation effectiveness may be than 1. The presentpaper reports on a study of warm air heating systems in an experimental room comparable to aresidential room during winter season. The ventilation system includes several combinationsof position of supply and return grills. Besides, a floor heating system was studied andcompared with warm air heating system. Evaluation of the possibilities and limitations arereported and discussed in the present work. Air temperature profiles, air velocity profiles, andventilation effectiveness were measured under different boundary conditions and compared tothe general criteria (ISO 7730, 2005) for a comfortable indoor environment.METHODSRoom set upThe experimental measurements were carried out in a full scale chamber, equipped with aradiant floor heating/cooling and a ventilation system, set up to reproduce a typical residentialroom (Figure 1a). The heat loss through windows was simulated through radiant coolingpanels located on one of the walls. Occupancy of the room was simulated using a thermalmanikin, which allowed for calculation of equivalent temperatures. The measurements weredone for various positions of supply/exhaust device, at different outside conditions, internalgains and air change rates. Additional setups with floor heating system were also measuredand compared with the warm air heating system.Figure 1. Pictures of: a) experimental residential room and b)  simulated pollution source inthe room.MeasurementsVertical air temperature, air velocity profiles, operative temperature in a reference point andsurface temperatures were measured in order to describe the thermal environment in the room.Contaminant removal effectiveness (CRE), air change effectiveness (ACE) and local airchange index were used to express the ventilation effectiveness.In CRE measurements a perforated table tennis ball covered by a sponge material served asa passive contaminant source in the room (see Figure 1b). The pollution was simulated usingtracer gas R134a. The CRE measurements have been carried out in steady-state thermal andtracer gas concentration conditions. The air change effectiveness measurements were based onthe mean age of air concept: step-up method was used, where constant tracer gas flow wasadmitted to the supply air duct and growth in tracer gas concentrations was continuouslyrecorded. Mean age of air is calculated as the sum of the mean travelling time and the meanpresence time (Breum, 1992). ACE is obtained as the ratio between the lowest possible meanage of air and the room mean age of air. An ideal piston flow is indicated by 100%, whereas50% suggests full mixing. Local air change index presents the ratio between the mean age ofair in the exhaust and the local mean age of air at the point of interest. At full mixing the ageof air in the exhaust is the same as the age of air in the room and the ratio is equal to 100%.Details on procedure and indicators of ventilation effectiveness are explained in detail in(Mundt E et al. 2004; Breum, 1992; Fisk, 1997).a) b)Room ventilation systemThree systems have been investigated. Positions of supply and/or exhaust and type of devicesare shown in Figure 2. In System 1, air supply and exhaust were located at the ceiling. System2 was designed with both, supply and exhaust device placed at the upper part of the back wall.Fresh air was supplied from the ceiling and removed by the exhaust device placed at thebottom level of the back wall in System 3.In Figure 2a are shown the locations of thermal environment measurements indicated by theblue points (S) and of ventilation effectiveness measurements, marked by the green crosses(C). Whereas CRE has been performed at three heights (0.6m, 1.1m and 1.7m above the floor)in all points (C1, C2 and C3), local age of air has been taken at least in the point C1 at 1.1mabove the floor. For case 3 of Systems 1 and 3 the local age of air has been taken in allpositions (C1, C2 and C3) at 1.1m and 1.7m above the floor.pollution souceS5C2desk lampmanikincooling panelsS4 S3 S2 S1S7S6C1 C3300300300250550300500500100065014502600 14008001300CB Aback wallFigure 2. a) Experimental room set up with locations of measurement points andsupply/exhaust devices; b) drawing of supply/exhaust used devices (A- supply at the ceiling,B – exhaust at the ceiling, C – supply/exhaust on the wall)Analyses of room ventilation performanceThe cases tested for particular systems are listed in Table 1. The air change rates werecalculated as the ratio between the air flow rate in m3/h and the volume of the chamber in m3.The air change rates represent theoretical values only corresponding to the air flow rates in theTable at full mixing conditions. Cases P5 and P6 represent the room conditions under acombination of radiant floor heating and mechanical ventilation, the rest of the performedcases is with warm air heating.Table 1. Simulated conditions for the experimental cases to be analysedP1 P2 P3 P4 P5 P6 P7Air flow rate (l/s) 5.6 11.2 5.6 11.2 5.6 11.2 16.8Air change (1/h) 0.5 1.0 0.5 1.0 0.5 1.0 1.5Heat loss (W) -120 -120 -220 -220 -220 -220 -220Heat gains (W) 0 0 100 100 100 100 100Overall heat balance (W) -120 -120 -120 -120 -120 -120 -120System 1 ? ? ? ? ? ? ?System 2 ? -- ? ? ? -- ?System 3 ? -- ? ? -- -- ?RESULTSRepresentative profiles of air temperature and air velocity for the occupied zone for the twoheating systems are shown in Figure 3 (System 1, case 3). For the other two systems and fora) b)all the cases the profiles had the same tendency, eventually with small modifications. Theresults show no significant risk neither due to draught, nor vertical temperature asymmetry.Results of segmental equivalent temperature obtained from the manikin shown slightlyincreased risk of thermal discomfort due mainly to exposition of the left foot to the coldenvironment and downdraught nearby the cooling panels.Figure 3. Air temperature and air velocity profiles for System 1, (5.6 l/s, 100W internal gains)An average of CRE for all the performed room ventilation systems, measured at threelocations in the occupied zone (C1, C2 and C3), is shown in Figure 4. The curves representaverage values of the three warm air heating cases with internal gains (P3, P4 and P7). Thetrend reported in the graphs is representative of CRE as a function of the air flow rate (from5.6 l/s to 16.8 l/s).Figure 4. CRE as a function of air flow rate for warm air heating/ventilation (cases P3, P4 andP7). The curves represent average values measured in points C1, C2 and C3In Table 2 the CRE (ec) for warm air heating system is reported compared with the CRE forfloor heating system. For System 5 the CRE of warm air heating is substantially higher thanfor System 1 presented in the Table.Table 2. Comparison of CRE (ec) for warm air heating and floor heating, System1q = 5.6 l/s q = 11.2 l/sev Location (warm airheating)(mixingventilation andfloor heating)(warm airheating)(mixingventilation andfloor heating)C1 0.65 0.95 0.6 0.9C2 0.7 0.95 0.6 0.9ecC3 0.65 0.9 0.55 0.8ecexp manikin 0.65 0.9 0.6 0.8Warm airheatingFloorheatingWarm airheatingFloorheatingIn Figure 5 are the results of air change effectiveness and local air change index for the threeventilation systems. The error bars in the left picture (local air change index) show uncertaintyof the mean travelling time. The uncertainty represents the difficulty in determining exactlythe time from molecules of tracer gas entered the space until it reached the measuring point.For case 2 ACE and local air change index were due to time reasons not thoroughlyinvestigated and are therefore not presented in the Figure.Figure 5. a) Local air change index for the three experimental ventilation systems at the pointC1, 1.1m above the floor, and b) Air change efficiency of the three SystemsIn case of ACE and local air change index, a set of measurements was performed with twofans present in the room, vigorously mixing the room air. The results at full mixing can becompared with the expected ideal values. They are briefly summarized in Table 3. One casewas accomplished with floor heating, the rest with warm air heating. Moreover, System 1 wasuninstalled and installed again later, and three experimental cases were repeated. Themaximum difference of 1 % for ACE and 8 % for local air change index was found.Table 3. Arithmetic mean of ACE with standard deviation for cases with full mixingAir change efficiency Local air change indexSystem1 System2 System3 System1 System2 System3Mean 49,8 48,7 50,1 97,6 94,7 98,1St. deviation 0,3 0,6 0,1 2,2 3,5 0,4No. of measurements 5 3 3 5 3 3DISCUSSIONWhereas temperature gradient occurs in warm air heating, in case of floor heating the airtemperature is evenly distributed. Slightly increased velocities have been found in lower partsfor warm air heating, caused by downdraught of warm air supplied in upper parts, descendingto the occupied zone nearby the cooling panels and moving above the floor. In case of floorheating the movement may be a result of downdraught and interaction of the room air with thewarm floor. If we compare CRE of warm air heating and floor heating systems, mainly forSystem 1 the CRE was better for floor heating, indicating conditions close to full mixing,whereas results for warm air heating indicate short-circuited flow, as could have beenexpected (Figure 4). This can be also seen looking at Figure 5. Best CRE was noticed forSystem 3 with the exhaust at the floor level, where the CRE was even higher than 1. System 2represented an alternative between the other two systems. However, in System 2 noticeable isthe decrease of CRE at higher air flow rates. Altogether, the results point out the importanceof the location of the air supply and also of where it is exhausted. Looking at Figure 5b, theresults of ACE for System 1 indicate a short-circuited flow pattern, and thus decreasedb)a)ventilation effectiveness. In other systems the results suggest conditions close to completemixing. However, looking at local air change index in Figure 5a, we can find differencesbetween particular cases. Systems 2 and 3 show impairing air distribution to the measuringpoint with increasing air flow rate. Lowest values have been noticed for System 1, indicatingmarkedly short-circuited air flow between supply and exhaust, both placed on the ceiling. Theexplanation of higher local air change index at lower air flow rates for Systems 2 and 3 mightbe a relatively higher contribution of buoyancy forces to the total air movement in the roomcompared to the forced ventilation. With increasing air flow rate more fresh air is introducedto the room, but the increase is not proportional to the increase in the amount of fresh airsupplied to the measurement point. Consequently, relatively more fresh air is exhausted athigher air flow rates. Influence of buoyancy forces on the air flow pattern is supported by thefact that for all Systems with warm air heating the results of ventilation effectiveness wereworse without internal gains. This might have been caused both by lower temperature ofcooling panels and missing effect of thermal plumes generated by the heat gains.CONCLUSIONSIn none of the Systems investigated significant risk of thermal discomfort due to verticaltemperature asymmetry or draught occurs, however, local cooling of certain body parts mightoccur nearby windows as a result of cold surfaces and downdraught. Whereas the effect ofwarm floor and cold air supplied to the room is conditions close to full mixing, for warm airheating systems it may be very important where the supply and the exhaust devices arelocated. Although the real conditions would differ to certain extent, the results suggest thatcold surfaces and internal gains have an important influence on the room air distribution. Evenin applications where the exhaust is not located in the same room, the fact whether the doorsare open or whether the air is exhausted through a leakage under the door could influence thedistribution of the air and ventilation effectiveness significantly.ACKNOWLEDGEMENTThis publication is supported by UPONOR Company of Denmark and by ICIEE/BYG ofTechnical University of Denmark.REFERENCESCEN. 2007. EN 15251, Indoor environmental input parameters for design and assessment ofenergy performance of buildings addressing indoor air quality, thermal environment,lighting and acoustics. Brussels: European Committee for Standardization.CEN. 1998. CR 1752, Ventilation for Buildings: Design Criteria for the Indoor Environment.Brussels: European Committee for Standardization.ISO. 2005. ISO EN 7730, Moderate thermal environments – Determination of the PMV andPPD indices and specification of the conditions for thermal comfort. Geneva: InternationalOrganization for Standardization.Breum, N. O. 1992. Ventilation efficiency in an occupied office with displacement ventilation– a laboratory study. Environment International, Vol 18, pp. 353-361.Mundt E, Mathisen H. M, Nielsen P. V, Moser A. 2004. Ventilation effectiveness. Brussels:Rehva.Fisk W. J, Faulkner D, Sullivan D. and Bauman F. 1997. Air change effectiveness andpollutant removal efficiency during adverse mixing conditions. Indoor Air 7, pp. 55-63.Simone, A, Olesen, B W, Kraj?ík, M., Hansen, J W. 2010.  Experimental study of airdistribution and ventilation effectiveness in a room heated by warm air and/or floorheating. In: Proceedings of the 10th International conference on Sustainable Energy Use inBuildings – Clima 2010, Antalya, Vol. 10, on CD.

Experimental evaluation of thermal environment and ventilation effectiveness in a room heated by warm air

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Experimental evaluation of thermal environment and ventilation effectiveness in a room heated by warm air

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