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The infl uence of opening windows and doors on the natural ventilation rate of a residential building David Marr Mark Mason Ron Mosley and Xiaoyu Liu U S EPA 109 TW Alexander Dr Research Triangle Park NC USA Corresponding author e mail david marr An analysis of air exchange rates due to intentional window and door openings in a research test house located in a residential environment is presented These data inform the development of ventilation rate control strategies as building envelopes are tightened to improve the energy effi ciency of residential structures Common physical processes in the building are evaluated for their ability to alter building air exchange rates through increased natural ventilation The impact on indoor air quality as related to contaminant concentrations is not considered Air exchange rates were determined for door opening frequency in addition to several window open areas using multiple zone tracer gas decay measurements Dataanalysisshowsthatwindowopenedarea dooropeningfrequency andindoor to outdoortemperature differences have the greatest effect on air exchange rates out of the measured parameters Introduction Infi ltration of outdoor air is reduced when building envelope tightness is increased most often for energy conservation purposes Infi ltration occurs due to leakage via cracks and crevices in the building envelope and when building windows and entrances are opened Indoor to outdoor tempera ture differences opening size and duration outdoor wind effects and window geometry Jong and Bot 1992 have been shown to directly affect the air exchange rate AER of a building Wallace et al 2002 Howard Reed et al 2002 Jordan et al 1963 Of these factors window and door openings can of ten be controlled by residents and have been shown to signifi cantly affect AERs Johnson et al 2004 measured a test house AER with multiple openings windows anddoors and foundthegeometric mean to change from 0 76 h 1for no openings to 1 51 Received December 15 2010 accepted April 18 2011 David Marr PhD Member ASHRAE is Mechanical Engineer and Post Doc Mark Mason is Environmental Scientist Ron Mosley PhD is Physicist Xiaoyu Liu PhD is Environmental Scientist h 1for one opening 2 30 h 1for two openings and 2 75 h 1for three or more openings During wind tunnel testing for a scaled building model Meroney et al 1995 found an order of magnitude increase in AER when both a door and window were opened compared to window only In 1945 Hartmann et al published data showing an increase by a factor of four in AER when windows were opened by a few centimeters in small apartment buildings Howard Reed et al 2002 presented a compilation of data including multiple open areas in two buildings demonstrating a maximum increase of greater than one air change per hour for the largest opening area OA and two to three air changes per hour for multiple window openings under a variety of different window opening combinations Vatistas et al 2007 found a signifi cant effect of indoor to outdoor temperature difference on AERs in a building where automatic doors cycled 195 HVAC R Research 18 1 2 195 203 2012 CopyrightC 2012 ASHRAE ISSN 1078 9669 print 1938 5587 online DOI 10 1080 10789669 2011 585423 196VOLUME18 NUMBERS1 2 FEBRUARY APRIL2012 between the open and closed positions Limited data exists for door openings of specifi c frequency The data presented here provides this information for a specifi c residential building and a direct comparison to the natural ventilation rate cause by opening a window to different heigths Presented here are residential AERs for an un occupied research test house RTH in the case of a window with multiple OAs and a repeatedly opened exterior door Measured AERs are independently discussed for each dataset due to the differences in measurement time of year and changes made to the building leakage area between 2000 and 2005 when the window and door data acquisition respec tively occurred The AER ratio fully open versus fully closed for the two datasets are compared as a means of determining the most effective method to increase the relative AER in a residential building and the primary parameters that impact residential natural ventilation These data points are important as they directly impact the use of natural ventila tion in existing standards such as ASHRAE 62 2 ASHRAE2007 whichdefi nesventilationrequire mentsforlow riseresidentialbuildings Thecontin ueddevelopmentofexistingstandardsreliesondata that include consideration of all factors impacting the natural ventilation rate The datasets presented donotincludeadiscussionofseasonalvariationdue to the time frame of the measurements Methods Measurements were acquired in the U S EPA RTH located in Cary NC shown in Figure 1 The RTH is a detached single fl oor residential property with a fl oor area of approximately 121 m2 1300 ft2 and a volume of 292 6 m3 10 330 ft3 excluding the garage crawlspace and attic The RTH is sur rounded by dense tree cover on the SE S SW sides andmoderatetonotreecoverontheW N Esides as shown an overhead view in Figure 2 Google 2010 The surrounding trees are nominally four times the height of the RTH Outdoor wind speeds were mea sured by a raised metrology tower using a Vaisala anemometer with a calibrated uncertainty of 0 20 ms 1 0 66 ft s 1 The tracer gas decay method with sulfur hex afl uoride SF6 was used to measure the building AER using concentration measurements every 6 min with injection via Tefl on tubing from a tank located in the attached garage A Bruel Kjaer model 1302 B K 1302 infrared photo acoustic multigas analyzer was used to sample the real time air concentrations of SF6in the den master Figure 1 Layout of the single fl oor EPA RTH located in Cary NC with circled locations of open door bottom and den window top HVAC R RESEARCH197 Figure 2 RTH left image indicated by marker overhead view with surrounding tree cover Wind rose data right shows the predominant wind direction s during AER data acquisition bedroom front middle bedroom and front corner bedroom This measurement system has an accu racy of 25 0 and a precision of 5 0 SF6was injected in the hallway near the return air grill and distributed throughout the house by the air handler ceiling fans and auxiliary fans used for additional mixing Window Tracer gas concentration data was acquired for the window open events during warmer outdoor conditions of June and July 2000 The scenario in volved the opening of a window in the RTH den indicated at the top of Figure 1 For a measure of building leakage during window AER data ac quisition blower door testing was conducted one monthaftermeasurementsprovidingavalueof14 5 ACH50 indicating a fairly leaky building envelope as discussed by Sherman and Chan 2004 This can be compared to the median values of 6 and 9 across 33 residential buildings in two climate zones for Madison and Knoxville respectively Antret ter et al 2007 As shown in Figure 2 the outdoor wind direction was predominantly from the south west during data collection raw data from Weather Underground2010 Effectsoflowtomoderateout door wind speeds on the RTH AER were not antici patedsincetheopenwindowwaslocatedintheback of the house adjacent to the predominant tree cover Tracer gas injection occurred every 6 h for a total of 65 injection cycles AER calculations in corporate a 2 5 h time frame beginning 1 h after measured peak concentrations repeating every 6 h following injection of SF6throughout the duration of the experiments Five different window openings were used width 89 cm 35 in height 2 5 5 10 20 and40cm 1 2 4 8 and16in corresponding to 226 452 903 1806 and 3613 cm2 35 70 140 280 and 560 in2 OAs respectively in addition to the closed window condition For context Offer mann 2009 showed that 108 occupied homes in California had a median window opening of 46 ft2 hrs equivalent to 1765 cm2of OA for an entire day with a wide range and seasonal variation Outdoor temperatures were measured using a local outdoor temperature sensor and indoor temperatures were maintained at 22 C 72 F by the HAC system us ing the auto setting during window related AER data acquisition The RTH does not have mechan ical ventilation capability Outdoor wind velocities were recorded during this measurement period al thoughmeasuredmagnitudeswerefrequentlyonthe order of the measurement system uncertainty av erage measured value 0 19 ms 1 0 62 ft s 1 com bined measurement uncertainty 0 20 ms 1 0 66 ft s 1 It is anticipated that these local velocity 198VOLUME18 NUMBERS1 2 FEBRUARY APRIL2012 measurementsaredirectlyaffectedbythesurround ing tree cover that acts as a natural wind barrier The hourly mean wind speed reported at the local Raleigh Durham International Airport RDU air port located ten miles north of the RTH during the same time period was 3 4 ms 1 11 ft s 1 which is suggestive of a tree cover impact on locally mea sured wind speed although there was also a poor correlation in time between wind speeds at the two locations r2 0 20 representing the square of the correlation Local wind and temperature measure ments were recorded every 5 min Door AERs were acquired for a series of door open ing frequencies in late April and early May of 2005 The main entrance of the RTH was used as indi cated at the bottom of Figure 1 located on the most northern side of the RTH The door has nominal dimensions of 90 cm 200 cm 35 80 in re sulting in an open area of 18 000 cm2 2800 in2 approximately fi ve times the largest window open area As shown in Figure 3 during data acquisition mostofthewindwasfromtheNNEandSSWdirec tionsasreportedbythelocalRDUairport Whilethe quantitativeimpactoflocalwindconditionsondoor openingAERisunknown itispossiblethatoutdoor wind conditions raised the indoor to outdoor pres sure difference Figure3 Windrosedatashowingthepredominantwindvectors during AER data acquisition via the opening door data from Weather Underground 2010 Figure 4 Example SF6concentration data for a door open test here opened once per minute following tracer gas injection and mixing period For each test the door was opened for an average of 6 2 sec at a steady rate of 3 4 6 12 and 60 openings per hour respectively The time in which the door remained open was recorded by a boolean switch and data logger An effort was made to open and close the door in a normal manner of build ing entry or exit To provide a measure of AERs the RTH was injected with SF6 followed by 20 to 30 min of HAC recirculation fan use to mix the gas throughout the structure after which point the HAC system was turned off for the remainder of the test Tracer gas mixing was followed by 2 h of door openings 2 h of closed door conditions 2 h of additional door opening and a fi nal 2 h of closed door conditions as shown in Figure 4 In this way an injection of tracer gas provided four measures of AER due to an opening and closing door based on an hourly AER analysis and another four mea sures of building AER during closed door condi tions AERs are based on tracer gas decay measure ments in six rooms of interest Indoor to outdoor temperature differential was relatively modest For this reason the HAC system was not used during door AER testing aside from the initial mixing pe riod as discussed Results and discussion Window AERs during open window conditions were found to be dependent on multiple parameters HVAC R RESEARCH199 Figure 5 Correlation of measured parameters with AER for all outdoor temperatures and window OAs for all outdoor tempera tures over the HAC set point including all window openings as defi ned in Equation 1 Figure 5 presents the correlation coeffi cient r determined using Equation 1 between the AER and the measured parameters including outdoor tem perature air exchange due to differences between the indoor and outdoor air densities window OA time of day and outdoor wind speed r 1 n 1 n i 1 xi x sx yi y sy 1 where n is the number of data points x and y are the datasets for comparison s is standard devia tion and an over bar represents the mean Sim ilar to results presented by Howard Reed et al 2002 the outdoor to indoor temperature differ ence had the greatest effect on the AER While window OA did affect the AER it was not as signif icant as outdoor to indoor temperature difference Cross correlations such as hour of day with out door temperature are not considered here Window OA hour of day and wind speed affected AERs to a lesser degree The increase in AER with increasing indoor to outdoor temperature difference supports the existing fi ndings that open windows can be used as a means of increasing natural residential venti lation although Offermann 2009 showed that in California occupants are less likely to open win dows during periods of increased indoor to outdoor temperature difference Room specifi c AERs were calculated from time dependent tracer gas decay measurements Figure 6 shows the AER in individual rooms for both the Figure 6 Average AERs hr 1 in individual rooms for both the fully closed and 3613 cm2 560 in2 window OA conditions where n is the number of measurements in each room Uncertainty bars represent one standard deviation MBR is the master bedroom FMBR is the front middle bedroom and FCBR is the front corner bedroom 200VOLUME18 NUMBERS1 2 FEBRUARY APRIL2012 Figure 7 a Left AER h 1 values for specifi c outdoor temperature ranges including all window OA as measured in the master bedroom temperature uncertainty 0 05 C 32 1 F b Right Room averaged AER values for locally measured outdoor wind speed uncertainty 0 2 ms 1 with average 35 standard deviation on each windowfullyopen 3613cm2 560in2 windowOA single window in den and closed conditions While the AER in the den is 5 greater than the mean of all four rooms due to its open window this 5 value is still signifi cantly less than the individual room standard deviation 0 08 hr 1 or about 20 so it is reasonable to consider individual room re sults as an indicator for the whole building AER excluding the crawlspace attic and garage which are not considered The high mixing level is likely due to the building interior doors being open during testing and the intermittent use of the HAC house fan associated with seasonal cooling The direct effect of outdoor temperature is seen in Figure 7a where AERs are presented for a series of outdoor temperature bins As expected increased deviation from indoor temperature settings 22 C 72 F resulted in the greatest AER As discussed previously the wind speed had a reduced correlation with the RTH AER given in Figure 7b when compared to that of the outdoor to indoortemperaturedifference Toremovetheeffects of wind speed on AER in the fully closed scenario exchangeratesarecorrectedforoutdoorwindspeed usingtheempiricalmodelofGuoetal 1995 based on the ASHRAE Handbook ASHRAE 2009 fl ow rate relationships given in Equation 2 with results presented in Table 1 along with uncorrected mea surements during open window conditions N A B T C 2 where N is the AER hr 1 v is the outdoor wind speed in ms 1 and A B and C are building spe cifi c values given for the EPA RTH as A 0 184 0 005 B 0 0129 0 004 and C 0 0882 0 0030 ThesevaluesforA B andC areonlyappro priate for use in the RTH under fully closed condi tions as they are directly dependent on the building open area and will vary if windows or doors are opened While an increase in AER is seen between the smallest and largest window OAs the pattern is not alwaysconsistentwitheachstepincreaseinwindow size This is likely due to local weather effects and the resulting number of measurements across the variable ranges Percent increases in AER shown in Table 1 are similar to those of Johnson et al 2004 where a near doubling of the building AER was found for a single building opening Results are of the same order as AERs presented by Howard Reed et al 2002 who found an AER increase from 0 30 to 0 56 between a 0 cm2and 3822 cm2 560 in2 window OA case in a California house These AERs can be compared to those reported by Guo et al 1995 who measured the air change rate in the EPA RTH with all windows open under two separate occasions in October of 1989 reported average outdoor temperature of 21 C 70 F and found AER values of 2 06 and 4 20 h 1under wind speeds of 0 33 and 0 83 ms 1 1 1 and 2 7 ft s 1 respectively HVAC R RESEARCH201 Table 1 RTH AER hr 1 measured in the master bedroom dueto an open window indoor temperature HAC setting 22 C 72 F cooling mode Data format AER number of measurements n one standard deviation Indoor to outdoor temperature difference C F Den window OA cm2 in2 0 2 32 35 6 2 4 35 6 39 2 4 6 39 2 42 8 6 8 42 8 46 4 8 10 46 4 50 00 17 6 0 030 20 1 NAb0 28 3 0 020 31 2 0 030 35 1 NA 0a0 15 6 0 030 20 1 NA0 26 3 0 020 27 2 0 010 31 1 NA 226 35 0 11 1 NANA 0 NA0 40 2 0 02NA 0 NA0 55 2 0 02 452 70 0 19 3 0 030 23 1 NA0 27 1 NA0 24 1 NA0 34 1 NA 903 140 0 22 3 0 030 27 1 NA0 34 1 NANA 0 NANA 0 NA 1806 280 0 32 1 NANA 0 NA0 45 1 NANA 0 NA0 67 2 0 02 3613 560 0 38 2 0 060 42 3 0 060 47 1 NA0 61 1 NA0 73 2 0 01 aCorrected for outdoor wind speed bNA not available Door AERincreasedwithincreasingdooropeningfre quency as shown in Figure 8 Due to changes made totheRTHtoreduceleakagepriortoAERmeasure ments for door openings see Mosley et al 2002 the results are normalized by AERs from the case of a fully closed door The building AER rate increase is minimal for three four and six door openings per hour increasing to nearly 400 of the closed door scenario in the living room when the front Figure 8 Increase in RTH averaged AER due to continued door openings with a given frequency with 10 uncertainty bars ASTM 2006 door is opened every minute as seen in Table 2 This door opening frequency is not realistic for the residential environment but may be more promi nent in population dense buildings such as high rise complexes AERs would be expected to differ for population dense properties due to mechanical ventilation stack effects and 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