Air Change in Low and High-Rise Apartments
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Article Air Change in Low and High-Rise Apartments Yonghang Lai 1, Ian A. Ridley 1 and Peter Brimblecombe 1,2,3,4,* 1 School of Energy and Environment, City University of Hong Kong, Hong Kong; [email protected] (Y.L.); [email protected] (I.A.R.) 2 Department of Marine Environment and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan 3 Aerosol Science Research Center, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan 4 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK * Correspondence: [email protected] Received: 8 April 2020; Accepted: 8 May 2020; Published: 13 May 2020 Abstract: Air exchange in tall apartment buildings is critical in controlling indoor environments in urban settings. Airtightness is relevant to energy efficiency, thermal comfort and air quality experienced by urban dwellers who spend much of their time indoors. While many air change measurements have been made in residential homes, fewer are available for high-rise apartments. The blower-door and CO2 exchange methods were used to measure air change in some Hong Kong apartment buildings, for comparison with those from other parts of the world. Hong Kong apartments are often small and typical rented apartments show a median of seven air changes per hour under a 50 Pa pressure difference, similar to Mediterranean houses, though much greater than the airtight buildings of Northern Europe. Extrapolation of blower-door measurements made at 50 Pa to the natural pressure difference measured for individual Hong Kong apartments provides an approximation (within 8%) of the natural air change rate measured with a tracer. Air flow is a function of the pressure difference DPnf and the exponent n was found close to the typical 0.6. There was a positive relationship between air permeability and construction age, but some of this also seems to reflect varying levels of maintenance by the building management companies. The median exchange in the apartments under naturally 1 ventilated conditions was 0.26 h− , not atypical of some houses on the US West Coast. Keywords: airtightness; blower-door; CO2 exchange effective leakage area; Hong Kong; indoor climate; indoor air pollution; permeability 1. Introduction Tall apartment buildings are increasingly a characteristic of high-density urban living and thus represent an environment where those dwelling in cities may spend a great deal of time. Air change is a key parameter in describing the exchange of room air with the ambient environment, so it affects indoor air pollution, yet is also relevant to other issues, such as thermal comfort and energy efficiency. The exchange of air is often expressed as the air change rate, i.e., the rate at which air is replaced. Conventionally, this is expressed as the number of air changes per hour (ACH), it is affected by factors such as the design of the building and occupant behavior, weather variation and building ventilation mode. There are three main ventilation modes [1]: (i) mechanical ventilation, (ii) natural ventilation driven wind or buoyancy-induced flow through open windows or doors, and (iii) infiltration through gaps and cracks in the building envelope. Unlike other mechanisms, infiltration cannot be controlled, as it represents the ventilation mode when all windows and doors are closed. In the Hong Kong Special Administrative Region, most residents will close all windows and doors during periods when air-conditioning is required, and during cold periods to prevent drafts. Airtightness thus becomes an important factor in building efficiency [2] and affects exposure to Urban Sci. 2020, 4, 25; doi:10.3390/urbansci4020025 www.mdpi.com/journal/urbansci Urban Sci. 2020, 4, 25 2 of 15 pollutants indoors [3]. There are few surveys of airtightness tests or comprehensive data sets for the region, and little in terms of standards for air change. Concern about airtightness has persisted for some decades [4,5], especially as Hong Kong is such a crowded city, and Guidance Notes now set ventilation rates for some building types [6]. In contrast, air change in residential dwellings has been well-studied in Europe and North America (some examples in Table1), although much research has focused on houses, with less work on the tall residential buildings that characterize the cities of Asia. In this paper we explore the air change rate in some Hong Kong apartments and compare these with other cities, aiming to establish a context for our measurements in apartment buildings and their relevance to regulating the built environment. Houses are very rare in Hong Kong and are typically in rural areas, so they have not been studied. Table 1. Previous work used. 1 1 2 Country Building k50/h− p/m s− AELA/cm Reference x, σ Korea High-rise Sn Shin and Jo [7] 3 0.5 ± x, iqr x, iqr China Low-rise e e Chen et al. [8] 6.5, 5.65 189,174 x, iqr x, iqr Russia High-rise e e Armstrong et al. [9] 7.5, 3.5 232, 143 x,σ US Public Bahnfleth et al. [10] 7.3 0.2 ± x, σ Canada Houses Sn Hamlin & Gusdorf [11] 3.1 1 ± x,σ x,σ Estonia Houses Kalamees [12] 4.9 3.5 4.3 3.5 ± ± x,σ Finland Houses Jokisalo et al. [13] 3.7 2.2 ± Denmark Houses - Tommerup et al. [14] x, σ Norway Houses Sn Granum & Haugen [15] 3.9 1 ± x, iqr x, iqr Italy Houses e e Alfano et al. [16] 6.6, 1.9 5.5, 2.8 x, iqr Greece Houses e Sfakianaki et al. [17] 7.6, 4.2 x, iqr x, iqr Ireland Houses e e Sinnott and Dyer [18] 10, 5 9.7, 4.7 x, iqr UK Houses e Johnston et al. [19] 13, 5 x, σ UK Low-rise Sn Pan [20] 5.3 0.3 ± x, σ UK Houses Sn Stephen [21] 11 8 ± x,σ UK Houses Hong et al. [22] 18 7 ± x, σ UK Houses Sn Grigg [23] 9 3 ± Notes: k50 air change rate at 50 Pa pressure difference, p is permeability and AELA is the effective leakage area with central tendency as median (ex,) or mean (x), and dispersion as interquartile range (iqr), standard deviation (σ) or estimated from Snedecor’s rule (σSn) or small (-). Note: Korean building values represent an average of four and the Canadian work only includes values of leakier building. Urban Sci. 2020, 4, 25 3 of 15 2. Materials and Methods Urban Sci. 2020, 4, x FOR PEER REVIEW 10 of 14 2.1. Data Sources This paper relies on data sets for air change change mostly mostly in in residential residential dwellings (Table 1 1),), inin additionaddition to using a small set from our own measurements made made from from apartments apartments in in Hong Hong Kong Kong (Figure (Figure 11).). Thousands of fan-pressurization measurements (i.e., blower-door tests), tests), usually usually made made at at a 50 Pa pressure didifference,fference, have been conductedconducted in US buildings [24] [24] and relate air exchange to the type of construction, region, the number of stories, floorfloor or basement type andand ageage ofof dwellings.dwellings. Previous studies, along with those shown in Table 1 1,, establishestablish thethe rolerole ofof design design andand construction,construction, buildingbuilding supervision and the quality of workmanship, workmanship, which may sometimes be more important than building age or resident and management behaviorbehavior e.g., [[8,18].8,18]. The junction of ceilingsceilings/floors/floors with external walls, the junction of separating walls with the externalexternal wall, gaps between electrical and plumbing installations or chimney and ventilation ducts, an andd gaps and cracks between windows and doors are all important placesplaces forfor leakage.leakage. InIn somesome countries,countries, especially especially those those in in Northern Northern Europe, Europe, airtightness airtightness is well-controlled,is well-controlled, but but elsewhere elsewhere it may it bemay lax. be China lax. mayChina not may meet not international meet international standards, standards, so increasing so increasingairtightness airtightness could reduce could residential reduce residential energy use energy by 12.6% use [8 by]. 12.6% [8]. Figure 1. PhotographsPhotographs of of some some Hong Hong Kong Kong buildings buildings that that formed formed part part of the of the study study for for(a) Site (a) Site4; (b) 4; (Siteb) Site 5; (c) 5; Site (c) Site 6; and 6; and (d) (Sited) Site 7. No 7. Nophotogra photographph of the of thebuilding building with with apartment apartment sites sites 1–3, 1–3, 8–10. 8–10. 2.2. Study Apartments in Hong Kong 2.2. Study Apartments in Hong Kong The current study adds measurements of air change at 50 Pa from 10 apartments located in HongThe Kong. current The study region adds has measurements a warm-humid of subtropical air change climate,at 50 Pa from with an10 apartments average temperature located in aboveHong Kong. The region has a warm-humid subtropical climate, with an average temperature above 30 °C 30 ◦C for the four warmest months, while winter months rarely drop to 10 ◦C. Hong Kong has more forthan the seven four million warmest inhabitants months, while crowded winter into months limited rarely areas ofdrop flat to land. 10 °C. Most Hong people Kong in thehas citymore tend than to sevenlive in million small apartments inhabitants in crowded high-rise into buildings limited due areas to of the flat lack land. of flat Most land, people which in leadsthe city to sometend to of live the inhighest smallpopulation apartments densities in high-rise in the buildings world (59,000 due to/km the2 inlack Kwun of flat Tong).