Methodology for the Evaluation of Natural Ventilation in ... - Cham

A range of air exchange rates was found and used to determine both when the façade heat lossdominated the energy balance and when airflow contributed an equal amount to heat loss. Thelow end of the air exchange rates was recorded on a day when only the upper vent windows oneach floor were open. This was done because there were few, if any, days on site when there waslittle to no wind outside. The upper end of the air exchange rate was measured on a similar day,but with the occupant-controlled windows open. On extremely windy days, the occupantcontrolledwindows were often left closed, to prevent drafts that may disrupt papers on deskswithin the office area. The air exchange rates for the building ranged from 0.6 air changes perhour (ACH) to almost 1 ACH during a site visit in early fall when the windows were mostlyclosed. The results of ventilation rates using the three different techniques are presented in Table15. The hot-wire data presented in Table 15 were determined using the effective opening area ascalculated using the bag device presented in Chapter 3.5.1.Table 15. Comparison of Hourly Air Change Rates (ACH) by Season and MethodHot-wireAnemometerCarbon DioxideDataEnergyBalanceInflated BagMethod*Summer 3 ACH(3.8 m 3 /s)2 ACH(2.8 m 3 /s)----- 2 ACH(2.6 m 3 /s)Fall0.6 ACH 0.6 ACH0.9 ACH -----(0.85 m 3 /s) (0.87 m 3 /s) (1.2 m 3 /s)*The inflated bag method is not directly comparable to the other measurements. It was done over aweekend with little internal loads, and is primarily wind-driven flow.The measurements taken during site visits to understand the airflow patterns and ventilationeffectiveness of Houghton Hall were also used in verifying air exchange rates calculated throughboth an energy balance for a snapshot in time and carbon dioxide measurements andcorresponding calculations. For the energy balance, the solar radiation from the weather stationwas 200W/m 2 , recorded electric load was 16 kW, and the number of people, each at 100W, was10. The outside air temperature was recorded at 14.9ºC and the exhaust temperature leaving thebuilding was measured at 22.5ºC. In this snapshot the heat loss through the façade has an equalcontribution to the heat loss due to airflow.Carbon dioxide measurements were recorded throughout the year and used to corroborate theestimated airflow calculation. Using 0.3 L/min/person (McQuiston 2000), a measured outsideCO 2 level of 464 parts per million (ppm), and a total of 120 people within the building, thisprovided an air exchange rate of 0.6 ACH with all of the windows closed. This is similar to theair velocity measurements that estimated 0.6 ACH with the windows closed. Several airflowbalances based on air velocity measurements and flow rate calculations taken over the period ofseveral days and several site visits, were determined to be within a 25 percent margin of errorbetween the amount of air entering and leaving the building. The resulting carbon dioxide levelsfor the summer conditions when all of the windows were open, and the winter when all of thewindows were closed is presented in Figure 22. The occupancy density that corresponds withthese CO 2 levels is approximately 15m 2 per person. This is on the low side for a commercialoffice building.63