Measuring the ventilation rate in occupied buildings and adapting the CO2 tracer gas technique

Measuring ventilation rates in occupied dwellings is challenging but represents the conditions that occupants experience. This paper explores the constraints of existing methods when measuring the ventilation rate of occupied buildings and proposes a new method addressing some of them.  
Ventilation rates in occupied buildings can change over short time scales due to changes in weather or window and door positions. PFT based methods measure average ventilation over an extended period. Similarly, an annualised ventilation rate based on the ‘20th rule of thumb’ can be approximated using pressurisation tests. It is unclear how the results relate to the ventilation rate under any specific conditions. Conversely, measurement periods of less than an hour are possible with tracer gas techniques.  
The spatial scale over which ventilation is measured is important. Whole dwelling estimates are obtained from pressurisation tests and tracer gas methods if the whole dwelling is dosed. In both cases internal doors must be open, affecting the airflow, which may or may not reflect the occupied configuration. Differences between whole dwelling and in-use ventilation could be important for IAQ and heat loss. Single zone tracer gas techniques have been applied to one room and assumed to represent the whole dwelling. Multi-zone methods using several different gases are not usually applied in occupied buildings because of the complexity of the experimental set-up.  Measurement techniques must be acceptable to occupants. Pressurisation tests are minimally disruptive, taking less than an hour. PFT measurements are unobtrusive, only requiring small sources and sorption tubes. Injection of tracer gases, such as SF6, and concentration measurement can require pumped gas sampling which is noisy and bulky, causing inconvenience to the occupants. Metabolic CO2 based methods do not require the injection of gas or pumped sampling and may be more acceptable to occupants.  
CO2 decay techniques require knowledge of occupancy; historically this has relied on occupant reports, assumed hours of occupancy, or hand-picked sections of data. Similarly, CO2 equilibrium or accumulation techniques require knowledge of the CO2 generation rate and occupant activities.  
A new approach has been developed to address some of these issues, based on metabolic CO2 tracer gas decay. An algorithm for identifying when a dwelling is occupied has been developed, agreeing with reported occupancy in 87% of cases, meaning that large volumes of data can be analysed automatically. CO2 is measured in each room every 5 minutes, meaning that sub-hourly variations in ventilation rate, and variation between rooms, can be explored. Proximity sensors are used to monitor windows and doors, so that ventilation can be calculated during periods with constant conditions, and variation due to different configurations can be investigated. The proposed method represents a step towards appropriate measurement of ventilation and its variation in occupied buildings.