Developing improved methods for measurement of ventilation rates in occupied dwellings
David Veitch, UCL
Ventilation accounts for around one third of space heating energy demand in dwellings built to current standards. Advances in airtight construction and renovation aim to reduce energy wastage from the uncontrollable background level of ventilation (otherwise known as infiltration); however, a suitable ventilation rate must still be maintained to conserve internal air quality and occupant health and to prevent damage to the fabric of the dwelling. Also, ventilation is the main mechanism to limit overheating during the summer months. It is therefore of increasing importance that the dedicated ventilation systems installed within airtight dwellings operate effectively.
The performance of ventilation systems is known to be highly variable (Sandberg 1993; Stymne et al 1994, Lowe & Johnston 1997; Lowe 2000; Wingfield et al. 2008; CLG 2010), and vulnerable to poor installation and maintenance and to variations in dwelling airtightness. Ventilation is also one of the least well understood aspects of dwelling performance, being difficult and relatively expensive to measure, particularly in occupied dwellings. More complex ventilation systems, which aim to reduce energy consumption, introduce additional failure modes that need to be taken into account in the evaluation of technologies and intervention programmes.Large-scale refurbishment of housing stocks has the potential to impact on the ventilation performance of dwellings both for good and ill, but in the absence of suitable monitoring methods, impacts may take many years to become apparent.
Current Methods of Measuring Dwelling Ventilation
The general basis of measuring the ventilation rate in occupied dwellings is introducing and monitoring the concentrations of gases not normally present. The most prevalent method uses Perfluorocarbon tracers (PFTs), which rely on chemical absorption and subsequent mass spectrometry analysis in a laboratory facility. The equipment placed in the dwelling consists of two components: a source and an absorber. The source allows a controlled quantity of tracer gas to diffuse into the dwelling, and the absorber traps a fraction of this in inverse proportion to the ventilation rate and dwelling size. Stymne et al (1994) report on an investigation of 1500 dwellings in Sweden using this approach.The main limitation of the approach is that it yields a single estimate of average ventilation rate over a single extended period (typically, a week), and that it requires the physical placement and recovery of the absorber tube before the results can be analysed.
AIMS and OBJECTIVES:
The aim of this project is to develop an improved method of measuring the ventilation rate in occupied dwellings which can capture the variations over time, with the potential for remote data collection. These are in addition to the process remaining cost effective, robust and non-intrusive.
(The benefits arising from these improvements are discussed within the ‘Expected Outcomes’)
It is envisaged that the work will be undertaken as a cross-UCL collaboration, involving the Energy Institute, BSGS, Chemistry and Computer Science. The overall project structure will be:
1. Reviewing the existing methods for measuring ventilation rates in occupied and unoccupied dwellings, in terms of cost, resolution, robustness and practicality;
2. Reviewing the physical principles at work to identify the most suitable for incorporation into the improved method;
3. Designing, building and evaluating in the laboratory and the field an improved approach to measuring ventilation rates in occupied dwellings.
The fundamental aspect of the proposed improvements is moving to a tracer gas that facilitates low cost, in-situ, electronic detection. The key initial work will be to establish the most suitable gas and sensor technology for this application. Criteria for this combination include:
health/ethical compatibility, both for the occupants (toxicity, odour) and wider environment (GWP and ODP);
the sensor must operate effectively at the compatible concentrations of the gas;
the arrangement should minimise potential interference from day-to-day household activities;
relatively low cost.
From preliminary research in the area, a number of candidate tracer gases have been identified for further investigation: PFTs with portable electron capture detector/chromatograph; low concentration ethanol vapour; ethane; hydrogen; helium; non-toxic VOCs; and low GWP refrigerants e.g. difluoroethane (R152a).
As well as the current approach of introducing gases specifically for the purpose of ventilation measurement, there is the potential to use gases which are naturally produced within the dwelling e.g. CO2 from respiration, water vapour or CO from gas cooking. Preliminary work in this area has been conducted within non-domestic buildings which demonstrates its feasibility (Roulet & Foradini 2002).
A further development for the method is the ability to model inter-zonal airflows. Currently this is achieved by using 2 or more PFTs released from different zones which then can be distinguished using gas chromatography. This may be reproducible using the realtime in-situ equipment proposed; however, an alternative approach has been theoretically demonstrated which uses a single tracer gas with pseudo random pulses from controllable sources (Balcomb, Martin & Littler 1996). This would have the benefit of simplifying the equipment required as well maximising the information available. Within the scope of the project, it is proposed to investigate practical application of this approach.
The chosen solution may use off-the-shelf gas sensors or potentially new sensors developed for the purpose in collaboration with UCL Chemistry. Following testing and development of the gas and sensor package, it is proposed to integrate the design within a dwelling level wireless monitoring network which may be remotely accessed by the researcher. This aspect of the design will be conducted in collaboration with UCL Computer Science.
As part of the validation process, it is envisaged that the new method will be benchmarked against the established PFT method through side-by-side tests both in the lab and field. Within time constraints, it is expected to conduct a small-scale field trial of the method and analyse the results both in relation to the method and underlying ventilation performance of the dwellings investigated.
The main benefit from the project is the ability to capture the time variation of the ventilation rate, in comparison to the long-term average measured by the PFT approach. The applications for this information are numerous:
Understanding the distribution of the ventilation rate across dwelling types and ventilation systems. Whilst the average ventilation rate within a dwelling may appear satisfactory, there may be significant periods of under and over ventilation resulting in poor IAQ and energy wastage respectively.
Correlation of ventilation control measures taken by occupants. Examination of the time variance of ventilation rate and correlating these to occupant action will help to unravel the complex interaction of people with their environmental controls.
Simplified method of estimating indoor air quality. Due to the range of pollutants present in dwellings, monitoring each individually can be laborious and expensive. Measurement of the time varying ventilation rate could provide a simpler method, as it may be construed as the defining input of indoor air quality.
Application to overheating analysis. With a warming climate, overheating is one of the major challenges facing new, highly insulated dwellings. Suitable ventilation is the main method of limiting its impact, therefore greater understanding of the real world variation in ventilation rates will aid robust design in this aspect.
In addition, remote data collection will allow analysis to begin whilst monitoring continues. This is of particular use in longer term field trials.The initial focus of the work is to identify the most suitable sensor and tracer gas combination, with the intention to leverage the latest developments in the respective fields. In the eventuality that an innovative sensor/tracer gas combination is not feasible, it is envisaged the project will pursue the use of existing sensors with either naturally occurring gases (e.g. respiration CO2) or existing tracer gases. The project is not considered to depend on future technology: it simply proposes to make best use of the latest developments in chemistry and electronics.
Year 1: Review of existing methods, sensors, electronics and propose lines of development
Year 2: Develop and lab test prototypes
Year 3: Demonstrate complete systems in occupied dwellings, evaluate and report