Air infiltration modelling based on low-pressure airtightness measurements

Vega Pasos, Alan Emmanuel (2020) Air infiltration modelling based on low-pressure airtightness measurements. PhD thesis, University of Nottingham.

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The building sector has been responsible for around 30% of carbon emissions. These emissions come from the construction materials, but mainly by the building operation. The largest energy user of a building usually is the Heating, Ventilation and Air Conditioning system (HVAC). The ventilation efficiency depends on a correct design; however there is an unintended part of the ventilation called air infiltration. One-third of the heating load of a building is related to infiltration. The fewer leakage paths the building envelope has, the less infiltration it will allow. These leakage paths define the airtightness of a building. Air infiltration mainly (but not only) depends on the airtightness.

Air infiltration measurements are taken through invasive and disruptive tracer gas methods. Infiltration energy losses are calculated via air infiltration predictions obtained using infiltration models. Examples of infiltration models are LBL, AIM-2 AIDA, CONTAM, or DOMVENT3D. Airtightness is typically measured at a high-pressure differential. Infiltration occurs most of the time in a lower pressure difference. Infiltration is then predicted based on those high-pressure airtightness measurements. This project found that there are no models predicting infiltration using low-pressure airtightness measurements.

The steady pressurisation method is the most common airtightness measuring test. It measures the building airtightness typically between 10-60 Pa pressure difference (quoted at 50 Pa). This method has shortcomings such as the disruption of building integrity, the potential creation of new leakage paths, and the uncertainty created when extrapolating from high to low pressure. Pulse is a low-pressure airtightness testing method. It measures the building airtightness typically between 1-10 Pa pressure difference (quoted at 4 Pa). Pulse maintains the building integrity and its measurements avoid the uncertainties created by extrapolation.

Pulse is used in this thesis to obtain infiltration predictions based on modified infiltration models. This project aims to review, calibrate, test and validate infiltration models based on airtightness measurements taken using the Pulse technique. The overall methodology is to do air infiltration and airtightness tests in various dwellings across the East-Midlands in the UK. Dwelling parameters and environmental conditions are also recorded. Measurements and predictions are compared and statistically analysed.

The first model tested was a leakage-infiltration ratio: the "divide-by-20" rule of thumb. The rule of thumb overestimated the infiltration rate for all dwellings. A divide-by-38 rule fitted better the data obtained. When using low-pressure airtightness measurements, a divide-by-8 rule fitted the data. It was concluded that leakage-infiltration ratios are not accurate when predicting infiltration.

The LBL model was used to predict air infiltration from high (LBL50) and low (LBL4) pressure airtightness measurements. Both LBL models delivered similar results: comparable standard errors and relative percentage difference. DOMVENT3D model was used to predict air infiltration using both airtightness methods' measurements (DOMVENT3D-50 when using the steady pressurisation method measurements and, DOMVENT3D-4 when using Pulse). DOMVENT3D-4 model turned to predict with and RPD accuracy of 3%±14%. Both models are comparable and can be used regardless of the airtightness inputs. A sensitivity analysis of all the models showed that the variables used interact similarly.

Comparing all models, DOMVENT3D-4 was the most accurate model (mean RPD 3%), whereas the LBL-50 was the one with the lowest standard error (3%). The least accurate is the divide-by-20 rule of thumb (95%). The best overall predictions were given by the DOMVENT3D-4 model: 3%±14%. The conclusion is that measurements from the Pulse method can be used to predict infiltration. Regardless of the infiltration model, when using the Pulse method, there is not added error or uncertainty compared with the standard steady pressurisation method. Recommendations regarding the creation or revision of legislation and the extension of Pulse's development are given.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Wood, Christopher J.
Gillott, Mark
Zheng, Xiaofeng
Keywords: Airtightness, Air infiltration, Air leakage, Modelling, Pulse, Blower door, Air permeability.
Subjects: T Technology > TH Building construction > TH7005 Heating and ventilation. Air conditioning
Faculties/Schools: UK Campuses > Faculty of Engineering > Built Environment
Item ID: 63440
Depositing User: Vega Pasos, Alan
Date Deposited: 14 Jan 2021 09:26
Last Modified: 14 Jan 2021 09:26

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