Page 59 - Build 152

Measuring air ows
Ventilation systems in New Zealand homes rarely have their air ows measured and adjusted during installation, resulting in poorly ventilated rooms. Using active ow hoods during commissioning could change this.
BY MANFRED PLAGMANN, BRANZ SENIOR PHYSICIST
Ventilation
VENTILATION has an important impact on living spaces, the most signi cant being:
● moisture control for building material
integrity
● indoorairqualityforhealth.
Building Code requires ventilation
The airtightness of a building indicates how much ventilation will occur simply by wind and temperature differences, without opening windows or doors. The more airtight a building becomes, the less air is exchanged between indoors and outdoors. This decreases the amount of fresh air replacing stale and potentially polluted indoor air.
The Building Code stipulates that ventila- tion has to be provided for each occupied room in a house. One method for rooms located on an external wall is having a net openable window area that is at least 5% of the oor area.
But are those windows or other openings such as external doors, opened often enough
to achieve the goal of providing good indoor air quality?
Poor indoor air quality common
Problems with high moisture levels and the associated poor indoor air quality have been observed in many homes. Although we do not have the stringent European venti- lation regulations to comply with, many New Zealand homes have been tted with some form of ventilation system
Ventilation systems in commercial buildings are maintained with the air ow delivered or extracted from each room through a di usor or vent. The air ow is then measured and adjusted to suit the needs of the building and its use.
In residential buildings, this is generally not the case. This can potentially cause under or overventilation and result in poor indoor air quality and unnecessary energy loss. Measuring the air ow of the ventilation system should be carried out as part of the commissioning of the system.
So what needs to be considered when measuring airflow through a ventilation system, and what does the accuracy of the result depend on?
Flow hoods measure air ow
There are many different devices and methods used to measure the airflow through vents. Generally, they are referred to as flow hoods, and they can usually measure air ow in both directions. Some devices or methods work well with low measurement uncertainties – less than 10% – while others show systematic errors of 50% and more.
Flow hoods with high uncertainty are not adequate for ensuring ventilation systems are delivering an appropriate amount of fresh air and should not be used.
The measurement device is not the only important thing for the accuracy of the measurement. The method and way it is placed over the vent also a ects the accuracy of the measurement.
Build 152 — February/March 2016 — 57
FEATURE SECTION