BASIX: Class 2 Buildings - Assessing Relief Air Intakes

Region: NSW

Assessor Question:

I have a few Class 2 jobs on at the moment which have come back for re-assessment at CC stage. All have details relating to the mechanical exhaust design from the same mechanical engineer. How should these be modelled in NatHERS?

TechLink Response:

• Class 2 construction does as a rule achieve high quality air tightness provided that windows are installed as they should be to withstand rain infiltration under high wind pressures.
• Accordingly, there needs to be a relief air pathway in order for exhaust fans to be able to expel air from cooking, bathrooms and laundries.
• Examples of serious condensation are known on external walls and windows where clothes are dried on airer frames in rooms rather than using clothes driers ducted to the outside. 
• The relief air grille and exhaust systems are usually considered to have sufficient resistance to air flow leakage to be considered as sealed vents when not running.
• This is especially so when the inlet and outlet are on the same side of the building with the same wind pressure on both.
• Where gas cook tops are installed there must be an exhaust system linked so that it is automatically activated if a gas burner is turned on so that combustion gases cannot accumulate.
• Occupant activated window opening would risk insufficient airflow to avoid asphyxiation.
• Exhausts other than cooktop units are usually activated by bathroom light switches and as a result are only operating for limited periods..
• The neighbour surfaces of walls and floor around the ceiling space plenum should be equal to the indoor conditions of the adjoining apartments.
• Accordingly, condensation potential should be reasonably low.
• Inlet vents from the plenum to the occupied rooms should have cleanable filters to ensure that fibres are excluded.
• Best practice engineering would have energy recovery heat exchangers modifying inlet air temperatures to match exhaust air as closely as possible. 
• NatHERS Technical Note provisions do not envisage all design complexities.
• For Class two designs like this it is appropriate to include wall heights to the underside of the ceiling and ceilings as ‘Neighbour’.