Frequently Asked Questions (FAQ)
1. Why is ventilation compliance not being met across the industry, and what is contributing to this non-compliance?
It is Not a matter of compliance. The right question is more like, “Why the systems are not performing as designed?”
Traditionally, the compliance issue has always been dealt with using deem to satisfied methods. As long as the duct manufacturers and installers only have to follow the prescribed methods and procedures for compliance. In the past, only special buildings may prescribe pressure testing of ductwork by the mechanical consultants.
NCC 2019 Vol 1 J5.6 states:
Ductwork in an air-conditioning system with a capacity of 3000 L/s or greater, not located within the only or last room served by the system, must be sealed against air loss in accordance with the duct sealing requirements of AS 4254.1 and 4254.2 for the static pressure in the system.(i) apply to ductwork located within the only or last room served by the system; and
It is unclear if this clause include the air leakage testing requirements of AS 4254.2 for all HVAC ducts. The NCC 2019 Guide to BCA Vol 1 come to the rescue.
The guide explicitly said the leakage test clause 2.2.4 of AS 4254.2 is part of the requirement.
2. Is poor duct sealing common, and where is it more likely to be found (supply air ducts, exhaust ducts etc)?
Poor sealing happens in all types of ducts, usually the larger the duct size, the more problems we expected to find. Our experience shows that most ductwork that we have tested leaks between 8% to 18% with some extreme cases, duct systems can leak 35%+ in the initial test. It is also worth to note that the above figures are obtained from pure random sampling of ducts to be tested on the day or during a feasibility study for energy efficiency retrofit works. Advanced notice on which sections of ducts will be tested, generally yield better performance as contractors usually put in extra effort to seal sections of sample ductwork.
3. Why are ducts not being effectively sealed? Is it skills, knowledge of the issue or care?
There are lots of factors contributing to the poor sealing of ducts. Usually the main issues are related to limited assessable work space around ducts. It can be caused by design issues or site constraints. Usually problems occur when ducts are installed too close to the underside of the slab or structure, which has restricted the installers ability to install cleats near the centre of the duct joints. In some cases, the installer can put the cleat in place with telescopic tools but accessibility issues disallow an inspection to understand how well the cleat is holding the joint together.
The other common issue is, not having enough room to install the custom pieces after all the standard sections have been installed, connecting all the ductwork to the riser. Contactors have to push the already installed ducts to the sides while slotting the custom piece in. This installation method can easily create situations where the foam seal on one side of the duct is “rubbed “ off or rolled up when the custom section is being slotted in place. As a result, the seal ends up being not effective. Not enough margin/tolerance, ends up leading to a situation were the contractor has to ’jam in’ duct pieces.
When ducts are in a subfloor, duct are damaged by trades walking on them. Improper transport and storing the duct usually ends up bending the corners of the joining rims or in some cases one side of the rim is deformed. These can easily create gaps that cannot be properly sealed by the typical foam seal used. In one instance, a gap caused by such deformation leaks 6% of the design air flow at test pressure.
4. Is a lack of compliance checking contributing to this (out of sight, out of mind)?
It is more like the contractor did not receive feedback on non-optimum practices, therefore unknowing carry on the poor practices. The other reason is that the industry focus on the volume of air being delivered at various outlets as a key performance indicator for HVAC ducts. As long as the HVAC contractor meets the air delivery specified, it’s a job well done. Regardless of the level of leakage. Sometimes the slight oversized air handler (safety margin) also reduced to incentive to improve duct sealing as the installers knows there is going to be enough slack.
5. What do the NCC and/or standards require to achieve compliance? How is duct sealing measured?
NCC refers to the AS 4254.2 2012 for Rigid duct sealing. There was a bit of confusion if the pressure testing of ducts is compulsory under NCC when it the explanatory not on the last section of ducts within the last enclosed room do not required to be sealed.
In addition, there are a few issues regarding details of the duct leakage test are not clearly defined.
AS 4254.2 2012 2.2.4
“Duct systems with a capacity of 3000 L/s or greater shall be tested for air leakage at a static pressure of a minimum of 1.25 times the calculated design operating pressure in the tested duct section. Leakage shall not exceed 5% of the design air quantity for the duct system.”
AS4254 calls for type-testing of at least10% of the system, including longitudinal seams, circumferential joints, floor distribution, riser and plant room duct, and each type of seam, joint and sealing construction. The standard does not indicate whether the 10% relates to total duct length, surface area or length to seams but overseas standards use duct surface area. Designers and contractors should agree as to which sections are to be tested having regard to design intent and practicability on site.
The standard does not define how to calculate the operating pressure of the test duct section. Another key issue is how to apportion the 5% system wide leakage allowance to the sample test duct sections.
It is not a problem if the entire system can be tested at one go. In larger system where 1 AHU serving a number of floors, there is no guidance for the tester to apportion the leakages to various sections of ducts.
For example, a System serving 3 floors. It is logical to break up to test into 4-5 parts namely
Ducts in the plantroom (directly connected to AHU and Riser:
- Ducts in riser
- Level 1 ducts
- Level 2 ducts
- Level 3 ducts
They all can have different operating pressure due to the frictional loss along ducts and the number of outlets on each floor.
If we assume we simply breakup an whole system test, then we test all sections using same test pressure (we use the arithmetic average of the pressure at the start and end of the duct system to estimate the operating pressure). Then, the leakage should be apportioned based on the duct surface area.
This method is simple but tends to under estimate the leakage in the plantroom section and risers and overestimate the leaks at floor levels especially the ducts downstream of VAV boxes.
The other method is use pressure adjusted area weighted method where the operating pressure difference between various parts of the duct systems are taken into account for apportioning the 5% leakage allowance. To use this method, a ‘pressure map’ along the duct system needs to be provided to the tester.
6. How should duct be sealed? What are the best methods?
The conventional method of combining foam seals in transverse joints and mastics can effectively seal the ducts. The issue is more on the inspection and verification of seal being applied. As the previous questions covered the theme that installers applied foam seals and mastics do not necessarily delivered sealed ducts. Visual inspection can only do so much, especially when space is limited or when the seal is covered by other materials such as insulted ducts or attenuators. Pressure testing ducts can reveal the issues but sometimes still hard to pin point the leaks even with the help of tracer smoke. In some cases, the timing for pressure testing of ducts means traditional method cannot be applied. Such as riser ducts enclosed in masonry shafts. Alternatively, hole seeking fully automated sealing system can be used to achieve the desired tightness level.
7. What are the implications of leaking ducts (energy loss, exhaust air re-entering supply air duct)?
2.1 Effect of leakage on energy and greenhouse gas emissions
Despite the view expressed in AS 4254:2002, a few simple calculations suggest that there is reason for concern about the impact of duct leakage. Consider a typical air conditioning system in which the designer follows AIRAH DA09  and assumes a supply duct leakage rate of 5%. To deliver the design air quantities to the spaces served, the fan must handle 1/0.95 times the sum of the room air quantities or 105.3% of the nominal air flow. Applying fan laws gives an increase in fan power of 117%, so the widely accepted leakage rate of 5% has added 17% to supply fan energy, for every hour the plant operates. At 10% leakage the extra fan energy is 37%.
This is not the end of the story because leakage also affects cooling and heating plant energy consumption. The size of the effect depends on where the duct is located. If the duct is in the conditioned space and the leakage percentage low, one might argue that nothing need be done, that is, that the fan can safely supply 100%, not 105.3% of design because the leaked air produces useful cooling or heating effect. This is not the case if the duct is in a ceiling return air plenum, as the leaked air will travel around the system producing minimal useful cooling and heating effect while increasing fan power and reducing return air temperature slightly.
If the supply duct is outside the conditioned space, such as in a ventilated roof space, the assumed leakage is simply lost and the 17% increase in fan power is compounded by 5% waste in cooling and heating effect and corresponding increase in greenhouse gas emissions.
The analysis for return air ducts also depends on where the return air duct is located. If the duct is in the conditioned space, leakage has little or no effect since the air leaking into the duct is the air that would have been returned anyway. If the return air duct is outside the conditioned space, the effect is more serious.
Assume that under normal (non-economy cycle) operation the plant handles 15% outside air, in which case return air will be 85% of design supply air. Leakage at the rate of 5% into the return air duct will thus be 5% of 85% or 4.3% of the design supply air. If the air that leaks in is from outside the building, it adds to the outside air load, the outside air percentage becoming 15% + 4.3% =19.3% of the supply air. Since the outside air load is pro rata, the outside air load increases by 4.3% / 15% = 28%.
For a typical comfort cooling plant in Sydney, 15% outside air would be about 18% of the peak cooling capacity so the leaked outside air will add 28% * 18% = 5% to the peak cooling load.
In summary, a 5% leakage rate implies 17% increase in fan power and fan energy on the supply side plus 5% additional cooling and heating energy if the leakage is to outside the conditioned space plus another 5% waste in heating and cooling energy on the return side if it increases the outside air percentage. The combined effects of these will depend on the detail of the system. It will have less effect on a VAV system with an economy cycle but more on a constant volume system with a lower percentage of outside air. For the example discussed, it is not unreasonable that a modest 5% leakage rate could add 10 or 15% to operating energy and greenhouse gas emissions.
We do not have published data for the effect of duct leakage in Australian systems but there have been of a number of overseas studies dealing with the issue. One  estimated the heating energy wasted by duct leakage in Belgium at 15 GW.h (0.054 PJ) per annum and 0.75 TW.h ((2.7 PJ) per annum for the rest of Europe (excluding the former Soviet Union). Another study of VAV systems in large commercial buildings in California  calculated that, compared to “tight” duct systems (2.5% leakage), systems with 10% leakage had annual HVAC system operating costs 9 to 18% higher, while those with 5% leakage used 2 to 5% more energy.
#Ecolibrium p.48 May 2013
Additional wastage in case if supply and return air risers are collocated in the same structural riser shaft. Short circuiting of conditioned air not only is direct energy wastage but it can also mess up the HVAC control due to the miss matching of room temperature and return air temperature. The thermostat in the conditioned space keep calling for more cooling while the return air temperature sensor indicating the chiller can be turn down.
Other problems such as poor distribution of air, uneven temperature across floor, increase occupant complains are all related to leaky ducts.
8. How big a problem can this have on performance and longevity of the duct?
In humid areas, the leaks in supply ducts can result in condensation which accelerates the deterioration of the sheet metal. The same applies to kitchen exhaust where the hot humid air leaks to cool spaces inside or outside of the building.
The major problem on the performance of the duct systems comes from air not being effectively delivered to the occupied space. Short circuiting of conditioned air is another common issue in leaky ducts where supply air entered the return stream by passing the occupied zone.
9. What are the difficulties of retrofitting or repairing non-complying ductwork? How costly can be this, particularly compared to doing it correctly the first time?
Space availability and accessability are the key problems. It is not easy to seal all seams and hole while the ducts are on the ground before installation. It is also tough to apply mastic in tight spaces afterward. In certain situations, it is virtually impossible to retrofit duct sealing using conventional methods, such as riser ducts inside speedwall risers as well as ducts above highly ornamented plaster ceiling.
Efficiency matrix has advance fully automated duct sealing system can provide cost effective solution for all duct sealing needs, sealing from the inside.
10. How can the industry improve its performance in this area?
- Improve factory fabrication
- Improve onsite touch up and quality control systems
- Advance duct joints
- Seal ducts from the inside, using a vapourised sealant.