Duct Work and Riser air leaks can compromise HVAC Energy efficiency and safety/health.
What are the Effects of leakage on energy use and even safety?
Ductless risers and even services risers are just big rectangles inside the building envelope that can become an avenue to feed the stack effect, turning tall buildings into giant chimneys.
Poor sealing happens in all types of ducts/builders’ risers, usually the larger the duct/riser size, the more problems we expect to find. Our experience shows that most ductwork that we have tested leaks between 8% to 18% with some extreme cases some duct systems can leak up to 35%+. It is also worth noting that the above figures are obtained from a random sampling of ducts tested during feasibility studies on energy efficiency retrofit projects. Advanced notice on which sections of ducts will be a part of a tested sample, generally yields better performance as contractors can inadvertently put in extra effort to seal areas of sample ductwork.
Leaky ductwork and/or builder’s risers can contribute to stair pressurization commissioning issues, which can be very time-consuming to resolve and potentially cause significant problems with a stair pressurization system functioning effectively should it be triggered for occupant evacuation on a windy day.
For buildings that have carparks below them, especially in cold climates, any risers that have connectivity to an underground car park can suck carbon monoxide straight into the conditioned space of the building above.
Energy Consumption of Leaky risers or ductwork
Despite the view expressed in AS 4254:2002, a few simple calculations suggest a reason for concern about the impact of duct leakage. Consider a typical air conditioning system in which the designer follows and assumes a supply duct leakage rate of 5%, to deliver the designed air quantities to the spaces served, the fan must handle 1/0.95 times the sum of the room air quantities or 105% of the nominal airflow. Applying fan laws increases fan power by 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 isn’t the end of the story because air leakage also affects cooling and heating plant energy consumption. The size of the effect depends on where the duct is located.
Suppose the duct is in a conditioned space, and the leakage percentage is low. In that case, one might argue that nothing needs to be done, that is, that the fan can safely supply 100%, not 105.3%, of the design because the leaked air produces valuable cooling or heating inside the building envelope. 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.
In a situation where the supply duct is outside the conditioned space, such as in a ventilated roof. In that case, the assumed leakage is lost, and the 17% increase in fan power is compounded by a 5% waste in cooling and heating effect and a 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 or riser 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. The effect is even more severe if the return air duct is outside the conditioned space.
Assume that under regular (non-economy cycle) operation, the plant handles 15% outside air, in which case return air will be 85% of the 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 the outside of the building, it adds to the outside air load.
The outside air percentage becomes 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 a 17% increase in fan power and fan energy on the supply side plus 5% additional cooling and heating energy if the leakage is going 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 economic cycle but more on a constant volume system with a lower percentage of outside air. For example, it is not unreasonable that a modest 5% leakage rate could add 10 or 15% of operating energy and greenhouse gas emissions.
We do not have published data on the effect of duct leakage in Australian systems, but some overseas studies have dealt 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 [6] 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.
In humid areas, the leaks in the supply duct can result in condensation, which accelerates the deterioration of sheet metal. The same applies to kitchen exhausts, where hot and humid air leakage moves into cool spaces.
The major problem with the performance of the duct systems comes from the air not being efficiently delivered to the occupied space. Short-circuiting of conditioned air is another common issue in leaky ducts where supply air enters the return stream by passing the occupied zone.
What are the best methods to solve air leakage?
The conventional method of combining foam seals in transverse joints and mastics can effectively seal ducts. The issue is more on the inspection and verification of a seal being applied consistently everywhere. Installers use foam seals and mastics but they do not necessarily deliver sealed ductwork.
Ductless risers can be quite challenging to troubleshoot, Efficiency Matrix has mining grade cameras for detailed visual inspections of builders risers to ensure no large holes are present, but to also audit the application of sealants to joins, and inspecting bracing for high pressure ductless riser systems.
Visual inspections can only do so much, especially when space is limited(Ducts installed close to the soffit) or when the seal/joins are covered by other materials, such as duct lagging or even other ducts or vermiculite. Pressure testing ducts can reveal the issues, but it can still be challenging to pinpoint air leaks, even with the help of a tracer gas or theatrical smoke. Sometimes, the timing for pressure testing of ducts means traditional methods of sealing ductwork cannot be applied. Especially riser ducts enclosed in masonry shafts, masonry or speedwall shafts, builders’ risers can be difficult to troubleshoot when they have finished off with plaster also. Alternatively, hole sealing with Aeroseal conducted by Efficiency Matrix’s automated sealing system can be used to achieve the desired air tightness level.
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