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 service risers are large rectangular cavities within the building envelope that can significantly contribute to stack effect—turning tall buildings into giant chimneys that unintentionally channel airflow.

Poor sealing is common across all types of ductwork and builder’s risers. In general, the larger the riser or duct, the more leakage problems we encounter. Our testing shows that duct systems typically leak between 8% and 18%, with extreme cases reaching over 35%. It’s important to note these figures are from random samples gathered during energy efficiency retrofit feasibility studies. When contractors are given advance notice about which duct sections will be tested, performance often improves due to extra care taken during sealing.

Leaky ductwork and risers can interfere with stair pressurisation system commissioning. These issues are time-consuming and dangerous during emergencies. On windy days, poor pressurisation could fail to protect occupants.

In buildings with underground car parks—particularly in cold climates—risers connected to the car park can draw carbon monoxide into the conditioned spaces above, posing serious health risks.

 

Energy Consumption Impact of Leaky Ducts and Risers

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 [4] 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%.

But that’s just the beginning—duct leakage also affects the energy used by heating and cooling systems. The impact depends on where the ducts are located:

  • If the ducts are within the conditioned space, low leakage might not be a major issue because some lost air still contributes to heating or cooling.

  • If the ducts are in a return air plenum or unconditioned space, the leaked air offers no useful benefit. It simply circulates inefficiently, wasting fan energy and reducing return air temperature.

  • If the supply ducts are outside the conditioned space, like in a ventilated roof cavity, leakage results in direct energy loss. In this case, the 17% fan energy increase is compounded by additional losses in heating and cooling—and increased 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 pronounced.

For example, let’s say the system uses 15% outside air. If there’s a 5% leakage rate in the return ducts (from outside), this effectively increases outside air intake to 19.3%—a 28% rise in outside air load. For a typical Sydney cooling system, this could add 5% to peak cooling demand. 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.

In summary a 5% leakage rate can contribute to the following:

  • +17% fan energy use (supply side)
  • +5% heating/cooling loss (supply ducts in unconditioned space)
  • +5% extra load from return duct leakage pulling in outside air
    → Combined, this can easily add 10–15% to HVAC energy use and greenhouse gas emissions, depending on system type.

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 [5] 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. 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.

Additional wastage occurs if supply and return air risers are collocated in the same structural riser shaft. This can cause short-circuiting, direct energy wastage and danger to HVAC control due to the miss matching of room temperature and return air temperature. The room will continue to call for cooling, but the return air sensor will be telling the chiller to reduce output.

 

 

 

What are the best methods to solve air leakage?

The conventional method of combining foam seals in transverse joints and mastics can effectively seal the ducts. However, the inspection and verification of seal being applied can cause issues. Ductless risers can be quite challenging to troubleshoot but Efficiency Matrix has mining grade cameras for detailed visual inspections of builder’s risers. This ensures there are no large holes, but to also allows us to audit the application of sealants to joins and inspect bracing for high pressure ductless riser systems.

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 and pinpoint the leaks with the help of tracer smoke. In some cases, traditional method cannot be applied, such as sealed masonry risers. A more advanced solution is automated aerosol-based sealing, which internally pressurizes the duct and seals leaks from the inside.