When a ductwork is leaky, part of the flowrate generated by the fan comes from (for extract ductwork) or goes through (for supply ductwork) leakages instead of air terminal devices (ATDs). If the fan compensates at least partially for leakage (with either higher fan power or a longer operating time) this will lead to an increase in fan energy use.

Moreover in countries where air is often the carrier of the thermal distribution, leakages also induce an increase of heating and cooling load.

Calculations and measurements performed in various studies (presented in a VIP dedicated to ductwork airtightness) show that improving ductwork airtightness may reduce the fan energy use from 30% to 75%. The impact of leakages on heating loads is estimated between 5% and 18% and between 10% and 29% for cooling loads; with also a high impact on the cooling design power that can be increased by 48% if leakages are considered.

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When a ductwork is leaky, part of the flowrate generated by the fan comes from (for extract ductwork) or goes through (for supply ductwork) leakages instead of air terminal devices (ATDs). Therefore, the fan needs to move more air to compensate for the extra flowrate and the extra pressure losses due to leakages. If air flows are not increased to compensate for leakage, then the required flowrates are not met at ATDs which may lead to a poor indoor air quality (IAQ). On the other hand, if the fan compensates at least partially for leakage (with either higher fan power or a longer operating time) this will lead to an increase in fan energy use.

Moreover in countries where air is often the carrier of the thermal distribution, leakages also induce an increase of heating and cooling load.

Additional fan energy use to compensate ductwork leakage

The fan power consumption depends upon the flowrate produced by the fan and the pressure difference on either side of the fan:

with:

• P_el (W): Electrical power of the fan
• Δp_f (Pa): Pressure difference at fan
• Q_f (m³/h): Flowrate at fan
• η_f : Efficiency of the fan (may depend on the pressure difference and flow rate)

The higher the pressure drop (resistance) in the ductwork, the higher the pressure difference the fan needs to produce to overcome this resistance and achieve the hygienic flow rate. So, leakages can be compensated by a higher fan power or by a longer operating time to achieve the same average indoor contaminant level. Both will increase energy use.

Pressure profiles along a simple extract ductwork are illustrated below [1] for three cases :

1. without leakages;
2. with leakages not compensated by the fan: the pressure drop is reduced at the ATD, inducing a lower airflow rate (poor indoor air quality);
3. with leakages compensated by the fan: same pressure drop as 1) at the ATD to meet a hygienic airflow rate which requires an increased fan pressure (increased energy use).

Calculation and measurements performed in various studies are summarised in [1] and show that reducing ductwork airtightness may reduce the fan energy use from 30% to 75%.

Additional heating and cooling loads due to losses of preconditioned air

In the US, air is often the carrier of the thermal distribution. A study from 2005 indicated that 10%–30% of the conditioned air in an average central air conditioning system escapes from the ducts [2]. Therefore, the main concern in the US, regarding ductwork leakages, is the loss of preconditioned air.

Indeed, leakages also induce an increase of heating and cooling loads as:

• when leakages occur in a conditioned space this may lead to over-ventilation;
• when the air is pre-conditioned and leakages of the supply ductwork occur outside the conditioned space, the pre-conditioned air is not fully used for the building (lost heated or cooled air);
•  when there is a heat exchanger, leakages of the extract ductwork in an unconditioned space decrease the energy recovery;
• when the air is pre-cooled, a secondary impact of the increased fan power is an increase in the cooling load associated with the heat generated from the increased fan power [3].

As both calculation (depending on the building's energy performance and on the climate) and measurements (with leakages mingling with conductions losses) are challenging, few studies estimate the impact of ductwork leakages on heating and cooling loads (summarised in the VIP). The impact of leakages on heating loads is estimated between 5% and 18% and between 10% and 29% for cooling loads. The highest impact seems to be on the cooling design power that can be increased by 48% if leakages are considered.

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References

[1] Leprince V., Hurel N. and Kapsalaki M.,2020. VIP 40 Ductwork airtightness – A review. AIVC, April 2020

[2] Srinivasan, K., 2005. Measurement of air leakage in air-handling units and air conditioning ducts. Energy and Buildings , 37, 273–277.

[3] Modera, M., 2005. Fixing duct leaks in commercial buildings. ASHRAE journal, 22-28.

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See also

• What is the impact of duct leakage on comfort, ventilation, indoor air quality and fire security?

Poor building airtightness results in excessive air infiltration and resultant uncontrolled energy loss. The energy use increase is dependent on the infiltration flow rate and the amount of conditioning of the air that is necessary to achieve thermal comfort. The infiltration flow rate depends on the building airtightness and natural driving forces by wind and stack effect. For the same building airtightness, the energy impact is larger when the building is more exposed to high wind speeds and large temperature differences, and when the building is higher.

As an approximation, an increase of the air permeability of the building envelope with 1 m³/(h.m²)@50Pa has a similar energy impact as an increase of the average building envelope thermal transmittance (U-value) with 0.02 W/(m²K).

Duct leakage is not only detrimental to energy efficiency, but also to indoor air quality (in terms of lower air change rates and ventilation efficiency in rooms), comfort, fire protection, noise, dust accumulation, moisture damage or even contamination issues. 

When the fan compensates for ductwork leakage by generating higher pressure and flow rates, energy losses are induced. When the fan does not (or only partially) compensate for leakage, the hygienic flow rate is not reached at every air terminal device, inducing indoor air quality (IAQ) issues especially in rooms located far away from the fan.

In a pandemic context, one can note that air leakage in extract ducts can also spread contaminants such as viruses to other parts of the building. Leaks located downstream the filter and upstream the fan can lead to polluted air bypassing the filter, leading to poor IAQ issues.

In Scandinavia good ductwork airtightness has largely been promoted together with indoor air quality benefits. Note that the Swedish VVS AMA guideline not only deals with energy issues related to ductwork airtightness but also with safety and indoor environment. [1].

In  [2], other effects than energy losses and IAQ issues are reported, such as changes in noise that tends to increase with increasing duct flows [3]. Leakages can have 3 noise related effects:

1. Increasing fan flowrate and pressure needed will increase the noise produced by the fan
2. Leaks can also increase the transmission of fan sound pressure
3. Leaks can create their own “whistling” noise

It is also believed that leakages can increase dust accumulation in filters [4], heat exchangers and ducts, as there is more flow rate going through.

Moreover, ductwork leakages lead to uncontrolled airflows that may induce depressurization causing backdrafting of combustion equipment or pressurisation causing moisture damage in walls [5]. This unbalance may also weaken contamination protection of sensitive areas (operating theatres, clean rooms, etc.)

Finally, fire-rated ventilation ducts can avoid fire and heat spread between two building compartments, but this can be compromised by ductwork leakage.

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References

[1] Guyot, G., and Carrié, F.R., 2010. Stimulation of good building and ductwork airtightness through EPBD. ASIEPI, 2010.

[2] Leprince V., Hurel N., and Kapsalaki M.,2020. VIP 40 Ductwork airtightness – A review.  AIVC, April 2020

[3] Richieri, F., et al., 2018. Ductwork design flaws and poor airtightness: a case study about a ventilation system reconditioning in a sealed building. Proceedings of the 39th AIVC-7 th TightVent-5 th venticool Conference, 18-19 September, Juan-Les-Pins, France. pp. 442-451.

[4] Dyer, David F., 2011. Case study: Effect of excessive duct leakage in a large pharmaceutical plant. Proceedings of the 32nd AIVC & 1st TightVent Conference, 12- 13 October. Brussels, Belgium. pp. 55-56.

[5] Modera, M., 2005. Fixing duct leaks in commercial buildings. ASHRAE journal, pp. 22- 28.

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See also

•  What is the energy impact of ductwork airtightness?

To improve the air tightness of existing air ducts, a method of sealing the ducts by applying a sealant using a spray atomizer was developed in the USA in the early 2000s and is now (since 2015) being used in Europe. This method [1] can reduce leakage in an installed air duct system by 66-86% and can therefore improve the air tightness after installation.  This can have other positive side effects, such as the impact on energy costs, indoor air quality & comfort or hygienic requirements.

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References

[1] MEZ-AEROSEAL. https://www.mez-technik.de/en/mez-aeroseal.html

Yes. The Building Research Establishment in the UK has published an experimental study showing energy savings achievable with improved airtightness [1,2].

[1] R. Coxon, “Research into the effect of improving airtightness in a typical UK dwelling,” The REHVA European HVAC Journal-Special issue on airtightness, vol. 50, no. 1, pp. 24-27, 2013.

[2] D. Butler and A. Perry, “Co-heating Tests on BRE Test Houses Before and After Remedial Air Sealing,” Building Research Establishment.