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.
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:
- Pel (W): Electrical power of the fan
- Δpf (Pa): Pressure difference at fan
- Qf (m3/h): Flowrate at fan
- ηf : 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 :
- without leakages;
- with leakages not compensated by the fan: the pressure drop is reduced at the ATD, inducing a lower airflow rate (poor indoor air quality);
- 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.
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.
See also
Posted in: Ductwork airtightness, Energy Aspects