Duct leakage, air tightness and efficiency
We know using airtight ductwork can have a positive impact on the Building Emission Rate (BER), but do we really know how airtight traditional ductwork is?
The performance gap
Since the lid has been lifted on the performance gap, the design vs as-built argument has been rife between energy modellers, SAP assessors, architects, engineers and others in the industry as they are now being asked to think more about the as-built performance of their buildings. The government has seen the gap as an important issue to address and is encouraging the use of programmes such as SAP and BIM in order to model buildings successfully.
Something we can be sure of is that air-leakage testing is not mandatory for low pressure ductwork according to the Non-Domestic Building Services Compliance Guide. However, research shows that low pressure ductwork can leak significantly more than previously thought, which can have a substantial impact on the as-built performance of a building. This can only have a negative effect on the BER and increase the performance gap further.
An independent study, the “Rationale for Measuring Duct Leakage Flows in Large Commercial Buildings” , showed that ductwork air-leakage can considerably increase the amount of energy required to run a fan for large commercial HVAC systems, and identified that the rate of air-leakage from the low pressure elements of ductwork systems can be far greater than many standard tests might suggest.
As previously mentioned, current practice is mostly based on pressurisation tests of high pressure ductwork, and tends to ignore low pressure ducts, even though the latter can represent a large proportion of the system. The study above measured the actual leakage flow for ten large commercial duct systems and compared the results with the estimated flow using standard industry methodologies. What the research revealed was that both the high pressure and low pressure sections of ductwork systems need to be tested to get a true picture of overall performance, and that a leaky system can result in an increase of as much as 35% in the energy required to run the fan.
“a leaky system can result in an increase of as much as 35% in the energy required to run the fan”
Of course, if a building design relies in part on the efficiency of its building services to achieve compliance, it is vital that the designed performance is met.
However, with low pressure ducts not being tested during the commissioning process, it is likely the as-built performance will be different than expected.
Performance of Pre-Insulated ductwork
So, the Zero Carbon Hub has put the onus back onto manufacturers, like us, asking that we develop products, which can be tested as a system so that the industry has a better idea of the in-application performance of their products. Manufacturing The Kingspan KoolDuct System has allowed us to test it as a system and find out exactly the air-leakage limits it reaches when fabricated correctly:
|Air-leakage standard||Air-leakage Classes Achievable with Different Coupling Systems|
|4-bolt||Aluminium Grip||Tiger Clip|
|BS EN 1507: 2006||Class D||Class C||Class D|
|BS EN 13403: 2003||Class C||Class C||Class C|
|B&ES DW/144||Class C||Class C||Class C|
By comparison it is extremely difficult to achieve low air-leakage using traditional sheet metal ductwork, even when it is installed with due care and attention to detail.
Furthermore, in a different case study the recorded air–leakage rate of an HVAC installation using The Kingspan KoolDuct System was 79% less than that using glass fibre insulated galvanised sheet steel ductwork.
This shows a substantial difference in performance and reinforces the importance of using products that have been tested as systems.
It has been proven that ductwork systems with low levels of air-leakage require smaller fans, and less energy to run them than leakier systems. By using pre-insulated ductwork instead of sheet metal, better levels of air tightness can be achieved, resulting in smaller fans, energy savings and an as-built performance which matches the design.
By using systems and products like this, that we know achieve or exceed their designed performance, we can ensure that buildings perform as designed. Once we have that nailed, we can be confident that the performance gap is being tackled.
If you would like more information about air leakage in pre-insulated ductwork take a look at our white paper on Air-leakage, Energy Usage & Running Costs.
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