Exemplary Energy Partners buildings director, Trevor Lee, identifies the hidden costs associated with dodgy ductwork and calls on the air conditioning industry to support better standards.
The general problem of non-compliant products in the building industry is now being tackled even in the relatively narrow field of duct insulation (e.g. Trans-Tasman certification scheme for insulation products, CCN October 2014, p16). And this, of course, is not just a problem of compliance because the economic optimum for ducts for HVAC systems is far better insulated than the Building Code requires.
Within the confines of multi-storey construction, the issue is primarily one of delivering most of the enthalpy in the conditioned air to the target space and not to the general structure or, worse, the return air stream.
But in this context, the temperature differences are modest (10K [15° to 25°] on average for cooling and 15K [35° to 20°C] for heating) so much of the energy saved is by way of the volumes of air through the ducts and thereby to reduced fan energy.
This also impacts as a cost via the sizing of all components to meet the peak design demand and that factor is often the most important driver in the selection of above-Code performing product. And, sadly, ignorance of these factors often drives the selection of sub-Code products in a context where compliance is indifferently policed.
The financial drivers are more obvious, however, in the low rise context where the majority of ductwork is installed outside the insulated envelope of the building itself.
This both increases the temperature differences driving the heat losses (or unwanted gains) and ensures that such energy is totally lost from the building.
Even in a roof space or under a suspended floor, the ambient temperatures that the ducts are exposed to range from sub-zero to over 50°C. For example, the financial costs of dud ductwork can easily exceed $1,000 per year on top of the added capital cost of the higher capacity
We first looked at this problem in the residential context in a report in July 2010 by CIE for the Master Builders called “Energy-efficiency: building code star-ratings What’s optimal, what’s not” i. There the MBA was putting the case for increasing HVAC efficiency in lieu of improving the building envelope beyond the current 6-star Energy Efficiency Rating (EER) standard for new homes.
Since then, focusing on the Capital Region (around Canberra), we developed a ready reckoner first published in the local MBA’s “Winning Homes” magazine of 2011 and it is still accessible via their websiteii.
It tabulates the running costs of all combinations of House EER Stars and Heater/Cooler EER Stars as well as the additional costs of duct losses/gains. Because much has happened to electricity and gas tariffs since then, we keep an updated running cost ready reckoner on our own websiteiii.
And, for the convenience of clients, we have made the house size user-selected for cases where the published area of 200 m² is inappropriate.
We invite the residential air conditioning industry to make use of this facility for client education and up-selling into a higher efficiency range for the systems they sell. And that goes for the plant as well as the ducts.