Ducts in any HVAC system are used for distribution of air which can be for the purpose of cooling, providing fresh air or for the exhaust purpose. Commonly the HVAC ductwork is fabricated using GI sheets, but there are different types of air conditioning ducts available in the market.
Below is a list of the most common forms of ducting with a description of what they are used for:
Reverse Cycle Air Conditioning Ducting:
Reverse cycle (refrigerant) air conditioners use duct that should be insulated with at least 2 inches of material surrounding the core.
Duct insulated with less than 2 inches will lose its cooling/heat quicker.
Evaporative Air Conditioning Ducting:
Evaporative duct systems usually use duct with only 1 inch of insulation surrounding the core.
This is because the air in an evaporative air conditioner is extremely fast moving and therefore there is little time for the heating/cooling effect to be removed.
Evaporative ducting is generally bigger in diameter than reverse cycle ducting.
Acoustic / Whispaflex Air Conditioner Ducting:
Acoustic ducting (or whispaflex as it is also known) is often used in the return air of a ducted reverse cycle system.
The inner core of acoustic ducting contains many tiny holes that reduce the noise inside the ducting.
Return air grille on a reverse cycle system can be quite noisy so this ducting helps reduce that noise.
Ducted Air Conditioning Ductwork Installation Method
This hvac method statement explains the steps in order to ensure that prefabricated hvac duct, bracketing, in line equipment installation, final connections and workmanship are correct and acceptable.
By implementing this method statement on a construction project air conditioning contractor shall make sure that the ductwork installation is conforming to the contract documents, shop drawings and hvac specifications.
All materials drawings and other documentation relevant to a particular section of works will be checked by the construction manager prior to the commencement of ac duct installation work ensuring that these are of the correct type, reviewed and accepted by the client/consultant.
Before commencing any mechanical hvac construction works, pre inspections will be carried out on all materials available in the storage area to ensure that these are of correct type and approved as per material submittals.
Prior to the commencement of any duct installation works, areas and access will be inspected by the construction team to ensure that these are safe and suitable for construction works.
Consultant/Client will be invited to attend this inspection at his discretion.
A close co-ordination with other trades will be carried out on site prior to commencement of ductwork.
A safe and protective storage area will be allocated on site, in close coordination with the main contractor/employer.
Ductwork Installation Steps
Supervisor will instruct tradesmen regarding the execution of the works and will distribute all necessary approved construction drawings of the latest revision for HVAC ducting installation.
The supervisor will also check that tools and equipment available are in compliance with the contract requirements.
Marking shall be done after which supports/brackets shall be installed using approved nut, bolts, threaded rods and other accessories.
Ensure horizontal ductwork is adequately supported at equal distances in accordance with construction drawings with all brackets fixed to the vertical face of ribbed beams of the structure.
Care will be taken during setting out, that ductwork runs between ceiling module lines so as to maintain maximum flexibility for any future relocation of lighting fixtures or other services.
Ductwork supports unless otherwise indicated will be installed to provide a minimum space of 15mm between finished covering and adjacent work, and 100mm clearance from suspended ceiling.
Supports or hangers will be placed within 300mm of any 45° – 90° bend.
Screwed drop rods will have thread adjustment in either direction available after leveling of ductwork and associated plenum box’s.
Air Duct will be supported within 300mm of mating flanges or joints.
As indicated on relevant approved construction drawings where more than one duct can be supported on one hanger, the size of the hanger will be assessed on the sum of the maximum widths.
Ductwork passing through non fire rated walls and floors will be adequately supported on either side.
All locknuts are tightened correctly.
At all open end’s check and remove all debris then cover with suitable material to prevent any ingress of debris vermin, or moisture.
Supervisor to ensure that there are no dents, distortion or buckling of ducting during installation being aware that damaged sheet metal work will be rejected.
Supports are to be fixed at correct levels checking the bracket material is compatible with the installation and the correct method used for thermal insulation.
Ducting to be installed allowing sufficient space for expansion/contraction and thermal insulation.
Continuously check horizontal and vertical alignment of ductwork and compliance with contractors drawings and specification requirements by using spirit level plumb or other suitable instrument.
In line equipment is to be independently supported from main structure and not from blockwork walls or sub-structure.
Silencers, fire dampers, balancing dampers & other plant items will be installed as per contractors drawings, and will be supported adequately and rigidly.
Supervisor to check before end of work day’s by recording in his diary (as necessary) that:
- The correct type supports installed
- Supports are adequately supporting the ductwork and associated plant equipment.
Test holes to be provided and plugged ready for commissioning and control equipment.
Ensure terminal equipment is installed as specified.
Supervisor to ensure that all completed installations look neat and tidy, and that there is adequate spacing between ductwork and in line equipment allowing access for operation and ease of maintenance, in accordance to approved contractors drawings and manufacture’s recommendations.
Ductwork Flanges Installation Requirements
Fixing of flanges to ductwork will be as per the approved details and in compliance with the specification requirements:
- Clean both flange faces to be joined.
- Add gasket jointing material to flanges to be mated.
- Using only undamaged rust free bolts nuts and washers join mating flanges together and tighten bolts.
- Clean any excess jointing material from flanges.
Site Quality Control
Ductwork supervisor in charge and mechanical QC inspector shall continuously monitor the activities (by recording the defects or snags into his House Process Inspection List) to ensure that all components indicated on the approved contractors drawings have been installed and that the installation is in accordance with the contract requirements.
Work inspection requests shall be issued for the approval of main contractor or consultant as per the approved inspection and testing plan, using approved transmittal forms etc.
After ductwork is approved duct insulation work starts and installation is done as per approved insulation method statement.
Types of Insulation for Air Conditioning HVAC Ductwork
Air conditioning ducting can be insulated using different materials.
These days polyester is a standard insulation material.
Polyester is the same fiber which is used in pillows so it is quite safe to use and handle it.
Fiberglass was also a common duct insulation type but its use is now minimized these days due to the high cost and the increased safety precautions needed to handle fiberglass products.
All in all both types of insulation are more than adequate for residential duct work.
Fire Rated Air Conditioning Ducting:
All ductwork should come with documents describing the products fire rating.
In general most insulations also act as fire retardant materials.
Warranties and Life Spans For Air Conditioning Ducting:
Most ductwork will have a warranty aging from 5-10 years.
After around 10 years ductwork starts to become quite aged.
However, as long as you are still getting adequate airflow and the ducts are clean and unbroken you can still use the system.
The biggest danger to ductwork is rodents like mice and rats who sometimes eat away the insulation and leave their waste inside the ductwork.
Ductwork can also be a major source of illness after long periods.
Bacteria can build up in ductwork over time that can make you and your family ill over long periods of time.
There are products available however that you can place in your ducting to disinfect the system.
You can also manually clean the inside of your ducting but this would be a long and tedious task.
Perhaps consider simply replacing the ductwork around every 7-8 years if your health is a major concern.