Sheet Metal Ductwork
Sheet metal designs for the residential market have evolved from the early systems that used a coal fired furnace with an “octopus” of round duct outlets branching off the furnace to the main level of a home. If the home had a second level there would be a grate in the floor of the upper level made of hardwood or cast iron to allow the heat from the main floor to rise through the floor. Furnaces were about as inefficient as they could possibly be back then, but coal was cheap. I can remember going to grampas’ house for Christmas and when I was old enough, being the one to stoke the furnace with coal 2 to 3 times a day. That was also when you would take a few coals from the furnace and put them in a long handled pot to warm your bed sheets before retiring. Cooking the Christmas turkey was done in a wood burning oven and the washroom facilities were found about 30 feet from the back door of the house. If you were like me, it was the best part of your life and who would have thought that stoking a coal fired furnace would lead to 36 years in a career in heating.
The old coal furnace was eventually replaced with a Kerosene space heater or an oil burning furnace and the coal chute to the house got boarded up for good. Oil furnaces had that unmistakable smell and they seemed to roar so loud that as a kid you would think you had a fire breathing dragon in the basement. Ductwork was now providing heat to every room of the house and all the ducts were rectangular in shape and formed by masters of sheet metal work. Unfortunately the ductwork on many of these old systems was insulated with asbestos that is now a costly health hazard, requiring removal by trained specialists.
The machinery for forming sheet metal is now so sophisticated that a computer tells a plasma torch what and where to cut for every fitting that can be made. One machine can start with a large roll of steel weighing several tons and cut the metal to length, fold it, form the locking edge, insulate the duct and stack it ready for shipment without being touched by human hand. This same machine can also change sizes of duct without stopping, simply by changing the computer instructions. Another machine makes the round pipe for branch ducts off the main trunk duct at such a speed that it is almost a blur. Seeing a modern shop, one has to wonder if the skills of the old masters are being lost to the computer geeks of today.
Ductwork is made from galvanized steel and coated with zinc. Ducts can also be made of aluminum and stainless steel depending upon the application. The thickness or “gauge” of the metal is determined by the size of the duct. To some degree it is sad what the minimum standard gauge is for residential ductwork. Some systems are made of metal so thin that when the fan of the furnace starts to operate the duct will “inflate” and make all kinds of noises that echo throughout a home. This is something any new homeowner may wish to address before finishing a basement and boxing in the ductwork. We are in an era driven by production, not quality and even an untrained eye can spot poorly installed duct systems. If you are a fan or detractor of Mike Holmes and his rants about the work of everyone else being sub- standard, except that done by him and his crew, relax…. Many contractors do good work and are not responsible for single handedly filling the local landfill site with useable material. Mr. Mike is correct in not cutting floor joists to install a duct, and taping of the joints in the perfect world may contain a few CFM for that farthest run of duct, but the seams and joint connections are made to fit well if installed correctly, with minimal leakage. If a joint should leak, just remember that the air leakage is still within the house. When it comes to finishing your basement it is best to contact your local HVAC contractor with your plans and provide the proper heat outlets (preferably low wall ) and return air inlets to make the space comfortable.
One thing to remember if you are a “do it yourselfer” is that venting of a clothes dryer in a residential application can be connected with the plastic or tinfoil material with what appears to be a coiled spring inside, but these materials are quite inferior for the task and can trap lint and moisture easily and restrict the flow of air. Any store selling hardware can provide these materials and they can only be used on electric clothes dryers. The only fastener that is used with this type of flexible vent is a gear clamp. Should you have a gas dryer, the venting must be in metal (aluminum) to combat corrosion. This is where the metal tape that Mr. Holmes likes to use is put to use in sealing all the joints, but more importantly, holding them together. This venting must not be screwed to connect various lengths of pipe or elbows. Metal tape is the only acceptable fastener. When suspending the pipe in the joist spaces of the basement you can use a coated strapping secured to the joists to cradle the pipe (no screws into the pipe). Be mindful to check the vent hood for your dryer several times a year to ensure the backdraft damper is not sticking or that the hood is choked with lint.
In commercial and industrial applications a wider selection of materials are available to the contractor. The fabricated metal is much heavier due to the increased thickness of the metal and the fittings can be large enough for a man to walk inside. Hollywood has always shown the criminals or the mission impossible agents crawling through ductwork in their quests for loot or the bad guys. The duct is always shined clean (maybe the day it was installed), never bongs or bangs in their travels and is surprisingly well supported to carry the weight of a small army throughout a building. I do have to laugh when seeing this, knowing that a good tug or an earthquake over a magnitude of 5 would probably cause most ducts to crash to the floor. Flexible ductwork made of aluminum that is self-supporting is permitted to connect a ceiling diffuser to the main trunk duct. There is a wide range of dampers for adjusting the airflow available and many styles of diffusers and registers to select from, all of which may be painted to blend in with the décor of the building. Sizing of ductwork is not random. The volume of air, the pressure of the system and the resistance of flow through certain fittings is all considered along with the length of the ducts. Commercial / industrial ducted systems may be insulated with many types of insulation that fall into two categories (Thermal insulation or Acoustic insulation). Thermal insulation is used to prevent heat transfer from the air in or outside of the duct and prevents condensation from occurring when the air inside the duct is cold and the air outside the duct is warmer and high in humidity. This type of insulation will also have a vapor barrier, typically made of foil. It is important to seal every tear or seam when installing this form of insulation as air leakage will result in condensation during cooling cycles. Acoustic insulation is fastened on the inside of the ductwork and dampens noise transmission throughout the ductwork. It can also provide a thermal break. The thickness of either insulation is dependent upon the ambient air conditions. There are situations where ductwork has to be insulated outdoors, for some roof top units.There are a few different material coatings that can be applied to the ductwork to make it weather tight. The sealant product can be a coating of mastic like tar that is trowelled onto the insulation that will require touchups, possibly annually or a heavy vinyl type coating that is applied much like wallpaper and lasts for ages. Proper design of fittings is always a consideration to ensure good air flow. When a 90 degree turn is made without standard radius allowance there are internal turning blades or vanes installed to channel the air through the restriction of the fitting. Vibration may be of concern with large commercial equipment so there are duct connectors made with a flexible canvas or neoprene material to stop the transmission of the vibration throughout the system. Some engineers will specify a sealer on the connections of ductwork, particularly on systems using a higher duct pressure and there are various materials to seal the joints from foil tape, brush on latex sealer or oil based sealers. In commercial applications where the ductwork is exposed in open ceiling concepts there may be a specification for the ductwork to be painted. For this application the metal used would have to be cleaned with a solution of vinegar and water to remove the oily film on the galvanized metal or the metal could be ordered “paint ready” by specifying “satin coat finish”. There are factory applications where large volumes of outdoor air is supplied by Make Up Air Units and distributed throughout a factory with a long plastic or canvas tube. This duct inflates like a balloon when the fan is operating and deflates when the fan is off. The tube is perforated with holes of specific size and spacing to match the air flow delivery of the Make Up Unit. This type of duct hangs by a cable stretched across the ceiling in much the same fashion as a curtain rod and is much less costly to install than rigid pipe.
Sheet metal is used for everything from the ductwork in your home or office to exhaust systems, chimneys, weather flashings, eaves trough and even the fuselage of the airplanes we fly the world in.