In forced air heating and cooling systems (such as furnaces, heat pumps, or central air conditioners), ductwork is responsible for distributing conditioned (i.e. hot or cold) air throughout a home or building. Well-installed and maintained ductwork enables your HVAC system to maintain consistent temperature and humidity control. Ducts are made from thin sheet metal, fiberglass, or other materials that conduct heat well. Not only does ductwork distribute hot or cold air, but it also returns air back to the equipment so it can be heated or cooled again.
In boilers and radiant floor heating, where water (or steam) is heated and distributed, piping and radiators are used instead of ductwork.
Your ductwork can accumulate dust over time, and this dust, in turn, can harbor mold or dust mites. Water can also condense inside poorly maintained ducts and can provide ideal growing conditions for mold. Some people have suggested that this can cause health concerns, especially for people with allergies to dust, dust mites, or mold.
However, you may want to approach these arguments with a degree of caution. The US Environmental Protection Agency (EPA) says: "Knowledge about air duct cleaning is in its early stages, so a blanket recommendation cannot be offered as to whether you should have your air ducts in your home cleaned." The EPA further estimates that duct-cleaning services typically "range in cost from $450 to $1,000 per heating and cooling system".
A light coating of dust in the ductwork is not generally considered a problem, and duct cleaning is rarely recommended as part of the yearly maintenance of your heating or cooling system. Before paying someone to clean your ductwork, ask your provider to show you lab results that prove that microbial growth is occurring in your ductwork.
Badly Installed or Insulated Ductwork
Badly insulated and sealed ductwork allows hot (or cold) air to escape from your heating system. If the conditioned air escapes into a non-insulated space in your home, such as a basement, crawlspace, or attic, your entire heating system becomes less efficient. Some estimates suggest that poorly installed or maintained ductwork can increase your heating and cooling bills by up to 30%. That, of course, is just the direct effect. There are indirect effects as well:
- It can make your home less comfortable, as conditioned air escapes before it gets to the rooms it was designed to reach.
- If you have an air filtration system integrated into your furnace or central A/C, it can reduce that system's ability to remove allergens from the air.
- Your furnace or air conditioner will need to cycle on and off more often, increasing wear and tear and decreasing efficiency.
You can inspect your ductwork yourself (although you may not find some of the more subtle problems. Begin by visually inspecting all of the ductwork. Many people have been surprised to find that entire runs of ducting have been disconnected in their house -- perhaps by someone bumping them accidentally! Look for damage to the ducts, such as rips in flexible ducts, dents or disconnected joints. Make sure that insulation is wrapped tightly and uniformly around the ducts.
Calculating Appropriate Duct Sizes
Since different rooms in a house need different volumes of air to keep the temperature even, it is important to match duct size with room size. If not, the house develops cold and hot spots. If the duct running from the attic to the rooms is too long, then this length can cause temperature loss -- hot air turning cooler and vice versa.
Accurate duct sizing in new construction (or in a retrofit) is done with the Air Conditioning Contractors of America "Manual D" calculation. The calculation varies based on whether your home needs a constant air volume design or a variable air volume design, different duct materials, the number (and function) of different rooms in your house, and many other factors. This calculation needs to be performed by a licensed HVAC contractor that is familiar with the procedure.
Sealing Leaks in your Ductwork
You can seal joint or leaks yourself (or you can hire a contractor to do it for you). For narrow joints (less than 1/4 inch) you can paint HVAC mastic over the opening. Mastic is a putty-like material that is designed specifically for this purpose. (Although there are solvent-based and two-component mastics, water-based mastic is generally easier to apply and every bit as effective.) For wider joints, you will first need to apply fiberglass tape, and then apply the mastic on top. Despite its name, don't use standard duct tape to seal your ducts -- it won't last very long, and you should be suspicious of a contractor that uses it.
No matter what the efficiency ratings of the heating and cooling systems are, they depend on a properly designed, installed, and insulated ductwork system to deliver heated and cooled air.