Good ductwork design in your new home and office is an important home building step that will pay off for years to come. If you follow a few guiding principles, you will see savings in energy usage, increased family comfort, and longer equipment life.
While your architect and general contractor are ironing out the details over blueprints, a mechanical contractor should be part of the conversation. Good ductwork design does not come about by wedging ducts in where they might fit and hoping for adequate airflow. The mechanical contractor can offer helpful suggestions about improving energy efficiency, selecting suitable heating, ventilation and air conditioning (HVAC) equipment, and shortcuts to installation.
Does Good Ductwork Design Matter?
According to the federal Energy Star program, 20 to 40 percent of your furnace’s output may be wasted on sloppy duct design and air leaks. A small investment in good ductwork design will return huge rewards for years to come. Other benefits:
- All rooms will heat and cool to the correct temperature, with no cold zones or hot spots.
- Airflow will be efficient and gentle—no odd breezes at furnace or air conditioner startup.
- Ducts will be quiet.
- Indoor air pollutants will be minimized.
- Furnace and central air conditioner will perform optimally and last longer.
- Annual cleaning and inspection will be simplified.
There really is no magic to good ductwork design. According to Manual J of Air Conditioning Contractors of America (ACCA), the amount of heating and cooling, room by room, that must be delivered by the equipment you select. Starting with equipment selection and then hoping you have the right size is an expensive mistake.
From Manual J’s calculations (usually through a computer software program), the mechanical contractor turns to equipment sizing with Manual S. Selecting the exact model furnace capable of efficiently producing the right amount of heat (in British Thermal Units, BTUs) is half the equation. The right central air conditioner for your unique space also comes from Manual S methodology.
Once the right heating and cooling load has led to the right equipment, the only steps remaining are good ductwork design and, possibly, zoning calculations.
Design Must Direct Installation
Manual D tells the mechanical contractor how large the profile of each main duct must be to move the right amount of air through your home. It also tells about the branch ducts, off to rooms large and small. Here some contractors skirt the calculations and simply install six-inch flexible ducts to speed up the job, minimize contractor time and take a cheap shortcut.
Do not let this happen. Using Manual J, each branch duct must provide adequate airflow and return for each room. Otherwise the small guest bedroom could be blasted with heat or chilled with icy cold air, while an upstairs master bedroom always feels chilly in winter.
Expect your HVAC installer to provide the right ducts, each sealed with mastic at all connections (including at registers and vents), for every room.
If necessary, your air conditioning service or HVAC Contractor will use Manual J to help design zoning systems (multiple areas independently controlled by programmable thermostats) for your new home or addition.
During the planning stage, discuss the relative benefits and costs for various air conditioning and furnace accessories that can be built into your new HVAC system:
- Thick-media air filters
- Electronic air cleaners
- UV germicidal lights inside ducts and the air conditioner
- Whole-house humidifiers
- Electrostatic air filters
If someone in your home or office suffers from a compromised respiratory system or seasonal allergies, these may not seem like accessories; they may be necessities.
Good ductwork design will save you many times the cost of the design work over the life of your new home or addition. Efficient airflow prevents your furnace from short-cycling or blowing too hard. Similarly, a central air conditioner that recirculates air efficiently does not work as hard as one that is fighting invasive heat through duct leaks or long runs through unoccupied attic space.