6 Ductwork Design Considerations & Their Importance
If you don’t get into your attic or crawlspace very often, it can be easy to forget the maze of ducting that gets your seasonally warm and cold air from the central system components to the rooms. There is a comfort component to ductwork design that you may notice when one or more rooms are not as comfortable as the rest of your home. Beyond comfort, you will spend more money for heating and cooling if your ductwork isn’t properly designed. There are 6 considerations that are important in ductwork design.
-1- Whole House Load Calculation
Your first consideration should be the proper sizing of your system. This requires a load calculation for your home, both for winter and summer operation. The amount of heat needed on the coldest days of winter and the amount of cooling needed in the heat of summer are calculated based on the transfer of heat into and out of your home. The load calculation determines the heat you need in winter and the cooling you need in the summer based on your home’s construction and materials.
-2- Properly Size the System for the Load
Proper sizing of the equipment is critical. You don’t want a system that’s too small for your needs, and too large is a problem as well. When properly sized, your system will run long enough to efficiently heat and cool components, including the ductwork, for comfort and efficiency.
-3- Match Ductwork to the System
One aspect of ductwork design is matching the size of ducts overall to the system size and the air it pushes. A duct system that’s too small overall will restrict the flow of air and put pressure back onto the blower. It will reduce comfort and efficiency. If the overall size of the ductwork is too large for the system airflow, the air slows down and will not adequately heat and cool areas of your home.
-4- Airflow Management
Once you know your system size and overall airflow necessary for comfort, the size of individual ducting to different areas is important. The farther a room is from the indoor blower unit providing the air, the more critical duct size becomes. The goal is to maintain pressure in the ducts, so the air gets to the end of the duct with enough speed to heat and cool the room.
In some cases, the size of a run of duct must be reduced part way along the run to increase the pressure and speed the flow of air. Another consideration is a smooth path for air, especially when flexible ducting is used. It’s easy to kink a flex run or turn it too hard around obstructions, reducing the airflow. Hard angles aren’t good for airflow.
-5- Air Return is Important as Well
Air leaving a room and going back to the system interior blower is important too. If a room is too tight, and it’s difficult for air to return to the system, the flow will be restricted, and the room’s comfort will suffer. The ideal solution is a return air duct in every room, but that’s not a common procedure due to space and cost.
More common is one return air duct, and it should be centrally placed where it helps with returning air from every room back to the central unit. Two returns can make a big difference in comfort and efficiency if they’re properly placed.
-6- Insulate, Seal, and Test Ducts for Leakage
Ducts in unconditioned spaces must be insulated to get all the warm or cool air to its destination, instead of losing it through heat transfer in those unconditioned spaces. Before insulating ductwork, be sure to test it for leakage as well. Poorly connected duct sections can condition your attic or crawlspace instead of the rooms in your home. Seal all connections to stop leakage, and then insulate the ductwork well.
Following these duct design rules will assure your comfort and reduce your costs of operation.