Most heating, ventilating and air conditioning (HVAC) contractors are unaware of the systems approach used in building science and green building. For this reason, they often oversize equipment because they fail to recognize the inherent efficiencies achieved when a whole-house design approach is employed. Even if they are aware of good design principles, for the reasons previously stated regarding inefficiencies in home design, they can’t employ them on the job so they just oversize the equipment and hope that extra capacity can overcome installation issues. The result is systems that are more expensive to install and less efficient to operate.
All HVAC systems should be sized and designed using the Air Conditioning Contractors of America (ACCA) Manual J 8th Edition or later software, the ducts should be sized to ACCA Manual D and the equipment selected to meet ACCA Manual S. Properly sizing your system is the key to purchasing the right system to begin with and to efficient, comfortable operations. This is not only wise and efficient, it is now a requirement of our national building codes.
As mentioned earlier, the air handler (what most people call the furnace; it houses the fan for both of the systems) should be located in the center of the conditioned living area that it will service. The shortest duct and pipe runs are found here. Conditioned air losses due to duct leaks over long runs contribute greatly to inefficiency, regardless of how efficient the equipment you install is rated. So, think about where to centrally locate the air handler so that each run of ductwork is as short and well sealed as possible. National building codes now require all duct systems to be tested and verified as airtight. Be sure your ducts are tested and that they pass code requirements. An average of 30% of your pricey conditioned air still escapes via ducts in many parts of the nation. That is very costly, as you can imagine, and leads to big comfort complaints, too!
It is also very important to think about and plan routes through the framing members (floor trusses and vertical chases) so that the ducts can be installed with as few twists, turns and compressions as possible to get air to each room or area of the home. If the duct design is done before construction begins, as it should be, the contractor can provide this layout to the truss company. The truss maker can then easily create large square openings in the trusses with the computer software used today to design and build engineered trusses. Often this can be done without an added charge.
If you plan to insulate at the attic floor, consider having the roof trusses designed like floor trusses at the base of each truss, so that the ductwork can run through the lower part of the truss web, inside the area that will be insulated. Or it might be necessary to drop the ceiling in areas of the home or frame a furrdown (enclose a boxed chase along the perimeter of the ceiling) to keep the ducts inside the thermal envelope. Chases, the chambers that house ducts running vertically between floors, should be kept within the thermal envelope by installing plywood, OSB or drywall and sealing and caulking them at the attic floor level (for a vented attic insulated at the attic floor). If these are located on an exterior wall, this means insulating and installing an air barrier on the inside of the outside wall within the chase, since that wall will not be covered by drywall. For horizontal duct runs, try to avoid running ducts all the way out to the exterior walls of the home as was once common practice. It’s best to terminate duct runs at an inside wall of each room or in the ceiling near an inside wall. This is a more efficient duct design model that has been created to take advantage of the better windows and insulated walls in our homes today. The US Department of Energy has termed this “compact duct design,” as opposed to the old way of running ducts out to near the exterior wall and then trying to throw the air back toward the center of the room or to wash the wall with conditioned air. The shorter duct run will reduce the initial cost of the system, and the reduced distance that the air has to travel will improve system performance and overall comfort.
Finally, make certain that your supply and return registers are working to support good airflow. Return air grilles should be sized according to the amount of total cubic feet per minute (CFM) of air being delivered to the space. Filter return grilles are (by ACCA Manual D criteria) sized at 200 CFM per gross square foot of grille, and open return air grilles are sized at 300 CFM per gross square foot of grille. Return grilles are very often undersized, leading to high bills, noisy registers and poor comfort. Supply grilles should be the curved-blade type. These will do a good job of distributing the air long distances to cause circulation, mixing it with the stale air before reaching the return air grille. Inexpensive stamped metal supply grills do a poor job of delivering the comfort that your system was designed and installed for. This is not the place to save money.
The term “pressure relief” refers to the ability to equalize pressure between rooms with closed doors, where air is delivered through supply vents for the HVAC system but has no way to get back to a return air grille to be circulated. If doors are closed and some significant portion of the delivered air cannot find its way back to the return, those rooms will be positively pressured, resulting in other rooms in the home experiencing simultaneous negative pressure because the air handler is trying to find the missing air that it sent out through the supply ducts. This negative pressure causes the system to find other sources of makeup air to meet its capacity. It’s possible to draw unwanted contaminated air from the path of least resistance, including combustion appliance vent pipes, fireplace chimneys or adjacent unconditioned spaces (attics and crawl-spaces). This introduces contaminants affecting indoor air quality.
This means that you should provide a means for the air supplied to a room to get back to a return air grille. This can be accomplished by placing jumper ducts (ducts that “jump” overhead from the enclosed room to an adjacent open space, like a hallway), transfer grilles (same concept as jumper ducts, only the grilles are through the wall) or dedicated returns (each room having its own return air back to the air handler of the HVAC system). All of these strategies add up to big savings on efficient operations over the life of your home as well as improved comfort.
