A forced-air central heating system is one, which uses air as its heat transfer medium. These systems rely on ductwork, vents, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs air from the central unit to the rooms, which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air-heating systems. Forced air heating is probably the type of central heating most commonly installed in North America. It is much less common in Europe, where hydronic heating predominates, especially in the form of hot-water radiators.
Schematic of a Forced-Air System
Advantages of Forced-Air Systems
Forced-air systems are extremely versatile
The network of ducts that distributes air around the house can be used for heating, air conditioning, fresh air distribution, dehumidification, and air filtration.
Forced-air heat is usually cheaper than a hydronic system. Efficient forced-air systems are well designed and carefully installed.
Air can be warmed in a variety of ways — with a hydronic coil from a geothermal heat pump or water heater, by electric resistance coils, by an air-to-air heat pump, or by a furnace that burns natural gas, propane, fuel oil, or firewood. This versatility, along with lower installation costs, makes warm-air furnaces the most common type of heating system in the U.S.
Some homeowners avoid forced-air systems, fearing they will be noisy, create uncomfortable drafts, or spread dust throughout the house. But a well designed and properly installed system should do none of those things.
Ductwork and Circulation
Ducts are the primary means of circulating air in a HVAC system. The key to an efficient system, a building, and a comfortable house, is careful duct design and installation. The two most common problems are routing ducts through an unconditioned space, such as an unheated attic or crawl space, and failing to provide a return-air pathway from each conditioned room to the furnace or air handler.
Poorly designed ductwork and circulation systems can result in pressure imbalances within the building envelope, making the house uncomfortable and contributing to moisture problems and high-energy bills.
Sealing ductwork to prevent air leaks is essential.
Duct Design and Layout
Efficient duct systems are compact. Ducts that run in unconditioned spaces significantly erode performance and waste money. Duct sizes should be the result of careful calculations, not guesswork or rules of thumb.
Some basics of good design:
Design by the book. Guides published by the Air Conditioning Contractors of America (ACCA) should be the foundation for sizing ducts. No contractor should specify a system without using the ACCA’s Manual J, which calculates heating and cooling loads, and Manual D, which tells the contractor how to size the ducts. (For more information on this topic, please refer to Saving Energy With Manual J and Manual D.)
A Florida study cited by the U.S. Department of Energy indicates an alarming number of contractors determine duct size solely by square footage of the house or other rules of thumb. In other words, they are flying by the seat of their pants. These systems will never be as efficient as ones designed by the book, and in fact can lead to oversized equipment that runs inefficiently and wastes energy.
Design early. Duct layout depends on floor and wall framing, the location of drop ceilings, the type of insulation installed in an attic or crawl space, and a variety of other design and construction details. The most efficient and most economical systems are likely to be those incorporated into overall house plans early in the game, when there’s still time for the builder, architect, and HVAC contractor to collaborate.
Keep supply ducts short and straight. Centrally located air handlers allow supply and returns to be as short as possible, minimizing energy losses and reducing the number of joints that are potential leaks. Ducts containing lots of twists and turns slow down airflow. Thus increasing energy losses and lowering the performance of the HVAC system. Tight, well-insulated building envelopes with triple-glazed windows allow registers to be located on interior walls with no loss of comfort. Locating registers on interior walls can dramatically shorten duct lengths.
Keep ducts in conditioned spaces. It makes little sense to install ductwork with R-6 insulation in an attic. During the summer, when the air conditioner is struggling to cool the house, the attic temperature will usually be significantly higher than the outdoor temperature, making the system work that much harder. Leaky ductwork makes the problem worse and contributes to a variety of air quality problems. Ducts and air handlers are best located in areas that are heated and cooled just like the rest of the house. Don't put ducts in exterior walls. That reduces the amount of insulation that can be placed in the wall cavity, and increases the risk of condensation.
Provide a return-air pathway from every conditioned room. The best forced-air systems include a return-air grille connected to return-air ductwork in every conditioned room. If the budget is too tight to allow this option, every bedroom will need a transfer grille or jumper duct connecting the bedroom with a room containing a return-air grille.
Use ducts, not building cavities. Some builders have used joist bays or other building cavities as supplies or returns. Since these areas are very difficult to seal properly, the use of panned joist bays in supply air systems is no longer allowed by building codes.
Sealing of the Duct System
Supply and return ducts are assembled on site from many individual pieces, and each connection is a potential air leak. Leaky ducts create a number of potentially serious problems in addition to wasting energy dollars. Unhealthy air from an attic or crawl space can be sucked into the system and distributed around the house. Leaks also can contribute to the growth of mold and mildew.
Leaks in a duct system do more than wasting energy.HVAC contractors argue it's impossible to seal ducts completely, but that attitude arises out of habit rather than logic. After all, plumbers have been installing leak-free pipes for decades. Duct manufacturers are beginning to address duct leakage by marketing new products with sealed seams. Examples include GreenSeam duct from Ductmate (East Monongahela, PA), Green Label duct fittings from Seal-Tite (Hillsboro, OH), and SpirAmir duct from SpirAmir (Dublin, CA). For detailed duct-sealing instructions, see “Duct Tape and Mastic.”
The best material to use for sealing ducts is mastic. Mastic is a gooey, non-hardening material with the consistency of smooth peanut butter. It is spread over duct seams with a paintbrush, trowel, or gloved fingers. Gaps in ductwork or plenums that are over 1/8 in. wide can be sealed with mastic as long as the gap is first reinforced with fiberglass mesh. Sections of galvanized duct should always be joined with sheetmetal screws before seams are sealed with mastic.
Special Duct Tapes
Although it's widely agreed that the best product for sealing duct seams is mastic, some HVAC installers find it convenient to seal some duct seams with tape. By now, everyone knows that gray hardware-store duct tape fails quickly and should never be installed on ducts. There are three categories of tapes worth considering:
Oriented polypropylene (OPP) tape is a smooth film-backed tape that resembles packing tape. Some brands have a shiny "metallized" plastic finish. Duct joints sealed with OPP should also be clamped. Three brands of OPP duct tape are Intertape Polymer Group's AC698 tape, Shurtape Technologies' DC-181 tape, and Venture Tape's 1599B tape.
Aluminum foil duct tape with acrylic adhesive performed well in test conducted by Lawrence Berkeley National Labortory (LBNL). Brands include Fasson 0800, Fasson 0810, Ideal Seal 587A/B, Shurtape AF100, Tyco Polyken 337, and Venture 1581. Foil-backed butyl tape is expensive but performs well, according to LBNL testing. A well-known brand is Hardcast Foil-Grip 1402-181BFX.
Duct Tightness Specifications
Specifications for a forced-air system should include a duct tightness specification. The specification usually requires Duct Blaster testing after the duct seams have been sealed with mastic.
One typical specification reads, "Ductwork should be sealed tightly with mastic and pass a level of duct tightness of no more than 5% of the floor area @ 25 Pa. For example, a 1,000-square-foot house can have no more than 50 cfm of leakage." Another specification reads, "Duct leakage shall not exceed 5% [or 7%, or 10%] of high-speed fan flow."
***More information can be found in Green Building Advisor***
The only way to verify that a duct system is tight is to test the duct system with a duct blower. The 2009 International Residential Code requires all residential duct systems except those located completely within a home's thermal envelope to be tested for tightness.
To learn more about Duct Leakage Testing, go to the top of the page, and click the Duct Leakage Testing tab.