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Traditional Forced Air System
Energy Efficiency, Comfort, and Health
Modern high efficiency furnaces use much less energy than older models. When properly sized to match the heating load and installed in conjunction with a tight building envelope and a well sealed duct system, total system efficiency and total home energy usage decreases since energy is not needlessly wasted. This means a more comfortable home environment while using less energy. Less energy means less CO 2 emissions. In addition, a well sealed home should have a heat or energy recovery ventilator integrated in the HVAC system, meaning that allergens can be filtered out of the home while the air entering the home will be fresher and cleaner.
Function and Optimal Operation
Often referred to as HVAC, a traditional forced air system provides centrally controlled heating, cooling, and ventilation to a home. This is the most common type of heating system found in the United States. Unfortunately, since it uses the generally leaky and poorly insulated duct system present in most homes, it can be difficult to reach optimal efficiency levels even when replacing an old inefficient furnace with a new high efficiency furnace. Indeed, duct systems, furnaces, and air conditioners are most often incorrectly sized and poorly installed leading to an under or over loaded forced air system which wastes energy, and can reduce indoor air quality. Installation, including system commissioning is very critical to a performance system.
Thus, while replacing an inefficient furnace or air conditioner with a newer, better rated model is often the first step a homeowner may consider when seeking to lower energy costs, this step should not be undertaken until the building envelope and ducts have been correctly sized, sealed, and insulated. Even at that point the correct household heating load and duct size need to be calculated and engineered using building science principles in order to achieve real efficiency gains. As with any temperature regulation system, a programmable thermostat or more sophisticated control device can be used in conjunction with a zoned duct system to assure that a traditional forced air system is as efficient and as effective as possible.
Components
Furnace | Recovery Ventilator | Thermostats and Controls | Duct System | Air Cleaner
These systems can be bought as complete systems or pieced together by selecting each component separately in order to maximize performance and efficiency. Each component plays an important role in the functioning of the system and can have a significant impact on system efficiency, depending on how they are integrated in design, installation, and commissioning steps.
Furnace
This is the most integral part of the system; centralized heat is distributed throughout the home through a duct system by a powerful fan and motor. The fan and motor component of the furnace is called the air handler. The air handler propels conditioned air through the duct system and back to the furnace through the furnace filter. If the home’s thermostat permits the fan to be turned on independently of the gas combustion, the air handler can be a component of a whole house ventilation and air cleaning system.
Most commonly the heat source is provided by gas combustion. Heat for centralized forced air may also be provided by:
- Electric resistance furnace
- Oil furnaces
- Hydronic air handler (heat from hot water)
- Air source heat pump
- Geothermal heat pump
Modern gas furnaces are much more efficient than older ones due to technology which extracts more heat from the same amount of fuel. [+] MORE.
Recovery Ventilator
When a home is thoroughly sealed to prevent losses of conditioned air and infiltration of pollutants, it becomes doubly important to ventilate the home with fresh outside air and to expel any occurring moisture or stale air. A recovery ventilator permits the exchange of fresh outdoor air for stale interior air in a heating (or cooling) system in the most efficient manner by first bringing the outside air to the temperature of indoor air. If indoor humidity is similar to exterior humidity, a heat recovery ventilator (HRV) is used; if humidity is a concern, an energy recovery ventilator (ERV) should be used. Recovery ventilators are very efficient because they use thermodynamics instead of mechanics to recover most heat (or cool) from outgoing stale interior air and transfer it to incoming fresh air. Typical HRV’s can recover 60-80% of heat via heat exchange; some ERV’s are up to 95% efficient. This technology can also be part of a whole house ventilation and air cleaning system.
Thermostats and Controls
This is the main point of interaction an occupant has with the systems which regulate the temperature and air quality of the living spaces. Whether the heat is delivered through a forced air or radiant heating system, thermostats and more sophisticated controls can allow the occupant to interact with the air regulation systems to tailor and automate temperature as well as humidity and ventilation. Options for increased comfort and energy efficiency include zoned heating and cooling. However, the energy efficiency and comfort levels attainable with a programmable or zoned thermostat are limited by the efficiency of the forced air or radiant heating system it controls, as well as the building envelope of the home in which it operates. [+] MORE.
Duct System
The duct system is a network of components which work together to supply warm or cool air to the living space and to return stale air to the furnace or hydronic air handler to be filtered. Due diligence is often not followed when installing a duct system and installation tends to be very error prone and leaky. Building science concepts must be applied with care to the design of repair of an effective long-lasting system. Such design and repair is among the most cost effective of all home performance retrofits because many homes have very leaky ducts: the average California home suffers from 30% duct leakage. In addition, a well sealed and functioning duct system can address issues of poor comfort (due to loss and poor distribution of conditioned air) and poor indoor air quality (due to infiltration of dirty crawlspace or attic air and particles) that are common with leaky ducts. [+] MORE.
Air Cleaner
Air cleaning is an integral part of maintaining healthy indoor air quality. The most efficient forced air systems can include an integrated air cleaner which can run continuously, using either filtration or electric charge to remove particles from the air. [+] MORE.