Forced-Air Heating

Throughout most of the country, forced air is the most common form of heating and cool­ing in new construction. Besides quick re­sponse time, the main advantage of forced – air heating lies in the opportunity it gives the homeowner to commission modifications and additions to standard equipment to cre­ate a healthy air-distribution system. A modi­fied system can control humidity, filter air, and introduce fresh, conditioned air from the out­side. Disadvantages of a forced-air system may include greater operating costs, noisy opera­tion, larger space requirements for equipment installation and ductwork housing, deple­tion of negative ions, and the need for regular maintenance and cleaning of ductwork to pre­vent mold and dirt buildup.

Disadvantages of a standard forced-air system can also include distribution of odors and particulate matter, and unwanted dehu­midification. Forced-air heating, which heats air, is considered to be far less comfortable than radiant heating, which heats objects. A forced-air system must be properly designed for appropriate balancing and distribution. Poor indoor air quality, energy inefficiency, and discomfort can result when system design is inadequate.

If forced air is your choice for heating and cooling (in much of the country this may be the only cost-effective choice), you can take advantage of the whole-house air distribution ducting that will already be in place to improve air quality by implementing the steps below:

• Use a fresh-air intake vent from the outside to the furnace to introduce and distrib­ute fresh, tempered ventilation into your home. Locate the vent so that it receives “fresh” air; do not place the vent near trash storage areas or where auto exhaust and other pollutants could be brought inside the house.

• Install enhanced filtration in your forced – air stream. (See the Air Filtration section below.)

• Choose a furnace with sealed combustion to avoid the entry of combustion byprod­ucts into the airstream.

Design of Healthier Forced-Air Ductwork

Care must be taken during the design, instal­lation, and maintenance of forced-air duct­work because the means of air distribution is often the source of allergies and other health problems associated with forced-air heating and cooling.

Ductless air plenums are a common source of air contamination associated with HVAC systems. Joisted floors, and wall cavi­ties without ductwork, act as pathways for contaminated attic or crawl space air to en­ter the building if air is forced through them. Fibers from wall and ceiling insulation are frequently sucked into the return side of the heating system and circulated throughout the building envelope. Furthermore, the plenums are inaccessible for cleaning and impossible to seal.

Floor registers should be avoided because debris will inevitably accumulate in them, not only during construction but also in the course of occupancy. For this reason, supply and return registers should ideally be located on walls or ceilings. Below-slab ductwork should be avoided because it can collect mois­ture and dirt, providing a breeding ground for microbes. Also avoid running ductwork through uninsulated spaces if at all possible. If this is unavoidable, the ductwork should be well insulated on its exterior.

Ductwork should be easily accessible for future inspection and maintenance. A good design should specify cleaning portals that will give access to all ductwork, especially points of probable condensation. Sheet metal is preferable to plastic flex ducts because the flex ducts are difficult to keep clean and are easily damaged. Ductwork may be coated with undesirable oils from the manufacturing process and should be cleaned of all oil prior to installation.

Installation of Forced-Air Ductwork Quality control during the installation of a well-designed ductwork system will help en­sure optimum efficiency and health. Ductwork should be well sealed with a nontoxic sealer. Ideally, an air distribution system should have a neutral effect on building pressurization.

A large amount of dust and debris is gen­erated during the construction process, and it frequently finds its way into the ductwork, becoming a source of air contamination once the system is in operation unless measures are

Forced-Air Heating

The Problem: Home investigation revealed that ductwork had not been sealed on the return side of this system causing contaminated air to be sucked in to the system and blown throughout the home. Recommendation: All ductwork should be thror – oughly sealed and tested for air leakage.

Photo: Restoration Consultants.

taken during construction to keep the duct­work clean.

In order to achieve an optimal ductwork system installation, we suggest the following specifications:

• Metal ductwork shall be free of all oil resi­dues prior to installation.

• Ductwork shall be well sealed with non­toxic compounds such as AFM Safecoat DynoFlex, RCD6, Uni-Flex Duct Sealer, Uni-Mastic 181 Duct Sealer, United Duct Sealer (Water Based), or approved equal. Mastics shall be water resistant and water-based, with a flame spread rating no higher than 25 and a maximum smoke developed rating of 50.

• During construction, the ends of any par­tially installed ductwork shall be sealed with plastic and duct tape to avoid the

introduction of dust and debris from con­struction.

• All forced air must be ducted. The use of unducted plenum space for the transport of supply or conditioned air is prohibited.

• Cloth duct tape shall not be used. (It has a high failure rate that can result in unde­tected leakage.)

• All joints, including premanufactured joints and longitudinal seams, shall be sealed.

• Gaps greater than Vs inch shall be rein­forced with fiber mesh.

• All ductwork running through uninsu­lated spaces shall be insulated to a mini­mum of R-io to prevent condensation problems and to save energy.

• Any ductwork requiring insulation shall have the insulation located on the outside of the ducts.

• Ductwork must be professionally cleaned prior to occupancy. The duct-cleaning

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