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

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

A retired couple contacted John Banta because they were experiencing eye irritation and diffi­culty breathing caused by dust in their home. In spite of frequent vacuuming and dusting, an un­usually heavy deposit of dust was noted on the furnishings during the house inspection. John suspected that the furnace system was the source of contamination because the heat registers in the home were lined with a fine dust and the clients’ symptoms worsened when the furnace was on.

John was puzzled, though, by the lack of dirt on the cold air return filter and the absence of air movement. He opened the cold air return and ex­amined the inside wall to see if there were any vis­ible obstructions. To his surprise, he found no duct at all. The cold air return was a dummy and went nowhere.

Further investigation revealed that the fur­nace and duct system were located in the crawl space under the home. John inspected the crawl space, where he discovered that there was no con­nection between the cold air return port on the furnace and the rest of the house. In fact, the fur­nace was taking cold air from the crawl space and blowing the unfiltered, contaminated air directly

can influence the ultimate outcome. For this reason we have provided specifications for the contractor where relevant. Finally, a regular cleaning and maintenance program is essen­tial for optimal efficiency. This task will ulti­mately fall to the owner and may influence your choice of HVAC system.

Choice of Fuel Source

Gas and other sources of combustion fuels can pollute the airstream if you do not plan care – into the house. Consultation with a heating and air conditioning company was recommended to cor­rect this construction defect.

Discussion

HVAC duct systems should always be leak-tested to ensure that they meet specified standards. The stated industry standard for a sealed duct sys­tem is less than 3 percent leakage, which is rarely achieved. The furnace itself will account for much of the leakage since it is difficult to seal. The fur­nace should be mounted in a clean, easily acces­sible area such as a mechanical room and not in an attic or crawl space.

Leakage also occurs at unsealed joints where the metal ducts fit together. Since the return side of the furnace is sucking air back into the furnace, it will suck contaminants through leaks in the duct­work. If the unsealed ducts pass through walls or attics containing fiberglass, fiberglass particles are sucked into the ducts and blown into the house. If the unsealed ducts are in a crawl space under the home, then moldy, pesticide-laden or dusty air can be sucked into the furnace system and blown into the house.

fully. Electric heat is often considered “cleaner” heat because combustion does not occur in the home. However, environmental pollution from electricity generation plants must be ac­knowledged. Moreover, electric heating ap­pliances generate electromagnetic fields, an invisible and often overlooked source of pol­lution. Whatever your choice of fuel source, there are several strategies that can be em­ployed in the mechanical room that will make heating healthier.

Mechanical Room Design

• The mechanical room should be a dedi­cated room, insulated and isolated from the living space either in a separate build­ing or in a well-sealed room that ventilates to the outside. It should be easily accessible for regular routine maintenance.

• The equipment in the mechanical room may produce elevated levels of electro­magnetic fields and should not be located adjacent to heavily occupied living spaces.

• Ensure the supply of adequate combustion air to the mechanical room.

• We recommend that you locate a fire alarm in the mechanical room.

• If there is a water source in the mechanical room, there should also be a floor drain.

Heating and Cooling Appliances

We recommend the following guidelines for choosing, locating, and maintaining heating equipment:

• Purchase equipment designed for back – draft prevention.

• Use sealed combustion units to prevent transfer of combustion byproducts into the airstream. This is especially important where the mechanical room must be ac­cessed directly from the living space.

• If you are using a forced-air system, we strongly recommend adding a good com­bination filtration system that will filter out both particulate matter and gas.

• If possible, choose a heating system that does not run hot enough to fry dust. Hy – dronic systems and heat pumps meet this requirement.

• Institute a regular maintenance program to clean components, change filters, and purge mold or mildew growth.

Hydronic Heating

Hydronic heating, delivered through hot wa­ter, is usually a wall-mounted baseboard or radiant floor system. Baseboard systems are usually made of copper tubes and aluminum radiating fins with painted steel covers. Base­board radiators can be noisy if not maintained, and they can become traps for dust and dirt. Some baseboard units are subject to outgas – sing at first, when the factory-applied paint on them gets hot. Verify with the manufacturer if this will be a problem with the model you are considering.

Hydronic radiant floor systems are usu­ally made of plastic, rubber, or copper tubing installed within or under the floor. Hot water circulating through the tubing heats the floor mass and the heat then rises through gentle convection. Radiant systems are silent and clean. Because this form of heating heats feet, occupants are comfortable at lower operat­ing temperatures. The water running through the piping is not hot enough to fry dust. Note that hydronic radiant floor heating should not be confused with radiant electric heating, in which the heat source is heated electrical wir­ing. We do not recommend this type of heat­ing because it will distribute a magnetic field throughout the home when in operation.

At one time, radiant floor heating used copper tubing almost exclusively but the ris­ing price of copper, combined with the intro­duction of plastic and rubber tubing, made this a less common option. Metal tubing, such as copper, can conduct electromagnetic fields through the structure if it becomes charged at any point along its route and for this reason we do not recommend it. Some in-floor systems use very odorous rubber products. While this is not a problem where they are embedded in concrete, it can be a source of indoor pollution where the tubing is exposed at access points. Wirsbo Hepex, a crosslinked polyethylene tubing, or Kitec, a crosslinked polyethylene tubing with an aluminum core, are odorless products for radiant floor heating.

The advantages of a hydronic system in­clude slightly lower operating costs, even heating, quieter operation, ease of zoning, and independent room-temperature control. Disadvantages of the hydronic system include slow response time and higher installation costs compared to forced air because of the number of mechanical components.

Methods of heating, cooling, and ventilating homes have many important health ramifi­cations that will affect us long after the initial building materials have outgassed and reached a neutral state. If we lived in a pristine natu­ral environment with low humidity and mild temperatures, we would be able to condition our homes without mechanical assistance by means of solar gain, shading, and cross­ventilation. Residents throughout most of North America do not have this luxury. Cold and cloudy winters, hot and humid summers, and polluted or pollen-filled air are realities from which homes must shelter occupants.

We have come to expect a level of comfort and temperature control in our homes un­dreamed of by our not-too-distant ancestors. Along with the increased comfort level, we have unwittingly come to accept many health problems associated with heating and cooling

systems. In fact, more than any other build­ing system or component, heating and cooling methods can be a major cause of sick building syndrome. Some of the problems include:

• toxic fumes from gas, oil, or propane fuels that work their way into the building enve­lope through leaky supply lines, from in­sufficiently ventilated or improperly sealed mechanical rooms, and from open com­bustion appliances

• backdrafting of hazardous and sometimes deadly gases into the living space from flues

• infiltration of pollutants from outside the building envelope resulting from depres­surization

• fried dust resulting from hot surface tem­peratures on heating appliances

• circulation of dust through an unfiltered forced-air heating system

• contamination from mold growing in the ductwork and air conditioning equipment

• fiberglass fibers from ductwork insulation that circulate in the living space

In the following section we focus on ways of reducing the need for mechanical heating and cooling. Later in this chapter we present guidelines for healthier heating and cooling installations, language for specifications, and maintenance suggestions that will help elimi­nate some of the problems mentioned above.

Reducing Heating and Cooling Loads Through Design Strategies

The application of a few simple design and planning principles can greatly reduce the amount of mechanical heating and cooling required to live comfortably, thereby improv­ing health and lowering energy consumption. In designing your home for energy efficiency, consider the following suggestions.

Create an Energy-Efficient Building Envelope

• Choose an exterior wall system with a high insulation value.

• Choose interior wall and floor systems with high levels of thermal mass to assist in

keeping things cool in summer and retain­ing heat in winter.

• Seal cracks and joints to prevent unwanted infiltration and exfiltration.

• Choose a high insulation value for the ceil­ing. This measure will be especially cost ef­fective because most heat escapes through the roof.

Consider the Surrounding Site as an Extension of Your Climate-Control Design

• Make use of deciduous trees to shade in summer and allow solar gain in winter.

• Observe prevailing wind patterns when planning for natural ventilation.

• Consider using trees as windbreaks to lower the heating load created by cold win­ter winds.

• Situate your home as far away from pollu­tion sources as possible so that the site can provide a quality air supply for home ven­tilation.

Take Advantage of Solar Heat

• Orient the home to take advantage of solar gain.

• Plan fenestration (arrangement of doors and windows) for the desired amount of heat gain.

• Make use of overhangs and sun angle in­formation to prevent overheating in sum­mer.

• Use light colors to reflect heat and dark colors to absorb and store heat.

• Provide thermal mass for heat storage.

• Provide cross-ventilation to facilitate nat-

ural air exchange and to provide cooling in summer.

• Use thermal window-shading devices to control heat loss.

• Use specialized window coatings to en­hance solar gain where desired and block unwanted heat gain.

Become a More Active Participant in Temperature Control

• Open and close windows to provide fresh air and control temperature.

• Open and close thermal shading devices to control heat gain and loss.

• Utilize automated thermostat controls to economize on heating and cooling when you are absent or asleep.

• Be willing to add and subtract layers of
clothing to allow for a greater range of ac­ceptable temperatures.

• Consciously acclimatize your body to a broader comfort range.

Healthier Heating and Cooling

Each heating and cooling system has advan­tages and disadvantages that you must weigh carefully when choosing a system that best fits your needs and budget. Once you have made a choice, there are several design, construction, and maintenance considerations that will op­timize performance and minimize the health risks of the system. In the preliminary design phase, you and your architect must consider factors such as the location of the mechani­cal room. During the construction phase, the choice of materials and installation procedures

CASE STUDY 15.1