HVAC Return Plenum Design

 

HVAC Return Plenum Design Issues

by David A. Andersen

 

Ø  To start with, let’s begin at the end; Improper HVAC Return Plenum design causes damage to the air conditioner and gas-fired furnaces.

 

Ø  Improper Plenum design causes reduced airflow which not only affects comfort and efficiency of the equipment but can significantly damage major components (some of which are a health issue).

o    Insufficient airflow through the HVAC Air Conditioning System does not deliver enough heat to use up all of the liquid refrigerant in the refrigerant circuit. The air conditioning compressor is designed to pump vapor only! Tight tolerances within the compressor do not allow for the presence of non-compressible liquid.

§  Even if the equipment is equipped with a scroll compressor or heat pump accumulator, excessive power consumption and stress on the compressor components result in excessive wear and reduced life expectancy.

§  In a reciprocal compressor; broken valves, bent crankshafts etc. result in mechanical failure.

o    Insufficient airflow to a gas-fired furnace causes excessive heat buildup in the heat exchanger resulting in a fractured heat exchanger. The heat exchanger is designed to separate combustion gases from the indoor air.

§  Dependent upon equipment design, significant amounts of combustion gases and byproducts may enter the living space of the house and become distributed throughout the house by way of the air duct system.

·         A cracked heat exchanger may also cause flame rollout from the firebox and cause interior damage to the electrical system of the furnace.

·         Insufficient airflow resulting in increased combustion chamber temperatures cause the equipment to shut down on a high temperature overload sensor. The sensor is not designed to operate on a continual basis (only for emergencies). Eventual failure of the safety device may result in a runaway condition where the unit will not shut off or will shut off and not turn back on, requiring a service call to repair the equipment.

 

Ø  What is correct Return Plenum design?

o    In spite of commonly observed procedures in the field, the return duct system design and balance is the most critical part of the air delivery system. The air in the house cannot be treated (by heating or cooling/removing moisture) unless air within the house is removed, treated by an appropriately sized HVAC system, and returned to the appropriate location based upon load requirements.

o    The equipment manufacturer designs equipment based upon certain criteria based upon airflow/static pressure. When an air duct system is attached to HVAC equipment, it operates under different pressure characteristics unpredicted by the equipment manufacturer. Through proper design by the installing HVAC contractor, airflow and pressure is controlled to not exceed equipment manufacturer’s specification.

§  Generally the negative static pressure should not exceed .03 inches of water column (or 75 Pascal). This is controlled by the size of the air duct system, its length and number of fittings incorporated in their design.

o    The Return Plenum is at the end of the return duct system (behind the filter).

§  The purpose of this Plenum is to create an equal flow and pressure across the filter (which is sized according to the cooling capacity of the air conditioning system).

·         This allows for adequate distribution of air across the filter providing even filtration.           

·         This also allows for a quieter operation as excessive airflow across the filter grill resonates significant noise when the airflow exceeds 500 ft./m.

o    A common practice of installing the return air duct directly behind the filter grill, by panning the 2 x 4 wall stud with metal and attaching the air duct, prevents equal distribution across the filter.

§  This causes the majority of the air to flow through the center (or directly in front of the air duct) only utilizing that part of the filter.

·         Airflow increases to that portion of the filter grill creating a noise situation.

·         Increased airflow restricted at a small section of the filter grill results in increased negative static pressure often exceeding manufacturer’s specifications.

·         Over time, utilizing only a small section of the filter and the proximity of the return duct to the backside of the filter causes a small section of the filter to become dirty. This sucks the filter media against the air duct further restricting airflow several hundred percent more than the simple design flaw pressures. Under these conditions equipment failure is eminent.

o    Friction loss calculations can be calculated by the HVAC contractor prior to installation.

§  Equipment specifications and the use of a Ductulator will determine appropriate sized air duct based upon its length and changes of direction.

§  These calculation should be verified with appropriate test equipment when the equipment “startup” process occurs. This is part of the “testing, adjusting and balancing” process (often never performed in a residential building).

o    The selected location of the Return Plenum does not always facilitate the appropriate end result due to building design and location of interior wall framing etc. Very often the first floor HVAC system on a multi-story house is located below the stair system. This is a very large void that is generally not used for any other purpose. There is no excuse or reason to construct an undersized return Plenum when there is sufficient space available (such as below a stair system or in the attic).

o    Other related issues:

§  Weatherization losses; occur to some extent in all air delivery systems. The amount of air leakage is based upon pressure differentials between the interior of the duct and the ambient atmospheric pressure. The greater the pressure (from restriction of its design) the greater duct leakage will occur.

§  Radon; radon gas enters the house from below. Many return duct systems are located in the crawlspace or basement below the house. Excessive pressure leakage in the return system pulls in greater amounts of radon gas which is distributed throughout the house through the air delivery system.

§  Allergies and dirty duct; natural contaminants enter the duct systems along with the entrained air. The greater the air leakage, the greater the concentration of these allergens within the system.

§  Inability to use high efficient air filters; 1 inch pleated filters are highly efficient but increase negative static pressure in the air duct system. If the return Plenum is not appropriately designed, these devices cannot be utilized without significant negative effect.

§  Psychometrics; there are two types of heat which the HVAC cooling system controls, latent heat (or moisture) and sensible heat (air temperature). Improper air duct design and pressure control results in a change in the sensible heat ratio away from the equipment design. This is commonly related with failure to control moisture conditions within the house.

·         Elevated moisture affects comfort levels by reducing the evaporation effect from our skin.

·         Elevated moisture results in an increased relative humidity within the building. This prevents things from drying out after activities such as cooking, bathing and simply the presence of the occupants residing within the building who perspire and transpire moisture into the space in significant amounts.

·         Elevated moisture produces the environment conducive to fungal activity associated with mold colonization and allergies.

·         Elevated moisture, or relative humidity within the building is more susceptible to condensation on cool objects within the house (cold water piping, toilets, refrigerators etc.) which changes moisture in the vapor state to a liquid state often resulting in building material degradation.

 

Ø  So who is responsible?

o    New construction: this is when this issue should have been addressed. The US Department of Energy, Manual D; ICC, Energy Star,  HERS, ASHRAE, the Equipment Manufacturers  (and numerous others) specifically address this situation.

§  Reality is that municipal code inspectors do not have the time or equipment to monitor system design performance.

§  Second reality is that HVAC installation is done by “accepted standards”! “This is the way we do it all the time!” But the real reality is that no matter how many times they do it wrong, it will never make it right.

 

o    Existing construction: unfortunately when these things get by they may sometimes fall upon the existing homeowner in a real estate sale.

§  Reality is, equipment damage takes time and goes unrecognized.

·         The conditions may not be identified in any home inspection or service inspection by an HVAC contractor.

·         Very often equipment failure occurs in new equipment which is installed without identifying or correcting what caused the equipment to fail in the first place.

§  Reality is, we find air duct restrictions and subsequently find equipment damage. We are responding to a homeowner complaint of comfort, indoor air quality etc. and find equipment failure and design flaws.

 

Ø  I am responsible!

o    Even if the equipment appears to be operating adequately, it is my responsibility to report my findings which may be associated with issues in the past, current issues or issues in the future which we cannot predict when they will occur.

o    A large majority of residential buildings are constructed in this manner, but not all of them perform at the same level of deficiency.

o    There is a difference in observing a potential situation and identifying an actual deficiency. If there is indication of previous equipment failure or actual testing measurements accompanying your report, the severity of the issue is elevated. If the construction is nonstandard but the equipment is operating within measurement parameters, you should be aware that how you operate the equipment (such as simply increasing the efficiency of the air filter) may have detrimental effects on your equipment and your indoor air quality.