The ductwork in your heating system consists of the warm air ducts, known as supply ducts, and cold air ducts, which function as returns. The importance of both supply and return ductwork imposes exact sizing and design considerations. Manual D, the industry standard software published by the Air Conditioning Contractors of America, is used by HVAC specialists to calculate duct sizing and compose a layout providing optimum comfort and energy efficiency. Cold air returns have their own set of critical parameters and stipulations. Problems specific to return ductwork usually result when these requirements are not met.
Keeping It Neutral
In a well-balanced heating system, warm air circulates into the trunk and branch supply ducts under positive pressure produced by the furnace blower fan. The warm air enters rooms through the supply registers, usually located near the ceiling. At a lower point on another wall, or at a central register in a common area such as a hallway, cold air is drawn into the return duct under negative pressure and conveyed back to the furnace. This critical balance of positive and negative air pressure in the ducts produces neutral air pressure in rooms, the optimum condition for heating and cooling comfort.
When cold air ducts go bad, significant efficiency and performance issues ensue. Leakage from cold air returns means that the volume of air entering a room through the supply ducts exceeds the amount going back to the furnace. This air surplus creates a positive pressure condition in the room instead of neutral air pressure. Rooms under positive pressure leak heated air through cracks and gaps in the structure. This heat loss causes the furnace to run longer to achieve thermostat settings, increasing energy costs and wear and tear on furnace components.
Good Air Out, Bad Air In
In addition to heat loss, defective cold air returns may jeopardize healthy air quality by drawing unconditioned air into the system. Returns often run through attics, crawl spaces and other unconditioned zones that may be contaminated by mold spores, bacteria or other pathogens. As the negative-pressure returns suck in tainted air through small leaks, these contaminants may circulate throughout living spaces in the home. This can cause allergic reactions or respiratory symptoms in susceptible individuals.
In homes built before high fuel costs raised energy consciousness, residential ductwork was often a place to reduce building costs. In many cases, installed ducts were not made to remain functional for the life of the home. This is particularly true of cold air returns. Instead of installing proper metal ductwork, residential builders often cut expenses by simply enclosing channels in wall voids or between ceiling joists to function as substitute return ducts. The wood construction of these surrogate ducts dried out and shrank over the years, leaking large amounts of return air and allowing airborne contaminants into the system.
- Photo Credit Comstock/Comstock/Getty Images