Duct leakage is more than just warm or cool air lost through the supply ductwork, never reaching the rooms. The problem can be far more serious, and may also involve air and pollutants that infiltrate the return ducts, or warm air escaping to the outside due to the pressurization of the house, or the reverse...
Leaks in the supply ducts
But how does all this happen? How do simple and relatively small leaks in the ducts, often hidden in inconspicuous places, be so powerful in their effects?
That’s something that comes from the inner dynamic of the forced air systems and their two typical air flows: the air that comes out of the supply ducts (propelled by the fan of the HVAC equipment) and the air that moves from the living space into the return ducts. Air leaks in the return ducts can cause unwanted gases and pollutants to enter the living space, namely if the ducts are installed in garages or basements with radon or other dangerous chemicals, or with moisture and mold problems.
If the amount of air delivered by the supply ducts is higher than the air that moves into the return ducts, the living space becomes pressurized. Part of the air (hot air, delivered by the furnace; or cool air delivered by the AC system) will escape to the outside through cracks or openings in the home’s shell. That’s a simple physics law.
Leaks in the return ducts
But the opposite may also occur: if the return ducts moves more air from the rooms than the air delivered by the supply ducts, the rooms will be somewhat depressurized and outside air will have to be brought into the building. In other words: comfort will be greatly affected; and behind all this are duct leaks or other problems in the ducts.
But the effects do not end here. Besides forcing the HVAC equipment to work longer and to use more energy, and besides the energy loss and the infiltration of outside air that it induces, duct leakage is also the ultimate cause of powerful side effects like dryness and drafts, not to talk of potential hazards coming from CO and pollutants entering the rooms, due to leaks in the return ducts.
Other problems with Ducts
Duct air leakage is particularly important, but problems with the design of the ducts, its size, or the lack of insulation (whenever the ducts are installed in unconditioned space) should also be carefully considered and fixed. They can aggravate leakage.
If the ducts are wrongly sized and designed, that may also lead to imbalances and to positive and negative pressures in the home’s living space.
Grilles and furniture
Sometimes, minor issues have a significant impact in terms of the functioning of the ductwork and the HAVC equipment: dirt, furniture and other objects blocking the airflow at the registers...
Anyway, there are other more powerful factors, involving inefficient windows, or low levels of insulation and air sealing. They too have a powerful and negative impact on the efficiency of the equipment and ductwork.
One of our Videos About Ducts: