There are several reasons you may need new duct work. The first is it is an extremely dirty and or mold-covered duct which can be a major health hazard. Second is your new high-efficiency system either will not operate, or it will suffer early failures and have increased energy usage. Let’s look at both of these.
If you have allergies or weird smells you may need a professional to come out and perform an inspection. These may come from the unit itself, the ductwork, or a combination of both. Your unit, if not properly maintained can build up dust, dirt, and dander, those when combined with a moist coil can create mold and other environments that cause odors. Sometimes these are referred to as dirty sock syndrome. These are the cause of most indoor air quality issues. If left long enough these contaminants can spread to the ductwork and multiply the effects to create air quality issues. An inspection will determine your level of contamination either by visual inspection or laboratory testing. Based on this inspection a recommendation will be made to either clean, replace sections, and or complete system and duct replacement. The last option usually requires a remediation company to properly remove the contamination before replacement begins. Without proper removal, the original contamination is likely to return. Recommendations should also be made to prevent the problems from returning.
The second reason you may need new ductwork is your new HVAC system will not function correctly without it. The federal government has implemented new energy standards for new ac systems. To meet these standards manufacturers have been forced to make their units very efficient. With these requirements, the systems use less brute force and more finesse. Your old system just forced the air into the duct and probably cooled your house. This method probably worked reasonably well but may have also contributed to issues mentioned above dealing with air quality issues. Other issues improper duct may cause are:
Reduced airflow is proven to reduce equipment life, problems with comfort, efficiency, and maintenance issues. Early equipment failure is also well documented. Compressors have been known to fail early from deceased airflow. Low airflow reduces air temperatures which doesn’t allow complete evaporation of the refrigerant in the evaporator coil. This liquid refrigerant flows back to the compressor and washes the oil from the components increasing wear.
Compressors are supposed to pump vapor not liquid. If you have a gas furnace low airflow causes the furnace to operate at increased temps which cause excessive expansion and contraction which leads to early failure. A failed heat exchanger can also cause death due to carbon monoxide poisoning. The poorly constructed duct that prevents the air required to operate cause excess stress on the new high-efficiency motors. It used to be thought that the new ECM motors would fix airflow issues. Time has proven that this is incorrect. The added stress on the motor causes the motors to run hotter than normal. Eventually, they fail. If you have had several motors fail in your system, it is time to look at the ductwork.
To summarize, what worked in the past won’t fly in the future. You should have a complete inspection performed on your system if you are experiencing any of these problems. Do not allow the company you chose to perform repairs without identifying the cause of the problem. The updated mandatory SEER2 (energy standard for equipment) and M1 testing is required by law now. Make sure you have a complete and proper evaluation before your next repair or replacement. Know where you stand.