HUH? This has to be nonsense, right? My ductwork is all metal —how does that go bad?

Technically, this ‘nonsense’ is just an oversimplification. More than a few HVAC websites make this claim but don’t explain how ductwork can deteriorate over time. So, there is a grain of truth here. 

First, let’s be honest. When it comes to homeowners and their home’s ductwork, they just don’t pay a lot of attention to each other. Unless you make a point of annually inspecting your home’s HVAC system, including the ductwork, you’re apt to assume that it’s just doing its thing and there’s nothing wrong with it. After all, it just lies there and nothing happens to it, right? 

Wrong. Your ductwork does move, and a lot can happen to it. Not that it books an extended vay-cay to Cannes once a year, but your ductwork does move around from thermal stress and vibration. To put it another way, winter heating causes the ductwork to expand while summertime cooling causes it to contract. This thermal heaving can pull ductwork and seams apart, causing leaks.

Sometimes, poorly joined ductwork that's not attached with screws or sealed with mastic or aluminum duct tape will pull itself completely apart. And similar disconnections can happen with the return ducts. So, if you’ve got a return duct running through a crawl space that pulls apart, then all that lovely, dank crawlspace air will circulate into your living room and bedroom.

Apart from leaky duct joints, uninsulated metal ductwork can rust due to condensation forming on the metal during the summer air conditioning season. Rust can eat holes into the metal and cause leaks or break down connections. Sections that may have been installed without adequate support may sag and buckle and block airflow. 

Of course, a homeowner can avoid most of these problems by air sealing and insulating older ductwork and then periodically inspecting, locating and plugging up any leaks that might develop. 

However, it’s not always that simple. 

Twists and Turns — Energy Efficiency 

As any HVAC professional will tell you, the shorter and straighter the ductwork runs, the better the airflow. That’s because as air moves through ductwork, it can slow down due to friction from running into the inside walls of the duct. If a flex duct is used, it should be stretched as much as possible during the installation to reduce the number of spiral ridges for air to run into. Another problem is that it curves and bends. Spiral ridges cause turbulence in the airflow, which also reduces the airflow’s velocity. In some older homes, the ductwork comes out from the HVAC air handler resembling octopus arms, with each branch bending and twisting around each other. 

So, too many twists, turns and even not following installation best practices can reduce your ductwork’s air movement efficiency. Rooms may not get cool enough in summer or fail to get warm in winter. In that case, the ductwork could need replacing. 

Lastly, you might need to replace old ductwork if you are replacing your 30-year-old HVAC system with a new high-efficiency unit. First, you’ll need to insist that your HVAC contractor performs a series of Air Conditioning Contractors of America (ACCA) calculations called Manual J and D. Manual J calculation helps determine the amount of heat loss or heat gain for each room and the amount of conditioned air needed. Manual D calculations help determine the layout of the ductwork that best balances the air flow requirements with the forces working against it. For example, old ductwork might be too small to work efficiently with new HVAC systems due to the higher airflow. These calculations are performed using software programs put out by a variety of companies. Most building codes now require Manual J and Manual D calculations. 

The idea behind these calculations is to prevent installing a new system that is too big for your home or forcing you to buy more equipment than you need. In an age when such calculations can be done easily, quickly, and accurately on a smartphone, why hire a contractor who just uses a rule of thumb? Plus, there are new ductwork best practices to consider: 

• Any room that has a supply vent should have a return vent. 
• Ductwork should be installed for air returns, not building cavities or joist bays. These setups can leak and pull in air from unclean sources such as garages or crawl spaces. 
• Ductwork needs to be in the conditioned space. This way, there will be less temperature loss. Similarly, they should run through interior walls. 
• Putting them on the exterior walls sacrifices space that should be insulated instead. 

Putting it broadly, perhaps the best way to explain aging ductwork is that your home’s ducts were fine for your home HVAC system when it was new. But after 30 years, what it was designed to do and how that system was designed to do it may no longer work with today’s highly efficient HVAC technology.