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Ductwork Diagnostics

There are three basic components to a standard heating and air conditioning system:

  1. The outdoor unit, or compressor — either a heat pump or an air conditioner (it’s really hard to tell the difference just by looking at them so don’t feel bad if you can’t tell.)
  2. The indoor unit — either an air handler in the case of a heat pump or a gas furnace and coil (it’s also difficult for a person off the street to tell the difference between these two. A furnace will have a vent pipe of some sort, usually either metal or pvc.)
  3. Ductwork

This is the story of ductwork.

Picture of a trunk line with one run. The trunk is metal pipe wrapped with insulation and the run is flexible duct, also covered with insulation.

Picture of a trunk line with one run. The trunk is metal pipe wrapped with insulation and the run is flexible duct, also covered with insulation.

Ductwork is the all-encompassing term for the system of runs that carry the conditioned air (either heated or cooled) from the air handler or furnace to specific rooms in your home. For most people, it’s out-of-sight, out-of-mind since it’s usually located in the attic or the crawlspace, neither of which are popular hang-out joints. But sometimes the reason you may feel more airflow in one room than another, have no airflow in one room, or even smell something foul isn’t always because there’s something wrong with your equipment. There could be something wrong with your ductwork.

What It’s Made From

Ductwork construction materials are defined by the NC Building Code. Most systems are metal, ductboard, and/or flexible duct (or flex).

Metal – It’s almost always galvanized steel. It is usually round pipe or square or rectangular-shaped made from a sheet of metal. It is used to build the supply and return boxes (plenums) that connect to the indoor unit, and the trunks, or main lines, that come off the plenums. If it’s located in an unconditioned area like a crawlspace, it should be wrapped with insulation. Sometimes it’s lined with insulation on the inside. If you knock on it gently, you should be able to tell.

onCall24There are great advantages to metal ductwork that is wrapped on the outside including:

  • Durability.
  • It can be cleaned.
  • It can be dried out and reused in the event that it gets wet or flooded.
  • It’s smooth on the inside so there are very few places where contaminants like dirt, bacteria and mold spores can take up residence.
  • The fact that it’s smooth also means that air flows better because there’s little resistance
  • It has a long lifetime.

The disadvantage is that it is a little more expensive. It takes special skills to build metal ductwork and can take a little longer to install properly which contribute to the higher cost. But because of its many benefits, we here at Canella Heating and Air Conditioning use metal almost exclusively for our plenums and trunks.

Ductboard – This product is made from tightly compressed glass fibers (fiberglass) that are bound together with a sealing agent. Like metal, it is used to build trunks and return boxes. It doesn’t require insulation over it because it is insulation.

The advantage to ductboard is that it’s less expensive than metal. The disadvantages are:

  • It’s difficult to dry thoroughly in the event that it gets wet or floods such as in the case of a plumbing leak.
  • It is difficult to thoroughly clean though there are companies that say they can do it. Check out www.epa.gov for more information about cleaning ductboard ducts.
  • The potential for the sealing agent to loosen over time or not completely hold the fibers means loose fiberglass fibers can blow through your system.
  • It is rough on the inside with small pockets that can provide a good home for dust, bacteria, mold spores and other contaminants.
  • The rougher surface inside creates some resistance to air flow especially over time.

Flex – Flexible duct is round and consists of a plastic liner with a layer of insulation surrounding it. Its name suggests its most important feature: it’s flexible and can bend around things like floor joists and plumbing pipes. It is used to carry the air from the main trunk to a register in a specific room. It comes in different sizes depending on how much airflow is needed for a particular room and the length of the line, among other things. Because of how it’s constructed, it’s rougher on the inside than metal and does offer some resistance to air flow. The good news is that it’s cheaper than ductboard or metal and can easily be replaced when necessary.

Now that the construction materials have been explained, it will be easier to understand what can go wrong.

Damage or Poor Connections

Flexible duct located in a crawlspace fell off the trunk line. In addition to the conditioned air that’s pouring into the crawlspace (translation: $$$), the hole allows crawlspace air, dirt, animals, etc. access to the inside of the house.

Flexible duct located in a crawlspace fell off the trunk line. In addition to the conditioned air that’s pouring into the crawlspace (translation: $$$), the hole allows crawlspace air, dirt, animals, etc. access to the inside of the house.

Over time things happen to ductwork. Humidity, bumping into it with things you store in the basement or attic, animals, or poor installation practices are all things that can damage ductwork and cause leaks or a complete disconnection of the duct from the system. Now the air you’re paying to heat or cool isn’t making it to the room you think it’s going to and in fact is being wasted by dumping into your crawlspace or attic.

Duct that has been crushed located in an attic. The correct amount of warm or cold air will never make it to the intended room.

Duct that has been crushed located in an attic. The correct amount of warm or cold air will never make it to the intended room.

Another common sight is ductwork that has been crushed or pinched off. This reduces the airflow, tends to make the airflow noisier, and can cause static pressure issues that bring a whole host of their own problems. You’ll feel it in the form of temperature differences between rooms.

Mold

Microbial growth inside duct constructed of ductboard.

Microbial growth inside duct constructed of ductboard.

Technically its “microbial growth” until it’s actually tested by a lab and confirmed to be mold. Whatever you want to call it, you know what it looks like. Under the right conditions it can grow inside ductwork or even the equipment itself. When you turn on the heat or the air conditioning, the air passes right over the mold and carries it through the rest of the system into your house. This can contribute to health issues for those with allergies, asthma or other respiratory problems.

Missing or Torn Insulation

Insulation around ductwork does two things: it helps keep the air inside the duct either hot or cold until it reaches the room it’s supposed to go to, and it keeps ductwork from sweating. Sweating ductwork, which happens when it’s humid outside and the air inside the duct is cold or vice versa, doesn’t sound too serious but it can make a mess. If the ductwork is in the attic and enough condensation forms, it can start to leak through your ceiling. And in your crawlspace or basement, moisture is one of the 3 things mold needs to start growing (besides a food source and heat).

Over- or Under-Sized

Size does matter. If your bedroom is 15 feet away from the air handler and you have a 4-inch duct that runs to that room, it will carry much less air than a 10-inch duct can carry. That will directly affect how much hot or cold air makes it to your bedroom. This is why it’s just as important for an HVAC contractor to correctly size the ducts as it is to correctly size the equipment. Too small and you won’t get enough hot or cold air. Too large and too much air will get to your room again making it either too hot or too cold and short-changing another room in your house.

This only begins to explain the science behind ductwork and what can go wrong. Looked pretty simple before you read this, didn’t it? Understanding what’s going on with your ductwork, whether it’s installed correctly, and whether it’s working like it should is a job only a qualified HVAC contractor can do.

We have the tools to show you what’s going on including cameras that can take pictures inside the ductwork. And if it’s bad, we have the expertise to fix it.