What to know
Where duct systems leak
Almost never along the middle of a run. Leaks happen at connections — the joint between two sections, the collar where a branch leaves the trunk, the boot beneath a register, and the point where flex duct meets sheet metal.
Those were sealed when the house was built, usually with tape. Tape dries out and lets go, and fittings work loose over years of the system starting and stopping.
The signs inside the house
- A room that used to keep up with the rest of the house and no longer does.
- Weak airflow at registers far from the furnace, with strong flow near it.
- The system running longer than it used to for the same result.
- Dust settling quickly around a register shortly after you cleaned it, which can mean unfiltered air is being drawn in on the return side.
- Rooms that are noticeably harder to keep comfortable in both summer and winter, rather than just one.
The signs outside the living space
If you can get safely into the attic or crawl space, that is where the evidence is. A supply leak makes the space warmer in winter and cooler in summer than it has any business being.
- Flex duct that has come apart at a collar or is hanging loose.
- Tape that has curled away from a joint, which you can often see from several feet.
- Insulation compressed or displaced along a run where somebody has walked or stored something.
- A section that has sagged into a low point, stressing the connections at both ends.
A rough check you can do
With the system running, hold your hand near accessible joints and connections. You will not find small leaks that way, but the ones large enough to matter are usually obvious once you are looking in the right place.
Compare the airflow at a register near the furnace with one at the far end of the house. A big difference between them, with a clean filter fitted, points at what is happening between the two.
What we would do
Look at the accessible ductwork and report on the condition of the connections, not just the amount of build-up inside. Those are different findings and they lead to different work.
If the ducts are basically sound and carrying accumulation, cleaning helps. If runs have come apart, cleaning them changes nothing about the room that will not warm up, and we would rather say so plainly than let you find out afterwards.