What to know
The half everybody pictures
A gap in a supply run lets conditioned air out where nobody lives. The system produced that air, you paid for it, and it went into a crawl space.
The room at the end of that run gets whatever survives the journey, which is why the far bedroom is usually the one that will not keep up.
The half nobody mentions
Returns are under negative pressure. A gap on the return side does not leak out — it draws in, taking whatever is in the attic, crawl space or garage and delivering it through the system to the whole house.
That air has not been through the filter, because it entered downstream of it. It is why dust sometimes reappears at a register within days of a clean, and why a musty crawl space can be noticeable in a bedroom.
What the national figure means
The Department of Energy's guidance puts losses in a typical duct system at roughly 20 to 30 per cent of the air moving through it. ENERGY STAR gives similar figures and recommends sealing and insulating accessible ducts as the remedy.
That range tells you the question is worth asking. It is not your number, and anybody quoting it as though they had measured your house is guessing.
Signs of each
- Supply-side: rooms far from the furnace falling behind, an attic or crawl space that feels conditioned, the system running longer for the same result.
- Return-side: dust arriving quickly after cleaning, smells from a garage or crawl space appearing indoors, a filter that loads far faster than it used to.
Why cleaning is not the fix
Cleaning removes what has accumulated inside the ducts. It does not close the gaps, so a system that is leaking will keep leaking and keep drawing in whatever is around it.
Both problems can exist in the same house, which is exactly why the useful first step is somebody establishing which one you have.
Where to start
Which of these applies to your system depends on how it was built and what shape it is in now, and that is not something a page can settle.
Our inspection is free. We will show you what we find, and if the answer is that your ductwork is sound and simply needs cleaning, that is what we will tell you.