This is the question I get asked more than any other, usually a week or two after somebody moves in, and often with a line from a home inspection report attached to it.
The short answer: do not book it automatically, but do look. Buying a house is the one point where the argument for looking is strongest, and it has nothing to do with the age of the house.
Why this moment is different
In a house you have lived in for ten years, you already know the history. You know whether there was a dog. You know the kitchen was redone in 2019 and what it was like. If something had been living under the floor you would probably have smelled it.
You have none of that in a house you just bought. The sellers knew, the previous owners knew, and the information did not come with the keys. A duct system is the one part of the house that stores its own history and does not show it to you from the hallway.
The things that actually make it worth doing
- Work done before the sale. This is the big one, and it is common. A house that was tidied up for market or flipped outright has had drywall sanded, floors cut, and trim replaced, usually with the heating system running. Drywall dust is fine enough to stay airborne for days, and it does not settle helpfully.
- Pets, if you are allergic. Not a hygiene issue. If somebody in your house reacts to cats and the previous owner had three, the ducts are one of the reservoirs and they are not one you can vacuum.
- Rodents. Around here this is a real one and not a scare story. Plenty of houses on the island have their supply runs in a crawlspace, and flexible duct in a crawlspace is not a difficult thing for an animal to get into. If something has been in there, cleaning is the least of it.
- A smoker. It is in everything else, and it is in the ducts too.
- Duct that has come apart. Also common, also crawlspace-related. A run that has pulled out of its collar is dumping your heated air under the floor and pulling crawlspace air into the system. That is not a cleaning job, it is a repair, but you find it the same way: by looking.
What is not a reason on its own
The age of the house. A forty year old system that has been in a dry, sealed, conditioned space and had a filter changed on time is often fine, and I would rather tell you that than take the work.
And a line in an inspection report is a prompt to look, not a verdict. Inspectors are describing what they could not fully see, which is exactly right of them, but it is not the same as a finding.
Do it before you move in, if you do it at all
This is the practical part nobody mentions.
An empty house is much easier to work in than a full one. Nothing has to be moved, nothing has to be covered, and access to every register and to the equipment is simply there. If you are going to have this done, the window between closing and the moving truck is worth using, and it is a window that does not come round again.
Checking it yourself first
Two things, and neither takes long.
Take the cover off a couple of registers, ideally a return, and shine a flashlight in. A layer of settled grey dust is normal and every house has one. What you are looking for is fuzz thick enough to hide the metal, anything growing, or anything that used to be alive.
Then, if there is a crawlspace, go under with the flashlight and follow the flex duct runs. You are looking for disconnections, sections crushed flat under something, and torn outer wrap. That inspection is free and it tells you more about the system than a duct camera will.
If either one turns up something you are not sure about, photograph it and text it to me. I would rather look at a picture and tell you it is fine than drive out and tell you the same thing.
For the general question of when a duct system is worth cleaning, separate from having just bought the place, I have written that up here. If what you found looks like growth rather than dust, start here instead.