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You cannot clean your way out of a duct in the ground
If the registers are in your floor, the ductwork is cast into the concrete. Most of those systems are fine. The ones that are not cannot be fixed by the service almost everybody asks for first.
A passage through the ground, built before anybody expected to inspect it
Houses built here from the nineteen-fifties through the seventies commonly have their supply ducts cast into the slab — sheet metal, transite, or in later houses a fibre or plastic pipe — running from a central plenum out to registers in the floor. It was a reasonable way to build. It is quiet, it delivers air low in the room where cool air wants to be, and when it is sound it works.
The difficulty is the ground. This is flat land with a high water table, subject to storm surge and to long periods of saturated soil. Where a joint has settled, a metal duct has corroded through, or a slab has cracked, groundwater gets in. Once it does, the system moves air across whatever is standing in those runs and delivers it to every room in the house, and nothing you do inside the house changes that.
None of this is a reason to assume the worst. A great many of these systems are dry, intact and will outlast their owners. It is a reason to look properly, once, with a camera, and to write down what is actually there — because it is the one part of the house that is invisible and the one part that is expensive to be wrong about.
What you have noticed, and what you were planning to do
Nothing is sent anywhere and nothing is recorded. This is what we would ask you on the telephone.
The instinct is understandable and on an ordinary attic duct system cleaning is sometimes reasonable. Under a slab it is different, for three reasons that are worth knowing before you pay anybody.
It does not address the water. If groundwater is entering through a failed joint or a corroded section, removing what is in there today changes nothing about tomorrow. The run refills. A system that has held water cannot be made dry by cleaning it.
It can make a failing duct worse. Aggressive brushing or high-pressure air in a run that has already corroded, cracked or partially collapsed can open it further, dislodge sections, or push debris into the parts that were still clear. A duct in the ground cannot be inspected from above afterwards to see what happened.
And the sales structure is against you. Duct cleaning is sold on a fear that is easy to create and hard to disprove, frequently at a low advertised price that grows once somebody is in the house. We do not offer duct cleaning at all, here or on attic systems, so we have nothing to gain either way from telling you this.
What to do instead: get a camera down the runs first, and decide from what is on the screen. If they are dry and sound, you need nothing. If they are not, the honest options are lining, partial replacement, or abandoning them and running overhead — and cleaning is not among them.
Something is wet down there, and masking it is the one thing not to do
A musty smell that is strongest at the floor and strongest when the system runs is coming from the duct rather than from the room. In an under-slab system that almost always means moisture — standing water in a low run, saturated soil against a failed joint, or condensate that has found its way in.
What we would do: put a camera down the runs from the registers, starting with the ones nearest the smell and the ones at the lowest point of the floor. An hour usually settles it, and you see the inside of your own ducts on a screen rather than taking anybody’s word for what is there.
What we would not do: fit an air freshener, an ultraviolet lamp or a scented cartridge into the system to cover it. Those change what you smell and nothing else, and they make the problem harder to find later.
And where the smell is genuinely mould: identifying and removing it is an assessor’s and a remediation contractor’s work rather than ours, and in Texas that is a licensed field of its own. What we do is find and remove the water that is feeding it, which is the part that actually settles the question.
Stop running the system on that side of the house and find out where it is coming from
Visible water, damp or silt in a floor register is not ambiguous. Something is open to the ground or to a leak. The first question is which, because the answers are very different: groundwater entering a failed duct, a plumbing leak under the slab, or condensate from the system itself going somewhere it should not.
What we would do, in order: camera the affected runs; check the condensate drain and pan, because that is the cheapest possible explanation and it happens; and look at whether the water level changes with the weather, which distinguishes groundwater from a leak. If it looks like a supply or drain leak under the slab, that is a plumber’s job and we will say so straight away rather than take the visit.
What to do today: close that register, and if the water is more than a trace, leave the system off until somebody has looked. Blowing air across standing water into the rest of the house is the part worth avoiding.
What this usually leads to: a duct that takes water is rarely worth repairing in place. The real decision is between lining and abandoning, and that is a decision to make with the camera footage in front of you.
A blocked or collapsed run — and it cannot be fixed from inside the house
One or two registers with weak or no air, while the rest of the house is fine, points at the run rather than at the equipment. Under a slab the usual causes are a collapsed section, a run that has silted up, debris from a previous repair, or a duct that has separated at a joint and is now discharging into the ground.
Rule the easy things out first: measure airflow at every register and static pressure on the system. If total airflow is fine and one register is starved, it is that run. If everything is low, it is the equipment, the filter or the return, and that is a much cheaper conversation.
Then look: a camera down that run will usually show you the blockage or the break. Knowing where it is matters, because the cost of dealing with it depends almost entirely on whether it is two feet in or twenty.
Be careful of the quick answer: a run that has separated into the ground is losing conditioned air into the soil continuously. Capping the register makes the symptom go away and leaves you paying to condition the earth under your house.
There is an opening to the outside, and that is the thing to fix
Insects or rodents coming out of a floor register means the duct system is connected to somewhere it should not be — a broken section against soil, a gap where a run passes the slab edge, or an abandoned opening that was never sealed. Pest control will deal with what is in the house; it will not close the hole.
What we would do: camera the runs to find the opening, and check the plenum and the slab edge, which is where these usually are. It is a locating job more than a repair job, and locating it is most of the cost.
Who does what: a pest control contractor for what is living in there, us for the duct opening, and occasionally a concrete contractor if it has to be opened up. We will say plainly which parts are not ours rather than quote for all of it.
One thing worth knowing: if there is an opening large enough for a rodent, there is an opening large enough for soil gas, moisture and ground air. The pests are the visible symptom of a duct that is no longer sealed from the ground.
Best possible time to look, and it is an hour’s work
If you have just bought or are about to, this is the cheapest hour in the whole transaction. A camera down three or four runs tells you whether you have a sound system that needs nothing, or a liability that will decide your next fifteen years of decisions about this house.
What a proper look produces: footage from several runs including the lowest ones, a note of what each shows, airflow at every register, static pressure, and the age of the equipment off its serial number. In writing, so that it exists whether or not you do anything with it.
Why nobody told you: a general home inspection almost never includes it. “HVAC appeared operational” is a statement about the equipment, not about a duct system nobody can see. It is not a criticism of inspectors — it is outside the scope they are working to.
And if it is all sound: that is a genuinely useful thing to know and to keep. Write the date on it. The next time somebody tells you your ducts need attention, you will have something to set against it.
About a musty smell
Note whether it is stronger when the system starts and whether it is worse after rain. Both answers narrow it considerably before anybody arrives.
About water in a register
Photograph it and note the date and the weather. Whether the level tracks the rain is the single most useful piece of evidence for telling groundwater from a leak.
About a weak register
Ask for airflow in cubic feet per minute at the weak register and at a good one. Two numbers separate a run problem from a system problem in minutes.
About insects or rodents
Ask for the opening to be located and photographed rather than for the system to be treated. Treatment without closing the hole is a subscription.
About a house you have just bought
Ask for footage from the lowest runs specifically, not the most convenient ones. Water sits at the low point, and that is where the truth is.
If you were going to have them cleaned
Read the note above before anything else on this page. In short: it does not address water, it can damage a run that has already failed, and we do not offer it. Get a camera down first and decide from what is on the screen.
If you were thinking of lining or sealing
A legitimate option and sometimes the right one. Three questions decide it: is the existing duct intact enough to line, which only a camera answers; is water still entering, because lining over an active leak traps it; and what is the warranty, from whom, and what does it actually cover. Ask for the footage before and after, and keep both.
If you were thinking of abandoning them
Frequently the most honest long-term answer, and the most disruptive. Running a new system overhead means attic space, soffits or bulkheads in some rooms, sealing the old openings properly so the slab is not left open, and designing the new distribution rather than fitting it into whatever space is left. Ask to see the proposed routes drawn before you agree a price, because that is what you will live with.
If you are selling soon
Two plain things. Replacing a duct system to improve a sale very rarely returns its cost, and a buyer’s inspector will not usually look under the slab at all. But if you know there is water down there, that is a different question from a comfort one, and how it is handled in a transaction is a matter for your agent and your attorney rather than for us. What we will do is give you a written, dated record of what is actually there. What we will not do is advise you on what has to be disclosed.
Four things to establish before any money is spent
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Footage from the runs, including the lowest ones
Not a description. Ask to watch the screen, and ask for a copy. Every decision on this page rests on what is actually in there.
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Airflow at every register and static pressure
Numbers, taken at your house. They separate a duct problem from an equipment problem before anybody proposes either.
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Whether water is still entering
Because it decides between lining and abandoning, and because sealing over an active leak is the one thing that makes matters worse.
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Who does which part
Concrete, plumbing, pest control, remediation and ductwork are four or five trades. A quotation that covers all of them from one contractor is worth reading carefully.