Duct smoke testing is one of those phrases that means two entirely different things depending on who is asking, and the two belong to different trades. This FAQ sorts that out first, then covers the building performance version in full: how the test runs, what it finds, what it cannot do, and the coordination it needs before anyone lights a smoke generator inside a building.
Table of Contents
First, the thing you may actually be looking for
Is this the same as duct smoke detector testing?
No, and this is the most important answer on the page. Duct smoke detector testing is fire alarm work: the periodic inspection and functional testing of duct mounted smoke detectors in air handling systems, performed under NFPA 72 by a licensed fire alarm contractor. It is about life safety devices, not about air leakage.
Do you perform duct smoke detector testing?
No. Stellar Building Performance is an energy code testing and building diagnostics company, not a fire protection contractor. If you need duct smoke detectors inspected or certified, you want a licensed fire alarm contractor and we would rather point you there than take the call.
So what is duct smoke testing in building performance?
It is a diagnostic. The duct system is sealed and pressurized, non-toxic theatrical smoke is introduced, and the smoke escapes wherever the system leaks, making every failure point visible. It exists for one reason, which is that a duct leakage test gives you a number and a number has no address attached.
How the test works
Walk me through it.
The duct system is isolated and sealed at every register and return grille, the same preparation a duct leakage test uses. A smoke generator then fills the system and holds it under slight positive pressure. Smoke takes the path of least resistance, which is the path the air has been taking, so it emerges at every failure in the assembly. After that it becomes a walking job: the attic, the chases, the mechanical closet and the conditioned space, documenting each finding with a location and a photograph.
What kind of smoke is it?
Non-toxic theatrical smoke, the same class used in entertainment and in fire service training. It is not combustion smoke and it is not a chemical tracer. It is designed to be visible, to stay suspended long enough to be followed, and to dissipate.
Is this the same as blower door guided smoke testing?
No. Those are two separate services and they are priced separately. Blower door guided smoke testing uses handheld foggers to pinpoint leakage in the building envelope while the house is under blower door pressure, so it is about walls and ceilings. Duct smoke testing pressurizes the ductwork itself. Either one can be useful without the other.
How is it different from a duct leakage test?
A duct leakage test measures, and a smoke test locates. Duct leakage testing uses a calibrated fan to determine how much air the system loses at a reference pressure, and produces a number in CFM25 that can be submitted for code compliance. Smoke testing produces no measurement and cannot be submitted for anything. In practice the sequence is usually test, fail, smoke, repair, retest.
Is this a tracer gas test?
No. Tracer gas methods introduce a detectable gas and measure its concentration, which is a quantitative technique used in specialized applications. Smoke testing is visual and qualitative. You are looking at where it comes out, not measuring how much.
Safety and coordination
Is the smoke safe?
It is non-toxic and widely used in entertainment and training environments. Even so, the building should be clear of occupants during the test and ventilated afterward, which is standard practice rather than an abundance of caution.
Will it set off the smoke alarms?
Yes, if nothing is done about it first. Visible smoke is exactly what smoke detection is built to respond to, and a duct system discharging smoke into occupied space will find a detector. With that said, testing can be coordinated after temporary sealing of the system to mitigate this.
What about a building with a monitored fire alarm system?
Then coordination is mandatory and it happens before we schedule, not on the day. The system has to be placed on test, the monitoring company notified, and a building contact identified who can authorize it. In a multifamily building this is usually the longest lead item in the whole job, which is why this service is arranged in advance rather than added to an existing visit.
Does it leave residue or a smell?
In normal use it dissipates without staining. In a home with completed finishes it is still worth a conversation about scope and duration before we start, because the right answer depends on how much smoke the system needs and how long it has to stay in there.
Can it be done in an occupied home?
It can, though the building may need to be ventilated afterward. Most of the work we do is on new construction before occupancy, which is simpler for everyone.
When it is worth doing
Our duct test failed. Is this the right next step?
Usually, and especially if the system missed by a wide margin. A system at 7 CFM25 per 100 square feet against a limit of 4 needs roughly 40 percent of its leakage removed, and sealing what looks suspicious and retesting is an expensive way to find out you guessed wrong. If the miss is small, sometimes the cheaper answer is to seal the known usual suspects first, and we will tell you when that is the case.
Do I need it if my duct test passed?
Generally no. If the system cleared its limit there is nothing to locate. The exception is a project on the performance path with a leakage target tighter than the code number, where you are working against the figure in the energy calculation rather than the prescriptive one.
Is it worth doing on multifamily?
This is where it pays best. Identical units built by the same crew fail in identical ways, so one thorough diagnostic on an early unit produces a correction list that applies to the whole building. Finding a detail problem on unit one is a change in method. Finding it on unit forty is rework.
What kinds of buildings do you do this on?
Multifamily, townhomes, duplexes and single family homes. That is the work we take, and it covers the great majority of duct systems that fail a code leakage test in Florida. If you have something outside that list, ask and we will tell you straight whether it is a fit rather than taking the job and figuring it out on site.
Can it be done at rough-in?
Yes, and that is often the best time for it, because the system is still accessible and repairs are cheap. Once the ceiling is closed and the attic is insulated, reaching a connection to seal it is a much bigger job than sealing it was.
Does it work on an existing home?
It does, and it makes the case visually in a way a number does not. A system dumping conditioned air into a 130 degree attic is a comfort problem and an energy bill at the same time, and watching smoke pour out of a plenum connection tends to settle any debate about whether the problem is real.
What the smoke finds
Where are the leaks usually?
The same handful of details, over and over. The air handler cabinet and plenum connection is consistently the largest single source in a residential system, including the filter slot and the cabinet seams. After that: boot to drywall joints, branch takeoffs and flex collar connections, return chases built from building cavities rather than sealed duct, trunk seams and joints, and anywhere the system crosses from conditioned space into an attic or soffit.
Will it find leaks inside walls and chases?
It finds them if the smoke has a route out, which it usually does, because a leak inside a chase is generally connected to something. What it will not do is show you the exact point of a leak buried in a sealed cavity with no path to open air. Those get identified by where the smoke does eventually emerge and by reasoning about the assembly.
Does it find flex duct problems?
Yes, particularly at the collar connections, which is where flex duct almost always fails. An inner liner that was pulled over the collar but never properly sealed, or sealed without the collar being fully seated, leaks steadily and looks completely acceptable from six feet away.
Does this also find envelope leaks?
Only incidentally. Boot to drywall joints are both a duct leak and an envelope leak, so fixing them improves the blower door number as well. For a real envelope diagnostic you want blower door guided infrared or blower door guided smoke testing instead.
Results, code and expectations
What do I get at the end?
A documented leak list. Each finding carries its location in the building, a photograph, and a note on what the failure is, so the repair can be handed to a trade without anyone having to interpret it. Where the same detail repeats throughout the system, it is called out as a pattern rather than listed thirty separate times.
Can I submit a smoke test for code compliance?
No. It produces no measured value. Duct leakage testing under Florida Building Code Energy Conservation section R403.3.2 remains the test of record, with limits of 4 CFM25 per 100 square feet of conditioned floor area for a postconstruction test or a rough-in test with the air handler installed, and 3 CFM25 per 100 square feet for a rough-in test without the air handler. You can however, add on a duct leakage test to your visit.
Will this guarantee my retest passes?
No, and anyone telling you otherwise is selling something. What it does is remove the guesswork, so the repairs are aimed at real leaks rather than plausible ones. Whether the retest clears depends on how much leakage was found, how much of it gets properly sealed, and how far the system was from its limit to begin with.
How long does it take?
It varies with the size of the system and how much of it is accessible. A single family home with one system in an accessible attic is a shorter job than a multi-system home or a building where the runs are buried. On a badly leaking system the major findings are obvious within minutes; the time goes into finding the small ones that add up.
Does the 2026 code change anything here?
Indirectly, and significantly. The 9th Edition takes effect December 31, 2026 and the R405 duct testing exception is removed, which means performance path projects that previously avoided duct testing will be carrying it. Systems that were never tested have never been sealed to a standard, so the first round of those tests is likely to produce failures on jobs that never had one. See 2026 Florida Building Code Energy Conservation: What Builders Need to Know.
Scheduling and coverage
Can you do it the same day as the duct leakage test?
It has to be quoted and authorized in advance, and in any building with fire detection it needs coordination, so it is scheduled rather than added on the day. If you already expect a system to fail or are uncertain of your expected result, say so when you book the test and we can arrange both.
What does it cost?
It is quoted per project rather than sold off a rate sheet, because the scope varies enormously between a single family retest and a multifamily building with a monitored fire system. Tell us what failed, by how much, and where the system runs, and you will get a number along with an honest read on whether this is the right next step or whether something cheaper gets you there.
Where do you work?
Brevard, Indian River, St. Lucie, Martin and Okeechobee counties, with travel beyond that for larger multifamily, townhome and affordable housing projects where the economics work. See the full service area.
Related reading
Duct smoke testing services · Duct leakage testing FAQ · Blower door testing FAQ · Infrared thermography FAQ
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