Quick answer
A plumbing smoke test is an inspection in which non-toxic smoke is blown into a drain, vent or sewer system under light pressure, so that it escapes and becomes visible wherever pipes are cracked, joints are open, traps are dry or storm drains are wrongly connected.
Cities use it to find where rainwater enters sewers. Plumbers use it inside buildings to trace sewer gas odors. It shows where gas or air escapes, not where water leaks.
Quick facts
The key facts about Plumbing Smoke Test, with sources:
- What it finds
- Defects that let in rainwater, storm-sanitary cross-connections, odor sources 1
- Smoke
- Non-toxic, but may irritate people or pets with breathing problems 2
- Typical city test length
- About 30 minutes per test, per Minneapolis 2
- Notice (NASSCO guideline)
- Door hangers at least 48 hours ahead 1
- Best conditions
- Dry soil; saturated or frozen ground limits results 1
- Building test pressure (former NYC code)
- Smoke held at 1 inch of water column during inspection 4
Key takeaways
- Smoke goes where air can escape, so it marks cracks, open joints, dry traps and roof drains tied into the sewer.
- Most smoke tests you hear about are city sewer surveys; plumbers also run them inside buildings to find sewer gas leaks.
- Fill rarely used drains with water before a city test so smoke does not come into the house.
- The smoke is non-toxic but can irritate; if it enters, open a window and report it.
- A smoke test finds air paths; a pressure test proves a system is tight, and a dye test proves where water goes.
What a plumbing smoke test finds
Drains and sewers are supposed to be sealed pipes that carry wastewater away and keep sewer gas out of buildings. When they crack, separate at joints or are connected where they should not be, two things happen: rain and groundwater get in, and sewer gas gets out. A smoke test makes those openings visible.
In a city survey, crews blow smoke into a sewer main at a manhole. Minneapolis explains that the smoke comes out through holes in manhole covers, plumbing vent pipes and other places where rainwater can enter the sewer 2.
NASSCO, the sewer rehabilitation trade association, lists the goals: confirm connections, find defects that let in inflow and infiltration (I&I), locate cross-connections between storm and sanitary sewers, track down odor complaints and record each defect 1.
Inside a building, a plumber may use a smaller smoke machine to find where sewer gas is leaking from drain and vent pipes behind walls.
Older codes spelled out the method: the former New York City Building Code required filling all traps with water, filling the system with thick smoke, closing the roof vents once smoke appeared, and holding a pressure equal to 1 inch of water column during inspection 4.
Smoke, dye and pressure tests
A smoke test shows where air escapes. A dye test shows where water goes, such as whether a downspout drains into the sewer. A pressure test proves a new system is tight before walls close. They answer different questions and are often used together.
How a city sewer smoke test runs, step by step
- Notify. Door hangers go out; NASSCO recommends at least 48 hours ahead, and re-notice if testing slips more than seven days 1. Fire departments are told too 1.
- Isolate a segment. Crews block a section of sewer between manholes.
- Blow smoke. A high-capacity blower over a manhole pushes non-toxic smoke into the pipe 1.
- Watch for exits. Smoke rises from roof vents (normal), and from cracks, yard cleanouts, downspouts, catch basins or the ground (defects) 21.
- Record and follow up. Each exit is photographed and logged; dye tests and camera work pinpoint the cause 1.
- NoticeDoor hangers 48+ hours ahead
- IsolateBlock a sewer segment
- Blow smokeBlower at the manhole
- ObserveVents normal; other exits are defects
- ConfirmDye test or camera to pinpoint
Smoke from a roof vent is expected. Smoke from a downspout, yard or foundation is a finding. Source: [1] [2]
Which kinds of smoke testing exist
| Type | Who runs it | Purpose | Where it happens |
|---|---|---|---|
| Sanitary sewer survey | City or utility crews | Find inflow, infiltration and cross-connections 1 | Street mains, laterals, roof vents |
| Building drain and vent test | Plumber | Find sewer gas leaks inside a building 4 | Drain, waste and vent piping |
| Odor investigation | Plumber or utility | Trace a persistent sewer smell 1 | Single building or block |
| Peppermint test (older alternative) | Plumber | Odor-based leak finding with oil of peppermint and hot water 4 | Building drains; rarely used now |
Example: what Newport, Rhode Island found with smoke testing
Newport, Rhode Island, smoke tested three priority sewer areas as part of its combined sewer overflow plan. Its 2007 report split the inflow sources it confirmed between private and public property 3.
| Finding | Count | Who acts | Recommended fix |
|---|---|---|---|
| Private sources (roof drains, yard drains) | 65 3 | Property owners, after a city notice 3 | Disconnect roof and yard drains from the sewer 3 |
| Public sources (catch basins, manholes, other) | 26 3 | The city 3 | Disconnect catch basins, fix manhole caps 3 |
| All sources identified | 91 3 | Both | Largest public sources first 3 |
The chart below shows the inflow the report estimated for each group: the 26 public sources carried roughly three times the water of the 65 private ones.
What this means for a homeowner: if smoke comes out of your downspout, yard drain or foundation area during a city test, the city may later ask you to disconnect a drain from the sewer.
Newport sent notices to owners and followed up with those who did not respond 3. The fix is usually plumbing work on your side of the property line; our sewer line repair page explains who owns what.
Where homeowners run into smoke testing
Most people meet smoke testing through a door hanger. Minneapolis, for example, mails a notice before testing, then hangs a second notice on the door one to three working days ahead 2. Crews watch from the street and alley and do not need to enter 2.
Inside, a plumber may suggest a smoke test when a sewer smell will not go away after traps are refilled.
It can find a cracked vent pipe in a wall, a loose wax ring, an uncapped pipe behind a finished wall or a failed air admittance valve, all places where the vent system is supposed to be sealed but is not.
On an inspection report or invoice, look for “smoke test of DWV,” “odor investigation” or “I&I survey.” A city may report a defect on your sewer lateral, the pipe from your house to the main, found by smoke surfacing in your yard.
What smoke testing does well and where it falls short
What it does well
Where it falls short
- Smoke alone does not show exactly where a connection or break is 1.
- Smoke can travel through soil and surface far from the defect 1.
- Saturated or frozen soil blocks smoke and limits accuracy 1.
- Pipes in poor shape may show only a few smoke points; cameras find more 1.
- Smoke can enter homes with dry traps and alarm occupants 12.
What a smoke test cannot tell you
It does not prove a pipe holds water. Smoke shows air leaks under very light pressure. A new drain system still needs its code test with water or air before the walls close.
It does not show pipe condition in detail. NASSCO warns that a sewer with only a few smoke defects can turn out to be in very poor structural condition when cameras go in, and cautions against repairs based only on smoke findings 1. A sewer camera run through the line fills that gap.
It cannot confirm where water flows. Smoke from a catch basin might be a direct connection or a leaky pipe nearby. NASSCO recommends dye flooding with camera work to pinpoint it 1.
Who pays for smoke testing and what drives cost
We do not list prices; fair numbers need dated local quotes.
City surveys are usually paid by the sewer utility. Minneapolis says its testing is funded by sewer fees and residents are not charged extra 2. Repairs found on private property are a different matter: the owner typically pays to fix their side.
For a plumber’s odor investigation, cost drivers are building size, how many stacks and vents must be sealed, roof access, whether walls or ceilings must be opened to reach the leak, and the repair itself. See our plumbing cost guide.
How to prepare for a city smoke test
These steps follow city guidance; they are things any homeowner can do.
- Read the notice. Note the date and the crew contact number 2.
- Fill rarely used drains. Pour a bucket of water down basement floor drains and any toilet, sink or shower not used in the last month 2.
- Check cleanout caps. Smoke can enter through missing or broken cleanout caps 2. Our sewer cleanout page shows where they are.
- Plan for sensitive people and pets. Minneapolis offers special notice for residents with breathing or mobility concerns 2.
- Stay calm if smoke appears. Open a window or door and call the crew contact 2.
- Fix what is found. If smoke entered, have a plumber check the trap, vent or pipe it came from.
How to keep smoke and sewer gas out of the house
- Run water in every drain every few weeks, including guest baths and floor drains.
- Add a trap primer or barrier device to floor drains that dry out.
- Keep cleanout caps on and intact.
- Do not connect roof drains, sump pumps or yard drains to the sanitary sewer; cities count these as inflow 3.
- Repair any pipe that let smoke in during a test.
Where did the smoke come from?
If smoke enters during a test, note the exact spot. Smoke from a floor drain that clears after you pour water in was a dry trap. Smoke from a wall, ceiling or a fixture with a full trap points to a broken pipe, vent or seal that needs a plumber.
Which signs suggest you need a smoke test
- A sewer smell that returns after every trap has been refilled.
- Odor near a wall, ceiling or closet with no fixture.
- Smoke entered your home during a city test.
- Your city reported smoke from your yard, downspout or foundation.
- Basement flooding or slow drains during heavy rain, which can point to inflow.
Smoke indoors and sewer gas
Minneapolis says to call 911 if you are not sure smoke comes from the city test; otherwise ventilate and call the crew 2. The test smoke is non-toxic but may irritate people or pets with breathing conditions 2.
A strong, lasting sewer gas smell is a different hazard: ventilate, leave if you feel unwell, and call a plumber. If you smell gas from a gas appliance, leave and call your gas utility or 911.
For a persistent odor, start with a plumber who can trace it; see our drain cleaning and repair page.
Which rules and guidelines cover smoke testing
Building plumbing: current model codes test new drain and vent systems mainly with water or air. Smoke and peppermint tests appear in some codes as final or alternative tests.
As one example, the former New York City Building Code (Title 27, §27-922) described a smoke test with filled traps, thick smoke, sealed roof openings and 1 inch of water column held during inspection, and a peppermint test using two ounces of oil of peppermint followed by ten quarts of water at 160°F or hotter 4.
That code has since been replaced; check the code your jurisdiction enforces.
Sewer surveys: NASSCO’s Specification Guideline for Smoke Testing (May 2021) is an industry guideline covering notification, blowers of at least 1,800 cubic feet per minute, dry-weather scheduling and records 1. It is not a law.
Utility rules: local sewer ordinances usually ban roof drains, sump pumps and yard drains from the sanitary sewer, and cities may require owners to disconnect sources found in testing, as Newport did 3.
Federal context: EPA’s 1999 sewer cleaning and inspection fact sheet cites an ASCE survey in which utilities smoke tested 7.8 percent of their systems per year on average 5.
Four numbers from these sources tell you what a city test looks like from the curb.
- 48 hoursMinimum resident noticeNASSCO guideline
- 1,800 cfmMinimum blower capacityNASSCO guideline
- 30 minutesTypical test at one locationMinneapolis
- 7.8%Share of system smoke tested per yearASCE survey, via EPA 1999
How a smoke test compares with dye, pressure and camera tests
| Test | What it shows | Best for | Main limit |
|---|---|---|---|
| Smoke test | Where air or gas escapes 1 | Odors, inflow sources, cross-connections | Does not pinpoint the defect alone 1 |
| Dye test | Where water flows 3 | Confirming a drain connects to the sewer | One source at a time |
| Pressure test | Whether a closed system holds water or air | New work before cover | Does not locate the leak |
| Sewer camera | Inside view of the pipe | Cracks, roots, sags, offsets | Needs access; does not show air paths |
| Peppermint test | Odor at leaks 4 | Older alternative for building drains | Rarely used; odor lingers |
What people get wrong about smoke tests
- Myth Smoke coming out of my roof vent means a problem.
- Reality Roof vents are expected exits; Minneapolis lists vent pipes among normal smoke outlets 2.
- Myth The smoke is dangerous.
- Reality It is non-toxic, though it may irritate people or pets with breathing conditions 2.
- Myth Crews need to come inside.
- Reality City crews observe from outside 2.
- Myth One smoke test tells the whole story.
- Reality NASSCO says it should be combined with flow monitoring, dye and camera work 1.
- Myth Smoke appears right above the break.
- Reality Smoke can travel through soil and surface far away 1.
How season and soil change when cities test
Season and soil decide when a city can test. NASSCO notes saturated or frozen soils keep smoke from migrating and recommends dry-season testing 1, so cold-climate cities usually test in summer and fall; Minneapolis runs its program from mid-August into the fall 2.
Older cities with combined or aging sewers, like Newport, test to cut overflows during storms 3. Rules on disconnecting private drains are set by each sewer utility, so read your notice for what applies.
When a smoke test matters to you and what to do
It matters when your city announces testing, when a sewer smell will not go away, when smoke came into your home during a test, or when the city tells you a downspout or drain is connected to the sewer.
Next step: before a city test, fill your traps. After one, fix anything that let smoke in. For an odor with no clear source, ask a plumber whether a smoke test of your drain and vent system makes sense.
- If Smoke from a floor drain, clears after water
- Then Dry trap: keep it filled or add a primer 2
- If Smoke from a wall, ceiling or full-trap fixture
- Then Broken pipe or vent: call a plumber
- If Smoke from your downspout or yard drain
- Then Likely tied to the sewer: plan to disconnect 3
- If Smoke from the ground in your yard
- Then Possible lateral defect: camera inspection 1
Questions about Plumbing Smoke Test
Is smoke coming out of my roof vent during the test a problem?
No. Roof vent pipes are open to the sewer by design, so smoke rising from them is expected; Minneapolis lists vent pipes among the normal places smoke comes out.
Smoke from a downspout, a yard drain, the ground or inside the house is what the crew is looking for, and those spots get logged as defects.
Is sewer smoke testing safe?
The smoke used is non-toxic, according to city guidance such as Minneapolis’s, but it may irritate people or pets with breathing conditions. Avoid unnecessary exposure. If smoke enters your home, open a window or door and call the crew contact; call 911 if you are not sure the smoke is from the test.
How do I prepare for a city smoke test?
Pour a bucket of water down basement floor drains and any toilet, sink or shower not used in the last month, so the traps are full. Check that cleanout caps are on. If someone in your home has breathing or mobility concerns, ask the city for special notice.
Why did smoke come into my house during the test?
Usually a dry trap, a missing or broken cleanout cap, or a defect in the plumbing. Minneapolis notes smoke can enter through broken cleanout caps or defective plumbing. If pouring water into the drain stops it, it was a dry trap; otherwise have a plumber check the pipe or vent.
Do I need to be home for a smoke test?
Usually not. Minneapolis says its crews observe from the street or alley and do not need to come inside. Each test lasts about 30 minutes. Being home can help if smoke enters, since you can ventilate and report the spot.
Can a plumber smoke test my house for a sewer smell?
Yes. A plumber can fill the drain and vent system with smoke to find where sewer gas leaks, such as a cracked vent behind a wall or a failed seal. Older codes described the method with traps filled, roof vents sealed once smoke appears, and light pressure held during inspection.
What is the difference between a smoke test and a dye test?
A smoke test shows where air escapes from pipes. A dye test shows where water goes: dyed water is put into a downspout or drain and crews watch the sewer for color. Cities often use smoke to find suspect sources and dye to confirm them.
What happens if the test finds my downspout is connected to the sewer?
The city may ask you to disconnect it. In Newport, Rhode Island, smoke testing found 65 private sources such as roof and yard drains, and the city notified owners to disconnect them. Roof drains add rainwater the sewer was not built to carry, which can cause overflows.
Why do cities smoke test sewers?
To find where rain and groundwater get into the sanitary sewer. That extra water costs money to treat and can overwhelm pipes during storms. Minneapolis says intense storms can trigger combined sewage overflows when too much stormwater enters the system.
Sources
- NASSCO, Specification Guideline for Smoke Testing (May 2021; industry guideline), retrieved .
- City of Minneapolis, Sewer Smoke Testing Questions and Answers, retrieved .
- City of Newport, RI, Phase 1 Part 2 CSO Control Plan, Chapter 6: Smoke Testing (Earth Tech, 2007), retrieved .
- Former New York City Building Code (1968), Title 27, Article 7, §27-922 Tests (archived copy; superseded), retrieved .
- U.S. EPA, Collection Systems O&M Fact Sheet: Sewer Cleaning and Inspection (EPA 832-F-99-031, September 1999; MWRD-hosted copy), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Survey purpose, limits, notification and soil conditions are from NASSCO’s 2021 smoke testing guideline (an industry guideline, not law). Resident guidance is from the City of Minneapolis.
Findings and inflow estimates are from Newport, Rhode Island’s 2007 CSO report.
The building smoke and peppermint test methods are quoted from the former New York City Building Code, labeled as superseded, because current model code text was not readable.
The ASCE utility figure is as cited in EPA’s 1999 fact sheet. No prices are given.
Suggest a correction. We fix errors and say what changed.