What an Airflow Reading Actually Means: How to Read Your Vent Report

At the end of every dryer vent cleaning we do, we hand you a number. Two numbers, really — what your vent measured before we started and what it measured after we finished — along with a written report of what we found along the way.

Most people have never been handed one of these before. That is not a knock on anyone; a great deal of vent cleaning in this city gets sold as an hour of work and a handshake, and a homeowner has no particular reason to know what feet per minute means. So people take the sheet, say thank you, and put it in a drawer.

We would rather you actually read it. We are Vent Pro NYC, a family-owned company working across Brooklyn and Deal, NJ, and the airflow reading is the single most useful thing we leave behind — more useful than the bag of lint, honestly, because the lint only proves that something came out and the reading proves that the vent works now. This is how to read it.

What we are actually measuring

When your dryer runs, it pushes hot, moist air down the duct and out of your home. An airflow reading measures how fast that air is actually moving when it leaves the termination — the hood or cap on your exterior wall, your roof, or your soffit.

We take the reading at the outlet with the dryer running, because that is the only place that tells you the truth about the whole system. A reading at the back of the machine tells you what the dryer can produce. A reading at the far end tells you what actually survives the trip — every foot of duct, every elbow, every crushed section, every ounce of lint bonded to the wall, and the flap at the very end that has to open against all of it.

That is the entire point. Your dryer and your home's ductwork are two different systems, and the reading measures the one you cannot see.

Feet per minute, and why it is not the same as cubic feet per minute

You will see one or both of these on your report, and they get used interchangeably in conversation even though they are different things.

Feet per minute (FPM) is a speed. It is how fast the column of air is traveling. If you could tag one particle of air leaving the vent, FPM tells you how far it would get in a minute.

Cubic feet per minute (CFM) is a volume. It is how much air is moving in total — how much of your dryer's hot, wet exhaust is actually leaving the building each minute.

They are related but they are not the same, and the thing that connects them is the size of the opening. Push air through a wide duct and a narrow one at the same speed and the wide duct moves far more air. A standard residential dryer duct is 4-inch round, so for most homes the two numbers track each other closely and either one tells a similar story. Where it matters is when a duct has been reduced somewhere along the run — a section pinched behind a wall, a transition hose crushed flat against the back of the machine, a reducer somebody installed to make two mismatched pieces fit. In those cases you can get a respectable speed reading through a very small effective opening while the actual volume of air leaving your home is poor.

This is one of the reasons we look at the vent as well as measure it. A number without an inspection can flatter a bad duct.

The pair matters far more than either number alone

Here is the part we most want people to take away. The before reading and the after reading are worth more together than either is on its own.

A single after-reading tells you the vent is moving air today. Fine. But it does not tell you whether the work did anything, and it does not tell you whether your vent was ever in trouble. The pair does both:

• The before number establishes what you were living with. It is the honest answer to "was this actually necessary?" — a question every homeowner is entitled to ask and very few vendors will answer with evidence.
• The after number establishes what you have now.
• The gap between them is the proof of work. It is the difference between a real cleaning and somebody running a shop vac at the wall for twenty minutes and telling you it looks much better.

This matters more in vent work than in almost any other home service, for a simple reason: the work happens inside a duct nobody can see. The measurement is the only thing standing between you and taking a stranger's word for it.

It is also the honest answer to the question of value. We wrote a whole piece on what dryer vent cleaning costs in Brooklyn, and the short version is that the price is not the interesting part — what you are buying is a measurable change in a system that affects your dry times, your energy use, and your fire risk. The reading is how you know you got it.

Why there is no single pass or fail number

People ask us for the magic number all the time. What should my vent read? What is normal? Is 900 good?

There is no universal answer, and any company that gives you one without looking at your vent is quoting an average rather than measuring your home. Four things move the baseline before a single strand of lint is involved:

• Run length. A dryer venting through 6 feet of wall is a different system from one pushing exhaust 40 feet up through a brownstone. Long runs are common in Brooklyn's older housing stock — we go deep on this in Brooklyn brownstone long vent runs — and a long run reads lower than a short one even when it is spotless.
• Elbows. Every bend costs you airflow. A run with four turns is meaningfully more restricted than a straight shot of the same length, and the equivalent-length penalty for bends is why two 20-foot runs can perform completely differently.
• Termination type. A wall hood with a light flapper, a roof cap, and a soffit vent do not behave the same way. Some terminations restrict more than others by design.
• The dryer itself. A high-end machine with a strong blower and a fifteen-year-old unit with a tired motor do not push the same air through identical ductwork.

So the right question is not "what should a dryer vent read" but "what should your dryer vent read, and how far is it from that." A vent that goes from restricted to strong is a good result whether the number ends up high or moderate. A vent with a short straight run that measures merely acceptable may actually be underperforming badly for what it is.

That is exactly why we measure your vent instead of quoting you an industry average.

Reading your after number

The interesting information is in how much the number moved. There are three broad outcomes, and each one means something different for what you should do next.

The number improved a lot

This is the common case and the happy one: the lint was the problem. The duct itself is sound, the run is workable, the termination opens, and what was choking the system was accumulation — which is now in a bag on our truck.

If this is your result, nothing further is needed. Note the date, keep the report, and plan on doing this again about once a year. The U.S. Fire Administration and the Consumer Product Safety Commission both recommend dryer venting be cleaned at least once a year, and that cadence exists precisely because lint comes back.

The number improved only a little

This is the result worth paying attention to, because it is telling you something a clean duct cannot fix. If we pulled real lint out and the airflow barely responded, something structural is limiting the system. The usual suspects:

• A crushed section — most often the transition hose, flattened when the machine was pushed back against the wall, but sometimes a duct pinched inside a wall or a floor joist bay.
• A disconnected or separated joint, where part of your exhaust has been dumping into a wall cavity, a crawl space, or the back of a cabinet instead of going outside.
• A flap or damper that will not open properly — painted shut, rusted, weighted down with old lint, or blocked from the outside.
• A run that is simply too long or has too many bends for the machine on the other end of it.

When we see this we tell you which one it is, and we show you. The fix is usually a repair rather than another cleaning, which is a different service and a quote-only one — new fireproof NYC-approved metal transition hose, a new exterior hood with a working damper, or a rerouted section. The fastest way to get a price on that is to text us a photo.

The number barely moved at all

Occasionally the reading is poor before and poor after, and very little lint came out. That combination is actually informative: it means lint was never the story. Something else is going on — a blockage we cannot reach from either end, a bird nest packed into a termination that needs opening up, a duct that was never correctly connected, a run to a termination we cannot access, or in some cases a dryer that is no longer moving air the way it should.

We are not going to charge you for a cleaning and walk away from that. We tell you what we found, what we think it is, what we can do, and — if it needs a trade we are not, like an appliance technician or a roofer — we say that plainly. A vendor who cannot tell you why your vent is still bad has not diagnosed anything.

What to do with the written report

The reading comes with a written report, and it has more uses than the drawer.

• Send it to a co-op board or a managing agent. If you live in a building that asks residents to demonstrate their vent has been serviced — and more Brooklyn buildings are asking every year — a dated report with before-and-after numbers is exactly the documentation they want. It is far stronger than a receipt, which only proves you paid somebody.
• Give it to an insurer or an adjuster. Documented maintenance on a system with a known fire risk is worth having on file, particularly if you ever need to show that the system was maintained.
• Hand it to a buyer or a tenant. It is a small, concrete piece of evidence that the place has been looked after.
• Keep it as your baseline. This is the one people underuse. Next year's reading only means something if you have this year's to compare it against. Over two or three visits you learn how fast your particular vent loads up, which tells you whether once a year is right for your household or whether you should be doing it more often. If you have pets, a big family, or a long roof run, you may find your numbers fall off faster than the standard advice assumes.

We keep a copy too, so when we come back we already know what your vent measured last time and what we found in it.

Be suspicious of a company that will not measure

We will put this plainly, and we understand how it sounds coming from us.

If a vent cleaning company cannot or will not give you a before-and-after airflow reading, ask yourself what they are able to demonstrate about the work. Not what they promise — what they can show. A photo of lint proves lint existed. A vacuum running for twenty minutes proves noise. Neither one proves the duct moves air now.

Measurement also constrains the vendor, which is the real reason it is not universal. A company that publishes a before number cannot pretend a healthy vent was an emergency. A company that publishes an after number cannot walk away from a job that did not work. Those are both good outcomes for you and slightly inconvenient ones for anybody selling on urgency.

If you are still working out whether your vent needs attention at all, the symptoms are the place to start — we list them in the warning signs your dryer vent is clogged, and the broader case for doing this at all is in why dryer vent cleaning matters.

Frequently asked questions

Do you take the reading before or after the cleaning?

Both. Before we start, so we know what we are dealing with and you know what you were living with, and again after the work is finished with the dryer running. Both numbers go on the report with the date.

What if my before number is fine? Do I still pay?

Yes — you booked an inspection and a cleaning, and both happened. But you also get the most valuable possible outcome, which is documented evidence that your vent is in good shape and a baseline for next time. We would far rather tell you your vent is healthy than invent a problem, and we will tell you if you can comfortably stretch the interval.

Can I measure my own vent?

Not meaningfully without an anemometer and a sense of what to compare the result against. What you can do is watch the symptoms — dry times creeping up on the same setting, a hot laundry room, a machine that shuts off mid-cycle, lint appearing around the door seal. Those are the household version of a falling airflow number.

Does the reading tell you whether my duct is the right size or material?

The number does not, but the inspection that goes with it does. While we are in there we are looking at the material, the connections, and the termination. Foil and white plastic transition hoses are not permitted in NYC, and if you have one we will tell you, because it is both a code problem and a performance problem.

Is this the same for AC ducts, hoods, and chimneys?

The written report and the photos are standard across our services, but the before-and-after airflow reading is specific to dryer vent cleaning, where the whole system is a single measurable exhaust path. On AC duct work we re-check airflow at the vents after ductwork replacement, which is a related but different measurement.

Book your dryer vent cleaning

A number you can check is a promise a company has to keep. That is the whole reason we measure — not because it makes a nicer-looking invoice, but because a duct is invisible and somebody has to be accountable for what happens inside it.

Vent Pro NYC cleans dryer vents for homes, apartments, and multi-unit buildings across Brooklyn and Deal, NJ. We are family-owned, licensed, and insured, and every visit ends with a before-and-after airflow reading and a written report you can keep. Book online or call or text us at (718) 541-5567.