A surprising amount of the “damp” I get called out to isn’t damp at all. It’s condensation: warm moist air from cooking, bathing and breathing hitting a cold bedroom wall because the extract fans that were supposed to remove it are moving almost nothing. The fan hums, the occupier assumes it works, and the mould behind the wardrobe says otherwise. The only way to know what a fan actually does — as opposed to what its box claimed in 2009 — is to measure it, and the tool for that is an anemometer.
An anemometer measures air velocity. Multiply velocity by the area the air passes through and you get volume flow, which for domestic ventilation we express in litres per second — the unit Approved Document F uses for every extract rate it sets. That arithmetic is the entire job. A bathroom fan is supposed to shift 15 l/s in intermittent use; I regularly measure fans doing 4. Nobody can see the difference by holding a hand up to the grille, which is why the measurement matters.
The instruments below are the ones I’d put in the bag today, from the smart probe much of the ventilation trade has standardised on down to a budget vane meter that’s perfectly capable of telling a landlord their fan is dead. What separates the good ones from the junk isn’t accuracy at 30 m/s in a wind tunnel — it’s whether the thing reads sensibly at the low velocities a domestic fan produces, whether it can average a reading rather than flicker at you, and whether it converts to volume flow without you doing sums on the van bonnet.
The one I’d buy
Testo 410i vane anemometer smart probe
A 40 mm vane head that talks to your phone over Bluetooth, made by the people who build most of the serious HVAC instrumentation in Europe. You enter the outlet dimensions once and it reads volume flow directly, does timed and multi-point averaging, and drops a logged record into a PDF for the report. For measuring extract fans against Part F it does everything a £500 instrument did ten years ago.
Typically £90–£110
Check price on AmazonHow I judge an anemometer
Low-end sensitivity. The marketing lists the top of the range — 30 m/s, gale force. Irrelevant. A domestic extract grille produces somewhere between 0.5 and 3 m/s across its face. What matters is the bottom of the range and the resolution down there. A meter that starts reading at 0.4 m/s with 0.1 resolution is useful; one that starts at 1.1 m/s will tell you a struggling fan is doing nothing at all, which is a different finding.
Volume-flow readout. Part F speaks in litres per second, not metres per second. An instrument that takes duct or grille dimensions and reads out volume flow directly saves the conversion and, more importantly, the transcription error at nine o’clock at night when you’re writing up.
Averaging. Airflow at a grille is turbulent and lumpy. A single spot reading is nearly meaningless; a timed average over 30–60 seconds, or a multi-point traverse, is a measurement. Good instruments do this for you.
Vane size against what you’re measuring. A big 100 mm vane averages nicely over a large grille but physically blocks a 100 mm fan spigot and disturbs the very flow it’s measuring. A small vane samples one point of a big grille and misses the rest. There’s no perfect head, which is why the cone matters — more on that below.
Survivability. This kit lives in a van and gets used at the top of a ladder over a bath. Vanes with exposed bearings and flimsy telescopic stems don’t last. Anything I recommend is built by a firm whose gear has a track record of surviving exactly that treatment.
The comparison
| Model | Type | Range | Typical price | Best for | |
|---|---|---|---|---|---|
| Testo 410i | 40 mm vane, Bluetooth | 0.4–30 m/s | ~£100 | Extract testing with logging | View |
| BTMETER BT-100 | Handheld vane | 0.3–30 m/s | ~£30 | Budget spot checks | View |
| Extech AN200 | 72 mm vane + IR thermometer | 0.4–30 m/s | ~£150 | The professional standalone | View |
| Testo 405i | Hot-wire, telescopic, Bluetooth | 0–30 m/s | ~£100 | In-duct and trickle-vent flows | View |
| Kestrel 1000 | Pocket impeller | 0.6–40 m/s | ~£80 | Site wind, survives anything | View |
| Proster handheld | Handheld vane | 0.3–30 m/s | ~£18 | Cheapest workable meter | View |
| Aranet4 Home | NDIR CO₂ monitor | 0–9,999 ppm | ~£180 | Whole-room ventilation checks | View |
The picks in detail
Testo 410i vane anemometer smart probe
Best overall — the extract-testing workhorse
The top pick above, in more detail. The 410i is a 40 mm vane on a compact probe body with no screen of its own — everything happens in the Testo Smart app on your phone. That sounds like a gimmick until you use it: you set the grille dimensions, choose timed averaging, hold the vane in place and let it do a proper measurement rather than squinting at a jittering display. The logged readings export straight into the job file, which for PAS 2035 ventilation assessments — where you need evidence, not recollection — is the feature that pays for it. The 40 mm head is small enough not to choke a ceiling grille and works well behind a cone.
For
- Reads volume flow in l/s directly, with timed and multi-point averaging
- Logged, exportable readings — proper evidence for reports
- Testo build quality at a hobby-tool price
Against
- Useless if your phone battery is dead
- No standalone display, which some people never forgive
Typically £90–£110
Check price on AmazonBTMETER BT-100
Best on a budget — the honest £30 answer
A basic handheld vane meter with a backlit display, max/min/average functions and a velocity range that starts low enough (0.3 m/s) to be genuinely useful at a grille. It won’t compute volume flow for you — you’ll be multiplying velocity by area yourself — and I wouldn’t certify anything off it. But for a landlord or homeowner who wants to know whether the bathroom fan is doing anything at all before the mould conversation escalates, it answers the question for the price of a takeaway. Owners report units surviving drops off loft ladders and still agreeing with meters three times the price to within sensible tolerance.
For
- Reads down to 0.3 m/s, which many cheap meters don’t
- Average function smooths turbulent grille flow
- Cheap enough to live in the glovebox permanently
Against
- No volume-flow mode — you do the arithmetic
- Calibration is nominal; treat readings as indicative
Typically £25–£35
Check price on AmazonExtech AN200
The professional option — standalone, no phone required
If the idea of an instrument that depends on your phone offends you, this is the grown-up standalone: a 72 mm vane on a lead, a proper display body, CFM/CMM volume-flow modes with programmable duct dimensions, and a built-in infrared thermometer that’s handier than it sounds for spotting cold spots around the grille you’re testing. Extech is a Teledyne FLIR brand and it shows in the build. The tethered vane is the underrated feature — you can hold the head flat against a ceiling grille while reading the display at eye level, instead of performing yoga on a ladder.
For
- Volume flow with entered dimensions, no app dependency
- Separate vane on a lead — much easier at ceiling grilles
- 20-point average mode built in
Against
- Two or three times the price of the smart probes
- Volume flow is in CFM/CMM — you’ll convert to l/s for Part F
Typically £130–£180
Check price on AmazonTesto 405i hot-wire anemometer smart probe
Best for ducts — the low-velocity specialist
The 410i’s sibling, with a heated-element sensor on a 400 mm telescopic stem instead of a vane. Hot-wire sensors excel exactly where vanes give up: very low velocities, and inside ductwork through a test hole, where a vane physically won’t fit. If you’re commissioning or fault-finding MVHR and MEV systems, checking flow inside a rigid duct run, or trying to demonstrate that a trickle vent is actually passing air, this is the right sensor type. It’s more delicate than a vane — the element is exposed at the tip — and it dislikes dusty loft air, so it complements the 410i rather than replacing it.
For
- Reads the very low velocities vanes can’t resolve
- Telescopic stem reaches into ducts and awkward voids
- Same app, logging and volume-flow maths as the 410i
Against
- Fragile sensor tip — not a tool for the bottom of the bag
- Point measurement; needs a traverse for honest duct averages
Typically £90–£120
Check price on AmazonKestrel 1000
Best for site conditions — the indestructible pocket meter
Not an extract-testing tool, and I won’t pretend otherwise — but it earns its slot. Airtightness testing has wind-speed limits (results get unreliable above about 6 m/s at the building), and a pocket impeller that tells you the actual wind on site beats arguing with a weather app about conditions three miles away. The Kestrel is waterproof to IP67, floats, is drop-tested to military standards, and has a user-replaceable impeller. Kestrels bought in the nineties are still in service, which is not something anyone says about £18 vane meters. If your interest is external conditions, draughts at openings and general site use rather than certified extract rates, this is the one that will still work in 2040.
For
- Genuinely rugged — waterproof, floats, survives drops
- Replaceable impeller instead of a replaceable meter
- Instant, honest wind readings for test-day decisions
Against
- No volume flow, no logging, no low-velocity finesse
- Costs more than meters with longer feature lists
Typically £70–£90
Check price on AmazonProster handheld anemometer
Best for almost nothing — the £18 sanity check
The cheapest meter here and the one I recommend to homeowners who ask what to buy. It reads wind speed and temperature, runs for years on a 9V battery, and its 0.3 m/s floor means it can distinguish “fan working poorly” from “fan working not at all” — which, held against a bathroom extract that hasn’t been cleaned since installation, is frequently the whole diagnosis. Don’t expect calibration certificates or repeatability between units. Do expect it to settle the “the fan’s fine, it must be rising damp” argument in about forty seconds.
For
- Costs less than the call-out it can save
- Simple enough to hand to a tenant over the phone
- Surprisingly consistent for spot comparisons
Against
- Indicative readings only — not for reports
- Small vane samples a small patch of a large grille
Typically £15–£20
Check price on AmazonAranet4 Home CO₂ monitor
Honourable mention — measures the outcome, not the fan
Not an anemometer, but it belongs in a guide about ventilation measurement. An anemometer tells you what a fan moves; a CO₂ monitor tells you whether the room is actually being ventilated, occupants and all. The Aranet4 uses a proper NDIR sensor (the kind that doesn’t drift into fiction after six months), runs about two years on batteries, and logs to an app over Bluetooth. Leave it in a bedroom overnight: readings sitting above 1,500 ppm by morning are a ventilation failure regardless of what the fans measured, and that’s often the finding that persuades a client the trickle vents shouldn’t have been painted shut. Cheaper NDIR units exist (Temtop makes serviceable ones) but the Aranet’s logging and battery life are why it’s the one worth paying for.
For
- Real NDIR sensor with sane calibration
- Weeks of logged data — shows the overnight pattern that matters
- E-ink display, two-year battery life
Against
- Expensive for what is physically a small plastic box
- Measures occupancy effects, so needs interpreting with care
Typically £170–£200
Check price on AmazonWhat actually matters — and what doesn’t
The cone matters more than the meter. Hold a vane against a fan grille freehand and you’ll get a different number every time — move it 20 mm and the reading changes by half. The fix is a rigid hood or cone that fits over the whole fan and channels every litre of extract through a known aperture where the vane sits. Testo sell a proper funnel set (the testovent range) for their vanes, and generic rigid anemometer cones for 100–150 mm fans are a modest add-on. Professional powered flow hoods with back-pressure compensation exist and are lovely; they also cost more than every meter on this page combined, which is why they stay in commissioning engineers’ vans. For survey work, a rigid cone plus a decent vane and a timed average is the repeatable, defensible method.
Sensor floor beats sensor ceiling. Ignore the headline top speed. Nothing in a house moves air at 30 m/s, and if it does you have a more interesting problem than ventilation. Look at the minimum reading and the resolution near it.
Averaging and logging are what make a reading a measurement. One glance at a flickering display is anecdote. Sixty seconds of timed average, recorded with the grille size and the date, is evidence — the sort that survives a disagreement with a fan installer.
What the salesmen push that you can skip: wind-chill and Beaufort modes (this is a house, not a yacht), enormous multi-parameter displays showing humidity to two decimal places from a sensor worth one, and “16-in-1” feature counts. A meter that does velocity and volume flow properly beats one that does fourteen things approximately.
One more note for anyone doing retrofit work: under PAS 2035, a ventilation assessment isn’t optional garnish, it’s part of the process — and it’s the part that stops insulation measures turning a slightly damp house into a mouldy one. Tightening a building without checking how it breathes is how most of my condensation callbacks were created, usually by someone else, usually years earlier.
Questions I get asked
How do you measure extractor fan airflow? Fit a rigid cone or hood over the fan so the whole extract stream passes the vane, run a timed average of the velocity for at least 30 seconds, then multiply the average velocity by the free area of the cone’s aperture. Velocity (m/s) × area (m²) × 1,000 gives you litres per second. Instruments like the Testo 410i do the conversion on screen once you’ve entered the dimensions. Without a cone, treat any grille reading as a rough indication, not a rate.
What’s a good airflow reading for a bathroom fan? Approved Document F asks for 15 l/s from an intermittent bathroom extract fan, or 8 l/s continuously from a dMEV unit running on trickle. In the field, an intermittent fan measuring 10–12 l/s is tired but arguable; anything under about 8 l/s is failing at its one job, and the usual culprits are a clogged grille, a kinked flexible duct in the ceiling void, or a gravity flap stuck shut at the outlet.
Vane or hot-wire anemometer — which do I want? Vane for grilles, fans and anywhere the air arrives over an area: the rotor averages the flow across its own face and shrugs off knocks. Hot-wire for inside ducts and for very low velocities: the probe fits through a 10 mm test hole and resolves flows a vane won’t spin at, but it measures a single point and the element is delicate. If you only buy one for domestic ventilation testing, buy the vane; the hot-wire is the second purchase, not the first.
What are the Part F ventilation rates, briefly? For extract in dwellings: kitchens need 30 l/s intermittent from a hood over the hob (60 l/s elsewhere in the room), utility rooms 30 l/s, bathrooms 15 l/s and WCs 6 l/s — or lower continuous rates (13, 8, 8 and 6 l/s respectively) if the fans run all the time. On top of that sits a whole-dwelling ventilation rate that scales with bedrooms, which is what trickle vents and background ventilators exist to serve — and why the trickle-vent conversation happens on every window replacement and every retrofit, whether anyone enjoys it or not.
If the fans, the mould or the trickle vents are the actual problem rather than the shopping, that’s the day job: the practice carries out ventilation assessments — measured extract rates, condensation and mould diagnosis, Part F advice — and PAS 2035 retrofit assessments where ventilation is checked before anyone tightens the building. Get in touch, or browse the rest of the field-kit guides.
Disclaimer. This guide is general information, not a specification or professional advice for your particular building; measured ventilation rates should be assessed by a competent person. Prices are indicative, move constantly and are checked at the time of writing; the linked page always shows the current price. As an Amazon Associate I earn from qualifying purchases.