Humidity-Tracking Extractor Fans: Why Landlords Are Replacing Standard Bathroom Fans
The standard bathroom fan in a rental property does almost nothing useful.
It comes on with the light. It runs for a fixed overrun — often three minutes, because that is where the dip switch was left in the box. It shifts a fraction of the air it is rated for, because it has been ducted through a crushed flexible hose into a soffit with a closed grille on the end. And it stops long before the moisture from a ten-minute shower has actually left the room.
Then the tenant reports mould, and everyone concludes the property has a damp problem.
Quick answer: A humidity-tracking (hygrostat) extractor fan continuously monitors relative humidity and runs at whatever speed is needed to remove moisture, ramping up when someone showers and dropping back to a trickle the rest of the time — rather than running blind on a timer. In rental property they substantially reduce condensation and mould complaints because they cannot be defeated by a tenant who does not use the light or who switches the fan off. Supplied and fitted, a Greenwood Airvac humidity fan is £330 + VAT and an EnviroVent filterless fan is £430 + VAT, including drilling through the external wall, ducting and certification. Call 0161 706 1360 to arrange a survey.
The Problem With Standard Timer Fans
An ordinary light-switch-linked fan fails in rental property for reasons that have nothing to do with the fan being cheap.
It only runs when the light is on. In summer, in a bathroom with a window, people do not put the light on. The fan never runs. Moisture accumulates all the same.
The overrun is far too short. A three-minute overrun after a hot shower removes a fraction of the moisture. The rest condenses on the walls and ceiling and evaporates back into the property over the following hours, moving into cooler bedrooms where it lands on cold walls.
It can be switched off. Pull-cord and isolator-switched fans get turned off — because they are noisy, because a baby is sleeping, or because a tenant is worried about electricity costs. Once off, they stay off.
Nobody knows when it has failed. A seized fan looks exactly like a working fan. We routinely find fans that have not moved air in years, in properties where the mould has been "treated" three times.
It is almost always undersized in practice. A fan rated at 15 litres per second on the box will deliver far less through four metres of concertina'd flexible ducting with a fly screen on the outside grille — and the rated figure is what appears on the specification, not what reaches the outside air.
Every one of those failure modes is a landlord problem, because the outcome is a mould complaint, a disrepair claim or a void period spent redecorating.
How Humidity-Tracking Fans Work
A humidity-tracking fan contains a hygrostat — a sensor reading relative humidity in the room — and control electronics that respond to it.
In normal conditions the fan runs at a low trickle rate, moving a small continuous volume of air and using very little power. When humidity rises — someone runs a shower or a bath — the fan detects the change and ramps up to its boost rate, extracting hard until the humidity comes back down, then settles back to trickle.
The important behaviours for a landlord:
It runs on humidity, not on the light switch. Summer showers with the light off are still extracted.
It runs for as long as it takes. Not three minutes — as long as the moisture is actually present. A long bath in a cold bathroom gets a long extract; a quick hand wash gets nothing.
Better models track the trend, not the absolute number. This matters more than it sounds. A simple humidistat set to a fixed threshold will either run permanently through a humid week or never trigger in a dry one. Intelligent models learn the property's background humidity and respond to a rise against that baseline, which is why they behave sensibly in a Manchester winter.
Constant low-level extraction prevents build-up. The trickle rate is doing quiet, continuous work between moisture events, which is where the real gain over a timer fan comes from.
What the Manufacturers Offer
EnviroVent produce a well-established range of humidity-tracking extract fans for domestic wet rooms, including filterless designs that avoid the maintenance item of a filter that nobody ever changes. Their continuous-running fans with intelligent humidity tracking are the specification we most often use for bathrooms in tenanted property.
Greenwood Airvac — long-established in UK domestic ventilation — produce continuous-running unitary fans with humidity sensing designed for exactly this application, alongside conventional intermittent extract models.
Both manufacturers also produce dMEV units (decentralised mechanical extract ventilation): fans designed to run continuously at a low rate as the primary ventilation strategy for the dwelling, boosting on humidity. For a property with an ongoing condensation problem, dMEV in the bathroom and kitchen is usually the right specification rather than intermittent fans.
Model ranges change, so we specify current models at survey rather than quoting a part number from a brochure. The comparison between the two manufacturers is covered in detail in our EnviroVent vs Greenwood Airvac guide.
Costs
| Item | Price | |---|---| | Greenwood Airvac humidity fan, supplied and fitted | £330 + VAT | | EnviroVent filterless fan, supplied and fitted | £430 + VAT |
What the price includes: the fan, core drilling through the external wall, the wall liner and ducting, the external terminal, the electrical connection, making good, commissioning and certification. Installed price, not supply-only with extras bolted on afterwards.
Running cost is separate and trivial — a few watts at trickle speed, roughly £8–£20 a year in electricity at typical rates.
Unusual wall construction, difficult access or a property needing new circuit work can move the price, and we confirm on survey before booking. Multiple properties booked together are priced on a schedule of rates — see our portfolio compliance guidance.
The Installation Is Most of the Job
We will say this bluntly, because it is the single most common reason a landlord has paid for a fan that does not work: the fan is the cheap part, and the ducting is what decides whether it works.
What a correct installation involves:
Ducting that is as short, straight and smooth as possible. Rigid duct beats flexible every time. Where flexible has to be used, it must be pulled taut, not concertina'd across the loft. Every bend costs airflow, and a crushed run can halve the delivered rate.
Correct duct diameter, maintained. A 100mm fan needs 100mm duct all the way, not a run that necks down at a joist or through an adaptor.
A proper external terminal. Grilles with tight fly mesh and stiff backdraught flaps strangle the airflow they are meant to pass. It must terminate outside the building — not into the loft, not into a soffit void. Discharging a bathroom fan into a cold loft is one of the most damaging things you can do to a roof structure, and we find it regularly.
Insulated ducting through cold spaces. Warm wet air moving through uninsulated duct in a cold loft condenses inside the duct, and the water runs back down into the fan and then through the ceiling. The stain that appears around a bathroom fan is very often this, not a roof leak.
Compliant electrical work. Bathroom zones under BS 7671 dictate what may be installed where and to what IP rating; a fan in the zone above a shower needs appropriate protection, and is normally an SELV (low voltage) unit with the transformer sited outside the zones. The fan needs a means of isolation for maintenance, and the work should be certificated. Adding a fan to an existing circuit in England is not notifiable under Part P, but it still has to comply with BS 7671 and you should receive a Minor Electrical Installation Works Certificate.
Commissioning and measurement. The extract rate should be measured after installation, not assumed from the box. Required rates are set out in Approved Document F — covered in our Part F guide.
Why This Is Now a Compliance Issue, Not Just Maintenance
Damp and mould in rented housing has moved from a maintenance grumble to a legal risk with real financial consequences.
Awaab's Law took effect for social landlords on 27 October 2025, imposing binding timescales on investigating and remediating damp and mould. Extension into the private rented sector is expected from 2027. Alongside it, damp and mould are Category 1 hazards under the Housing Health and Safety Rating System, giving local authorities enforcement powers now — and Greater Manchester councils have been notably active since the Rochdale case.
What that means practically for a landlord:
- Blaming tenant lifestyle is no longer a viable defence on its own
- You need to be able to evidence that adequate ventilation is installed and working
- A documented ventilation upgrade with a certificate is a materially stronger position than a record of repeated mould washes
- Fitting effective ventilation is cheaper than a disrepair claim by a wide margin
Frequently Asked Questions
Are humidity-tracking fans noisy?
Modern continuous-running units are very quiet at trickle speed — most people stop noticing them within a day. They are audible when boosting, which is the point. If a tenant complains about noise, it is usually a badly mounted fan transmitting vibration into the ceiling, or duct noise from an undersized run, both of which are fixable.
Do they cost more to run than a normal fan?
Slightly, in electricity terms, but the numbers are trivial — a continuous fan at trickle typically costs well under £20 a year to run. That is far less than the cost of redecorating a mouldy bedroom once.
Can a tenant turn one off?
That is exactly the design intent — they are normally wired to be permanently live so they cannot simply be switched off, with isolation provided for maintenance only. Where a tenant has been switching fans off, this alone often solves the problem.
Will a better fan fix a serious mould problem on its own?
Usually, where the moisture is being generated in the bathroom and kitchen — which is the large majority of condensation cases. Continuous extraction at source stops the moisture spreading into cooler rooms in the first place, which is why mould in a back bedroom often clears once the bathroom is extracting properly. Where mould is tied to a building defect — a leaking gutter, failed pointing, a plumbing leak — no fan will fix it, and we will tell you that at survey rather than selling you one.
Can you replace fans across a whole portfolio?
Yes, and it is one of the more cost-effective programmes we run. Fans are quick to swap, we can do them alongside EICR or remedial visits so properties are attended once, and pricing is on an agreed schedule of rates.
How long does a fan replacement take?
A like-for-like swap with usable existing ducting is typically under an hour. Where new ducting, a new external terminal or new wiring is needed, allow half a day. Both are usually achievable in a tenanted property with minimal disruption, though void periods are the easiest time to do it — see void period compliance.
Arrange a Ventilation Survey
Manchester Compliance installs EnviroVent and Greenwood Airvac ventilation across Greater Manchester for landlords, letting agents, block managers and housing providers. EnviroVent filterless fans, Greenwood Airvac humidity fans, dMEV and kitchen extraction — installed through the wall with proper ducting, measured on commissioning and fully certificated.
Call 0161 706 1360 Email: hello@manchestercompliance.co.uk
Covering Manchester, Salford, Stockport, Tameside, Oldham, Rochdale, Bury, Bolton, Wigan and Trafford.