Introduction
Foggy mirrors, musty smell, and clients complaining the “new fan doesn’t do anything”? You’re not alone. Many contractors find that a fan rated well on the box delivers a fraction of the airflow once it’s squeezed through long, kinked duct runs and flimsy soffit vents. Here’s what’s going wrong, why it matters (mould, call-backs, warranty headaches), and how to fix it for good. We’ll walk through fast diagnostics, proven upgrades, commissioning that sticks, and prevention on future jobs. Along the way, I’ll show where a quick on-site capture with photos and voice notes can help you turn findings into a clean, signable proposal fast.
Table of Contents
Key Takeaways
- Real airflow is often much lower than the box rating. It’s common for installed fans to deliver less than half the claimed flow due to duct losses and poor terminations.
- UK Building Regulations Part F calls for 15 litres per second (l/s) intermittent extract in bathrooms with a bath or shower. Design to meet this at the grille, not just at the fan.
- In general, every tight 90-degree bend and each extra metre of flexible duct can noticeably cut airflow. Straight, short, rigid 125 mm ducting lifts performance dramatically.
- Upgrading from 100 mm to 125 mm ducting commonly raises delivered flow by 20–40% and reduces noise, because pressure losses drop.
- Commissioning with a simple vane anemometer and setting proper overrun/humidity control reduces call-backs and mould complaints.
The Problem: Box Ratings Vs. Reality
Fan boxes shout big numbers measured in ideal lab conditions. On site, fans fight long runs, crushed flex, undersized grilles, and sticky backdraft flaps. The result? Mirrors fog, ceilings spot, clients call.
- Industry practice shows installed flow is commonly well below rated flow once duct losses bite.
- In general, a sharp 90-degree bend can reduce airflow by around 10–30% depending on duct size and fan curve.
- Flexible duct that isn’t fully stretched can add significant resistance; every extra metre often knocks more flow off.
Why It Matters
- Part F compliance (bathroom intermittent: 15 l/s; continuous systems follow lower trickle with boost figures) is there to prevent damp and mould.
- Poor airflow means condensation, peeling paint, and warranty visits that eat your week.
Example Scenario
A 1930s terrace with a 100 mm wall fan and 3 m of loose flex to a soffit grille. Rated 90 mÂł/h on the box (~25 l/s). Delivered at the grille? Tissue barely holds. Client complains of mould after showers. Classic case of duct and termination losses.
Fast Diagnostics That Dont Lie
Quick Checks You Can Do In Minutes
- Tissue test at the grille: holds firm or flutters? Weak hold hints at low flow.
- Visual duct check: crushed flex, sagging loops, tight bends, diameter reductions.
- Termination: louvred soffit packed with fluff? Gravity flap stuck half-closed?
Measure Airflow Simply
- Vane anemometer at the grille: take multiple readings and average. You want to see around 15 l/s for intermittent extract in bathrooms with a bath or shower.
- If you can’t measure, assess indirectly: with door closed and an adequate undercut, a good fan should pull noticeable make-up air beneath the door.
Capture Findings For A Decision-Ready Proposal
- Talk through what you see while you’re still on site. Many contractors find that 60 seconds of voice notes and a few photos are enough to explain the problem and the fix clearly to the client.
- With Donizo, you can capture voice, text and photos and instantly turn them into a professional, branded PDF proposal (paid plans remove watermarks), ready to e‑sign. Clients often decide faster when they can see the issues you’ve photographed.
Fixes That Restore Real Airflow
Prioritise The Air Path
- Go rigid where you can. Smooth rigid duct keeps losses low. Keep runs short and straight.
- Upsize the duct. Moving from 100 mm to 125 mm commonly boosts delivered flow by 20–40% with the same fan because static pressure drops.
- Minimise bends. Where unavoidable, use large-radius bends. In general, each tight 90-degree bend can cost a noticeable chunk of flow.
Terminations That Don’t Strangle Flow
- Use a quality external grille or roof cowl sized to the duct. Tiny louvred soffit vents can choke a good fan.
- Fit a low-resistance backdraft damper. Many plastic flaps stick; an inline butterfly damper with a light spring is more reliable.
Choose The Right Fan For The Route
| Option | Current State | Improvement |
|---|
| Wall/panel fan (100 mm) | Easy swap, struggles with long or bent duct | Fine for very short, straight runs. Keep the path clean and short |
| Inline mixed-flow fan (125 mm) | Fan remote in loft, quieter in room | Handles longer runs with better pressure capability; lower noise at grille |
| Through-roof kit (125 mm) | Neutral pressure path upwards | Often the best performance; specify a proper cowl to avoid wind-driven backdraft |
Seal And Support
- Tape and mastic every joint. Leakage in lofts can easily halve effective flow to the outside.
- Support flex every 600–900 mm, fully stretched, no bellies. Keep flex sections as short as practical.
Real-World Example
Back to that 1930s terrace. We replaced the 100 mm panel fan with a 125 mm inline mixed-flow fan, swapped 3 m of sagging flex for 1.8 m rigid plus a single smooth-radius bend, and installed a low-resistance soffit cowl. Commissioned airflow went from “tissue flutters” to a steady 16–18 l/s at the grille. Client reported mirrors clear and paint staying dry after a week. Noise at the room grille dropped notably because the fan was now remote and the duct was larger.
Controls= And Commissioning That Stick
Controls= That Actually Get Used
- Overrun timer: set 15–30 minutes so moisture clears after the shower. Many contractors find clients won’t run fans unless it’s automatic.
- Humidity sensor: aim for a sensible trigger point (commonly around 60–70% RH) to avoid constant cycling in humid climates.
- Light-link vs independent: in family homes, a light-linked boost with a proper overrun usually gets the best compliance.
Commissioning: Make It Measurable
- Target Part F figures at the grille (bathrooms: 15 l/s intermittent). Record readings in your job notes.
- Check door undercut or transfer path. No make-up air = poor extract.
- Verify backdraft damper movement and termination flap action.
Document The Result
- Snap the anemometer reading and the final install. With Donizo, attach photos and a brief voice summary, then generate a proposal-turned-work-report on paid plans with work report exports. Clients see proof, you reduce disputes.
Prevention On Future Jobs
Design Rules That Save You Call-Backs
- Size up ducting by default (125 mm) unless the route is very short and straight.
- Keep total equivalent length low. In general, every extra metre of flex and each sharp bend adds up fast.
- Prefer inline mixed-flow fans for longer or complex routes; keep the fan accessible for maintenance.
- Specify quality terminations and low-resistance dampers.
Set Expectations With Plain Language
- Explain that fans need door undercuts and regular cleaning of grilles.
- Note that very cold lofts can encourage condensation in ducts; specify insulated duct where routes pass through unheated spaces.
Turn Site Notes Into Fast Approvals
- On site, speak your findings and snap photos. In minutes, create a clear, line-by-line proposal in Donizo using voice, text and images.
- Send a branded PDF (paid) with a client portal link. They can accept with a legal e‑signature, and you can convert to an invoice in one click when the job’s done. Contractors often report this cuts back-and-forth by half and saves a couple of admin hours a week.
Frequently Asked Questions
What Airflow Do I Actually Need In A UK Bathroom?
Part F specifies 15 l/s intermittent extract for bathrooms with a bath or shower. Continuous systems have different trickle/boost values, but for a simple intermittent fan, aim for 15 l/s measured at the grille after installation.
Is 100 Mm Duct Ever OK?
Yes, for very short, straight runs with a low-resistance termination and a capable fan. In general, once you add bends or length, 100 mm struggles. Upsizing to 125 mm typically lifts delivered flow and cuts noise.
My Fan Is Noisy But Weak. Why?
Usually back-pressure. Long or crushed flex, multiple tight bends, or a tiny termination can create high resistance. The fan works harder (noise increases) yet moves less air. Straighten and upsize the path, then consider an inline mixed-flow fan.
Do Humidity Sensors Help With Mould Complaints?
Commonly, yes. A well-set humidity sensor keeps the fan running until moisture is cleared, even if someone turns the light off. Pair it with proper ducting and you’ll see fewer condensation issues and call-backs.
Measure at the grille with a vane anemometer and record the value. Take photos of the setup and the reading. Use Donizo to compile a clean PDF report (paid plans support custom branding and no watermark) and get it e‑signed. That transparency builds trust and reduces disputes.
Conclusion
Underperforming bathroom extract isn’t about the badge on the fan. It’s the air path, the termination, and commissioning. Keep it short, straight, and preferably 125 mm; choose inline where runs get long; set overrun or humidity control; and verify 15 l/s at the grille. Do that and you’ll cut mould complaints and call-backs dramatically.
On site, capture photos and a quick voice summary, then turn it into a professional proposal with Donizo. Clients can e‑sign on the spot, and when the job’s accepted, you can convert the proposal to an invoice in one click. Less admin, faster yes, better outcomes.
