Bathroom extractor fan upgrade for Airbnbs: the 15-minute overrun timer that fixed my damp reviews

The damp complaint that changed my thinking wasn't the first one. It was the third. Two reviews in three months mentioning 'slight condensation on the windows' and 'a faint musty smell in the bathroom' — both in my Edinburgh Newington flat, both in autumn. I'd dealt with both by opening windows and using a spray bleach on the grouting. They came back.

An electrician who came to sort out a different job looked at my extractor fan, switched it on, held a tissue at the intake grille, and the tissue barely moved. The fan was running — the motor was fine, the light on it was on — but the ductwork had a 90-degree bend and a three-metre run to the external vent that was reducing airflow to near nothing. The fan had been installed by the previous owner and had probably never extracted properly. I'd owned the property for two years without knowing.

New fan, new ductwork routing, timer overrun switch. Three autumn seasons since then. No more damp complaints.

What the regulations actually require

Approved Document F — the part of UK Building Regulations covering ventilation — sets minimum extraction rates for bathrooms in dwellings. For an intermittent extract fan (one that runs only when switched on), the minimum is 15 litres per second. For continuous background ventilation — rarer in domestic settings — it's 8 l/s.

The other requirement most hosts miss: a timer overrun. An intermittent bathroom fan must continue to run for at least 15 minutes after the light is switched off to clear residual humidity. A humidity-sensing fan — one that runs until the relative humidity drops below a set threshold — is an acceptable alternative to a fixed timer. Both work. Both are better than a fan that switches off the instant the light goes off, which is what most older properties have.

These rules apply when you replace an existing fan, not just when you install new. If the replacement fan is rated below 15 l/s, it's technically non-compliant — even if you're just swapping like for like. Most modern bathroom extractor fans from reputable manufacturers meet the 15 l/s threshold, so in practice it's rarely an issue, but it's worth checking the spec sheet.

What an underpowered fan actually looks like

The tissue test is real and useful. Hold a sheet of tissue about 5cm from the grille intake of a running extractor fan. On a properly functioning 15+ l/s fan, the tissue should be noticeably pulled toward the intake and hold there. If it barely moves, the airflow is insufficient — either the fan is undersized, the ductwork is obstructed or poorly routed, or the external vent is blocked (birds' nests, insulation having been pushed over it in a loft, a cap that's stuck shut).

The external vent is the part that gets missed. I had one property where the fan worked perfectly and the ductwork was clear, but the external louvre vent had a broken flap that was stuck partially closed. The flap had been installed to stop draughts coming back in and it was doing its job very well in both directions.

Replacement options: what to buy

For a standard bathroom retrofit, I've settled on two options:

Xpelair DX100HTS — humidity-sensing fan with timer overrun, 15 l/s rated, 100mm duct connection, white front panel that fits standard square cutouts. Around £45–55 depending on the retailer. The humidity sensing is genuinely useful in an STR context because you can't rely on guests to leave the fan running long enough — it runs until the bathroom is dry, then stops.

Vent-Axia Solo Plus BT — slightly quieter at 25 dB(A), also humidity-sensing, 100mm, similar price bracket. Good for bedrooms with en-suites where noise matters more.

Both are available at Screwfix and Toolstation. Both are straightforward retrofits assuming your existing ductwork is 100mm. If your ductwork is the older 4-inch (102mm) standard, it's compatible. If it's 125mm, you need a reducing collar.

A like-for-like replacement is a DIY job if you're comfortable with electrical work — it's a switched live and neutral, that's it. If not, an independent spark will do it in under an hour: expect to pay £60–90 in Brighton or Edinburgh for the call-out and installation, not including the fan itself.

Timer switches: if you're keeping an existing fan

If the fan itself is adequate but it has no timer overrun, the cheapest fix is a replacement timer isolator switch. This is the switch on the bathroom wall or pull-cord that controls the fan independently of the light — or a dual function switch that runs the fan for a set period after the light goes off.

Greenbrook and MK Electric both make 4-in-1 or 6-in-1 bathroom timer switches in the £20–35 range. These allow you to set overrun times of 1–30 minutes. They fit in a standard single back box. Again, it's a like-for-like wiring swap — straightforward if you're comfortable, an hour's work for an electrician if not.

This single change — adding a 20-minute overrun timer to an existing functional fan — solved the condensation problem in my Lake District cottage without touching the fan or ductwork. The fan had always been adequate; it had just been switched off too quickly every time.

The inspection I now do at every property in October

Every October, I do the tissue test at every bathroom in every property. I also check the external vent (visual from outside, or from the loft if it's a roof vent). I check the timer overrun is working by switching off the light and watching the fan continue to run. Takes ten minutes per property.

It's not a glamorous maintenance job. But 'slight damp smell' in a November review, from a guest who arrived when the heating had been off for four days and the bathroom hadn't been ventilated properly — that's recoverable. 'Mould by the window' in the same review is harder to come back from, and the cost of fixing it properly goes up fast.

Check the fan. Check the vent. Check the timer. It costs very little to get right and very much to put right once it's gone wrong.