Extract fan, trickle vent, PIV or MVHR — which ventilation does my house need?

Ventilation advice tends to arrive as a list of products. It is more useful as a hierarchy, because the options are not alternatives — they do different jobs, at different points, at different costs, and the cheap ones are also the effective ones. The expensive systems are worth considering only once the cheap layer is genuinely working.

Step by step

1. Start with extraction at source.A moisture problem is easiest to solve at the point the moisture is made. A bathroom fan that actually moves air, ducted on a short route with a fall to the outside, with an overrun timer and the door kept shut, removes the shower's water before it reaches the rest of the house.
2. Test the fans you already have.Hold a sheet of paper against the grille. If it does not hold, the fan is not moving useful air — usually because the duct is long, kinked, uninsulated, crushed in the loft, or terminates at a closed backdraught shutter.
3. Sort out the background ventilation.Trickle vents open and clean, airbricks clear, and any purpose-provided vents unblocked. This is the continuous small air change the house needs when nobody is opening a window.
4. Change how you air the house.Windows wide for a short period beats a window ajar all day, because you exchange the air without cooling the fabric. Mornings in bedrooms, and after cooking or bathing, are when it matters.
5. Consider a dehumidifier for a specific problem.It is the only option that removes water without removing heat. It suits the room where washing dries, or a whole flat where nothing else is practical, and it costs electricity while it runs.
6. Consider PIV when the whole house is humid.A single-point loft or wall unit that dilutes the indoor air continuously. Effective on whole-house humidity, adds a heating cost, and needs somewhere for the displaced air to leave.
7. Consider continuous mechanical extract as the alternative retrofit.A central unit extracting at a low continuous rate from the wet rooms. Similar scale of work to PIV, works by removal rather than dilution, and suits houses where a loft unit is not practical.
8. Treat MVHR as a new-build or deep-retrofit measure.It recovers most of the heat from the extracted air, which is genuinely valuable, but it needs ducts to every wet and habitable room and an airtight envelope. Retrofitted into a leaky house it is expensive and underperforms.
9. Reduce the moisture load in parallel.None of these are as cheap as not putting the water into the air in the first place. Lids on pans, washing dried outside or in a vented dryer, and the bathroom door shut during a shower.

Tips

• The commonest ventilation fault in UK houses is not an absent system — it is a bathroom fan discharging into a loft, or into a long crushed flexible duct that has filled with condensate.
• Humidity-sensing fans that run automatically when the room gets damp are a good match for households that forget, and for bathrooms without a window.
• Whatever you fit, ventilation and warmth work together. A well-ventilated cold house still grows mould, because the surfaces are still below the mould threshold.

Common mistakes

• Buying a whole-house system before checking the extract fans — It is common to find that the bathroom fan has never worked properly, and fixing it costs a small fraction of a whole-house unit and solves the problem at source.
• Retrofitting MVHR into an ordinary leaky house — Heat recovery depends on the extracted air being the main route out of the building. In a leaky house most of the air leaves through the fabric, so the unit recovers heat from only a fraction of it and the payback never arrives.

Alternatives

• Extract fans at source: Cheapest, most targeted, and the first thing to get right. Needs short well-fallen ducting, an overrun timer and a working backdraught shutter.
• Trickle vents and airbricks: Continuous background air change for almost nothing. Only works if they are open and clear, which is the usual failure.
• Positive input ventilation: Single-point retrofit that dilutes whole-house humidity. Good record on persistent whole-house condensation, adds a heating cost.
• Continuous mechanical extract: Central continuous low-rate extraction from the wet rooms. Works by removal rather than dilution; a good alternative where a loft unit will not fit.
• Mechanical ventilation with heat recovery: Removes the moisture and keeps most of the heat. Requires airtightness and full ducting, so it belongs in new build or deep retrofit rather than a straightforward upgrade.
• Dehumidification: Removes water without removing heat, portable, no installation. Running cost while operating, and it manages the symptom rather than ventilating the house.

Questions people ask