No statutory interval governs the servicing of a ventilation system in an occupied dwelling, and that is the first thing to tell a client asking which rule applies. The duties bite at other moments. On a new dwelling, regulation 42 of the Building Regulations 2010 requires the mechanical ventilation air flow rates to be tested and notice of the results given to the local authority within five days of the final test. Regulation 39 requires the person carrying out the work to give the owner written information on the system and its maintenance requirements within five days of completion. Everywhere else the only defensible benchmark is the flow rate itself: the Approved Document F figures, measured with a powered flow hood rather than judged by eye.
What the rules actually require, case by case
Three regimes that get confused
Ventilation servicing is not one subject. It splits into three regimes with different triggers, different deadlines and different paperwork.
| Case | Rule | When | Document produced |
|---|---|---|---|
| New dwelling | Building Regulations 2010, regulation 42 | once, at final test | notice of measured air flow rates to the local authority within 5 days |
| Mechanical system installed or extended | Approved Document F, section 4 | once, at commissioning | commissioning notice to the building control body |
| Occupied dwelling, system in service | no statutory interval | contractual | service sheet with measured flow rates |
New dwellings: a completion test, not a service
On a new dwelling the air flow rates must be measured, not estimated, and the results notified. The person responsible for commissioning completes air flow measurement and commissioning sheets carrying at minimum the information set out in Part 3 of the example sheet in Appendix C of Approved Document F. This applies to intermittent extract fans as well as to continuous systems, which is the part most often missed on a small job. The test fixes the starting condition; it discharges nothing afterwards.
Occupied dwellings: nothing imposed, everything contractual
Once the dwelling is in use, no rule sets a frequency. Approved Document F does require reasonable access for maintaining the system, including access to replace filters, fans and coils and access points for cleaning ductwork, which is a design duty rather than a servicing one. In practice that means your maintenance contract defines its own scope, and the only thing that makes it defensible is what you measure. Check before quoting on a block: a system serving gas appliances or a communal riser carries obligations of its own that a domestic contract does not cover.
The flow rate, the only benchmark that holds in service
The figures to recover after an intervention
Approved Document F Table 1.1 gives the minimum intermittent extract rates and Table 1.2 the continuous high rates. These are the values you measure back to, and they alone support a claim that a system is fouled.
| Room | Intermittent extract | Continuous, high rate |
|---|---|---|
| Kitchen, hood extracting outside | 30 l/s | 13 l/s |
| Kitchen, no hood extracting outside | 60 l/s | 13 l/s |
| Utility room | 30 l/s | 8 l/s |
| Bathroom | 15 l/s | 8 l/s |
| Sanitary accommodation | 6 l/s | 6 l/s |
The whole dwelling rate runs alongside them, at 19 l/s for one bedroom rising to 43 l/s at five, and never below 0.3 l/s per m² of internal floor area. The French equivalent, built the same way but on rooms rather than bedrooms, is set out in our reading of the regulatory ventilation flow rates.
What the flow hood shows and an inspection does not
A clean terminal on a leaking duct run gives a correct rate at the grille and no useful ventilation at all, because the air is entering through the joints instead of through the dwelling. Conversely, a blackened terminal can still hold its rate. Room-by-room measurement is therefore the only way to know what is happening, and it is also what turns a visit into a service you can defend. Duct losses are quantified separately: see ductwork airtightness.
What to look for when the rate is short
A rate below target traces back to three causes, in that order of frequency. Terminal obstruction, at the grille or the background ventilator, which cleaning resolves. Failure of the air path, missing or blocked background ventilators and door undercuts below the 10 mm equivalent free area, which no amount of work at the terminal will fix. And duct failure, crushed runs, disconnected joints or a fall holding water, which needs remedial work. Terminals are sized, not adjusted by ear: see background ventilators and extract terminals.
How a billable visit actually runs
Isolation and baseline before anything comes apart
Isolate at the consumer unit, prove dead, then record the starting condition before removing anything. Photograph the terminals in place and note their identification: a grille refitted in the wrong room on a humidity-controlled system is enough to unbalance a dwelling, and nothing in the measured result will explain why.
The work, by system type
On intermittent extract, clean the grilles and background ventilators, check that the shutters open, and leave the unit closed while the measurement holds. On a humidity-controlled system, dry cleaning only, soft brush and vacuum: water sticks or falsifies the sensing element, and the modulating slots are never taped over. On heat recovery, change the filters and check the heat exchanger, the condensate tray and the casing seal. A loaded filter drops recovery efficiency, as our article on heat recovery ventilation sets out.
What damages a system rather than maintaining it
Three practices come up repeatedly in reports written after a failed intervention.
- Bleach, solvents and strong degreasers on plastic components, which embrittle and crack them.
- Compressed air blown into the unit, which drives dust into the motor and wears the bearings.
- Adjusting terminals by ear, which ends in under-ventilation that stays invisible until the first complaint.
What gets recorded, and what gets sold
The service sheet: minimum content
With no rule imposing an interval, your service sheet is the only proof the work happened and produced a result. It carries the date, the terminal identification, the measured rates room by room with the instrument used, the Approved Document F target for that room, the actions taken and the faults left unresolved. It is also what supports a later return visit: without a baseline measurement, a client who disputes the work is always right.
The maintenance contract: what it actually commits you to
A ventilation maintenance contract on an occupied dwelling is an obligation of means that you define yourself. So write down what you measure, how often, and what triggers an additional quotation. Where a communal system is involved the logic reverses and the scope is set for you, with a certificate the managing agent has to be able to produce. Tied to a single client record holding the dwelling, the equipment and the history of visits, the contract renews without re-keying.
When replacement takes over from servicing
The switch is decided on the measurement, not on the age of the kit. If the rate stays below target after the terminals are cleaned and the ductwork made good, if the unit no longer holds its rates at boost, or if the terminals are cracked or painted over, servicing will recover nothing. Price a replacement instead, which does fall within scheme scope where the technology installed justifies it, and keep the pre-works measurement on file: it is what justifies the work to the funder and to the client alike.
