How often should infrequently used outlets be flushed?

Infrequently used outlets should be flushed at least once a week. HSG274 Part 2, the HSE technical guidance for hot and cold water systems, treats any outlet unused for seven days or more as infrequently used (Table 2.1), and sets weekly flushing as the standard control, run until the temperature at the outlet stabilises. Two exceptions sit either side of the rule: higher-risk settings may need flushing more often, and an outlet nobody needs is better removed than flushed.

Why weekly is the frequency

A week is roughly how long still water can be left before the conditions legionella needs start to assemble. Water standing in a branch drifts towards room temperature — in a heated building, straight into the 20°C to 45°C growth range — residual disinfectant decays, and biofilm begins to establish on the pipe wall. HSG274 paragraph 2.78 notes that weekly flushing of outlets not in regular use, sustained for several minutes, can significantly reduce proliferation, and adds the warning that gives the regime its teeth: once started, the procedure has to be sustained and logged, because lapses can result in a critical increase in legionella at the outlet. How long legionella takes to grow explains the kinetics behind the seven-day line.

What counts as infrequently used

The test is use, not appearance. An outlet that looks clean but is drawn off less than about once a week is infrequently used: the guest bathroom, the cleaner's sink, the outside tap in winter, the shower in a room between occupants, the emergency eyewash point, the tap left live on a branch after the fixture it served was removed. Building the list is the same walk-round exercise as finding dead legs, and the two overlap: a little-used outlet is a dead leg that still has a working fitting on the end of it. Every outlet you cannot honestly describe as used at least weekly goes on the flushing schedule, named individually in the written scheme of control.

How to flush: to temperature, and gently

The full method lives in flushing infrequently used outlets; the essentials are these. Run each outlet until the temperature stabilises — hot reaching at least 50°C (55°C in healthcare), cold dropping below 20°C — because the stable temperature is the proof that the standing water in the branch has been replaced with fresh supply water. Open the outlet gently, and remove shower heads and spray inserts first, because flushing a colonised outlet is itself a release of aerosol. If you want a structured timer for the round, the flushing timer walks you through outlet by outlet, and how long to run taps covers the timing question in detail.

When weekly is not enough

Weekly is the baseline for ordinary premises, not the universal answer. HSG274 paragraph 2.78 is explicit that where there are high-risk populations — it names healthcare and care homes — more frequent flushing may be required, as indicated by the risk assessment. In augmented care settings the water safety plan sets the regime, and outlets serving susceptible patients may be flushed daily or taken out of service altogether. The same logic applies anywhere occupants are more susceptible: the assessment tightens the baseline, and the written scheme records what was decided and why.

When the better answer is removal

Flushing manages a stagnation risk; removal ends it. HSG274 asks you to consider removing infrequently used showers, taps and associated equipment first, cutting redundant supply pipework back as far as possible towards a common supply. An outlet that serves no real purpose — the disused sink in a converted store room, the shower nobody has used in two years — costs a recorded flush every week for the life of the building and still stands stagnant between visits. Removal is a one-off plumbing job that permanently deletes the risk and shortens the flushing round. Flush the outlets that are still wanted; remove the ones that are not.

Empty properties and void periods

A vacant property is a building full of infrequently used outlets. HSG274 paragraph 2.144 sets the general principle that outlets should be used at least once a week to maintain a degree of water flow, and advises managing vacant dwellings carefully: implement a suitable flushing regime, or take other measures such as draining the system if it will be empty for a long period. For landlords between tenancies, holiday lets between guests and schools over the summer, a documented weekly flush through the void avoids a large recommissioning exercise at the end; longer closures may call for a full flush-through, disinfection or sampling before reoccupation, as set out in reopening buildings after closure.

The record is the control

HSG274's sustained-and-logged warning makes the log inseparable from the flush. Record the date, the outlet, who flushed it and the temperature reached, in the log book or on a flushing log sheet. If an outlet is later linked to a case, or an inspector or insurer asks how a known little-used outlet was controlled, the flushing log is the evidence the control happened — an unrecorded flush cannot be evidenced, so on paper it is no control at all.