An empty rental still has a live water system, and the day the taps stop running, the water in those pipes begins to stagnate and drift towards the warm, still conditions Legionella favours [2]. You have two defensible ways to hold a void: keep it flushed, or drain it down. Both finish the same way — a deliberate flush-through and temperature check before the next tenant gets the keys.

This is a turnaround problem, not a seasonal one. A holiday cottage that closes every winter follows a predictable pattern you can plan around, the kind covered in Seasonal buildings: managing intermittently used properties. A void is open-ended: you rarely know on handover day whether the property re-lets in three weeks or sits for five months. That uncertainty is what your procedure has to absorb.

Most domestic rental systems are lower-risk than a hospital or a cooling tower, and HSE is clear that the duty to assess and control still sits with landlords and their agents [1]. The fix is not complicated. It is a short, repeatable routine that someone actually completes and records.

Drain down or keep flushed?

The first decision is the holding strategy, and it turns on how long the void is likely to last and who can realistically attend.

Keeping the system flushed means leaving it charged and running every outlet regularly — commonly weekly for outlets that would otherwise sit unused — so water never stands long enough to matter [1]. It suits short, predictable voids, and properties someone visits anyway for viewings or works. The weakness is obvious: it relies on the visit happening. Miss a fortnight and you have the stagnation you were trying to avoid. This is the same logic used for Building shutdowns: flushing and monitoring during low use.

Draining down means isolating the incoming supply and emptying the system so there is no standing water to colonise. It suits longer voids, winter voids where you also want frost protection, and properties no one will reliably attend. The catch: a drain-down is rarely complete. Traps, low points, the base of a hot water cylinder, washing-machine and dishwasher fill hoses and electric showers all hold water. Refilling then reintroduces fresh nutrients and disturbs any biofilm. So you flush and check at re-let regardless of which strategy you ran.

No rule says one is correct. Your risk assessment weighs void length, the building, the likely susceptibility of the next occupants, and who can attend — and records the decision [1][4].

The void-to-relet field checklist

Work through this in four stages. Tick and date each line; the record is the point.

When the property goes void

  • Record the date the property became empty and the chosen holding strategy.
  • Confirm the drain-down versus keep-flushed decision against the current risk assessment.
  • If keeping flushed: schedule the flush visits and name who carries them out.
  • If draining down: isolate at the stop tap, open all outlets to drain, and empty the cold water storage cistern and hot water cylinder where fitted.
  • Leave shower heads off to drain and dry, or remove and bag them.

While it sits empty

  • Run the strategy you actually chose — do not half-do both.
  • If flushing: run every outlet, hot and cold, for long enough to clear the standing water and let the temperature settle, showers included.
  • Log each visit: date, outlets run, and any temperatures taken.
  • Maintain heating or frost protection as the season needs; see Managing water temperatures during building downtime.
  • Watch the calendar. A void that overruns its planned window is a trigger to re-assess, not to carry on regardless.

Before re-letting

  • If drained, refill slowly and vent trapped air.
  • Flush every outlet thoroughly, hot and cold, working outward from the outlet nearest the incoming main. Minimise spray — remove the shower head or run it at low flow, and open a window.
  • Confirm hot water reaches the temperature your scheme works to and cold stays below your cold figure, to the values written in your scheme of control [2].
  • Clean and descale shower heads and tap aerators; replace any that are scaled or perished.
  • Decide on pre-let disinfection. If the system has stood for a long period, or the risk assessment or a sample points to it, arrange disinfection by a competent person. There is no single legal “X weeks empty equals mandatory disinfection” line — it is a risk-based call [1][3].

Re-occupation sign-off

  • Record the re-occupation date and that the bring-back flush was completed.
  • Note the temperatures achieved and attach any disinfection certificate.
  • Give the incoming tenant the basics: flush outlets after any time away, keep shower heads clean, and report scalding or persistently tepid water.
  • File everything with the property’s risk assessment so the next void starts from a known baseline.

What void teams skip

The failures cluster in the same few places. Drain-down gets treated as a guarantee when it never fully empties a system. The forgotten outlets — the outside tap, the electric shower, appliance fill hoses, a cloakroom basin no one thinks about — never get run. A “we’ll flush weekly” plan is logged that nobody attends. And the overrun void, planned for three weeks and still empty four months later, sails past its review point because no one owns the calendar. Housing providers running voids at scale should tie this into their wider programme; Legionella compliance for social housing and housing associations covers the governance around it.

A note on scope

This is general guidance to help you build a turnaround routine; it is not legal advice or a substitute for a competent, property-specific risk assessment. Buildings differ — a converted flat with a stored hot water system and a long pipe run is not the same animal as a small modern flat on a combi boiler — and the standing time, holding strategy and disinfection decision are yours to make through that assessment, with a competent person where the system is more complex [1][4].

Common questions

How long can a rental sit empty before it becomes a Legionella risk?

There is no fixed legal limit. Stagnation risk builds over days rather than months, and warm, still water is the concern, so your risk assessment sets the point at which a flush or drain-down is required — not a number borrowed from another building [1][2].

Do I have to disinfect a void before a new tenant moves in?

Not automatically. For a simple domestic system, a thorough flush and a temperature check are usually enough. Disinfection becomes the answer when the system has stood a long time, the occupant is more susceptible, or a sample or the risk assessment indicates it [1][3].

Is draining down better than keeping it flushed?

Neither wins every time. Draining suits long or winter voids and properties no one will attend; flush-retention suits short voids with reliable visits. The deciding factors are void length, who can attend, and frost risk — all recorded in the assessment.

Do this next

Pull your last quarter of voids and check each one has a logged bring-back flush with temperatures against it. The properties with a gap — re-let with no record of being flushed or temperature-checked — are the ones to fix first, and they show you exactly where your turnaround process is leaking.

Sources

[1] HSE, “Legionella and landlords’ responsibilities”. https://www.hse.gov.uk/legionnaires/legionella-landlords-responsibilities.htm [2] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [3] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [4] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm