Office Reopening: Water Systems Checklist for UK Sites
If your office has been closed, part-empty or running on a skeleton staff, the water inside it has been sitting still. Before people return, you need to review the Legionella risk assessment, flush and where necessary disinfect the system, confirm hot and cold temperatures, and start monitoring and record-keeping. This guide sets out that sequence for UK offices and similar workplaces, and shows when a competent contractor should take over.
TL;DR
Stagnant water lets bacteria such as Legionella multiply, so an office that has been quiet needs its water system recommissioned before staff return, not after.
Start with paperwork: confirm who the responsible person is, then review and update the Legionella risk assessment to cover the period of reduced use.
Flush every outlet, check temperatures (hot stored at 60°C or above, distributed at 50°C or above, cold below 20°C) and disinfect where the system needs it.
Keep monitoring after reopening, sample sentinel outlets and record who did what, when.
Schools, dental surgeries and healthcare sites carry extra duties because of vulnerable users and specialist equipment.
Table of Contents
Why a quiet office is a water hygiene problem
Water that does not move warms up in cold pipework and cools in hot pipework. Both trends push it toward the temperature range where bacteria thrive. UK government guidance says dormant water systems result in bacterial growth, especially in warmer weather, and that Legionella, which occurs naturally in water systems, causes Legionnaires’ disease, which is fatal in around 10% of cases. The same guidance applies to organisations with a water supply, offices included.
The usual culprits in an office are predictable:
Little-used outlets: kitchenette taps, showers in cycle-to-work facilities, washrooms on empty floors, and cleaners’ sinks.
Dead legs: redundant or capped pipework that still holds water but no longer carries flow.
Storage: cold water tanks and calorifiers where turnover has slowed.
Fittings that make aerosols: showerheads, spray taps and hoses, because Legionella causes harm when droplets are inhaled.
Reduced occupancy counts too. Hybrid working means many offices never fully closed but are far less used than the system was designed for. The risk is the same: water that moves rarely.
Step 1: Confirm responsibility and review the risk assessment
Before anyone touches a tap, settle who is in charge. The HSE’s L8 Approved Code of Practice says the dutyholder is responsible for ensuring a suitable and sufficient Legionella risk assessment is carried out by a competent person. In a leased office that may be the landlord, the managing agent or you as tenant, depending on the lease. Settle it in writing. Our guide to what a responsible person must do covers the roles in more detail.
Next, look at the assessment itself. Local authority reopening guidance says the risk assessment should be reviewed and updated to include periods when water use was reduced or shut down, and that a competent contractor should be identified where needed to help with recommissioning. L8 also says the dutyholder must review the assessment regularly and whenever there is reason to believe it is no longer valid. A long closure, a change in occupancy or building alterations are all such reasons.
Questions to ask when you open the document:
Is the schematic of the system still accurate? Have floors been stripped out, kitchens moved or outlets removed?
Does it say how the building was left during the closure or low use?
Are the control measures (flushing regime, temperature checks, tank inspections) actually being carried out, and is there evidence?
Is the assessor competent? L8 expects the dutyholder to make reasonable enquiries that staff and consultants are trained and properly equipped.
If the assessment is missing, out of date or does not match the building, commission a new one. Our Legionella risk assessments start from £185, and our guide to reviewing a Legionella risk assessment shows what a sound review looks like.
Step 2: Map the system and decide what state it is in
Walk the building with the schematic. Mark every outlet, tank, calorifier, thermostatic mixing valve (TMV), shower and any equipment that takes a water feed, such as humidifiers, coffee machines, water coolers and air conditioning. Then sort the outlets into three groups:
Group | Examples | What it usually means |
Used regularly throughout | Main washrooms, kitchen | Flush and check temperatures; routine regime resumes |
Used rarely or not at all | Spare floors, showers, cleaners’ sinks | Flush first, sample, consider disinfection |
Isolated or mothballed | Capped sections, closed wings | Needs specialist assessment before reconnection |
For buildings that were deliberately mothballed, Ministry of Defence technical guidance notes that pipework should not be drained down, but that sources of heat should be removed and the system locked off with clear signage. That guidance is written for defence sites, but the principle of a clearly controlled shutdown applies to any building. If nobody recorded how your office was left, treat it as the worse case.
Air handling and cooling equipment deserves its own check. Our article on reducing Legionella risk in air conditioning systems explains where those systems fit into the picture.
Step 3: Flush and recommission outlets safely
Flushing replaces stagnant water with fresh supply and brings temperatures back into range. Do it in a planned order, not tap by tap at random. The MoD guidance says a structured flushing process must be instructed and agreed by the responsible person before it begins, which is good practice in any workplace.
A workable approach:
Check the supply and storage. Inspect cold water tanks for debris, dead insects and damaged lids. HSE guidance recommends lids and insect screens on tanks and insulated pipes and tanks.
Bring hot water up to temperature. Refill and reheat calorifiers so hot water reaches 60°C.
Flush from the supply inwards. Open outlets in sequence, hot and cold, starting nearest the incoming main or storage and working outwards, so you do not draw stale water back through freshly flushed pipes.
Control the spray. Run taps gently and keep showerheads from creating mist while staff flush. Avoid scalding at hot outlets, particularly where there is no TMV.
Treat showers and hoses separately. Remove, clean and descale showerheads and hoses before use. HSE guidance says these should be cleaned and descaled at least quarterly in normal operation.
Record everything: outlet, time, duration, person, temperature reading.
Use trained people. Whoever flushes should understand what aerosols do and what to do if an outlet runs discoloured, smelly or lukewarm. Our Legionella awareness training is available on site or online for facilities and estates staff who carry out these tasks.
Step 4: Check temperatures against the limits
Thermal control is the main defence. HSE guidance gives the targets: hot water stored at 60°C or higher and distributed at 50°C or higher, cold water stored and distributed below 20°C.
Point | Target |
Hot water storage (calorifier) | 60°C or above |
Hot water at outlets | 50°C or above |
Cold water in tanks and at outlets | Below 20°C |
If readings are outside these limits, do not simply carry on. Possible causes include a failed heat source, a faulty thermostat, poorly insulated cold pipework running next to heating, or a tank warmed by a plant room. Escalate to the responsible person, note the fault, and have a competent contractor investigate. Our remedial works and TMV servicing cover this kind of fault-finding and repair, and TMVs themselves need servicing so that they protect against scalding without becoming a bacterial reservoir.
For sites that struggle to keep up with manual readings, automated water temperature monitoring logs temperatures continuously and flags excursions, which is useful where a building is staffed only part of the week.
Step 5: When to disinfect, sample or call a specialist
Flushing is not always enough. If temperatures have been out of range for a prolonged period, tanks are dirty, or the system has been standing for a long time, disinfection becomes the next step. Government guidance is blunt that if a system requires disinfection, a water consultant will be needed.
The reopening guidance from Wiltshire Council describes a typical recommissioning method: disinfect the cold-water system to all outlets at 50 ppm free chlorine or an equivalent biocide for at least an hour, then refill and reheat the calorifier to 60°C throughout before flushing outlets. Dosing, contact time, neutralisation and safe discharge are not a job for office staff with a bottle of bleach. Our tank cleaning and system disinfection service handles the work, and the method of works page shows how we structure it.
Call a specialist before reopening when:
the building has been closed or mostly empty for a long period and nobody recorded how it was left;
temperatures will not come into range;
tanks are visibly dirty, uncovered or contain debris;
there are dead legs or alterations the risk assessment never covered;
anyone on site has been diagnosed with Legionnaires’ disease or a cluster of pneumonia-like illness is reported.
What sampling tells you
Sampling is evidence, not a substitute for control. HSE explains that the primary method of control is thermal management, supported by cleaning, flushing and periodic sampling to show the system is under control. After recommissioning, sample from sentinel outlets, typically the first and last on a circuit and the most remote. Our Legionella water testing and analysis starts from £55 per sample, and covers Legionella alongside TVC, Pseudomonas, potable and chemical testing where the building needs them. If you want the basics of how tests work, see how to test for Legionnaires’ disease.
Step 6: Keep monitoring after people return
Reopening day is the start of the process. The council guidance suggests monitoring temperatures and, where applicable, biocide levels for at least 48 hours, then sampling sentinel outlets. After that, return to a regular regime:
Weekly flushing of infrequently used outlets. HSE guidance says these should be flushed at least weekly. MoD guidance similarly aims for water movement through outlets at least once a week and turnover of stored water every 24 hours.
Regular temperature checks at calorifiers, sentinel outlets and tanks.
Quarterly cleaning of showerheads and hoses.
Review of the risk assessment whenever circumstances change, including a shift in how many people use the floor.
Keep the logbook. The record should show who carried out each check, with dates, times and signatures. That is what you produce if an inspector, insurer or landlord asks how you managed the risk. We supply a bespoke logbook system and can run the regime for you under an ongoing maintenance contract.
Schools, dental surgeries and healthcare settings
Offices are the simple case. Where users are vulnerable or equipment is connected directly to water, the standard rises.
Dental surgeries: dental unit waterlines are a recognised concern. Professional guidance for dental teams, based on Health Technical Memorandum 04-01, notes that the risk grows where water sits at 20 to 45°C, is stored or recirculated, or carries biofilm. Waterlines need a managed disinfection and flushing routine before the first patient.
Healthcare sites and care settings: patients with weakened immunity are at greater risk, so reopening a ward, clinic or treatment room needs a stricter sign-off. See our article on Legionella risk management for immunocompromised patients.
Schools and colleges: long holidays leave outlets, showers and sports facilities unused for weeks. Termly flushing and checks at the start of each term sit naturally in the same regime as reopening an office.
Housing and mixed-use premises: where staff and residents share a system, see Legionella prevention in social housing.
In each case the dutyholder should rely on the sector’s own technical guidance and a competent assessor who knows the setting.
Our perspective
The most common mistake we see is treating reopening as a plumbing chore: someone runs a few taps, and the building is declared fit. The real failure is usually in the paperwork before and after. The risk assessment describes a building that no longer exists, nobody knows who the responsible person is, and the flushing records stop the week after reopening.
Our approach is to keep it simple and proportionate. We review the existing assessment against the building as it stands, agree a flushing and temperature plan the site team can actually carry out, test where results are needed, and leave a logbook that survives a change of facilities manager. We would rather tell you that a quiet office needs a tidy flushing regime than sell disinfection you do not need, and we will say plainly when a system does need specialist work. For multi-site organisations, a consistent method across every building matters more than a perfect one at head office.
— Sammi
Get your office water system checked before staff return
We carry out Legionella risk assessments, water sampling, disinfection, TMV servicing and temperature monitoring for commercial, education and healthcare sites across the UK, with reports and quotes turned around quickly. If you are planning a reopening or a return to higher occupancy, start with a Legionella risk assessment from £185, or send us the site details through our main site.
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FAQ
How long can an office be closed before the water system needs recommissioning?
There is no safe universal number. The CIEH reopening guidance says that where water has not moved for more than a week, the risk of growth increases. Treat any period of reduced use as a reason to review the risk assessment and flush, and escalate to a specialist as the closure gets longer or the system more complex.
Do I need a Legionella test before staff come back?
Not always. Temperature control, flushing and a current risk assessment come first, and sampling confirms the system is under control. If the system has been disinfected, or temperatures were out of range, sampling sentinel outlets afterwards is sensible.
Who is legally responsible for the water system in a leased office?
The dutyholder, as defined in L8, which may be the landlord, the managing agent or the tenant depending on the lease. Agree it in writing and name the responsible person and the competent person who supports them.
Can we flush the taps ourselves?
Trained staff can flush outlets if the responsible person has agreed a plan, records are kept and spray is controlled. Disinfection of tanks and systems should go to a competent contractor.
Sources
A safe water supply: information for all organisations (GOV.UK)
Legionnaires’ disease: The control of legionella bacteria, L8 (HSE)
Where should I start in order to re-open my business? (Wiltshire Council)
TB 2021-01 Water hygiene guidance for closed or reduced-occupancy buildings (GOV.UK)
Legionnaires’ disease: lockdown risks and reopening safely (CIEH)
Legionella and dental unit waterline management (CPD for dental hygienists)
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