Legionella Monitoring Frequency: How Often Each Check Must Happen
TL;DR: Legionella monitoring frequency for a UK hot and cold water system comes from Table 2.1 of HSE guidance HSG274 Part 2. Sentinel outlet temperatures and calorifier flow and return are monthly. Subordinate loop returns are quarterly. Little-used outlets are flushed weekly. Cold water storage tanks are inspected annually. Two duties carry no fixed interval at all, and your scheme can set anything more frequent, never less.
Last updated: 19 July 2026
Key takeaways
- Monthly is the dominant Legionella monitoring frequency: sentinel outlet temperatures plus calorifier flow and return.
- Quarterly covers subordinate loop returns and descaling showers, not a general sample of outlets.
- Infrequently used outlets are flushed weekly, and once started that regime must be sustained and logged.
- Annual is the lowest Legionella monitoring frequency in the table: tank and calorifier inspections.
- HSG274 Part 2 gives no interval for reviewing a risk assessment. It lists triggers instead.
On this page
What sets Legionella monitoring frequency in a UK building?
Legionella monitoring frequency is set by your written scheme of control, built on Table 2.1 of HSG274 Part 2 and adjusted by your risk assessment. HSE gives minimum intervals for a system already under control: weekly flushing, monthly temperatures, quarterly loop checks, annual inspections. Complexity and user susceptibility push those higher. The table is a floor, never a ceiling. The legal layers above it are in our guide to Legionella monitoring requirements in the UK.
The HSG274 Part 2 Legionella monitoring frequency table
Every frequency below is taken from HSG274 Part 2, Table 2.1. Where HSE gives no interval, this table says so rather than inventing one.
| Check | Frequency | What is measured | Why it matters |
|---|---|---|---|
| Calorifier flow and return | Monthly | Flow close to 60°C without dropping below, return not below 50°C | The sterilising step for the hot side |
| Hot water sentinel points | Monthly | Nearest and furthest outlets, or each principal loop return, at 50°C (55°C healthcare) | The extremities run coolest |
| Cold water sentinel taps | Monthly | Below 20°C within two minutes | Above 20°C the cold side enters the growth range |
| Subordinate loop returns | Quarterly, ideally on a rolling monthly rota | Return leg, or last outlet above 50°C | The easiest circulation failure to miss |
| Representative selection of other outlets | Rotational, over a defined period. No interval given | A temperature profile of the whole system | Reveals localised heat gain |
| Infrequently used outlets | Weekly, or as the risk assessment indicates | Flush until temperature matches supply, then purge | Anything unused for seven days stagnates |
| Cold water storage tank | Annually | Condition, lid, screens, insulation, summer stored temperature | The tank feeds every cold outlet |
| Calorifier internal inspection | Annually, or as the rate of fouling indicates | Internal condition, or debris purged where there is no hatch | Sludge is a nutrient bed |
| Showers and spray taps | Quarterly, or as the rate of fouling indicates | Dismantle, clean and descale heads, inserts, hoses | These generate the aerosol |
| TMV strainers and filters | Annually, or as the risk assessment sets | Inspect, clean, descale, disinfect | A blending valve sits in the growth range |
| Risk assessment review | No fixed interval. Regularly, and whenever it may no longer be valid | The whole assessment, against the listed triggers | A stale assessment invalidates everything below it |
Which Legionella checks are monthly?
Three run monthly: calorifier flow and return, hot water sentinel points, and cold water sentinel taps. They confirm the temperatures the whole regime rests on, which is why monthly is the Legionella monitoring frequency most estates teams quote. Sentinel points represent the system, not convenience. The 60°C, 50°C and 20°C targets sit in the HSE hot and cold water guidance, and point selection on our sentinel outlet monitoring page.
What does the quarterly Legionella monitoring frequency cover?
Two things: the return legs of subordinate loops on circulating hot water systems, ideally on a rolling monthly rota, and descaling showers and spray taps. It is not a general quarterly sample of other outlets, which is a common misreading. Subordinate loops branch off a principal loop, are invisible on most rounds, and are where circulation quietly fails. Set your Legionella monitoring frequency from the monthly line alone and they go a year unread. Our HSG274 Part 2 monitoring page maps each check to its asset.
Which duties have no fixed Legionella monitoring frequency?
Two. The representative selection of other outlets is checked on a rotational basis over a defined time period, so the whole system is profiled rather than the same taps each month. The risk assessment is reviewed regularly, and whenever there is reason to suspect it is no longer valid. You set both periods and justify them.
These are the only entries in the Legionella monitoring frequency regime where HSE lists triggers instead of a period: a change to the system or its use, a change of building use, new information on risks or controls, checks showing control is lost, changes to key personnel, and a linked case of the disease. The two-year interval widely quoted is convention, not guidance.
What does a monthly Legionella monitoring frequency actually prove?
It proves the temperature was in range at the moment someone stood at that tap. Twelve readings per point per year, each about a minute of a system that runs 8,760 hours. A circulation pump can fail in week two and be found in week six. A cold main can climb over 20°C in a hot spell and settle before the next round. The frequency was met exactly, and the system was not under control.
How does continuous monitoring change the Legionella monitoring frequency question?
It changes no frequency at all. Your scheme names the same checks at the same cadences, and the physical tasks, flushing, descaling, tank inspection and calorifier drain-off, still need doing. What changes is the record: a continuous trace instead of twelve annual points, and an alert when a threshold is crossed rather than at the next round.
AQUAIOT Legionella monitoring places wireless sensors at the points your scheme already names: cold water tanks, calorifier flow and return, loop circulation, TMVs and sentinel outlets. Installation is retrofit-friendly with no pipe cutting, so an occupied building keeps running. Readings travel over encrypted cellular or LoRaWAN telemetry to the AQUAIOT Cloud, are checked against your thresholds, and escalate by SMS, email and app. Records are timestamped and exportable to BMS, CAFM and AIMS platforms. None of that alters a published Legionella monitoring frequency. Where an estate needs chemistry as well as temperature, the iSPA-T monitoring system covers water quality.
Missed flushing becomes visible rather than assumed, which is the point of Legionella flushing monitoring on void and little-used outlets. Start at the automated Legionella monitoring hub.
Frequently asked questions
How often should Legionella temperature checks be carried out?
The core Legionella monitoring frequency is monthly: calorifier flow and return, hot water sentinel points, and cold water sentinel taps. Subordinate loop returns are quarterly, ideally on a rolling monthly rota. A representative selection of other outlets is profiled on a rotational basis over a period you define.
Is there a single legal Legionella monitoring frequency in the UK?
No. No regulation states an interval. The legal duties are to assess and control the risk under COSHH and the Health and Safety at Work Act, and the intervals sit in HSE guidance, principally Table 2.1 of HSG274 Part 2. Departing from it means showing you achieved equivalent control another way.
How often must little-used outlets be flushed?
Weekly, or more often where the risk assessment indicates, and HSE treats anything unused for seven days or more as infrequently used. Flush until the temperature stabilises and matches the supply water, then purge to drain. Once started, the regime must be sustained and logged, because a lapse can cause a critical increase at that outlet.
Does continuous monitoring replace the scheduled checks?
No. It evidences temperature between the scheduled checks and alerts on drift, but the physical tasks still have to happen and the written scheme remains in force. It changes the resolution of your record, not your legal position. The duty holder retains responsibility at all times.
Talk to an engineer about your monitoring schedule
The useful conversation is specific: which assets you hold, what Legionella monitoring frequency your scheme sets, and where the record has gaps. Speak to an expert and we will scope it against your scheme. Full guidance sits on the HSG274 publication page and the HSE Legionnaires’ disease pages.
By Gianbattista Porru, Digital and IoT lead at AQUAIOT.