Short answer: a sentinel outlet is a tap chosen because it represents the worst case of the system. On the hot side of a simple system that is the tap nearest the cylinder and the tap furthest from it. On the cold side it is the tap nearest and furthest from the storage tank or the incoming supply. You monitor those, not every tap in the building.
The word does most of the work: a sentinel stands watch. If the two extremes of a system are within their temperature targets, the outlets in between are very likely to be as well.
Why the extremes
The two ends of a hot water system fail for opposite reasons, and each end catches a different fault.
- The nearest hot tap should be hot almost immediately. If it is not, the problem is at the cylinder: thermostat setting, a failed element, or stratification.
- The furthest hot tap is the one the heat has to travel to. If it fails, the problem is distribution: a long dead leg, poor insulation, or a circulation pump that is not doing its job.
The cold side works the same way in reverse. The tap nearest the tank or main should be coldest. The furthest is the one whose pipe has had the most opportunity to pick up heat from a warm loft, a heated riser or a hot pipe running alongside it.
Picking them in a real property
A flat or small house with a cylinder. Usually two hot sentinels and two cold sentinels, and in a very small flat the nearest and furthest may be only a couple of metres apart. That is fine. Record which taps you chose and stick to them, because a trend at the same tap is worth more than a single reading at a different one each month.
A house with a combi boiler and no stored water. There is no calorifier to check and no tank. The system is simpler and its risk profile is different, which is exactly the sort of thing the property’s risk assessment settles. Do not assume the full cylinder-based regime applies; work from the assessment.
A circulating hot water system. This is the case people get wrong. Where hot water circulates around a loop, HSG274 points to the return legs of the principal loops as the hot sentinels, rather than the nearest and furthest taps (para 2.63, p72). A circulating system is designed so every leg stays hot; the return is where a circulation failure shows up first, and it is the honest measure of whether the loop is working.
A block or an HMO. The nearest and furthest are per system, not per building. If a block has several risers or several cylinders, each system has its own sentinels.
What the sources say
| Point | What it is | Source |
|---|---|---|
| Hot sentinels, non-circulating | Nearest and furthest hot taps from the cylinder | para 2.62, p72 |
| Hot sentinels, circulating | Return legs of the principal loops | para 2.63, p72 |
| Cold sentinels | Nearest and furthest taps from the tank or incoming supply | para 2.59, p72 |
| Other outlets | A representative selection considered on a rotational basis | para 2.66, p73 |
Page numbers are the printed ones in HSG274 Part 2, second edition 2024.
The targets at a sentinel
Sentinels are checked against the ordinary temperature targets:
- Hot: at least 50 °C within one minute of running the tap.
- Cold: below 20 °C within two minutes of running the tap.
Note that 50.0 °C meets the hot target and 20.0 °C does not meet the cold one, because the guidance says “a minimum of 50” and “below 20”. Our guide to hot water temperatures covers all four thresholds including the cylinder itself.
The rotation is not optional padding
Alongside the sentinels, HSG274 asks for a representative selection of other outlets checked on a rotational basis, so that over time the whole system is profiled and not just its two ends. A middle-of-the-run tap that has quietly become a dead leg after a bathroom was reconfigured will never show up in a sentinel reading.
Here the guidance deliberately does not give you a number. It names the practice but sets no period, so the interval is a judgement for the property. Legionella Logger reflects that: rotating outlets have no default cadence and the app asks you to set one rather than inventing a figure that is not in the document.
Record which is which
Whatever you choose, write down which tap is which sentinel, in enough detail that someone else could repeat the measurement: “first floor bathroom, hot basin tap, furthest from cylinder”. HSG274 asks for records that carry the name of the person, the date, and enough location detail to repeat the reading (para 2.54, p70), and L8 asks for records to be signed or otherwise authenticated (para 73).
That level of detail is what turns a column of numbers into evidence. It is also what makes a trend readable a year later, when the person who set the system up has moved on.
Scope and sources
From HSG274 Part 2, second edition 2024, published by the Health and Safety Executive, read with the Approved Code of Practice L8. Download them free: HSG274 on hse.gov.uk. Frequency for a given property comes from its risk assessment; Table 2.1 is described in the document as an indication of frequency.
Checked against the archived HSE PDF of HSG274 Part 2 on 10 September 2026. Contains public sector information licensed under the Open Government Licence v3.0. This is general information, not a legionella risk assessment, and Plan CD is not affiliated with the Health and Safety Executive.