Temperature, scald protection and water hygiene at the outlet
Hygiene control keeps hot water hot and cold water cold, while scald protection limits the temperature delivered at the outlet, and the blended water that results sits between the two in the pipework after the mixing valve. How much of it there is depends on where the valve is placed, which is why temperature control and flushing have to be designed together rather than separately.
- German technical rules
- 60 and 55°C
- At the heater outlet and in circulation.
- BS 8300 outlet maximum
- 43°C
- Basin taps and shower fittings.
- North American model codes
- 49°C
- Point of use, through ASSE 1016 and ASSE 1070 devices.
In this section
Two requirements pulling in opposite directions
The hygiene rules are consistent in direction if not in detail. Hot water is stored and circulated hot and cold water is kept cold, so that the bulk of the system stays out of the range where growth accelerates. The figures differ by market and by guidance document: German technical rules give 60 degrees Celsius at the trinkwassererwärmer outlet and at least 55 degrees in circulation, while UK guidance expects hot water to reach the outlet device at 50 degrees within a minute and 55 degrees in healthcare premises, with cold water kept below 20 degrees.
Scald control pushes the other way at the point of use. UK guidance identifies a scalding risk for vulnerable users at bath and shower outlets above 44 degrees. BS 8300, the UK standard for accessible and inclusive design, sets a maximum of 43 degrees at basin taps and shower fittings while requiring the circulating pipework behind them to be kept hot. In North America, model plumbing codes require shower and tub-shower control valves to include a means of limiting the maximum setting to 49 degrees, using valves conforming to ASSE 1016, and limit tempered water to public hand-washing facilities to the same figure through a device conforming to ASSE 1070. Healthcare guidance in some jurisdictions sets lower limits again for specific settings and fixture types.
The system is deliberately hot, the outlet is deliberately not, and something sits between them.
These figures come from different documents with different scopes and they should not be read as a single scale.
What the mixing valve standards govern
The device that resolves the conflict is a thermostatic mixing valve, and the standards for it are well established.
BS EN 1111 covers thermostatic mixing valves at normal pressure and BS EN 1287 covers low pressure valves. The TMV2 scheme certifies valves to those standards for general domestic and commercial use, while the TMV3 scheme, aligned with the NHS D08 specification, applies to healthcare settings and requires more of the valve in fail-safe performance and durability. UK accessibility guidance points directly at these standards, requiring outlet temperature to be controlled by a blending valve conforming to one of them.
ASSE 1017 covers the master or source mixing valve, which is explicitly not for delivering water directly to a user. ASSE 1016 covers automatic compensating valves at individual showers and tub-shower combinations, providing both scald and thermal shock protection. ASSE 1070 covers water temperature limiting devices at the point of use, providing scald protection without addressing thermal shock, and its test regime rejects a device whose outlet exceeds 49 degrees during temperature variation.
What all of these govern is the valve: its accuracy, its response, and its behaviour when a supply fails. What none of them governs is what happens to the water after it leaves the valve.
The blended leg
Downstream of a mixing valve the water is neither hot nor cold. Depending on the setpoint, the residence time and how quickly the pipe loses heat, it can sit within the range that favours the growth of Legionella and other waterborne organisms, and it sits there whenever the outlet is not in use.
The blended volume is small, and a draw exchanges it quickly.
The blended volume stands between uses, and it is the water the user receives first.
On a well-designed installation that volume is small, because the valve is close to the outlet. Where a single valve serves several fixtures through a run of pipework, the blended volume can be substantial and it stands between uses. The valve itself also accumulates deposits, which is why maintenance regimes cover the valve as well as the pipework.
Where the temperature is measured matters
One detail from UK guidance is worth carrying into any specification, because it changes what a measurement means.
Guidance expects hot water to reach the outlet device at 50 degrees within a minute of opening. Where the outlet device is a mixing valve, that temperature is measured at the hot inlet to the valve rather than at its outlet, and a TMV2 valve requires a hot supply of at least 55 degrees to work correctly.
So a compliant installation can present tepid water at the tap while satisfying the hot water requirement upstream. Anyone verifying temperatures needs to know which side of the valve they are measuring, and any monitoring arrangement needs to sense in a position where the reading means something.
Accessibility pulls the outlet temperature down further
Where a fixture is designed for accessibility, the outlet limits tighten, and in some markets the requirement sits inside the accessibility standard rather than beside it.
UK accessibility guidance sets the outlet maximum for basin taps and shower fittings and simultaneously requires the circulating pipework to be kept hot, which is the same conflict stated in one document. In Australia, AS 1428.1 addresses tapware directly, including anti-scald provision, so temperature limiting is part of accessible design rather than a separate plumbing decision.
The practical consequence is that accessible fixtures tend to have the lowest permitted outlet temperatures in a building, which means the largest gap between the hygiene target and the delivery target. How often those fixtures are used is a question about the building rather than about the label on the door. Accessibility requirements for taps, showers and flush controls covers the operating side of them.
What this means for flushing
Four consequences follow, and they are what a control at the outlet has to deal with.
A blended draw takes water from both legs, and may fully exchange neither
Running a mixed outlet draws part of its volume from the hot side and part from the cold, in a ratio set by the blend. Whether either leg is exchanged depends on that ratio, on each branch volume, and on the flow and duration of the draw. The first step is to work out how much water the blended draw actually removes from each leg. Separate actuation of the hot and cold paths is one architecture that addresses the shortfall where the mixed path cannot demonstrate the necessary exchange; some systems treat cold line flushing as a distinct function for that reason. It is not an automatic requirement.
Temperature is a practical endpoint, and it is used in more than one country
UK guidance advises flushing until the temperature at the outlet stabilises and is comparable to the supply, on the reasoning that a comparable temperature indicates the standing water has been replaced. German practice uses the same criterion, expressed as flushing to temperature constancy. The technical value is that a stable outlet temperature is observable evidence that water from the supply has reached the outlet, which a fixed duration on its own is not. Detecting it rather than estimating it implies a temperature sensor.
Thermal disinfection has to get past the valve
A disinfection regime that raises system temperature is obstructed by the device whose purpose is to stop hot water reaching the outlet. Whatever arrangement is used, a valve with a disinfection mode, a bypass, or manual intervention, the required outcome is that normal use cannot occur while the outlet is delivering water above its usual limit. That is a product and installation safety function, engineered with fail-safe behaviour considered, and a control state alone does not deliver it.
Verification needs the right measurement point
Following from the section above, a temperature reading taken after a mixing valve does not verify the hot supply, and one taken before it does not verify what the user receives.
What the control has to be able to do
These are conditional functions rather than a required set, and which of them applies depends on the architecture.
- Where the hot and cold legs have to be exchanged independently, the control needs to actuate them separately rather than only through the blended path.
- Where the endpoint is a measured temperature rather than a fixed duration, it needs a temperature input and the ability to run a cycle to a condition.
- Where the site operates thermal disinfection, it needs to be commanded into and out of a disinfection state, within an arrangement that prevents normal use while that state is active.
- Where records are required, it needs to distinguish a commanded cycle from a measured thermal response, and report accordingly.
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