Legionella — frequently asked questions

Does temperature monitoring replace the annual lab swab?

No. It reduces the frequency of swabs (in jurisdictions that allow that, such as Germany under DVGW W551) but does not replace them. The lab swab confirms there is no Legionella; temperature monitoring evidences that conditions remain unfavourable for growth.

OpenSense generates a quarterly PDF showing 50/60/50 compliance; most EU health authorities accept this as evidence to move from monthly to annual sampling. Confirm with your local authority before committing.

Where do I install the sensors?

Five places, in priority order:

1. Cylinder top — top of the hot-water storage tank. The hottest layer. Must be ≥ 60 °C.
2. Cylinder bottom — coldest layer. Where Legionella grows first if the heater is undersized.
3. Ringline return — the return loop just before re-entering the cylinder. Must be ≥ 50 °C; the diagnostic that the recirc pump works.
4. Distant outlet — the tap farthest from the cylinder. Must reach ≥ 50 °C within 30 s of opening.
5. Cold supply — the cold-water inlet. Must be ≤ 25 °C. (Below this temperature Legionella does not thrive.)

For most small buildings, sensors at positions 1, 2, 3 (storage top

• bottom + ringline return) are the minimum. Position 4 requires a flow-actuated probe; position 5 is cheap and worth adding.

How do I install a probe on a closed pipe without cutting it?

Use a clamp-on PT100 (the strap-on contact probe with thermal paste, wrapped under insulation). Vendor: Sensata, OMEGA, or any DIN-rail PT100 transmitter with a contact probe.

Accuracy is ±1.5 °C against a wet probe in the same line, dominated by the quality of the insulation around the contact. For monitoring the 50/60/50 thresholds at ±2 °C precision this is sufficient.

For high-stakes compliance (a hospital), use a thermowell (cut a T-piece into the pipe, install a brass thermowell with thread-seal, insert a PT100 probe in the well). Expensive (€80–€150 per point installed) but rated for compliance audit.

What about the thermal-disinfection cycle (70 °C nightly)?

OpenSense supports a window in the rule that raises the acceptable temperature during the cycle. See Alerts API → time-of-day windows.

Be cautious of two things during the cycle:

• Some old thermostatic mixing valves (TMVs) cannot tolerate 70 °C on the supply for an hour. Newer DVGW-certified TMVs can. Check yours.
• People in the building can scald themselves at 70 °C taps. If the cycle runs while the building is occupied, post warnings.

What is the "delta" channel?

flow_delta = supply − return. A healthy ringline has a delta of 5–10 °C — the loop loses some heat in the run but the return is still > 50 °C. A delta > 15 °C means too much heat is being lost in the loop (broken insulation, oversized pipe) or the pump is too small for the flow demand. A delta < 3 °C means the pump is too strong — minor concern, but it can mean you are heating more water than the building uses.

OpenSense computes the delta automatically when both flow_supply and flow_return channels exist on the same device or same site.

A guest scalded themselves and is threatening to sue.

OpenSense's temperature log shows the actual outlet temperature at the time of the incident. If the tap was at 38 °C (you have a TMV) the scald claim is unsustainable; if the tap was at 65 °C (you ran a disinfection cycle in occupied hours), the log is evidence of operator negligence.

The log cuts both ways. We do not tell you it always saves you. It is honest evidence.

My building manager says all this is overkill.

For a single-family home, it is. For any building serving the public — hotels, spas, care homes, hospitals, schools, dormitories, gyms, public swimming pools — Legionella is the waterborne risk. Periodic outbreaks happen in EU buildings; insurance and statute now require demonstrable management. OpenSense is one component of that management; an annual risk assessment by a competent person and a written water-safety plan are the others.