Vaillant (myVaillant/sensoNET): evcc DHW control leaves the weekly anti-legionella disinfection unable to run

[Johannes-del]

Describe the bug

What happens

To let evcc take exclusive control of domestic hot water (DHW) for PV self-consumption, the Vaillant DHW operation mode is set to operationModeDhw: OFF in the myVaillant app. Otherwise the appliance's own DHW time program keeps charging the cylinder and competes with evcc. With DHW mode OFF, evcc does all DHW heating itself, on demand, via hot-water boosts (StartHotWaterBoost / StopHotWaterBoost in the vaillant charger).

The catch: the appliance still reports its native weekly thermal disinfection (anti-legionella) as scheduled. In my case isLegionallaProtectionActivated: "WEDNESDAY". But that setting is only a stored preference. With operationModeDhw: OFF the cylinder is never charged by the appliance's own logic, so the scheduled Wednesday disinfection has nothing to act on and never actually heats the tank to disinfection temperature. The device believes it is protected while, in the evcc-controlled state, it effectively is not.

evcc's hot-water boosts target the normal setpoint (60 °C here) and include no disinfection run, so the periodic high-temperature hold (65 °C or more) that Legionella control requires simply never happens. Nothing in evcc surfaces this. Observed on my system; I would appreciate confirmation of the appliance-side behavior.

To scope this precisely: it is specific to the myVaillant cloud / sensoNET integration (the vaillant charger). That is the only path that sets operationModeDhw: OFF; operationModeDhw and StartHotWaterBoost live only in charger/vaillant.go. The EEBUS integration never touches the DHW operation mode (it only writes load-control power limits), so EEBUS-controlled Vaillant appliances keep their native DHW schedule and the legionella cycle intact.

The reason this isn't just "use EEBUS instead": remote deployments have no choice. If evcc doesn't sit on the same LAN as the heat pump (e.g. a hosted/cloud VM), EEBUS isn't reachable and the myVaillant cloud path is the only option, which is exactly where this gap appears.

Why it matters

Users who let evcc manage Vaillant DHW for PV self-consumption must set operationModeDhw: OFF, and thereby silently lose the manufacturer's legionella protection even though the appliance still reports it as scheduled. This is a safety-relevant side effect with no warning and no built-in mitigation. I currently work around it with an external weekly systemd timer that forces a high-temperature boost (setpoint to 70 °C, hold 65 °C or more) and then restores evcc's normal mode, which shouldn't be necessary out of band.

Possible directions (open for discussion)

1. Warn when evcc takes over Vaillant DHW with operationModeDhw: OFF, i.e. surface that native legionella protection can no longer execute.
2. Built-in periodic disinfection: an optional scheduled high-temperature forced boost for heat-pump DHW loadpoints (config: weekday, target temp, min hold), so evcc owns the whole DHW lifecycle safely.
3. Investigate whether DHW control can coexist with the appliance keeping its own disinfection schedule effective (a mode other than full OFF).

Happy to provide more detail on the observed myVaillant API behavior and to test any change on a real aroTHERM plus setup.

Steps to reproduce

1. Integrate a Vaillant heat pump (aroTHERM plus, VRC720 controller / sensoNET) via the vaillant charger template.
2. In the myVaillant app, set DHW operation mode to OFF so evcc has exclusive DHW control (required, otherwise the appliance's own DHW schedule fights evcc's boosts).
3. Run DHW as an evcc loadpoint in mode: pv (evcc issues StartHotWaterBoost on surplus, StopHotWaterBoost otherwise).
4. Leave the appliance's isLegionallaProtectionActivated at its default (e.g. WEDNESDAY).
5. Observe for one week or more: on the scheduled disinfection day the cylinder is not heated to disinfection temperature. currentSpecialFunction stays REGULAR and currentDhwTemperature stays around the PV/tapping setpoint, because with operationModeDhw: OFF the native disinfection does not execute.

Configuration details

evcc (`evcc.yaml`, credentials redacted):

chargers:
  - name: vaillant_wp
    type: template
    template: vaillant
    user: <redacted>
    password: <redacted>
    realm: germany

loadpoints:
  - title: Warmwasser
    charger: vaillant_wp
    mode: pv
    enable:  { threshold: -1300, delay: 5m }
    disable: { threshold: 0,     delay: 5m }


Appliance side (set once in the myVaillant app, not in evcc):
- `operationModeDhw: OFF`   (required so evcc controls DHW exclusively)
- `tappingSetpoint: 60`
- `isLegionallaProtectionActivated: WEDNESDAY`  (native default, still shown as active)
- Controller: VRC720, firmware 404.x, sensoNET.

Log details

Produced per the evcc FAQ (https://docs.evcc.io/en/docs/faq, "How do I create a log file for error analysis"): either via the WebUI Logs view (filter area `vaillant`), or by adding to `evcc.yaml` and restarting:

levels:
  vaillant: trace


Redacted excerpt of the appliance state evcc reads each cycle (`GET .../v1/systems/<SYSTEM_ID>/tli`), trimmed to the DHW-relevant fields:

[vaillant] TRACE GET https://api.vaillant-group.com/.../v1/systems/<SYSTEM_ID>/tli
[vaillant] TRACE {
  "configuration": {
    "dhw": [ { "index": 255,
               "operationModeDhw": "OFF",
               "tappingSetpoint": 60.0 } ]
  },
  "properties": {
    "dhw": [ { "index": 255, "maxSetpoint": 70.0, "minSetpoint": 35.0 } ],
    "system": { "controllerType": "VRC720",
                "isLegionallaProtectionActivated": "WEDNESDAY" }
  },
  "state": {
    "dhw": [ { "index": 255,
               "currentDhwTemperature": 43.5,
               "currentSpecialFunction": "REGULAR" } ]
  }
}

Key point: `operationModeDhw: OFF` (evcc control) coexists with `isLegionallaProtectionActivated: WEDNESDAY`, while `currentSpecialFunction` never leaves `REGULAR` and the cylinder never reaches disinfection temperature.

What type of operating system or environment does evcc run on?

Docker container

External automation

• I have made sure that no external automation like HomeAssistant or Node-RED is active or accessing any of the mentioned devices when this issue occurs.

Nightly build

• I have verified that the issue is reproducible with the latest nightly build

Version

0.310.1

[andig]

/cc @WulfgarW

[WulfgarW]

@Johannes-del
So you have set the opMode for the Dhw to 'off'. But evcc with the vaillant charger can only start (and stop) hotwater boosts. it does not initiate a legionalla protection routine.
I would suggest, that you change the opMode to 'time-controlled' and set up a timer schedule, that only produces hotwater on Wednesdays between 12:00 and 14:00, no hotwater production on the other days. The time window from 12:00 to 14:00 is one, where you normally should have power from your PV, and 2 hours should be enough to reach the legionalla protection temperature.
Give it a try and report back, if that solves your problem.

[Johannes-del]

Thanks a lot @WulfgarW, that really helps to hear it confirmed. So evcc's Vaillant charger only triggers and stops hot water boosts and never runs the legionella routine itself.

Your time-controlled idea is a nice workaround and I did think about it. The main reason I went a different way is that a scheduled window guarantees that hot water is produced, but not that it actually reaches the temperature needed for legionella protection. On a cloudy, PV throttled day the 12:00 to 14:00 slot can fall a bit short. I also quite like keeping off for the rest of the week so there are no unwanted reheats.

In the end I set up a small weekly systemd timer that forces a boost, holds until the water is hot enough, and then restores the previous state. The first real run went through last Wednesday and worked nicely (water went from 43 to 65 °C in about 112 minutes, with a clean restore afterwards).

The one thing I wanted to flag with this report is the underlying behaviour: when operationModeDhw is set to off, the heat pump's own weekly legionella protection seems to be suppressed silently. I set that mode myself to let evcc handle hot water via PV, so this is my own doing, but there is no warning that legionella protection stops along with it. Would it maybe be worth a short note in the docs about this side effect or even at a more visible place?

[WulfgarW]

@Johannes-del
Did you find it anywhere in the evcc documentation of evcc or the vaillant charger, that you should set the opMode to 'off'?
Why did you expect, that a legionella protection schedule would still be running with opMode='off', when even the circulation pump is affected by the opMode?
The vaillant charger can only start or stop hotwater boosts and zone quick vetos. The heat pump accepts hotwater boosts (independent if they are initiated by the MyVaillant app or by evcc) even when the opMode is 'off', but I would advise not to set the opMode to 'off', because then you have to do it all on your own:

• Providing power to and scheduling the circulation pump.
• Legionalla protection
• Making sure, that you don't run out of hotwater, when you don't have enough sun for some days, or if evcc gets stuck.

I would suggest to set the opMode to 'time-controlled' and to define a schedule, that produces hotwater every day around 12:30 to 14:00 local winter time (13:30 to 15:00 daylight saving time), even if there is not enough PV production.
I'm quite sure, that this has nearly no negative effect on your autarchy.

[Johannes-del]

Thanks @WulfgarW, fair point and taken.

No, the docs don't tell anyone to set opMode: off, that was my own choice to let evcc handle DHW purely from PV surplus (mode: pv + limitSoc). So this is self-inflicted, ok. The only surprise was that it stops completely and silently, with nothing in the UI or logs.

I know off means I own the rest, and I've covered your three points: a weekly job that forces a boost, holds until ~65 °C and restores the previous state (first real run went fine this week); a hysteresis plus a smartCostLimit safety net for cloudy spells or if evcc gets stuck; and the circulation pump, which we don't rely on here.

On autarky I agree for winter: there's basically no surplus to lose from November to February, so a daily 12:30 to 14:00 window costs almost nothing and I may well switch to time-controlled for the cold months.

So I'm not asking evcc to change anything. I'd just gently suggest one line in the Vaillant docs, e.g. "setting operationModeDhw: off also disables the heat pump's own legionella protection and circulation scheduling." That would have saved me the surprise. Otherwise I'm happy to close this.

[andig]

Please feel free to update the template to include this. Happy to take a PR.

[Johannes-del]

Thanks @andig — opened #31421 adding the one-line warning to the Vaillant template help text (de/en). It notes that setting the appliance's DHW operation mode to OFF disables the native weekly legionella protection, so users must handle disinfection heating themselves.