Viessmann E8 error (DHW sensor fault) — diagnosis and fix

The Viessmann error code E8 points to a domestic hot water (DHW) temperature sensor fault. Your heating usually keeps working, but hot water becomes unreliable — either cold, scalding, or cut off entirely.

What does E8 mean?

E8 signals a problem with the DHW temperature sensor (hot water). This sensor is specific to configurations with a separate hot water tank — not present on pure combi boilers.

Symptoms

• No hot water, or hot water at uncontrolled temperature (too hot/too cold)
• Heating usually works normally
• E8 displayed on the Vitotronic
• Sometimes intermittent — sensor at its working limit

Affected models

• Vitodens 100-W, 111-W (very common on these combi variants)
• Vitodens 200-W, 222-W
• Vitopend 100 combi
• Vitola 200 with integrated hot-water tank

Common causes

• Aged NTC sensor — 8–10 years of service (60% of cases)
• Oxidised connector at the sensor plug
• Damaged wiring — chafing inside the boiler casing
• Limescale on the sensor — thermal insulation affects reading
• Control board fault on the sensor read side (rarer)

Diagnostic checks

1. Is the fault permanent or intermittent? (intermittent suggests connector issue)
2. Is DHW demand active in Vitotronic settings?
3. Does the DHW pump start at tap-open? (if not, sensor is reporting « already hot »)
4. Note the output water temperature (too hot = sensor reads too low)

Replacement cost

• DHW NTC sensor part: on request
• Labour: 30 min to 1 hour
• Total typical: on request

On some compact models (e.g. 111-W), sensor access requires partial disassembly of the DHW module — allow 1.5 hours.

Frequently asked questions

Can I keep using hot water while waiting for repair?

Heating still works fine. For hot water, temperature is no longer controlled — scalding or cold shower possible. Fix quickly if you have children or vulnerable adults at home. Use a thermostatic mixer (already required on modern Luxembourg installs) as a safety.

Does descaling fix E8?

In roughly 20% of cases, yes — if scale buildup is the cause. A full descaling (boiler + sensor) can be tried before replacement.

Where is the sensor located?

It depends on the model: on the DHW flow pipe, on the tank, or on the secondary heat exchanger. Our technician identifies the correct sensor on arrival.

Need an English-speaking Viessmann specialist in Luxembourg?

Weber & Fils is certified on all Viessmann boilers and heat pumps installed in Luxembourg (Vitodens 100–300, Vitopend, Vitocal, Vitola). Their technicians speak English fluently and stock the most common parts.

For quotes and emergency call-outs: weberetfils.lu/en or call +352 20 60 22 22 — 24/7.

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