Viessmann A9 error on Vitocal heat pump — condensate sensor guide

Error A9 is specific to Viessmann Vitocal heat pumps. It indicates a fault with the condensate temperature sensor — a safety sensor that monitors compressor discharge temperature and protects the refrigerant circuit.

Symptoms

• Heat pump stopped, no automatic restart
• No heating (heat pump mode) — sometimes falls back to backup electrical resistance if configured
• Fault LED on the Vitocal controller
• Sometimes abnormal compressor noise just before lockout

Affected Vitocal models

• Vitocal 100-A / 100-S (air-air, air-water)
• Vitocal 200-A / 200-S
• Vitocal 222-S / 242-S (combi thermal)
• Vitocal 300-A / 300-G (ground-source)
• Vitocal 333/343-G (ground-source combi)

Common causes

• Failed condensate sensor — the most common cause
• Oxidised sensor wiring — moisture in the outdoor unit
• Disconnected connector — after intervention or vibration
• Real compressor overheating — serious fault, needs deeper diagnosis
• Low refrigerant charge — refrigerant circuit leak

Important — don’t take this lightly

Unlike classic boiler codes, A9 on a heat pump can hide a refrigerant circuit problem (compressor, pressure, refrigerant). A forced reset without diagnosis can damage the compressor — the most expensive part of the unit.

Always diagnose before rearming, especially if the heat pump is more than 5 years old or has a history of refrigerant-circuit interventions.

Professional diagnosis required

Unlike a simple boiler, a heat pump requires a technician who is F-Gas certified for refrigerant handling. The diagnosis must measure:

• Compressor discharge temperature
• High and low pressures on the refrigerant circuit
• Refrigerant charge (R32, R410A, R290 depending on model)
• Sensor vs actual-temperature consistency

Intervention costs

• Full heat-pump diagnostic: on request
• Condensate sensor replacement: on request
• Refrigerant-side intervention if circuit involved: on request
• Compressor replacement (worst case): on request 500–on request 000

Frequently asked questions

Is my Vitocal under warranty?

Viessmann heat pumps are typically warranted 5 years on parts and 10 years on the compressor on some models — conditional on annual maintenance by a certified installer. Check your service log.

Can I switch to backup electric heating while waiting?

Yes, most Vitocals have an electric backup that takes over. It’s enabled in Vitotronic settings. Warning: electricity consumption spikes 3–4× compared to heat pump mode.

Typical Luxembourg response time for heat pump?

24–48 hours for common models. In deep cold spells (heating outage), priority intervention. Ground-source models take longer (48–72 hours) due to complexity.

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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