Viessmann
The Viessmann Vitocal 151-A is a 7.3 kilowatt monobloc air source heat pump using R290 refrigerant, featuring an integrated 190-litre cylinder and a maximum flow temperature of 70 degrees.
The Viessmann Vitocal 151-A is a high-temperature monobloc air-source heat pump designed for domestic heating and hot water. Utilizing R290 natural refrigerant, it achieves flow temperatures up to 70°C, making it suitable for retrofit applications. This floor-standing indoor unit integrates a 190-litre DHW cylinder, providing a compact footprint for internal installations. Engineers can access technical specifications, electrical requirements, and common fault codes like A.11 to assist with commissioning and maintenance.
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Full technical specifications extracted from the manufacturer brochure. Empty sections are hidden.
The Viessmann Vitocal 151-A represents a significant shift toward natural refrigerants in the UK domestic market. By employing R290 (propane), which has a Global Warming Potential (GWP) of just 0.02, the unit operates efficiently while meeting strict environmental standards. The system is a monobloc design, meaning the entire refrigerant circuit is contained within the outdoor unit, simplifying installation by requiring only hydraulic connections between the indoor and outdoor components.
Performance is a core attribute of the Vitocal 151-A range. The nominal 7.3kW variant delivers a Seasonal Coefficient of Performance (SCOP) of 4.825 at a 35°C flow temperature, earning an ErP rating of A+++. Even at higher flow temperatures of 55°C, the unit maintains a SCOP of 3.7. With a maximum flow temperature of 70°C, the system can often utilize existing radiator networks without extensive upgrades, provided the heat loss calculations support the output. The outdoor unit is engineered for quiet operation, with a sound power level of 56 dB(A), which is critical for meeting MCS-020 noise assessment criteria in high-density housing.
The physical configuration consists of a substantial outdoor unit weighing 197kg and a floor-standing indoor unit. The indoor module stands 1900mm high and incorporates a 190-litre stainless steel DHW cylinder. This integration reduces the need for separate external components, though installers must ensure the floor can support the 386kg filled weight. Hydraulically, the system operates at a maximum pressure of 3 bar and requires a minimum water flow rate of 16.67 litres per minute.
Control is managed via the HMI programming unit or the ViCare app, with built-in WiFi and Modbus compatibility for smart home integration. For maintenance, the Vitocal 151-A requires an annual service interval. Key tasks include cleaning the filter in the ball valve, checking the fan in the outdoor unit, and cleaning the evaporator and condensate system. Diagnostic codes are displayed on the HMI; for instance, fault code A.11 indicates a system pressure deviation, often caused by leaks or expansion vessel failure. Communication failures between the HPMU electronics and the HMI will trigger a connection error lockout, requiring a check of the wiring and plug connections.
Every fault code listed in the manufacturer brochure, with likely causes and engineer actions. Each code has a permalink — paste a URL like https://www.boilertag.com/heat-pumps/viessmann/awot-m-e-ac-af-151-a04-sp#fault-a.11 to jump straight to the entry.
| Code | Meaning | Details |
|---|---|---|
| A.11 | System pressure deviation Show causes & actionsThe system pressure value deviates for a determined period by more than the given range. Likely causes
Engineer action Check the system for leaks. Check the expansion vessel. Restore system pressure. | Details |
| Connection errorLockout | Communication failure Show causes & actionsLoss of communication between HPMU electronics module and HMI programming unit. Likely causes
User action Check connecting cable and plug between HPMU electronics module and HMI programming unit. Engineer action Verify wiring integrity and plug seating. | Details |
Patterns that group across the manufacturer's fault-code table. Each entry links to the full code listing further down the page.
User: Check connecting cable and plug between HPMU electronics module and HMI programming unit.
Engineer: Verify wiring integrity and plug seating.
Triggered by fault code Connection error.
Engineer: Check the system for leaks. Check the expansion vessel. Restore system pressure.
Triggered by fault code A.11.
Common questions from engineers and homeowners about the Viessmann AWOT-M-E-AC-AF 151.A04 SP.
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Specifications on this page are extracted from the manufacturer brochure. Always cross-check against the latest datasheet before commissioning or quoting. Last updated 30 April 2026.