- Heat pumps
- Air-to-water heat pump
81-00
Daikin Altherma 4 H F 81-00: Exit water temperature thermistor after BUH abnormality
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
81-00 means “Exit water temperature thermistor after BUH abnormality” on the scoped Altherma 4 H F family. Daikin records this trigger: Exit water temperature thermistor after BUH is malfunctioning. Effect: Heat pump and backup heater will stop operating for space heating and domestic hot water. The table marks the reset as automatic.
Safety first
Stop and check this first
- Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Safe checks you can perform
- Preserve the diagnostic state
- Let the automatic reset logic decide
- Do not substitute a sensor diagnosis
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured handoff for 81-00
What a technician must be able to justify
A technician is coming
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Measured handoff for 81-00
What they doGive the technician 81-00, both model numbers, the source trigger “Exit water temperature thermistor after BUH is malfunctioning”, the effect “Heat pump and backup heater will stop operating for space heating and domestic hot water”, reset result and recurrence conditions. They should follow Daikin's sensor and measured value branch and compare live measurements with the service values before replacing parts.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Checks and evidence, step by step
Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
- 1
Preserve the diagnostic state
No special tools
Procedure
Photograph 81-00 and the complete display. Record both unit model numbers, operating mode, requested temperatures, weather, system pressure shown to the user and the moment the code appeared.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- 2
Let the automatic reset logic decide
No special tools
Procedure
Daikin marks this code as automatic reset. Do not cycle breakers or force repeated acknowledgements. Record whether the controller clears it and the exact operating state in which it returns.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- 3
Do not substitute a sensor diagnosis
No special tools
Procedure
Note the displayed temperatures and whether they are stable or implausible. Do not unplug, bridge, heat, cool or resistance-test any sensor and do not open either unit.
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Daikin
- Product type
- Air-to-water heat pump
Known code variants
- Altherma 4 H F 81-00
- code erreur Daikin Altherma 81-00
- codice 81-00 pompa di calore Daikin
- código 81-00 bomba de calor Daikin
- Daikin 81-00
- Daikin Altherma error 81-00
- Daikin Altherma Fehler 81-00
- Daikin heat pump code 81-00
- Daikin Wärmepumpe Fehlercode 81-00
- défaut 81-00 pompe à chaleur Daikin
- error Daikin Altherma 81-00
- errore Daikin Altherma 81-00
What it means
In guide 4P773391-1C (2026.02), Daikin assigns 81-00 to “Exit water temperature thermistor after BUH abnormality”. The documented trigger is “Exit water temperature thermistor after BUH is malfunctioning” and the operating effect is “Heat pump and backup heater will stop operating for space heating and domestic hot water”. This is a precise controller decision for Daikin Altherma 4 H F — EPSK(S)04~14A + EPVX07~14S18+23A; it does not by itself prove which component failed.
Warnings and stop conditions
- Highsource verified
Stop and isolate the installation for smoke, burning smell, sparks, unusual heat, water near electrical parts, suspected refrigerant release or repeated protective trips. Do not open the R290 refrigerant circuit or live electrical compartments.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- What the controller establishedConfirmed
Daikin's table defines the condition as “Exit water temperature thermistor after BUH abnormality” and gives this exact trigger: Exit water temperature thermistor after BUH is malfunctioning.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- What the unit does nextConfirmed
Documented effect: Heat pump and backup heater will stop operating for space heating and domestic hot water. Reset classification: Automatic. A cleared display is not proof of repair.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Virtual technician
Does 81-00 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Daikin Europe N.V. · 2026 · official_doc
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