- Heat pumps
- Air-to-water heat pump
8H-09
Daikin Altherma 4 H F 8H-09: backup heater relay check active
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Daikin Altherma 4 H F units, 8H-09 means backup heater stuck detection is ongoing while the backup heater relay is checked during unit initialisation, stopping operation with an automatic reset. Note whether a setting change or power reset just happened and call your installer if 8H-09 stays.
Safety first
Stop and check this first
- Do not repeatedly turn the circuit breakers off and on while 8H-09 is displayed: Daikin states that the backup heater relays are checked during unit initialisation after a power reset, so cycling power restarts the stop period instead of clearing it.
- Never open the indoor unit switch box to check backup heater relays, terminal X41M, or thermal protector Q1L yourself: the backup heater operates on dedicated high-voltage mains power and must only be serviced by an authorised installer with all power supplies disconnected.
Safe checks you can perform
- Check whether a setting edit or power outage immediately preceded 8H-09
- Observe whether 8H-09 clears automatically in [11] Malfunctioning
- Review [11] Malfunction history for repeated 8H-09 entries or AA-01
Leave these tasks to a qualified professional
- Do not attempt this professional task: Verify [10.8] Backup heater grid configuration and maximum capacity settings
- Do not attempt this professional task: Inspect backup heater relays, PCB A6P, and X41M wiring and run [7.1.6] Backup heater test
What a technician must be able to justify
Technician visit
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.
Verify [10.8] Backup heater grid configuration and maximum capacity settings
What they doHave the installer verify that [10.8] Backup heater (or [5.5] Backup heater) matches the installed backup heater power supply (Single phase, Three phase 3x400V+N, or Three phase 3x230V; Maximum capacity 4 kW, 6 kW, or 9 kW; and Fuse >10A setting) and confirm a clean initialisation without 8H-09 locking up.
They must show youthe [10.8] Backup heater screen showing Grid configuration, Maximum capacity, and Fuse >10A alongside a clean [11] Malfunctioning screen after initialisation.
In writingthe before-and-after values recorded in installer table [10.8] Backup heater and confirmation that initialisation completed automatically.
Inspect backup heater relays, PCB A6P, and X41M wiring and run [7.1.6] Backup heater test
What they doWith mains power disconnected for more than 10 minutes and capacitor voltage verified below 50 V DC, the authorised installer inspects the indoor unit switch box, multistep backup heater PCB A6P, main terminal X41M, overcurrent fuse F1B, and thermal protector Q1L, then runs [7.1.6] Backup heater test in Maintenance mode and follows the Daikin service manual.
They must show youthe result of the [7.1.6] Backup heater test on the screen and photos of the inspected relay and X41M connections.
In writingthe relay contact test results, the [7.1.6] test outcome, and the part reference of any relay or A6P board replaced.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Check whether a setting edit or power outage immediately preceded 8H-09
- Who:
- You (the homeowner or user) can review recent activity without tools.
- How:
- Recall whether someone just changed a parameter in the user interface menus (which temporarily stops operation until returning to the home screen) or whether household electricity briefly went out before 8H-09 appeared.
- Result that matters:
- Whether 8H-09 was triggered by normal unit initialisation after a setting change or power reset.
- If a setting change or power restoration occurred right before 8H-09 appeared, that confirms the controller is running its normal protective backup heater relay check during initialisation.
- If neither a menu edit nor a power interruption happened before 8H-09 appeared, that suggests the unit is re-initialising unexpectedly or encountering a relay check issue.
- Evidence to keep:
- Write down the exact time 8H-09 appeared and note any menu edits or power interruptions that preceded it.
Do not switch the unit circuit breakers off and on to hurry 8H-09; every power reset starts a brand-new initialisation relay check.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Observe whether 8H-09 clears automatically in [11] Malfunctioning
- Who:
- You, watching the indoor unit user interface.
- How:
- Wait without touching the breakers and check the [11] Malfunctioning screen and the LED status indicator to see whether 8H-09 disappears on its own and normal operation resumes.
- Result that matters:
- Whether the automatic reset finishes once the backup heater relay check completes.
- If 8H-09 disappears from the Malfunctioning screen and the status LED returns to blue, that confirms the backup heater relay stuck detection passed and no service visit is needed.
- If 8H-09 stays on the screen and the unit remains stopped, that indicates the relay check cannot finish and requires an installer inspection via [6.2] Dealer information.
- Evidence to keep:
- Record both the start time and the time 8H-09 cleared, or photograph the [11] Malfunctioning screen if the code persists.
Never open the indoor unit switch box to inspect the backup heater relays or terminal X41M yourself.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Review [11] Malfunction history for repeated 8H-09 entries or AA-01
- Who:
- You, checking the 8H-09 log on the indoor touchscreen with advanced end user permission enabled.
- How:
- Open [11] Malfunction history to see whether 8H-09 is logging repeatedly on days without power cuts or setting edits, and check whether AA-01 (Backup heater overheated or BUH power cable not connected) is also listed.
- Result that matters:
- Whether 8H-09 is an isolated startup notification or part of a recurring backup heater fault pattern.
- If Malfunction history shows repeated 8H-09 logs or an accompanying AA-01 code, that points to a persistent backup heater relay, thermal protector Q1L, or power supply issue for the installer.
- If 8H-09 appears only after known power resets or installer configuration sessions, that rules out a recurring fault and confirms expected protective behaviour.
- Evidence to keep:
- Capture a clear photo of the [11] Malfunction history list showing the dates and codes.
Do not enter installer Maintenance mode [7] from the menu, as doing so disables protective functions for 12 hours.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Still not solved
If the error persists, stop here. See what the technician must do and show you.
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 8H-09
- code erreur Daikin Altherma 8H-09
- codice 8H-09 pompa di calore Daikin
- código 8H-09 bomba de calor Daikin
- Daikin 8H-09
- Daikin Altherma error 8H-09
- Daikin Altherma Fehler 8H-09
- Daikin heat pump code 8H-09
- Daikin Wärmepumpe Fehlercode 8H-09
- défaut 8H-09 pompe à chaleur Daikin
- error Daikin Altherma 8H-09
- errore Daikin Altherma 8H-09
What it means
In Daikin installer reference guide 4PEN773391-1C (Chapter 14, section 14.4.4, page 202), code 8H-09 is titled Backup heater stuck detection ongoing with the trigger The backup heater relay is currently being checked. During initialisation of the unit (for example after a setting change on the user interface or a power reset), the controller tests the backup heater relays for protective reasons, stops the unit while the check runs, and resets automatically afterward. Seeing 8H-09 during startup does not by itself prove a backup heater relay or multistep backup heater PCB A6P is faulty, unless the check fails to finish or repeats without a prior reset.
Warnings and stop conditions
- Mediumsource verified
Do not repeatedly turn the circuit breakers off and on while 8H-09 is displayed: Daikin states that the backup heater relays are checked during unit initialisation after a power reset, so cycling power restarts the stop period instead of clearing it.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Highsource verified
Never open the indoor unit switch box to check backup heater relays, terminal X41M, or thermal protector Q1L yourself: the backup heater operates on dedicated high-voltage mains power and must only be serviced by an authorised installer with all power supplies disconnected.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
Symptoms and causes
Probable causes · in order
- Routine protective relay check during unit initialisationCommon
Whenever the indoor unit re-initialises after a configuration setting edit on the touchscreen or after mains power returns, the controller checks the backup heater relays for protective reasons and displays 8H-09 until the check completes.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
- Backup heater relay or multistep PCB A6P not passing stuck detectionPossible
If 8H-09 remains active indefinitely or reappears without any setting change or power interruption, the controller may be unable to complete stuck detection on the backup heater relay circuit connected to terminal X41M and PCB A6P.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C
- Mismatch in backup heater configuration settings or supply wiringPossible
A discrepancy between the actual electrical supply at terminal X41M (such as 4 kW, 6 kW, or 9 kW) and the configured grid configuration or maximum capacity under [10.8] Backup heater can prevent normal backup heater initialisation.
Evidence: Daikin Altherma 4 H F installer reference guide — 4PEN773391-1C · Daikin Altherma 4 H F+W+ECH2O operation manual 4PEN773378-1C
Virtual technician
Does 8H-09 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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