Daikin AA-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AA-00 fault code mean?
The AA-00 fault code on a Daikin Altherma heat pump indicates that the back-up heater's thermal protector (Q1L) has tripped open, meaning the water temperature at the back-up heater has exceeded 90°C. This is a safety shutdown — the system stops operating to prevent damage and will not restart until the cause has been identified and the fault cleared. Because this fault involves dangerously high water temperatures, it should never be repeatedly reset without properly investigating the root cause.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Air trapped in the water circuit or back-up heater Common
Air pockets in the heating circuit or within the back-up heater itself restrict water flow locally, causing that section of water to overheat rapidly. This is one of the most common triggers of AA-00, particularly after commissioning, a recent service, or any work on the system pipework.
- Low system pressure or insufficient water flow Common
If the system pressure has dropped below the recommended level, or if the flow rate through the heat pump is too low (Daikin typically requires at least 12 litres per minute), the back-up heater cannot transfer its heat away quickly enough and the water temperature rises until the thermal protector trips. Check the pressure gauge on the unit or buffer vessel — it should typically read around 1.5–2 bar when cold.
- Faulty thermal protector Q1L Sometimes
The Q1L thermal protector is a resettable safety device. Over time it can become faulty and trip at lower temperatures than intended, or fail to reset correctly after a genuine overheat event. If the fault recurs immediately after a reset without any obvious cause, the Q1L itself may need replacing.
- Faulty thermistor (R2T or R1T) Sometimes
The R2T thermistor monitors water temperature leaving the back-up heater, while R1T monitors the leaving water temperature from the heat exchanger. If either is giving an inaccurate high reading, the control PCB (A1P) may respond as though the water is overheating when it is not — triggering AA-00 as a false alarm.
- Faulty back-up heater assembly Sometimes
Internal failure of the back-up heater element itself — such as a partial short, degraded element, or failed internal component — can cause localised overheating that trips the thermal protector even when water flow and pressure are normal.
- External heat source on the same water circuit Rare
If another heat source (for example, a solar thermal system or a second heat pump) is connected to the same water circuit, it may push water temperatures above the back-up heater's safety threshold. The hydraulic circuit would need to be reconfigured to prevent this.
How to fix it
- Switch off the heat pump at the controller and allow the system to cool DIY safe
Before doing anything else, turn the unit off at the main controller and leave it for at least 20–30 minutes. The thermal protector trips at over 90°C, so the system needs to cool before any meaningful check can take place. Do not attempt to force a reset while the unit is still hot.
- Check and restore system water pressure DIY safe
Locate the pressure gauge on the indoor unit or buffer vessel. If the reading is below 1.0 bar (cold), the system is likely under-pressurised, which can restrict flow through the back-up heater. Top up via the filling loop until the gauge reads approximately 1.5–2.0 bar. Refer to your system's installation guide for the exact target pressure — if you are unsure how to use the filling loop safely, contact an engineer.
- Check for and clear any obvious blockages around the outdoor unit DIY safe
While the system cools, check that the outdoor unit's air intake and discharge are not obstructed by leaves, snow, or other debris. Although the outdoor unit is not the direct cause of AA-00, good airflow is important for overall system health.
- Attempt a manual reset of the thermal protector Q1L and then restart the unit DIY safe
On some Daikin Altherma models, the Q1L thermal protector has a small reset button accessible on the unit (refer to your model's user guide for its location). Once the system has cooled, press this button firmly. Then restart the heat pump from the controller in the normal way. If the fault clears and does not return within a short period, monitor the unit closely over the next few hours.
- If the fault returns, do not continue resetting — call an MCS-certified heat pump engineer heat pump engineer
A recurring AA-00 fault means the underlying cause has not been resolved. An engineer will check water flow rates, purge air from the circuit and back-up heater, test the Q1L thermal protector, measure thermistor resistance values (R1T and R2T), and inspect the back-up heater assembly and hydro PCB (A1P). Because this involves internal electrical components operating at high temperatures, this work must only be carried out by a qualified engineer. Note that no F-Gas or refrigerant work is typically required for this specific fault unless a separate refrigerant circuit issue is also identified.
Parts you may need
- Thermal protector Q1L · from £35
- Thermistor R2T (back-up heater outlet) · from £45
- Thermistor R1T (leaving water) · from £45
- Hydro PCB A1P · from £280
- Back-up heater assembly (complete) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£500, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the AA-00 fault code myself?
You can attempt a single reset once the system has fully cooled, but you should not keep resetting it without finding the cause. The thermal protector trips because water reached over 90°C — a genuine safety event. Repeatedly overriding it risks damaging the back-up heater, the PCB, and potentially the wider heating system. If the fault comes back after one reset, call an engineer.
Is the AA-00 fault covered under my Daikin warranty?
Daikin typically offers a 5–7 year parts and labour warranty when the system was installed by an MCS-certified Daikin-approved installer and registered at the time of installation. If your unit is within this period, the repair may be fully covered. Check your warranty documents or contact your original installer before paying for any repairs. Warranty cover can be voided if the system has not been serviced annually by a qualified engineer.
How much does it typically cost to fix AA-00 if it is not under warranty?
Most homeowners pay somewhere between £180 and £500 to resolve an AA-00 fault, depending on whether the cause is straightforward (such as air in the circuit or low pressure, which requires less labour) or involves a component replacement such as a thermistor, thermal protector, or back-up heater assembly. In more complex cases where the hydro PCB A1P requires replacement, costs can rise considerably higher — this is worth discussing with your engineer before authorising work. Because Daikin Altherma systems are somewhat specialised, labour rates for experienced engineers may be higher than for generic heat pump brands.
Why is my Daikin Altherma showing AA-00 after recent plumbing work?
This is very common. Any time the system pipework is opened — for example, during a boiler removal, cylinder replacement, or pipe modification — air can enter the water circuit. Air pockets near or inside the back-up heater prevent proper heat dissipation, causing the water to overheat and trip the Q1L thermal protector. The solution is usually to bleed the system thoroughly, check the pressure, and reset the fault. If you had a heating engineer carry out recent work, ask them to return and purge the circuit correctly before calling a heat pump specialist.