Daikin 7H-04 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 7H-04 fault code mean?
The 7H-04 code indicates that the Daikin Altherma heat pump has detected an abnormal or insufficient water flow specifically during domestic hot water (DHW) production. When this fault is triggered, the unit stops operating and requires a manual reset before it will restart. This code is distinct from other 7H-series faults: 7H-01 relates to general water flow problems, 7H-05 to heating or sampling mode, and 7H-06 to cooling or defrost. If you see a 7H code on a non-Altherma Daikin product, this may point to a malfunctioning main PCB rather than a genuine flow fault.
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
- Low system water pressure Common
If the pressure in your heating circuit has dropped below around 1.0–1.5 bar, there may not be enough water flow for the heat pump to operate the DHW cycle safely. The pressure gauge on the indoor unit or buffer cylinder should read between 1.5 and 2.0 bar under normal conditions.
- Blocked or dirty water strainer/filter Common
A partially or fully blocked strainer on the return pipe to the heat pump is a very frequent cause of restricted flow during DHW operation. Debris, scale or sludge can accumulate over time and significantly reduce flow rate.
- Air in the water circuit Common
Air pockets trapped in the system — particularly at high points in the pipework — can interrupt flow and trigger a flow abnormality code. This is especially likely if the system has recently been filled, vented or had work carried out on it.
- Closed or partially closed isolation valve Sometimes
An inadvertently closed or only partially opened valve somewhere in the DHW or primary water circuit can restrict flow enough to trigger the 7H-04 alarm. This includes valves that may have been closed during maintenance and not fully reopened.
- Faulty or stuck 3-way valve Sometimes
The 3-way diverter valve directs water between the space heating and DHW circuits. If it is stuck, seized or electrically faulty, it may prevent adequate flow reaching the DHW heat exchanger, causing the fault to appear specifically during hot water production.
- Failing or failed water pump (circulator) Sometimes
If the internal or external circulation pump is wearing out or has seized, it will not be able to maintain the flow rate required during DHW operation. This can be intermittent in the early stages of failure.
- Faulty flow sensor Sometimes
The flow sensor monitors water movement through the system. A sensor that has become clogged, corroded or has drifted out of calibration may report a flow problem even when physical flow is adequate.
- Missing or incorrectly set bypass Rare
Where the system design requires a bypass to maintain minimum flow, the absence of one — or a bypass set too restrictively — can prevent the heat pump achieving the required flow rate during DHW mode.
- Faulty hydro PCB (A1P) Rare
The hydro PCB controls the pump and monitors flow signals. If this board is faulty or is not the correct replacement part for the model, it may incorrectly report a flow fault or fail to drive the pump correctly.
- Freezing conditions affecting pipework Rare
In very cold weather, poorly insulated external or exposed pipework can partially freeze, restricting flow specifically when the system switches to DHW operation.
How to fix it
- Check and note the fault code on the controller DIY safe
Before doing anything else, confirm the code displayed is 7H-04 and note any other codes shown alongside it. Take a photo of the display if helpful. This information will be useful if you need to call an engineer.
- Check the system pressure gauge DIY safe
Locate the pressure gauge on the indoor unit, cylinder or buffer tank. It should read between 1.5 and 2.0 bar. If it is below 1.0 bar, the system is likely undercharged with water. Use the filling loop to top up pressure to approximately 1.5–2.0 bar. If you are unsure how to use the filling loop, refer to your user manual or contact your installer — topping up pressure is a homeowner task on most systems, but only if you are confident doing so.
- Check all isolation and service valves are fully open DIY safe
Walk around the system and check that all visible isolation valves on the pipework to and from the heat pump are fully open. A valve that is only partially open is a common and easily missed cause of this fault. Do not close or adjust any valves you are unsure about.
- Check for visible leaks DIY safe
Look around the indoor unit, cylinder, pipework joints and the outdoor unit's water connections for any signs of dripping water, staining or corrosion. A slow leak can cause persistent low pressure and repeated triggering of this fault.
- Clear any debris from around the outdoor unit DIY safe
Ensure the outdoor unit's air intake and surroundings are free from leaves, snow, ice or other obstructions. While blockage of the outdoor unit is not a direct cause of this fault, it is good practice to rule it out.
- Attempt a system restart DIY safe
Once you have completed the above checks and corrected anything obvious (such as low pressure), turn the heat pump off at the controller, wait at least 3–5 minutes, then restart it. The 7H-04 fault requires a manual reset via the user interface — simply powering off and on may not clear it; follow the reset procedure in your user manual. If the fault clears and does not return, monitor the unit over the next 24–48 hours.
- Do not repeatedly reset without finding the root cause DIY safe
If the fault returns after resetting, avoid continuing to reset the unit without investigation. Repeated restarts while the underlying flow problem persists can cause wear to the pump and other components, potentially turning a minor repair into a more costly one.
- Have a qualified engineer inspect the water strainer and 3-way valve heat pump engineer
A trained heat pump engineer should remove and clean the system strainer/filter on the return pipework, and test the 3-way diverter valve for correct operation and positioning. These tasks involve working on the water circuit connections and should not be attempted without the proper tools and training.
- Have an engineer test the flow sensor and water pump heat pump engineer
An MCS-certified engineer can measure flow rate directly, test the flow sensor's output and check the circulator pump for correct speed and pressure. A faulty sensor may need replacement; a failing pump will need to be serviced or swapped out.
- Have the hydro PCB (A1P) inspected if all else is normal heat pump engineer
If no mechanical cause is found, the engineer should check the hydro PCB is receiving power, is the correct part number for your model, and is functioning correctly. PCB diagnosis and replacement must only be carried out by a qualified engineer.
- Call an MCS-certified heat pump engineer heat pump engineer
If the fault persists after your own checks, or you are not comfortable with any of the above steps, contact an MCS-certified heat pump engineer. Daikin Altherma systems are specialised and benefit from engineers with Daikin-specific training. Refrigerant circuit work, if ever required alongside this fault, must only be carried out by an F-Gas registered engineer.
Parts you may need
- Water flow sensor (Daikin Altherma compatible) · from £65
- 3-way diverter valve assembly · from £120
- Domestic circulation pump (hydronic) · from £180
- Hydro PCB A1P (model-specific) · from £320
- System strainer/filter element · from £25
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 £150–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the Daikin 7H-04 fault myself?
Yes, a manual reset via the user interface is required to clear this fault after the underlying cause has been fixed. You can attempt a reset yourself, but if the fault returns — especially within a short time — it means the root cause has not been resolved and you should contact an MCS-certified engineer rather than continuing to reset the unit.
Why does this fault only appear during hot water production and not during heating?
The 7H-04 code is specifically triggered during DHW (domestic hot water) operation because the heat pump monitors flow separately in different modes. During DHW production the 3-way valve switches to direct water through a different circuit path. If that valve is sticky, the DHW side of the system has a blockage, or the flow rate drops when switching modes, the fault appears in DHW mode only — while the heating circuit may appear to work normally. This is a useful diagnostic clue that points engineers toward the DHW circuit and 3-way valve.
How much will it cost to fix the Daikin 7H-04 fault?
Most people with this fault pay somewhere between £150 and £500, covering an engineer call-out, filter cleaning, valve adjustment or a flow sensor replacement. A 3-way valve replacement typically adds up to around £400–£550 all-in, and a water pump replacement is often £350–£600 including labour. If the hydro PCB (A1P) turns out to be the cause, parts alone can reach £300–£400, with total costs potentially £600–£900. In rare cases where the fault has caused collateral damage to other components, costs can be higher — your engineer will be able to give a firm quote after diagnosis.
Is the 7H-04 code covered under Daikin's warranty?
Daikin's standard warranty covers manufacturing defects on residential Altherma systems for three years on parts, provided the unit has been serviced annually by a suitably qualified engineer. If your system is within warranty and the fault is caused by a defective component such as the flow sensor, 3-way valve or hydro PCB, it may be covered. However, faults caused by installation issues (such as a missing bypass or incorrect settings), system contamination, or lack of maintenance are typically not covered. Contact your installer or Daikin UK directly to check your warranty status before commissioning any repairs.