Daikin 7H-01 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 7H-01 fault code mean?
The 7H-01 code appears on Daikin Altherma heat pump controllers when the system's flow sensor detects insufficient water circulating through the heat exchanger. In plain terms, the unit cannot confirm that enough water is moving around the heating circuit to operate safely, so it raises this alert to protect itself. This code is specific to the Altherma air-to-water and water-to-water ranges — on other Daikin product lines, a 7H reading is not a recognised fault and may instead point to a malfunctioning main PCB misreporting its status.
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
- Closed or partly closed zone valves Common
If one or more zone valves have shut — perhaps after a power cut, a thermostat calling for no heat, or a manual adjustment — the circulation pump has nowhere to push water. The flow sensor picks up the resulting drop in flow rate and triggers 7H-01. This is one of the most frequently overlooked causes, especially in systems with multiple zones or motorised valves.
- Low system pressure Common
Heating circuits need to be pressurised to circulate water effectively. If the system pressure has dropped below around 1.0 bar — due to a slow leak, a recent bleed, or natural pressure loss over time — the pump struggles to maintain adequate flow. Daikin Altherma systems generally run best between 1.5 and 2.0 bar.
- Blocked strainer or filter Common
Most Altherma installations include an inline strainer or mesh filter on the return pipework to protect the heat exchanger. Over time this can accumulate debris, scale, or sludge — particularly in older systems with magnetite contamination — restricting water flow enough to trigger the 7H-01 alarm.
- Circulation pump failure or poor performance Sometimes
The internal or external circulation pump is responsible for driving water around the heating circuit. If the pump is failing, seized, or running at reduced speed due to an electrical or mechanical fault, flow rates will fall below the sensor's acceptable threshold and the fault will appear, often intermittently at first.
- Faulty flow sensor Sometimes
The flow sensor itself can fail or drift out of calibration, causing it to report low flow even when water is circulating normally. If all other checks come back clear and the fault returns quickly after a reset, a defective sensor is a likely culprit requiring engineer investigation.
- Freezing or very cold weather conditions Rare
In extreme cold, water within outdoor pipework or poorly insulated sections of the circuit can partially freeze, restricting flow. This is more likely in systems without adequate antifreeze protection or where buffer tanks or pipework are exposed to frost.
How to fix it
- Check all zone valves are open DIY safe
Locate each zone valve on your heating circuit — they are typically found near the manifold or in an airing cupboard. Confirm they are all in the open position. Some motorised valves have a manual override lever; consult your system's documentation if you are unsure how to operate them. Once any closed valves are opened, try restarting the heat pump from its controller.
- Check and top up the system pressure DIY safe
Find the pressure gauge on your indoor unit or buffer vessel. If the reading is below 1.0 bar, the system needs topping up. Use the filling loop (a braided flexible hose with an inline valve, usually located near the indoor unit) to slowly introduce mains water until the gauge reads between 1.5 and 2.0 bar. Close the filling loop valve fully once done, then reset the unit. If pressure keeps dropping, there is a leak that needs professional attention.
- Clear any debris from around the outdoor unit DIY safe
Ensure the outdoor unit is free from leaves, snow, ice, or anything else that could be impeding airflow or, in freezing conditions, contributing to ice forming around pipework connections. Gently clear any blockage without using sharp tools or excessive force on the unit's fins or casing.
- Reset the heat pump from the controller DIY safe
After completing the checks above, perform a system reset using the Altherma controller — this is typically done through the settings or alarm menu. A genuine one-off fault (such as a momentary air lock or brief pressure dip) will often clear at this point. If the 7H-01 code returns within minutes or hours, a deeper underlying cause needs investigation.
- Inspect the strainer or inline filter heat pump engineer
The return-side strainer needs to be isolated, removed, cleaned, and refitted correctly. This involves shutting isolation valves and managing pressurised water. While not a refrigerant task, it does require familiarity with wet heating systems. A heating engineer or MCS-certified heat pump engineer should carry this out to avoid introducing air locks or causing leaks.
- Have the circulation pump tested and replaced if needed heat pump engineer
Testing whether the pump is running, drawing the correct current, and delivering adequate head pressure requires diagnostic tools and electrical knowledge. Pump replacement on an Altherma system also involves draining sections of the circuit and correct commissioning afterwards. This must be carried out by a qualified engineer.
- Have the flow sensor and wiring inspected heat pump engineer
An engineer can test the flow sensor's electrical resistance and output signal to confirm whether it is reading accurately. Loose wiring, corrosion at the connector, or a degraded sensor element are all possible findings. Sensor replacement must be completed by a suitably qualified engineer to ensure the system is correctly recommissioned.
- Contact an MCS-certified heat pump engineer heat pump engineer
If the fault persists after your own checks, arrange a visit from an MCS-certified engineer with Daikin Altherma experience. They will carry out a full system diagnostic, test the pump, sensor, and wiring, and identify any blockages or pressure-related issues. Daikin's own service network offers fixed callout rates (with reduced rates for customers on a maintenance contract) and will handle any warranty repairs through registered D1/D2 installers.
Parts you may need
- Daikin Altherma flow sensor · from £85
- Inline strainer / Y-filter (22mm or 28mm) · from £25
- Grundfos or Wilo circulation pump (compatible replacement) · from £180
- Zone valve motorised head (replacement) · from £45
- Filling loop assembly · from £20
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 fix the Daikin 7H-01 fault myself without calling an engineer?
In some cases, yes. If the cause turns out to be low system pressure or a zone valve that has inadvertently closed, topping up the pressure via the filling loop or reopening the valve and resetting the unit costs nothing and takes only a few minutes. However, if these simple checks do not clear the fault — or if it keeps coming back — you will need a qualified engineer to investigate the pump, strainer, and sensor. Do not attempt to open any pipework or work on internal components yourself.
Why does the 7H-01 keep coming back after I reset it?
An intermittent fault that clears briefly after a reset but returns within hours is a strong sign that the root cause has not been addressed. Common culprits include a circulation pump that is starting to fail (it may run for a while before losing head pressure), a strainer that is partially blocked and gradually restricting flow again, or a flow sensor that is drifting. Each reset just masks the underlying problem. An engineer visit is the appropriate next step to get a lasting fix.
Does the Daikin 7H-01 fault affect my hot water as well as heating?
Yes, potentially. The Altherma system uses the same water circuit and flow sensor for both space heating and domestic hot water production (via the heat exchanger or cylinder). If the fault is active and the unit locks out, it will typically suspend both heating and hot water functions until the alarm is cleared. Resolving the flow issue restores both.
Will the repair be covered under warranty?
If your Daikin Altherma was installed by a registered D1 or D2 Daikin installer and you registered the product for the extended warranty within 30 days of installation, you should be entitled to parts and labour coverage — typically for five years. Contact Daikin UK or your original installer to log a warranty claim. If the system is out of warranty, Daikin's own engineer network charges a fixed callout rate (reduced for customers with a maintenance agreement). For major component replacements such as a new circulation pump or PCB, costs can exceed £600, so it is worth confirming warranty status before authorising any work.