Remeha H.01.05 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha H.01.05 fault code mean?
The H.01.05 code appears on Remeha boilers — including the Tzerra Ace and Quinta Ace series that use the dot-notation fault code system — when the boiler detects an unusually large gap between the temperature of water leaving the heat exchanger (the flow) and the temperature of water coming back to it (the return). In a healthy system, this difference should be modest; when it becomes too wide, it usually means hot water is not moving around the system efficiently. The boiler responds by shutting down as a protective measure to avoid overheating internal components. The root cause is almost always a circulation problem, though a faulty temperature sensor giving a false reading can also trigger the code.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Seized or failing circulation pump Common
The circulation pump is responsible for pushing water around the central heating system. If it has seized, is running slowly, or has failed entirely, water sits in the boiler rather than circulating, causing the flow temperature to rise sharply while the return stays comparatively cool. This is the single most common trigger for H.01.05.
- Air locked in the system or radiators Common
Air trapped in the pipework or radiators blocks water from flowing freely. Even a partial airlock can reduce circulation enough to create an excessive flow-to-return temperature difference. Airlocks often develop after system maintenance, a pressure drop, or if the boiler has not been used for an extended period.
- Low system pressure Common
If the system pressure drops below around 0.8–1.0 bar, the boiler may not be able to push water around effectively. Modern boilers will also lock out to protect the pump from running dry. The pressure gauge on the boiler front panel will show a reading below the normal 1.0–1.5 bar operating range.
- Sludge or debris blockage in the system Sometimes
Over time, rust particles, scale, and magnetite sludge build up inside radiators and pipework. This restricts water flow, causes uneven heating, and can prevent sufficient water from returning to the boiler. Systems without a magnetic filter or inhibitor are especially prone to this.
- Closed or restricted radiator valves Sometimes
If thermostatic radiator valves (TRVs) across the system have closed simultaneously, or a lockshield valve has been accidentally shut, the hydraulic resistance in the circuit rises sharply and water flow is severely reduced, triggering the fault.
- Faulty flow or return temperature sensor Rare
If either the flow NTC or return NTC thermistor is giving an inaccurate reading, the boiler's control board may calculate a false temperature differential and raise the H.01.05 fault even when circulation itself is fine. This is less common but worth considering when all mechanical checks pass.
- PCB or pump wiring fault Rare
A fault on the printed circuit board, or a wiring fault between the PCB and the pump, could cause the pump to run at incorrect speed or not at all, producing circulation failure without any mechanical pump failure. A Gas Safe engineer is needed to diagnose this safely.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. The reading should normally sit between 1.0 and 1.5 bar when the system is cold. If it is below this, use the filling loop — usually a braided hose with a valve beneath the boiler — to top it up slowly until the needle reaches around 1.2–1.3 bar, then close the valve firmly. Consult your boiler manual if you are unsure where the filling loop is located.
- Bleed the radiators and any air vents DIY safe
Work through each radiator in the house, starting on the ground floor. Use a radiator bleed key to crack open the bleed valve at the top corner of each radiator until water dribbles out steadily with no hissing air. Catch drips with a cloth. After bleeding, recheck the boiler pressure as described above and top up if it has dropped below 1.0 bar.
- Check that all radiator valves are open DIY safe
Walk around the property and confirm that every thermostatic radiator valve (TRV) and lockshield valve is open. If all TRVs have closed at the same time (common in warmer weather), the system may have no water path to circulate through. Fully open at least some of them to give the boiler a circuit to push water around.
- Reset the boiler once DIY safe
Press and hold the reset button on the boiler's front panel for the time specified in your user manual (typically 3–5 seconds) and then release. Wait at least 60 seconds to see whether the boiler fires up and runs normally. If it locks out again with H.01.05, do not keep resetting — repeated resets without resolving the underlying fault can mask the problem and potentially cause further damage.
- Listen and observe during the restart attempt DIY safe
When the boiler restarts, listen for the circulation pump running (a faint hum from inside the boiler casing) and check whether radiators begin to warm evenly. If radiators are slow to heat, heat unevenly, or some stay cold while others are hot, this points towards a circulation or sludge problem that an engineer needs to investigate.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault returns after a single reset, or if your DIY checks did not find an obvious cause, a Gas Safe registered engineer is needed. They will be able to test the circulation pump (freeing it, repairing it, or replacing it), carry out a power flush if sludge is restricting the system, test both the flow and return NTC temperature sensors, inspect pump wiring, and test the PCB. Do not attempt to open the boiler casing, touch wiring, or work on any gas components yourself.
Parts you may need
- Circulation pump (suitable for Remeha Tzerra Ace / Quinta Ace) · from £130
- Flow NTC temperature sensor · from £35
- Return NTC temperature sensor · from £35
- Magnetic system filter (e.g. Adey MagnaClean) · from £65
- Central heating inhibitor (1 litre) · from £15
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–£420, depending on the underlying cause.
Frequently asked questions
Can I fix Remeha H.01.05 myself, or do I always need an engineer?
Some of the checks — topping up system pressure, bleeding radiators, and a single reset — are perfectly safe for a homeowner to carry out and do occasionally resolve the fault if it was caused by a minor airlock or pressure drop. However, if the fault returns after these basic steps, the cause is likely a seized pump, sludge blockage, faulty sensor, or wiring issue, all of which need a Gas Safe registered engineer. Never open the boiler casing or interfere with gas components yourself.
How much does it cost to fix Remeha H.01.05 in the UK?
For the most common repairs, expect to pay roughly £150–£420. A circulation pump replacement typically costs £150–£380 all-in (parts and labour), and a temperature sensor swap usually comes in at £100–£200. A power flush to clear sludge from the system costs £300–£500 but often solves persistent circulation faults without the need for further component replacement. If the PCB turns out to be at fault, that is a more significant repair — usually £300–£500 for parts and labour alone — though PCB failure is a less common cause of this particular code.
Why does my Remeha keep showing H.01.05 after I reset it?
A recurring H.01.05 after resetting almost always means the underlying circulation problem has not been resolved. The most likely culprits are a pump that is on the verge of seizing, a significant build-up of sludge restricting flow, or a faulty temperature sensor giving repeated false readings. Each time you reset without fixing the cause, the boiler is detecting the same problem and shutting down again to protect itself. A Gas Safe engineer should carry out a proper diagnosis rather than relying on resets.
Could a power flush fix H.01.05 on my Remeha boiler?
Yes — if the fault is being caused by sludge, magnetite, or scale restricting water flow around the system, a power flush can clear the blockage and restore normal circulation, which will resolve the H.01.05 code. Signs that sludge may be the culprit include radiators that are cold at the bottom but hot at the top, noisy pipework, or dark/dirty water when radiators are bled. A power flush costs around £300–£500 in the UK but also extends the life of the pump and heat exchanger, so it can be good value if the system is heavily contaminated.