Remeha H.01.08 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha H.01.08 fault code mean?
The H.01.08 code appears on Remeha boilers — most notably the Quinta Ace range — when the central heating flow temperature is climbing far more rapidly than the control system expects. The boiler labels this condition 'CH Temp Grad. Level3', meaning the temperature rise rate has breached the third and most serious internal threshold. Because heat is building up in the heat exchanger faster than circulating water can carry it away, the boiler enters a blocking state: it suspends operation temporarily as a precaution rather than performing a full lockout. The underlying message is almost always the same — water is not flowing through the system at the rate it should be, whether due to a pump problem, low pressure, trapped air, or a physical blockage.
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
- Circulation pump failure or seizure Common
If the pump has stopped turning — or is running but not generating adequate flow — heat accumulates in the heat exchanger almost immediately after the burner fires. This is the single most common trigger for a Level 3 temperature gradient alarm. Pumps can seize after a long idle period (e.g. at the end of summer), lose impeller efficiency through age, or fail electrically.
- Low system pressure Common
A sealed system running below roughly 1 bar may not hold enough water volume to absorb and transport heat effectively. The pump can also start to cavitate, reducing flow further. Check the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold.
- Airlock in the system Common
Trapped air pockets in radiators, pipework, or the pump itself can partially or fully block water circulation through a circuit. Even a partially airlocked system can cause localised overheating that triggers the temperature gradient protection.
- Sludge or scale blocking the heat exchanger or pipework Sometimes
In older systems or those without adequate inhibitor, magnetite sludge and limescale deposits can narrow internal passageways significantly. A blocked heat exchanger or restricted pump inlet reduces flow to the point where temperature gradients become extreme.
- Closed or restricted isolation valve Sometimes
An isolation valve on the flow or return inadvertently left closed, partially closed, or seized in a restricted position will dramatically reduce circulation. This is easy to overlook but quick to check.
- Faulty flow or return temperature sensor Rare
If the NTC temperature sensor on the flow or return is giving erratic or falsely high readings, the boiler's control board may register a temperature gradient that does not reflect reality. Loose wiring connections to the sensor can produce the same effect.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. For most Remeha sealed systems the cold fill pressure should sit between 1.0 and 1.5 bar. If it reads below 1 bar, repressurise slowly using the filling loop — usually two braided hoses with quarter-turn valves located beneath the boiler — until the gauge reaches about 1.2–1.3 bar, then close both valves. Do not overfill beyond 2 bar.
- Bleed your radiators to release trapped air DIY safe
Starting with the radiator furthest from the boiler and working back towards it, use a bleed key to open the bleed valve at the top corner of each radiator. Hold a cloth underneath and release air until a steady trickle of water appears, then close the valve. Re-check the boiler pressure afterwards and top up again if it has dropped below 1 bar.
- Check that all visible isolation valves are fully open DIY safe
Inspect any lockshield or service isolation valves on the pipework around the boiler and on radiators. A valve is fully open when its slot or handle is in line with the pipe. If you find one that is partially or fully closed and you are confident it is a water-side isolation valve (not a gas valve), open it carefully.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the Remeha control panel as described in your user guide. Wait a full 60 seconds after the boiler attempts to restart before judging whether the fault has cleared. If H.01.08 returns straight away or within a short time, do not reset again — repeated resets when a genuine flow problem exists can cause heat exchanger damage. Note down the code and call an engineer.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
An engineer will check whether the pump is energised, test for correct voltage at the terminals, and attempt to free a seized shaft. If the pump has failed electrically or mechanically, replacement is the most likely outcome. A like-for-like pump swap on a Quinta Ace is a straightforward repair for a competent engineer.
- Have an engineer check all temperature sensor connections and test sensor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the flow and return NTC sensors across the expected temperature range and compare the readings to the manufacturer's specification. A sensor reading outside tolerance, or one with corroded or intermittent wiring, will be replaced.
- Commission a power flush if sludge is suspected Gas Safe engineer
If the engineer finds restricted flow throughout the system — evidenced by cold spots on radiators, noisy pipework, or heavy sludge in the pump body — a power flush using a specialist machine and chemical cleaner will clear debris from the heat exchanger, pipework, and radiators. A magnetic system filter and inhibitor dose should be fitted afterwards to prevent recurrence.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the steps above have not resolved H.01.08, or if you are not comfortable carrying out the DIY checks, contact a Gas Safe registered engineer. Provide them with the exact code (H.01.08), the boiler model (e.g. Quinta Ace 65), and a description of any symptoms such as cold radiators, noise, or how long the boiler runs before blocking. Do not attempt to access internal gas or electrical components yourself.
Parts you may need
- Central heating circulation pump (compatible replacement) · from £85
- Remeha NTC flow temperature sensor · from £25
- Remeha NTC return temperature sensor · from £25
- Inhibitor dose (e.g. Fernox F1 or Sentinel X100) · from £15
- Magnetic system filter (e.g. Adey MagnaClean 22mm) · from £65
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Is H.01.08 a full lockout or will the boiler try to restart on its own?
H.01.08 is classified as a blocking fault, not a hard lockout. This means the boiler suspends operation temporarily and may attempt a restart automatically once temperatures normalise. However, if the underlying cause — such as a failed pump or low pressure — has not been resolved, the fault will return quickly. You can try one manual reset, but if the code keeps coming back, treat it as requiring an engineer visit rather than continuing to reset.
My Remeha boiler shows H.01.08 but the pressure looks fine — what else could it be?
Adequate pressure is necessary but not sufficient for good circulation. Even with the pressure gauge reading correctly, a seized circulation pump, an airlock in the system, or a partially closed isolation valve can all prevent water from flowing freely enough to carry heat away from the heat exchanger. It is also worth checking whether your radiators heat up evenly — cold radiators or slow warm-up times alongside H.01.08 are a strong indicator of a circulation or sludge problem rather than a sensor issue.
How much does it typically cost to fix a Remeha H.01.08 fault in the UK?
For the most common cause — a circulation pump replacement — expect to pay roughly £200–£350 including the part, labour, and VAT. A sensor replacement is generally cheaper at around £120–£200 all-in. If the system needs a full power flush to clear sludge, that adds roughly £300–£500 to the bill, though this is often worthwhile on an older system. In rare cases where the PCB is involved, the repair can climb to £400 or more; if you are facing a PCB quote on a boiler over 10 years old, it is worth getting a new-boiler comparison as well.
Can I prevent H.01.08 from recurring once it has been fixed?
Yes. Keeping your system pressure between 1 and 1.5 bar, fitting a magnetic filter (such as an Adey MagnaClean) if you do not already have one, and maintaining a correct dose of corrosion inhibitor in the system water are the three most effective long-term measures. An annual service by a Gas Safe engineer will also catch early signs of pump wear or sensor drift before they develop into a blocking fault. If the original fault was sludge-related, a power flush followed by inhibitor dosing should prevent a repeat for many years.