Remeha H.01.06 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha H.01.06 fault code mean?
The H.01.06 code is a blocking fault on Remeha boilers, meaning the boiler temporarily suspends operation as a protective measure rather than locking out permanently. The display text 'Max Delta TH-TF' tells you that the temperature difference between the heat exchanger (TH) and the flow water (TF) has grown too large for the boiler's safety system to allow continued normal operation. In plain terms, the heat exchanger is running significantly hotter than the water actually leaving the boiler, which points to a problem with how heat is being transferred into the circulating water. Remeha's control system will first attempt to modulate the burner down to bring the gap back within limits; if that doesn't resolve things, the blocking fault is logged and the boiler halts. Unlike an E-prefix lockout, an H-prefix blocking can sometimes clear on its own or with a reset once the underlying condition improves, but a recurring H.01.06 always warrants investigation.
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
- Poor or absent water circulation Common
The most frequently reported root cause. If the circulation pump has seized, is wired incorrectly, or is running in the wrong direction, water moves too slowly through the heat exchanger. Heat builds up in the exchanger faster than it can be carried away, widening the TH-TF gap and triggering the block. A faulty pump or a stuck zone valve starving the primary circuit of flow produces identical symptoms.
- Low system pressure Common
Remeha sealed systems should sit between 1.0 and 1.5 bar when cold. Below this, the volume of water circulating is reduced, thermal stratification worsens, and the heat exchanger temperature climbs disproportionately relative to flow temperature. A pressure gauge reading below 0.8 bar is a strong indicator this is contributing to the fault.
- Air trapped in the system Common
Air pockets in radiators or pipework disrupt the smooth flow of water around the circuit. Even partial airlocks can prevent the pump from circulating water effectively through certain zones, creating localised hot spots in the heat exchanger and pushing the TH-TF difference above the permitted threshold.
- Dirty or scaled heat exchanger Sometimes
Limescale deposits in hard-water areas, or magnetite sludge in older systems, coat the internal surfaces of the heat exchanger and act as an insulating layer. This slows the transfer of heat from the burner into the water, causing the heat exchanger itself to overheat relative to the flow. Systems that have never had a power flush or inhibitor treatment are particularly susceptible.
- Faulty NTC temperature sensor Sometimes
The boiler relies on dedicated NTC sensors to measure both the heat exchanger temperature and the flow water temperature. If either sensor has drifted out of calibration, has a damaged wire, or has a loose connector at the PCB, the control unit may calculate a false temperature difference and trigger H.01.06 even when actual hydraulic conditions are fine.
- Restricted pipework or closed isolating valve Rare
A partially closed service valve, a heavily sludged filter, or a kinked flex connector anywhere in the primary circuit can throttle flow sufficiently to reproduce the same effect as a failing pump. This is less common but easy to overlook during diagnostics.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top up the system using the filling loop (the braided flexible hose or built-in lever beneath the boiler) until the gauge reads around 1.2–1.3 bar, then close the filling loop securely. Never overfill beyond 1.5 bar.
- Bleed the radiators to remove trapped air DIY safe
Work through each radiator in the property, starting on the ground floor and moving upward. Use a radiator bleed key to open the bleed valve at the top corner of each radiator until water — rather than a hiss of air — flows out steadily. Re-check the system pressure after bleeding, as releasing air can cause a small pressure drop, and repressurise if needed.
- Reset the boiler DIY safe
Once you have checked pressure and bled the radiators, attempt a reset using the button or menu on the boiler's control panel (consult your user guide for the exact method on your model). The H-prefix blocking may clear if the underlying conditions have improved. Limit yourself to two or three reset attempts; if the fault returns promptly, further DIY action is unlikely to resolve it and an engineer is needed.
- Inspect all visible isolating valves and the system filter DIY safe
Walk the visible pipework and check that all lockshield valves on radiators and any inline service valves are fully open. If your system has an inline magnetic filter or strainer, check whether it has an accessible indicator showing it is full of debris — an excessively clogged filter can restrict flow enough to contribute to this fault. Do not attempt to dismantle pipework yourself.
- Engineer to inspect and test the circulation pump Gas Safe engineer
A Gas Safe registered engineer should verify the pump is energised, spinning in the correct direction, and delivering adequate flow. Pump speed settings, capacitor condition, and wiring integrity all require professional assessment. If the pump is found to be seized or underperforming, a like-for-like replacement is required.
- Engineer to test the heat exchanger and flow NTC sensors Gas Safe engineer
The engineer should inspect all sensor wiring harnesses and connector plugs for signs of corrosion or looseness. Using a multimeter, they should measure the resistance of both the heat exchanger NTC and the flow NTC against the manufacturer's temperature-resistance curve. Any sensor reading outside tolerance should be replaced.
- Engineer to clean or power flush the heat exchanger and system Gas Safe engineer
If scale or sludge build-up is identified, the engineer may recommend a chemical flush of the heat exchanger or a full system power flush to remove magnetite and debris from the pipework and radiators. A corrosion inhibitor should be dosed into the system afterwards and a magnetic filter fitted if not already present.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If H.01.06 returns after the above DIY checks or cannot be cleared by a reset, a qualified Gas Safe engineer is required to carry out a full hydraulic and component diagnostic. Continuing to run the boiler with a recurring blocking fault risks accelerated wear on the heat exchanger and pump, potentially turning a moderate repair into a much more costly one.
Parts you may need
- Circulation pump (Grundfos or equivalent) · from £85
- Heat exchanger NTC temperature sensor · from £25
- Flow temperature NTC sensor · from £22
- Inline magnetic system filter · from £55
- Automatic air vent · from £12
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–£450, depending on the underlying cause.
Frequently asked questions
Is H.01.06 a lockout or a blocking fault — do I need to reset it manually?
H.01.06 is a blocking fault, indicated by the H prefix in Remeha's fault code system. This is less severe than an E-prefix lockout: the boiler first tries to modulate the burner down to bring the heat exchanger and flow temperatures back into line, and only suspends operation if the situation does not improve. You can attempt a manual reset (up to two or three times), and the boiler may resume if the cause was transient — for example a brief pressure drop. If it blocks again shortly afterwards, the underlying cause needs professional attention.
Can low boiler pressure really cause H.01.06?
Yes, it is one of the more common contributing factors. When system pressure drops below around 1.0 bar, the volume of water available to absorb heat from the exchanger is reduced. The heat exchanger temperature climbs more rapidly than the flow temperature can follow, and the control unit detects the excessive difference. Topping up via the filling loop to around 1.2 bar is a straightforward DIY check worth doing before calling an engineer.
How much does it typically cost to fix H.01.06 in the UK?
Most homeowners resolve this fault for between £150 and £450 once labour is included. Replacing an NTC temperature sensor tends to sit toward the lower end (around £100–£200 all-in), while a circulation pump replacement typically costs £200–£400 including parts and labour. A full system power flush to clear sludge from the heat exchanger usually runs £300–£600 depending on the number of radiators and the engineer's location. A heat exchanger replacement is the more expensive outlier and can reach £500–£700 or beyond — if your boiler is over ten years old and facing that quote, it is worth getting a boiler replacement comparison at the same time.
Will annual servicing help prevent H.01.06 from occurring?
Significantly, yes. During a Remeha annual service an engineer checks system pressure, tests the circulation pump, inspects sensor readings, and measures inhibitor concentration in the water. Catching a pump that is beginning to struggle, a sensor drifting out of range, or a system with low inhibitor levels before they cause a blocking fault is far cheaper than dealing with the fault reactively. A service also gives the engineer a chance to clean the heat exchanger if early scaling is detected, preventing the more costly scenario of a full strip-down clean or replacement.