Remeha A.01.21 Fault Code: Causes, Fixes & Repair Costs
What does the Remeha A.01.21 fault code mean?
The A.01.21 code is a warning generated by the boiler's control system when it detects that the domestic hot water (DHW) temperature is climbing far too rapidly during a heating cycle. In Remeha's fault classification system, 'A' prefix codes are warnings — the boiler continues to operate, but the underlying problem must be investigated promptly. Left unattended, a warning can escalate into a blocking or full lock-out. The 'Level 3' designation means the control system has hit the most severe rate-of-rise threshold it monitors for DHW temperature, indicating the water passing through the hot water circuit is being heated far more aggressively than it should be. This code appears on Remeha Quinta Ace, Gas 220 Ace, and Gas 320/620 Ace boilers.
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
- Scaled or blocked DHW plate heat exchanger Common
In UK hard water areas, limescale accumulates inside the domestic hot water plate heat exchanger over time, narrowing the internal channels and reducing the flow of cold water through the unit. With less water to absorb the same burner output, whatever water does pass through heats up much more steeply and quickly than normal, triggering the temperature gradient warning. This is the single most common cause of A.01.21, particularly in boilers that have not been serviced or descaled for several years.
- Restricted cold water flow or partially closed isolation valve Sometimes
If the cold water isolation valve serving the boiler's DHW circuit is not fully open, or if the inlet strainer or filter on the DHW side has become clogged with debris, the flow rate through the heat exchanger drops. The result is the same as scaling: a small volume of water is exposed to a high heat input and its temperature rises too fast. Checking that the cold water supply valve is fully open is one of the few things a homeowner can safely check themselves.
- Faulty DHW NTC temperature sensor Sometimes
The NTC sensor monitors the temperature of the outgoing hot water in real time and feeds that data back to the boiler's control board. If the sensor is degrading or has developed a fault, it can report temperatures that are climbing faster than the water actually is, causing a false A.01.21 warning. Alternatively, a genuinely failing sensor may give erratic readings that confuse the control system. Replacing the sensor is a straightforward job for a Gas Safe engineer and the part itself is inexpensive.
- Gas valve modulation fault Rare
During DHW production, the gas valve should modulate the burner output downwards to avoid over-firing. If the valve is sticking, worn, or not responding correctly to control signals, the burner may deliver more heat than intended into the plate heat exchanger, causing the water temperature to spike rapidly. This is a less common cause but should be investigated if the plate heat exchanger and sensor have already been checked and cleared.
How to fix it
- Check that the cold water inlet isolation valve is fully open DIY safe
Locate the cold water supply valve feeding the boiler — usually found on the cold water pipework close to the unit. Ensure it is turned fully open (handle in line with the pipe). A valve that is even slightly restricted can reduce DHW flow enough to cause the temperature gradient warning.
- Run a hot water tap and check flow rate is normal DIY safe
Open a hot water tap fully and observe the flow. If the flow is noticeably weaker than usual from multiple outlets, this points to a supply restriction or a badly scaled heat exchanger. A single tap with poor flow may indicate a localised blockage rather than a boiler fault.
- Check boiler system pressure and top up if needed DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop to top up to around 1.2 bar. While low pressure is not a direct cause of A.01.21, it is worth eliminating before calling an engineer.
- Reset the boiler once and observe DIY safe
Press the reset button on the boiler's front panel once, then wait at least 60 seconds. If the boiler fires up normally and the warning clears, monitor it over the next few hours. If the A.01.21 warning returns quickly, do not keep resetting — repeated resets without fixing the root cause can mask a worsening problem and risk escalating it to a lock-out.
- Have a Gas Safe engineer inspect and descale or replace the DHW plate heat exchanger Gas Safe engineer
A qualified engineer will remove the plate heat exchanger and assess the degree of scaling. Mild to moderate scale build-up can often be cleared by circulating a descaling solution such as dilute citric acid through the unit. If the exchanger is heavily fouled, corroded, or physically damaged, replacement is the correct course of action. This single step resolves the majority of A.01.21 faults.
- Have the DHW NTC temperature sensor tested and replaced if faulty Gas Safe engineer
The engineer will use diagnostic equipment to check whether the DHW NTC sensor is reading accurately and consistently. If the sensor is giving erratic or false readings, it should be replaced. The part is inexpensive and the swap is relatively quick, making this a sensible early check if the plate heat exchanger is found to be clean.
- Have the cold water inlet strainer cleaned Gas Safe engineer
The engineer should remove and inspect the inlet strainer or filter on the DHW side of the boiler. Even a partially blocked strainer can reduce cold water flow enough to cause rapid temperature rises. Cleaning or replacing this filter is a simple task during a service visit.
- Have the gas valve tested for correct modulation Gas Safe engineer
If the plate heat exchanger, sensor, and inlet strainer have all been checked and are not the cause, the engineer should verify that the gas valve is modulating correctly during DHW demand. A valve that over-fires without throttling back will drive the DHW temperature up too steeply. Gas valve work must only ever be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the warning persists or returns Gas Safe engineer
If you have completed the homeowner checks above and the A.01.21 warning is still showing, or keeps returning after a reset, contact a Gas Safe registered engineer without delay. A warning code in Remeha's system can progress to a blocking or lock-out if the root cause is not addressed, and commercial Remeha models such as the Quinta Ace or Gas 220 Ace may benefit from Remeha's own service team being involved.
Parts you may need
- DHW plate heat exchanger · from £180
- DHW NTC temperature sensor · from £20
- Cold water inlet strainer/filter · from £15
- Gas valve · from £220
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Is A.01.21 dangerous — should I turn my boiler off?
A.01.21 is classified as a warning in Remeha's fault system, meaning the boiler continues to run rather than shutting itself down immediately. It is not an emergency in the way a gas leak or carbon monoxide alarm would be, but it should not be ignored. The warning indicates the hot water circuit is not behaving correctly, and if the root cause — most often a scaled plate heat exchanger — is left untreated, the code can escalate to a blocking or lock-out that leaves you without hot water or heating entirely. Get it looked at by a Gas Safe engineer within a few days.
Why does A.01.21 keep coming back after I reset the boiler?
A reset clears the displayed warning but does nothing to fix whatever is causing the DHW temperature to rise too fast. If the plate heat exchanger is scaled, the NTC sensor is faulty, or cold water flow is restricted, the same conditions will retrigger the warning as soon as the boiler next attempts to produce hot water. Resetting repeatedly without investigation is not recommended — it can mask a deteriorating situation. One reset to confirm whether the boiler recovers is reasonable; if the warning returns, call an engineer.
How much does it cost to fix a Remeha A.01.21 fault?
For the most common cause — a scaled DHW plate heat exchanger — a descale typically costs between £100 and £200 including labour. If the exchanger needs replacing rather than cleaning, expect to pay roughly £200 to £350 all-in for a standard Remeha residential model. A faulty DHW NTC sensor is one of the cheaper fixes, usually £80 to £150 including parts and labour. Gas valve replacement is less common but sits in the £200 to £350 range. In rare cases where the fault stems from a PCB or modulation control issue, costs can exceed £400, though this is not the typical outcome for A.01.21. Prices run around 15–25% higher in London and the South East.
Can limescale really cause this fault, and how do I prevent it coming back?
Yes — limescale is the most frequent trigger for A.01.21 in UK hard water areas (broadly, the Midlands, South East, and East of England). Scale builds up inside the narrow channels of the DHW plate heat exchanger, restricting flow and causing the water that does pass through to overheat rapidly. The best prevention is an annual boiler service, during which the engineer can assess scale build-up before it becomes severe. Fitting a whole-house or point-of-use scale inhibitor on the cold water inlet can slow future accumulation. Some homeowners in very hard water areas also ask their engineer about installing a DHW water softener, though this is a more significant investment.