Ariston 722 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 722 fault code mean?
Fault code 722 on Ariston boilers has two distinct meanings depending on which model you own — always check the data plate on your boiler before drawing any conclusions. On most multi-zone and SenSys-type Ariston systems, code 722 signals that the water temperature in heating Zone 2 has climbed above the safe operating threshold, causing the boiler's safety system to shut the appliance down in a protective lockout. On Clas One, Clas Net One, E-Combi, and E-System models, the same code instead points to a defective or disconnected return temperature sensor on Zone 2 — the boiler cannot read a valid temperature signal from that zone and locks out as a precaution. In both cases the boiler will stop heating until the underlying fault is resolved and the unit is reset.
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
- Blocked or restricted flow in Zone 2 pipework Common
If Zone 2 radiator valves are partially closed, or if sludge and debris have built up in the pipework, water cannot circulate freely around that zone. Heat builds up in the local pipework rather than being distributed, and the boiler's overheat sensor trips. This is the most frequent trigger for a genuine Zone 2 overheat.
- Circulation pump failure or reduced output Common
The pump that pushes water around the heating circuits may be seized, worn, or running at insufficient speed. When flow rate drops, heat accumulates faster than it can be carried away, and Zone 2 temperatures spike. A pump that runs but is noisy or intermittent can still cause this fault before it fails completely.
- Zone 2 NTC temperature sensor fault (Clas/E-Combi/E-System models) Common
On models where 722 indicates a sensor defect rather than a true overheat, the Zone 2 return NTC probe may have developed a short circuit, an open circuit, or a high-resistance fault due to age, corrosion, or physical damage. Loose wiring connections at the sensor terminals can produce the same symptom without the sensor itself being faulty.
- Limescale build-up in the heat exchanger Sometimes
In hard-water areas of the UK, limescale accumulates inside the heat exchanger over time. The restricted waterways cause localised hot spots and reduced flow, meaning temperatures in a connected zone can exceed safe limits. Boilers in hard-water regions without water softeners or scale inhibitors are particularly susceptible.
- Air trapped in Zone 2 circuit Sometimes
An airlock in the Zone 2 pipework prevents water from circulating properly through that part of the system. The boiler fires but water barely moves in Zone 2, causing rapid temperature rise and an overheat lockout. This often follows a recent system drain-down, component replacement, or pressure loss.
- Faulty or stuck Zone 2 motorised valve Sometimes
If the motorised zone valve for Zone 2 is not opening fully — or is stuck closed — water simply cannot enter that zone. The boiler may continue firing with no load on Zone 2, driving local temperatures up quickly. Zone valve heads wear out and can fail electrically or mechanically.
- PCB or wiring harness fault Rare
In some cases the printed circuit board misreads the sensor signal or generates a spurious fault due to a failing component or damaged wiring loom. This is less common but should be considered if all sensors and mechanical components check out correctly.
How to fix it
- Check that all Zone 2 radiator valves are fully open DIY safe
Walk around your home and confirm that every thermostatic radiator valve (TRV) and lockshield valve on the Zone 2 circuit is open. Partially closed valves are a surprisingly common cause of restricted flow and localised overheating. Also check that no Zone 2 radiators feel cold at the bottom (indicating sludge) or cold overall (suggesting an airlock or closed valve).
- Bleed any air from Zone 2 radiators DIY safe
Using a radiator bleed key, open the bleed valve on each Zone 2 radiator until water — not air — runs out, then close it. Work from the lowest radiator upward. Trapped air prevents water from circulating and is a common cause of localised overheating.
- Check and top up system pressure if needed DIY safe
Look at the pressure gauge on the boiler. It should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop to top up to approximately 1.2 bar, then close the loop securely. Low pressure reduces flow throughout the system and can contribute to overheating faults.
- Attempt a single reset of the boiler DIY safe
Press and hold the reset button for around 5 seconds as described in your boiler's user guide. Allow the boiler to run for a few minutes and observe whether code 722 returns. If it clears and stays clear, monitor the system over the next 24 hours. Do not reset the boiler more than two or three times — repeated resets without fixing the root cause can damage internal components and mask a potentially serious fault.
- If the fault persists, call a Gas Safe registered engineer Gas Safe engineer
A recurring code 722 requires a qualified professional to diagnose safely. The engineer will test the Zone 2 NTC flow and return sensors for correct resistance readings, inspect and test the circulation pump, check the motorised zone valve operation, and examine the heat exchanger for scaling or blockage. Any replacement of sensors, pumps, zone valves, or PCB components must be carried out by a Gas Safe registered engineer. Do not attempt to access or adjust gas components, the PCB, or internal wiring yourself.
Parts you may need
- Zone 2 NTC return temperature sensor · from £25
- Zone 2 NTC flow temperature sensor · from £25
- Motorised zone valve (including head) · from £55
- Circulation pump (compatible replacement) · from £85
- PCB (printed circuit board) · from £180
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–£380, depending on the underlying cause.
Frequently asked questions
My Ariston shows code 722 but I don't have a multi-zone system — why?
On Clas One, Clas Net One, E-Combi, and E-System models, code 722 does not necessarily mean a true overheat. On these boilers it typically indicates a fault with the Zone 2 return temperature sensor — the probe has failed, corroded, or become disconnected. The fix is usually sensor replacement or a wiring repair rather than any work on the heating circuit itself. Check your model name on the data plate and let a Gas Safe engineer confirm which meaning applies to your appliance.
Can I keep resetting the boiler to get heating back while I wait for an engineer?
One or two resets are reasonable to check whether the fault clears on its own, but repeatedly resetting a boiler that keeps displaying code 722 is not advisable. If the boiler is genuinely overheating, the safety lockout is there to protect it from damage and to keep your home safe. Forcing repeated restarts without fixing the cause can accelerate wear on components and may void your warranty. If the code returns after one or two attempts, leave the boiler off and arrange for an engineer promptly.
How much does it typically cost to fix Ariston fault code 722 in the UK?
Most homeowners pay somewhere between £120 and £380 to resolve this fault, depending on what is actually wrong. A sensor replacement with labour usually falls in the £100–£200 range, while a zone valve or pump replacement typically costs £150–£350 all in. If the heat exchanger needs descaling or a full system power flush is required (usually when sludge is found throughout the radiators), costs can rise to £350–£500. PCB replacement is rare for this code but can cost £300–£500 if needed — worth mentioning to your engineer before authorising that repair on an older boiler.
Will an annual boiler service help prevent code 722 from coming back?
Yes, significantly. An annual service by a Gas Safe engineer includes checking sensor readings, inspecting the pump, verifying zone valve operation, and assessing limescale build-up — all of which directly relate to the causes of code 722. Regular servicing also keeps your Ariston manufacturer warranty valid, which can cover parts and labour costs entirely if your boiler is still within its warranty period (typically 2 to 12 years depending on the model and registration). A service costs around £80–£120 and is far cheaper than an emergency repair.