Ariston 706 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 706 fault code mean?
Fault code 706 appears on Ariston E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System boilers when the second heating zone has exceeded its safe temperature threshold. The boiler performs a safety lockout to protect the system from heat damage and to prevent scalding. Until the underlying cause of the overheating is resolved, the boiler will not restart normally. Note: if you own an older Ariston model such as the Clas One, Clas Net One, E-Combi (non-Evo), or E-System (non-Evo), the equivalent fault is code 722, which indicates the same Zone 2 overheat condition on those platforms.
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
- Restricted water circulation in Zone 2 Common
If water cannot flow freely around the Zone 2 circuit, heat builds up rapidly and triggers the overheat sensor. Common culprits include a partially closed isolating valve, stuck or closed thermostatic radiator valves (TRVs), or a significant airlock in the pipework or radiators.
- Circulation pump fault or incorrect setting Common
The pump is responsible for pushing heated water around the system. If it has seized, is running at too low a speed setting, or has developed an internal blockage, flow through Zone 2 will be reduced and temperatures can spike beyond safe limits.
- Sludge or limescale buildup in the system Common
Magnetite sludge and limescale deposits accumulate over time in pipework, radiators, and the heat exchanger. These deposits insulate components and impede water flow, causing localised areas to overheat well before the rest of the circuit reaches temperature.
- Blocked or scaled heat exchanger Sometimes
When the heat exchanger becomes heavily scaled or partially blocked, the heat it generates cannot be transferred efficiently into the circulating water. Temperatures on the boiler side rise faster than the sensor expects, triggering a Zone 2 overheat fault.
- Faulty Zone 2 NTC temperature sensor Sometimes
The NTC thermistor monitors the Zone 2 flow temperature. If this sensor has degraded or developed an intermittent fault, it may report temperatures far higher than actual, causing the boiler to lock out even when the real water temperature is within acceptable limits.
- Zone valve fault Rare
A motorised zone valve that is stuck partially open or closed, or that is not responding correctly to the controller, can disrupt the balance of flow across zones and cause Zone 2 to overheat.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on the control panel (on most Evo models it is clearly labelled and may be behind a small flip cover). Press and hold for roughly five to ten seconds until the boiler attempts to restart. Watch to see whether the 706 code clears or returns. One reset attempt is reasonable; if the fault reappears within a short time, do not keep resetting as this can mask a more serious problem and may cause further damage.
- Check that all Zone 2 radiator valves are open DIY safe
Walk round every radiator in the Zone 2 circuit and confirm that TRVs are turned up and any manual lockshield valves are not closed down too far. Restricted valves are one of the most common reasons for localised zone overheating and are easy to overlook after decorating or furniture rearrangement.
- Bleed the Zone 2 radiators DIY safe
Turn the boiler off and allow the system to cool slightly. Using a radiator bleed key, open the bleed valve on each Zone 2 radiator (usually at the top corner) until water — not air — flows out steadily, then close it. Air pockets in the circuit reduce flow and can cause temperature spikes. After bleeding, check the boiler's system pressure gauge; if it has dropped below 1 bar, top it up via the filling loop until it reads between 1 and 1.5 bar, then close the filling loop valves.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler fascia. Healthy operating pressure is typically between 1 and 1.5 bar when the system is cold. Pressure consistently above 2.5 bar or dropping below 0.8 bar both warrant investigation. Topping up via the filling loop is a straightforward homeowner task, but if pressure keeps dropping or rising without explanation, an engineer should investigate further.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
If the fault persists after the checks above, a qualified engineer should test the pump for correct operation, confirm speed settings are appropriate for the zone configuration, and look for internal seizure or wear. Replacing or servicing the pump requires draining down part of the system and is not a DIY task.
- Have a Gas Safe engineer test the Zone 2 NTC sensor Gas Safe engineer
An engineer can use a multimeter to verify whether the Zone 2 flow temperature sensor is reading accurately across the expected resistance range. A drifting or failed sensor is relatively inexpensive to replace but must be fitted correctly to avoid ongoing false lockouts.
- Have a Gas Safe engineer assess and flush the system if sludge or scale is suspected Gas Safe engineer
If the engineer finds evidence of sludge, magnetite, or heavy limescale, a power flush will clean the entire circuit. This involves connecting a flush machine to the system, circulating specialist descaling and cleaning chemicals, and flushing until the discharge runs clear. A magnetic filter should be fitted afterwards to prevent future buildup.
- Call a Gas Safe registered engineer if you are unsure or the fault returns Gas Safe engineer
If the 706 code keeps returning after your initial checks, or if you are not confident in any of the steps above, contact a Gas Safe registered heating engineer. Persistent Zone 2 overheating that is left unresolved can cause damage to the heat exchanger, pump seals, and other internal components, increasing the eventual repair bill significantly.
Parts you may need
- Zone 2 NTC flow temperature sensor · from £25
- Motorised zone valve (22mm or 28mm) · from £45
- Central heating circulation pump (e.g. Grundfos UPS2 15-50) · from £85
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
- Inhibitor and descaler chemicals (power flush pack) · from £35
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 Ariston fault code 706 the same as code 722?
They indicate the same Zone 2 overheat condition but apply to different generations of Ariston boilers. Code 706 appears on the newer Evo range — E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System. Code 722 is the equivalent on older models such as the Clas One, Clas Net One, E-Combi, and E-System (non-Evo). The diagnostic approach and likely causes are the same for both codes.
Can I fix Ariston fault 706 myself, or do I always need an engineer?
Some initial checks are safe and sensible for a homeowner: resetting the boiler once, checking that TRVs and lockshield valves are open, bleeding Zone 2 radiators, and confirming system pressure is between 1 and 1.5 bar. These steps resolve the fault in some cases where the cause is simply trapped air or a closed valve. However, if the fault returns after a reset, or if you suspect a pump fault, a failed sensor, or sludge in the system, a Gas Safe registered engineer should carry out the diagnosis and repair. Working on the pump, sensors, or any internal boiler components is not a DIY job.
How much does it typically cost to repair an Ariston 706 fault in the UK?
Most homeowners with this fault pay somewhere between £150 and £450 all in. Replacing an NTC sensor sits at the lower end of that range once you include the call-out and an hour or so of labour. A pump replacement including parts and labour typically falls in the £200–£350 range. A full power flush to clear sludge or scale is the most expensive common remedy and can reach £400–£600 depending on the number of radiators and the engineer's location. Costs are generally higher in London and the South East. If you find the fault is covered under your Ariston warranty (which runs from two to twelve years depending on the model and whether annual services have been carried out), the parts and labour may be covered at no cost to you.
Could the 706 code be a sensor fault rather than a real overheat?
Yes, this is worth considering — particularly if the boiler locks out quickly after a reset but the pipework does not feel unusually hot. The Zone 2 NTC thermistor can develop a drift over time, reporting temperatures above the actual value and causing a spurious lockout. An engineer can test the sensor's resistance against the expected curve in a few minutes. If the sensor is the culprit, it is one of the more affordable repairs associated with this code, with the part costing around £20–£40 and total repair typically under £150.