Chaffoteaux 722 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 722 fault code mean?
Fault code 722 appears on Chaffoteaux multizone boilers — such as the Niagara Delta range — when the control board detects that the water temperature in Zone 2 has climbed beyond its safe operating threshold. The boiler triggers a protective shutdown to prevent damage, and the fault code is displayed to alert you to the problem. In essence, hot water in Zone 2 is not circulating properly, causing heat to build up rather than being distributed around the heating circuit. For reference, fault code 723 signals the same overheating condition but in Zone 3, confirming that Chaffoteaux monitors each zone independently.
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
- Faulty or stuck zone valve Common
The zone valve controlling flow to Zone 2 may be seized, stuck open, or failing to respond correctly to thermostat signals. When a valve cannot regulate flow properly, water circulates without adequate control and heat builds up in that zone. This is the most frequently identified cause of a 722 fault.
- Failing or seized circulation pump Common
The pump is responsible for moving heated water around all zones. If it has seized, is running intermittently, or cannot maintain sufficient flow, hot water stagnates near the heat exchanger and overheating follows. Pump failure often presents alongside unusual noise or a complete loss of heat across multiple zones.
- Faulty NTC thermistor (temperature sensor) Sometimes
The NTC thermistor in Zone 2 monitors water temperature and feeds readings back to the control board. If it has drifted out of calibration due to age, limescale coating, or physical damage, it may report false high temperatures and trigger the 722 fault even when actual temperatures are acceptable — or it may fail to warn the board in time, allowing genuine overheating to occur.
- Limescale or sludge blockage in Zone 2 pipework Sometimes
In hard-water areas or older systems, scale deposits and magnetite sludge can narrow pipework and heat exchanger passages. Restricted flow in Zone 2 means the water cannot carry heat away efficiently, and temperatures rise. A power flush or chemical clean is typically required to resolve this.
- Trapped air in Zone 2 radiators Sometimes
Air pockets in Zone 2 radiators reduce the volume of water circulating in that loop, which can cause localised overheating as the remaining water absorbs more heat than normal. Bleeding the radiators in that zone is a straightforward first step.
- Incorrect zone wiring or configuration Rare
On multizone installations, wiring errors between zone sensors, delivery probes, and the control board can cause the boiler to misread Zone 2 conditions. This is most likely to occur after a recent installation, modification, or replacement of zone controls.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button as described in your Chaffoteaux user manual. A single reset is worth attempting in case the fault was triggered by a brief, one-off temperature spike. Avoid resetting more than two or three times — repeated lockouts suggest a persistent underlying fault that needs professional attention.
- Check and restore system pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top up via the filling loop until you reach the correct level, then reset. Low pressure reduces water flow and can contribute to overheating in individual zones.
- Bleed the radiators in Zone 2 DIY safe
Using a radiator bleed key, open the bleed valve on each radiator served by Zone 2 until water trickles out steadily with no air. Start with the radiators furthest from the boiler. Once done, recheck system pressure and top up if needed, as bleeding can cause a slight pressure drop.
- Check that the gas supply is active DIY safe
Confirm that the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check whether other gas appliances in the property are working normally. If there is no gas supply to the property, contact your gas supplier.
- Have a Gas Safe engineer inspect the zone valve Gas Safe engineer
An engineer will test whether the Zone 2 valve actuator is opening and closing correctly in response to thermostat demand. If the valve head is seized or the actuator motor has failed, the engineer will replace the actuator or the full valve assembly to restore proper zone control.
- Have the NTC thermistor tested and replaced if faulty Gas Safe engineer
Using a multimeter, the engineer will measure the thermistor's resistance at a known temperature and compare it against Chaffoteaux's specifications. A reading that deviates significantly indicates a failed sensor that should be replaced to restore accurate temperature monitoring.
- Have the circulation pump inspected Gas Safe engineer
The engineer will check pump speed, flow rate, and whether the pump shaft has seized. In many cases a seized pump can be freed, but if the motor has failed it will need to be replaced. Restoring full pump output is essential for correct heat distribution across all zones.
- Consider a power flush if sludge or scale is suspected Gas Safe engineer
If the engineer finds evidence of restricted flow — such as cool spots on Zone 2 radiators, discoloured system water, or a heavily scaled heat exchanger — a power flush using specialist equipment and chemical cleaners will dislodge deposits and restore circulation. A magnetic filter should be fitted afterwards to protect the system going forward.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the steps above do not resolve fault code 722, or if the boiler continues to lock out repeatedly, you need a Gas Safe registered engineer to carry out a full diagnostic. Do not continue operating the boiler if it keeps overheating, as persistent high temperatures can damage the heat exchanger and other internal components.
Parts you may need
- Zone valve actuator (22mm or 28mm) · from £35
- Zone valve full assembly · from £75
- NTC thermistor / temperature sensor · from £20
- Grundfos or equivalent circulation pump · from £85
- Magnetic system filter (e.g. Magnaclean Pro2) · from £75
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
Can I fix a Chaffoteaux 722 fault myself?
A few basic checks are safe for homeowners: resetting the boiler, topping up system pressure to 1–1.5 bar, and bleeding Zone 2 radiators to release trapped air. However, anything involving the zone valve, thermistor, pump, or pipework inside the boiler casing must be carried out by a Gas Safe registered engineer. Opening the boiler casing without the correct qualifications is dangerous and will void any remaining warranty.
What is the difference between fault codes 722 and 723 on a Chaffoteaux boiler?
Both codes signal the same type of fault — zone overheating — but they refer to different zones. Code 722 indicates overheating in Zone 2, while code 723 indicates the same condition in Zone 3. If you see 723 on your display, the diagnostic process and likely causes are identical, but the engineer will focus their checks on the Zone 3 valve, sensor, and pipework rather than Zone 2.
How much does it cost to fix a Chaffoteaux 722 fault in the UK?
Most repairs for a 722 fault fall in the range of £120–£400, depending on the cause. Replacing a zone valve actuator or thermistor typically costs £100–£250 including labour. Pump replacement usually runs £150–£400. If a full power flush is required to clear sludge or limescale, expect to pay £300–£600 on top of any component costs. In rare cases where the PCB is involved, costs can reach £400–£500. Prices tend to be higher in London and the South East.
Why does my Chaffoteaux boiler keep showing fault 722 after I reset it?
If the fault returns shortly after a reset, there is an underlying problem that a reset alone cannot clear — most likely a stuck zone valve, a failing pump, or a faulty thermistor. Repeated overheating can cause progressive damage to the heat exchanger, so it is important to have a Gas Safe engineer diagnose and fix the root cause rather than continuing to reset the boiler and hope it clears.