Intergas 1 Fault Code: Causes, Fixes & Repair Costs
What does the Intergas 1 fault code mean?
Fault code 1 on an Intergas boiler signals that the boiler's water temperature has climbed beyond its safe operating limit — an overheat condition. When this happens, the boiler locks out and the red indicator flashes alongside the '1' on the temperature display. The root cause is almost always poor water circulation: if hot water cannot move freely around the system, heat builds up inside the boiler rather than being transferred to your radiators and hot water. Note that on some Intergas models the digit '1' during normal operation simply indicates the burner is firing — this is not a fault. A genuine fault code 1 is always accompanied by a flashing red indicator and a lockout, meaning the boiler stops producing heat until the underlying problem is resolved and the boiler 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
- Airlock in the Heating System Common
Pockets of trapped air act like soft blockages, preventing water from circulating properly. The boiler then overheats because it cannot shed heat into the system. Airlocks are especially common after the system has been drained and refilled, or if pressure has repeatedly dropped and been topped up.
- Faulty or Seized Circulation Pump Common
If the pump has seized, has worn bearings, or is internally blocked with heating sludge, water flow slows or stops entirely. A telltale sign is a pump that feels hot to the touch while the boiler flow pipe remains cool — the pump is straining but moving little or no water.
- Sludge or Debris Blockage Common
Over time, corrosion products and scale build up inside pipework, radiators, and the heat exchanger. This restricts flow, creating hotspots and triggering the overheat sensor. Systems without a magnetic filter are particularly vulnerable.
- Incorrect Pump Speed Setting Sometimes
Most modern pumps offer multiple speed settings. If the pump is set too low for the size of the heating circuit, water moves through the boiler too slowly and the temperature rises beyond acceptable limits.
- Closed or Restricted Radiator Valves Sometimes
If thermostatic radiator valves (TRVs) or lockshield valves are fully closed on too many radiators, the boiler has nowhere to push heat and will overheat quickly. This is more common after decorating or when furniture has been moved against radiators.
- Low System Pressure Sometimes
Insufficient water in the system (below around 1.0 bar) reduces the volume of water available to absorb heat, making overheating more likely and sometimes triggering pressure-related lockouts alongside the temperature fault.
- Faulty Overheat Thermostat or Temperature Sensor Rare
If the overheat thermostat or NTC sensor is giving incorrect readings, the boiler may lock out even when temperatures are within a safe range. This is a less common cause but should be considered if all circulation checks come back clear.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on 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 via the filling loop (usually a braided hose or a pair of valves beneath the boiler) until the needle reaches 1.2–1.3 bar, then close the filling loop securely. Do not overfill beyond 2.5 bar.
- Bleed your radiators to remove trapped air DIY safe
Switch the heating off and allow the system to cool for at least 30 minutes. Starting with the radiator furthest from the boiler and working back towards it, use a radiator bleed key to open the bleed valve (a small square fitting at the top corner of each radiator) until water dribbles out steadily — this confirms the air has been purged. Close the valve, wipe away any drips, and move to the next radiator. Re-check the system pressure afterwards and top up if it has dropped below 1.0 bar.
- Check that all radiator valves are open DIY safe
Walk around the property and make sure the thermostatic valve (the numbered plastic head) and the lockshield valve (the capped valve on the opposite side) of every radiator are open. Fully closed valves on several radiators at once can restrict flow enough to cause an overheat lockout.
- Reset the boiler (once or twice only) DIY safe
After completing the checks above, press and hold the reset button on the boiler for 3–5 seconds (refer to your Intergas manual for the exact location — it is typically a dedicated button or a combination press). The boiler should attempt to restart. If it locks out again quickly, do not keep resetting — repeated resets without fixing the root cause can damage internal components. Limit resets to two or three attempts maximum.
- Have a Gas Safe engineer inspect and adjust the circulation pump Gas Safe engineer
If the fault persists after the homeowner checks above, a Gas Safe registered engineer should assess the pump. They will check whether it is running, whether the speed setting is appropriate for your system, and whether it is blocked internally. The engineer can bleed air directly from the pump body (there is usually a bleed screw on the pump head) and adjust the speed setting if needed.
- Arrange a powerflush or chemical flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge or scale — discoloured water, cold spots at the bottom of radiators, or a sluggish pump — a powerflush or chemical flush of the whole system will be recommended. A powerflush uses specialist equipment to force clean water through the system at high velocity, dislodging debris. A chemical flush is a lighter option suitable for systems with mild contamination. Both must be carried out by a qualified heating engineer.
- Have the overheat thermostat and temperature sensors tested Gas Safe engineer
If circulation appears fine but the boiler continues to lock out on fault code 1, a Gas Safe engineer should test the overheat thermostat (also called the high-limit stat) and the NTC temperature sensors using a multimeter. A faulty sensor may need replacing to restore reliable operation.
- Contact a Gas Safe registered engineer if the fault code keeps returning Gas Safe engineer
A recurring fault code 1 that cannot be resolved by pressure top-up and bleeding indicates a mechanical or component-level problem that requires professional diagnosis. Book a Gas Safe registered engineer who has experience with Intergas boilers — they will carry out a thorough assessment and advise on whether repair or replacement is the most cost-effective route.
Parts you may need
- Circulation pump (Grundfos or Wilo equivalent) · from £110
- Overheat thermostat / high-limit stat · from £35
- NTC temperature sensor · from £25
- Magnetic system filter (e.g. Magnaclean) · from £75
- Radiator bleed key · from £3
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 just reset my Intergas boiler to clear fault code 1?
A reset will clear the lockout temporarily, but the boiler will almost certainly lock out again unless the underlying cause has been fixed first. Repeatedly resetting without addressing the problem risks damaging the pump or other components. Try the homeowner checks — bleeding radiators, checking pressure, and opening radiator valves — before resetting, and limit yourself to two or three reset attempts. If the fault keeps coming back, call a Gas Safe engineer.
How do I tell if the display is showing a normal '1' or a fault code 1?
The key difference is the red indicator light. During normal burner operation, some Intergas models display '1' with no alarm indication. A genuine fault code 1 is always accompanied by a flashing red indicator light and the boiler will be in lockout — it will not be producing heat or hot water. If you see a steady or absent indicator alongside the '1' and the boiler is working normally, the display is simply showing an operating status.
How much does it cost to fix an Intergas fault code 1?
For most households, the repair cost falls between £120 and £400. Simple jobs — bleeding, pressure top-up, and pump speed adjustment — typically sit at the lower end and may be covered within a standard call-out charge of around £80–£120. Replacing a circulation pump usually costs £150–£380 including parts and labour. If the system needs a powerflush due to heavy sludge, expect to pay £350–£850 depending on the number of radiators. In rare cases where the heat exchanger is damaged, costs can be significantly higher — at that point, replacing the boiler may be more economical.
Why does my Intergas boiler keep overheating even after bleeding?
If bleeding and re-pressurising the system does not stop the fault code 1 from returning, the most likely culprits are a failing or incorrectly set circulation pump, a sludge blockage restricting flow through the heat exchanger, or — less commonly — a faulty temperature sensor giving false readings. A Gas Safe engineer can carry out a flow-rate test and inspect the pump directly to pinpoint the problem. Regular annual servicing and fitting a magnetic system filter are the best ways to prevent repeat overheating faults.