Glow-worm Temperature 70 Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm Temperature 70 Flashing fault code mean?
When your Glow-Worm boiler's display shows a flashing '70' or a flashing temperature/radiator symbol at 70°C, it is most often a normal part of how the boiler operates — not a fault code or lockout. It indicates the boiler has reached its set flow temperature of 70°C and has cut the burner, while the pump continues to run in overrun mode to distribute residual heat around the system. You may also see this during anti-cycling, where the boiler's control logic waits for the return water temperature to drop sufficiently before allowing the burner to re-fire. This is especially common in summer when thermostatic radiator valves (TRVs) close off rooms that are already warm. However, if the heating is genuinely not working, the flow pipe feels excessively hot to the touch, or the boiler stays locked in this state for a prolonged period, there may be an underlying circulation or component issue worth investigating.
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
- Normal pump overrun or anti-cycling operation Common
The boiler has reached its 70°C target flow temperature, shut the burner off, and is circulating water to dissipate residual heat. The control logic then waits for the return temperature to fall before re-firing. This is entirely normal behaviour and will resolve itself once the system cools slightly.
- Thermostatic radiator valves (TRVs) closed or restricted Common
If most TRVs in the property are closed — common in summer or when rooms are already up to temperature — water has nowhere to circulate freely. The boiler reaches temperature quickly and sits in this flashing state repeatedly. Opening a TRV or two (or ensuring one radiator is always left without a TRV) can help significantly.
- Low system pressure reducing circulation Common
If the system pressure has dropped below around 1.0 bar, the pump cannot circulate water effectively. The boiler overheats rapidly, hits 70°C, and cuts out before the heat is properly distributed. Check the pressure gauge on the boiler front.
- Sludge build-up restricting water flow Sometimes
Over time, black iron oxide sludge accumulates in the pipework and radiators, partially blocking the system. This reduces flow rate, causes the boiler to reach target temperature too quickly, and can lead to repeated anti-cycling. A severely sludged system will feel very hot at the boiler flow pipe but cool further round the circuit.
- Circulation pump seizing or underperforming Sometimes
A pump that is worn, seized, or running at too low a speed will not move water around the system fast enough. The boiler heats the same small volume of water repeatedly, triggering the overheat cut-off and leaving it in a flashing state. Listen for a humming sound from the pump — silence when the boiler is calling for heat can indicate pump failure.
- Faulty flow or return temperature sensor Rare
If one of the NTC thermistors monitoring flow or return temperature is giving inaccurate readings, the boiler's control board may believe the water is hotter than it actually is, causing premature burner shut-down and persistent flashing at 70°C even when heating demand hasn't been met.
- PCB or control board fault Rare
In rare cases, the control board itself misinterprets sensor data or fails to correctly manage the anti-cycling logic, keeping the boiler in a false 'temperature reached' state. This is usually only suspected once sensors, pump, and circulation have been ruled out by an engineer.
How to fix it
- Check the flow pipe temperature and the boiler pressure gauge DIY safe
Locate the large pipe leaving the top of the boiler (the flow pipe). Carefully touch it — it should be warm to hot but not scalding. If it is extremely hot, there may be a circulation restriction downstream. Also check the pressure gauge; it should read between 1.0 and 1.5 bar when the system is cold. Note both readings before doing anything else.
- Top up system pressure if it is below 1.0 bar DIY safe
Find the filling loop — usually a silver or grey braided hose with one or two valves beneath the boiler. Slowly open the valve(s) while watching the pressure gauge until it reaches around 1.2 bar, then close the valve(s) firmly. Never overfill above 1.5 bar (cold). If the pressure keeps dropping, there may be a leak that requires an engineer.
- Check that thermostatic radiator valves are open and radiators are not air-locked DIY safe
Walk around the property and make sure TRVs are not turned to their lowest setting. If any radiators feel cold at the top but warm at the bottom, they likely have trapped air. Bleed them using a radiator key, holding a cloth under the bleed valve until water appears. Recheck the boiler pressure afterwards and top up if needed.
- Verify the flow temperature is set to 70°C on the boiler controls DIY safe
On the boiler's front panel, locate the central heating temperature dial or button and confirm it is set to 70°C. If it has been inadvertently turned down, the boiler will reach its set point very quickly and appear to flash constantly. Ensure at least one radiator in the property is left without a TRV so there is always a path for water to circulate.
- Reset the boiler DIY safe
On most Glow-Worm models, turn the CH control knob to the reset position (marked with a power symbol), hold for a moment, wait approximately 30 seconds, then turn it back to your desired temperature setting. Allow the boiler a full heating cycle to see whether it fires and heats properly. Do not reset more than two or three times — repeated resets without resolution suggest an underlying fault.
- Call a Gas Safe registered engineer if the issue persists Gas Safe engineer
If the boiler continues to flash at 70°C without providing adequate heating, or if the flow pipe remains excessively hot suggesting a circulation restriction, the system needs professional diagnosis. An engineer will check the pump operation, inspect for sludge (and advise on a powerflush or magnetic filter if needed), test the temperature sensors, and examine the PCB if required. Do not attempt to work on the gas valve, pump internals, sensors, or PCB yourself.
Parts you may need
- Grundfos or Wilo circulation pump (compatible replacement) · from £85
- NTC flow or return temperature sensor · from £18
- Magnetic system filter (e.g. Adey MagnaClean Pro2) · from £75
- Printed circuit board (PCB) · from £130
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 £90–£280, depending on the underlying cause.
Frequently asked questions
Is Temperature 70 flashing on my Glow-Worm boiler a serious fault?
In the majority of cases, no. A flashing 70°C display simply means the boiler has hit its target flow temperature and the burner has cut out while the pump finishes circulating the heat — this is normal behaviour called pump overrun or anti-cycling. It becomes a concern only if your radiators are not getting warm, the boiler stays in this state for a very long time, or the flow pipe feels dangerously hot, which could indicate a blockage in the pipework.
Why does my Glow-Worm boiler keep cutting out at 70 degrees and not heating the house?
The most common reasons are: system pressure that is too low (should be 1.0–1.5 bar), TRVs that have closed off most of the radiators leaving the pump nowhere to push water, or sludge build-up restricting circulation so the boiler overheats a small volume of water very quickly. Check the pressure gauge, open some TRVs, and bleed any cold radiators first. If those checks don't help, a Gas Safe engineer should inspect the pump and check the system for sludge.
Could a failing pump cause the Temperature 70 flashing indicator on a Glow-Worm boiler?
Yes, a worn or seized pump is a fairly common culprit — particularly on boilers that are seven years or older. If the pump cannot move water efficiently, the boiler repeatedly heats the same small volume of water, trips at 70°C, and sits in anti-cycling. Listen carefully when the boiler calls for heat: you should hear a faint hum from the pump. Complete silence usually points to pump failure. A replacement pump fitted by an engineer typically costs £180–£280. If a PCB fault or heat exchanger problem is ultimately diagnosed, costs can rise further, so it's worth getting a full assessment.
What are the D-codes on Glow-Worm boilers and can they affect the flashing Temperature 70 display?
Glow-Worm (and related Vaillant-platform) boilers have a set of installer-level parameters called D-codes, accessible via a PIN in the service menu. D1 and D2 specifically influence pump overrun timing and anti-cycling behaviour. If these have been set to very conservative values, the boiler may appear to sit in the flashing 70°C state for longer than expected between firing cycles. Adjusting D-codes is an advanced procedure intended for qualified engineers; if you suspect a D-code misconfiguration, ask your Gas Safe engineer to review these settings during a service visit.