Glow-worm Temperature 30 Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm Temperature 30 Flashing fault code mean?
On Glow-Worm boilers, a flashing temperature reading of around 30°C is most often a completely normal display behaviour rather than a fault. It typically appears during the pump overrun phase — the short period after the burner shuts off when the circulation pump keeps running to dissipate residual heat from the heat exchanger. You may notice it more in warmer months when thermostatic radiator valves (TRVs) have closed across much of the system. However, if the flashing is persistent, the boiler is cutting out repeatedly, or your heating and hot water are not performing as expected, it can point to an underlying issue such as restricted flow, a failing NTC thermistor (temperature sensor), or a circulation problem. In those cases, the boiler may be struggling to dissipate heat properly and could be approaching its overheat limit.
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 — no fault present Common
After the burner switches off at the set flow temperature, the pump continues to circulate water to protect the heat exchanger. During this phase the display temperature may flash or read low (around 30°C) as the water cools. If heating and hot water are working correctly and the flashing stops within a few minutes, this is entirely normal behaviour and requires no action.
- Faulty NTC thermistor (temperature sensor) Common
The NTC thermistor measures flow or return water temperature and feeds readings to the PCB. When it begins to fail or its wiring becomes loose or corroded, it sends inaccurate signals, causing the boiler to cut out prematurely, short-cycle, or display unexpected temperature readings. A faulty thermistor is the most frequently replaced component in temperature-related Glow-Worm faults.
- Sludge or limescale restricting the heat exchanger Common
A build-up of magnetite sludge or limescale narrows the waterways inside the heat exchanger, reducing flow and trapping heat. The boiler detects excessively high flow temperatures and shuts down on overheat protection. This is often accompanied by a kettling noise and is more prevalent in hard-water areas or older systems without a magnetic filter fitted.
- Low system pressure or trapped air affecting circulation Sometimes
If system pressure has dropped below around 1 bar, or if air is trapped in the radiators or pipework, water cannot circulate efficiently. The boiler may overheat locally or read inaccurate temperatures. Bleeding radiators and topping up the system pressure to between 1.0 and 1.5 bar can resolve this.
- Circulation pump failure or undersized pump Sometimes
A weak, seized, or undersized circulation pump cannot move water through the system fast enough. This raises the temperature differential (delta-T) between flow and return beyond acceptable limits, causing the boiler to reduce output or lock out on overheat protection. You may notice some radiators not heating evenly or the pump being unusually noisy or silent.
- PCB or wiring fault Rare
Damaged, loose, or corroded wiring between the thermistor and the PCB can cause the same symptoms as a thermistor failure — erratic temperature readings and nuisance cut-outs. In rarer cases the PCB itself develops a fault affecting how temperature signals are processed. This is typically diagnosed only after other components have been ruled out.
How to fix it
- Check whether the flashing is normal pump overrun DIY safe
Observe the boiler for a few minutes after it has reached temperature. If the display temperature flashes briefly, the burner is off, the pump is still running, and your heating and hot water are working normally, this is the pump overrun phase — completely expected behaviour. No action is needed. Only proceed further if the flashing is persistent, the boiler is cutting out, or performance is poor.
- Check your gas supply is on DIY safe
Confirm that other gas appliances in your home (such as a gas hob) are working normally. If they are not, contact your gas supplier rather than attempting a boiler reset.
- Check and top up system pressure if needed 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 reads below 1 bar, use the filling loop to slowly top up the pressure until it sits around 1.2 bar, then close the filling loop valve fully. Refer to your boiler's manual for the location of the filling loop on your specific model.
- Bleed the radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve slightly until you hear hissing air escaping. Once a steady trickle of water appears, close the valve. Work through all radiators. Afterwards, recheck system pressure and top up again if it has dropped.
- Reset the boiler DIY safe
On most Glow-Worm models, turn the central heating control knob to the reset position (marked with a power or reset symbol), hold for a few seconds, then return it to your desired temperature setting. On some models you press and hold the power button for around five seconds until a symbol flashes on screen. Allow the boiler to complete a full start-up cycle. Do not reset more than two or three times — repeated resets without identifying the cause can mask a developing fault or cause further damage.
- Check whether overheating is linked to sludge or restricted flow DIY safe
If the boiler repeatedly cuts out on high temperature and you can hear a kettling or rumbling noise from the heat exchanger, sludge or limescale build-up is likely. Do not continue resetting the boiler — sustained overheating can permanently damage the heat exchanger. Note any symptoms to report to your engineer. A professional powerflush and fitting of a magnetic system filter is the appropriate remedy.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault persists after the homeowner checks above, an engineer will need to test the NTC thermistor(s) and wiring with a multimeter, inspect the heat exchanger and pump, and assess the PCB if required. Thermistor replacement, pump replacement, and powerflushing are all tasks that must be carried out by a qualified professional. When booking, describe the flashing behaviour, how often the boiler cuts out, and any noises you have noticed.
Parts you may need
- NTC thermistor (flow or return temperature sensor) · from £25
- Circulation pump (e.g. Grundfos UPS replacement) · from £80
- Magnetic system filter (e.g. Magnaclean) · from £55
- Overheat thermostat / high-limit stat · from £20
- 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 £100–£300, depending on the underlying cause.
Frequently asked questions
Is the temperature flashing on my Glow-Worm boiler always a fault?
No — in many cases it is entirely normal. When the boiler reaches its set flow temperature the burner shuts off, but the pump continues to run for a short period to circulate residual heat. During this pump overrun phase the temperature display can flash or show a lower reading such as 30°C. If your heating and hot water are working properly and the flashing stops within a few minutes, there is nothing wrong. It is only worth investigating further if the boiler is cutting out repeatedly, performance has dropped, or the flashing never seems to stop.
How much does it cost to fix a temperature sensor fault on a Glow-Worm boiler?
Replacing an NTC thermistor on a Glow-Worm boiler typically costs between £100 and £175 including labour, making it one of the more affordable boiler repairs. If the root cause turns out to be a sludge-blocked heat exchanger requiring a powerflush, expect to pay £300–£600 depending on the size of the system. A circulation pump replacement usually falls in the £180–£280 fitted range. In rare cases where the PCB is at fault, parts and labour can reach around £400–£500 — if that is the case it is worth comparing the cost against a new boiler installation.
Can I fix a flashing temperature fault myself?
There are a few safe checks a homeowner can carry out: verifying the gas supply is on, topping up system pressure to between 1.0 and 1.5 bar via the filling loop, bleeding radiators to remove trapped air, and resetting the boiler up to two or three times. Beyond that, any internal work — including testing or replacing the thermistor, inspecting the pump, or working on the PCB — must be done by a Gas Safe registered engineer. Attempting these yourself is unsafe and could invalidate your boiler warranty or home insurance.
Why does my Glow-Worm boiler keep cutting out on temperature even after a reset?
If the boiler resets and then cuts out again on temperature repeatedly, a simple reset is not solving the underlying problem. The most likely culprits are a failing NTC thermistor sending false readings, restricted water flow caused by sludge or limescale in the heat exchanger, or a weakening circulation pump that cannot move water quickly enough. Continuing to reset without investigation risks permanent heat exchanger damage. Book a Gas Safe engineer to diagnose the root cause — describe how frequently it cuts out and whether you hear any kettling or gurgling noises, as this will help them narrow down the fault quickly.