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Glow-Worm Boiler F11 Fault Code

The Glow-Worm fault code F11 relates to the NTC thermistor on the flow or return and here we have Q & A or require an engineer for an emergency boiler repair.


The Glow-Worm Boiler Fault Code F11 Error Common Q & A

Just below, we have a list of common questions relating to the F11 fault code on the Glow-Worm Boiler.

Glow-Worm boiler error code F11
  • What does the F11 fault code mean on my Glow-Worm boiler?
  • If I reset my boiler, will this clear the F11 fault code?
  • Is there a DIY fix for the F11 fault code on my Glow-Worm boiler?
  • What are thermistors?
  • What causes the PCB to notice a problem with the NTC thermistors on my Glow-Worm boiler?
  • What can cause problems with the NTC thermistors?
  • How will my boiler engineer know if the NTC thermistors are faulty?
  • Can the PCB be tested for faults in a similar way to the thermistors?
  • If I need to replace the PCB on my boiler, how much is it going to cost?
  • What could have caused the PCB to fail?
  • If my PCB and pump have failed, should I replace my boiler?
  • What can cause sensing issues in my boiler, besides a faulty thermistor?

What does the F11 fault code mean on my Glow-Worm boiler

When your Glow-Worm boiler develops a problem, you’ll see a fault code on the display panel. These fault codes help a boiler repair company to narrow down where the problem stems from, making the fix quicker, and cheaper.

In the case of the F11 fault code on Glow-Worm boilers, there’s a problem with the NTC thermistor on the flow or return.

If I reset my boiler, will this clear the F11 fault code

A reset may potentially clear the F11 fault code, but only for a few minutes. Once the boiler recognises that the problem is still present, it will lock out.

And for good reason. Some potential causes of the F11 being displayed could mean that continued operation of the boiler, contributes to further and expensive boiler damage.

So, don’t reset your boiler until the problem has been fixed. A Gas Safe boiler repair company will be able to find and fix the problem. And, when they have, they’ll reset the boiler on your behalf.

Is there a DIY fix for the F11 fault code on my Glow-Worm boiler

Unlike other fault codes (such as low boiler pressure), there isn’t a DIY fix for the F11 fault code. You’ll need to call someone that’s qualified to work on gas boilers; a Gas Safe engineer.

What are thermistors

Your boiler has various sensors. They monitor everything from air pressure, to water temperature. All these sensors feed back their readings to the printed circuit board (PCB). Based on these readings, the PCB knows when parts need to be operating, or not, and gives them the relevant command.

Thermistors are one of these sensors. They monitor resistance. The higher the resistance reading is, the lower the temperature reading. The PCB uses these readings to determine if water needs to be cooler, hotter, or if there’s a problem.

There’s a thermistor monitoring flow temperature (water leaving the boiler after it’s been heated). And, there’s also one on the return (water entering the boiler after it’s travelled around the heating system).

What causes the PCB to notice a problem with the NTC thermistors on my Glow-Worm boiler

Your boiler is looking for signs of anything unusual. That includes:

  • Flow water not increasing in temperature
  • Flow water getting too hot
  • Return water isn’t hot enough
  • The differential between the flow and return water is too high

What can cause problems with the NTC thermistors

Your boiler repair company will need to start with the two most obvious parts; the thermistors, and the PCB. If one of these parts aren’t working correctly, neither is going to be able to do their job.

How will my boiler engineer know if the NTC thermistors are faulty

Like all electrical components, your boiler engineer can test the thermistors for power using a multi-meter. For a more accurate test of their operation, they’ll need to test their resistance readings. Typically, thermistors on modern boilers will usually measure 12ohms when at 20C.

If they’re not measuring this resistance reading, the signals they’re giving to the PCB are incorrect. That’s going to cause the boiler to think the flow and return water temperature is higher/lower than it is. And, that’s going to lead to the boiler locking out, and displaying the F11 fault code.

If the thermistors are giving the incorrect resistance readings, they’ll need to be replaced.

Can the PCB be tested for faults in a similar way to the thermistors

Like the thermistors, the PCB will need to be tested. Alongside a multi-meter test, your boiler engineer will want to check the wiring harness between the PCB and the thermistors. If there’s even one loose connection, or a simple case of damaged wiring, the signals being passed back and forth, aren’t going to be correct.

And, as the PCB controls all electronic components in your boiler, as soon as this circuit is broken, your boiler is going to work intermittently. That’s going to mean it does things like:

  • Cycles on and off randomly
  • Doesn’t heat hot water or radiators to the correct temperature

If I need to replace the PCB on my boiler, how much is it going to cost

Unfortunately, not all PCB faults are easily fixed; some can’t be fixed at all. If your boiler engineer suggests a PCB replacement, you should expect to pay £400-500.

What could have caused the PCB to fail

If old age isn’t to blame, the culprit is likely to be water damage. Boilers are prone to leaking. And, once they start leaking, the leak can cause damage to a whole host of electronic components.

Have you noticed drops of water beneath your boiler? If so, there’s a good chance that it’s a leak that’s causing the issue.

Leaks typically come from old and tired heating pumps. Over time, the seals on the pump can degrade. Even the smallest of leaks can wreak havoc. Older boiler’s electronics aren’t as well sealed as they are on newer boilers. And, that means even the smallest bit of condensation can ruin parts.

Whilst the pump can be fixed with a new set of seals, it’s usually a better idea to invest the cash in a new pump instead. Typically, a pump replacement is going to be in the region of £250. At that price, you’ll be getting a pump from a top brand such as Grundfos.

If my PCB and pump have failed, should I replace my boiler

The combined cost of a PCB and pump replacement could be up to £750. And, whilst a new boiler might cost more than £1,800, it’s worth considering all the options. If water damage is the cause of the PCB failure, it’s likely that other components have been exposed too. And, it makes no sense to spend that amount of money on an old boiler, that’s inefficient, and out of warranty.

Before deciding, consult a Gas Safe engineer. They’ll be able to give a general overview of your boiler’s health, by checking the heat exchanger, pump, PCB, fan and even the air pressure switch. If multiple parts are in poor condition, a replacement is likely to be more cost-effective in the long-term, rather than a repair.

What can cause sensing issues in my boiler, besides a faulty thermistor

If there’s a combined problem between the PCB and thermistor, the issue might stem from the water temperature, rather than either of these parts. And that’s because, fault codes aren’t always 100% accurate.

When your boiler engineer takes apart your appliance, they’ll want to see how your central heating system is operating. Are radiators getting up to temperature? Is the flow pipe on your boiler, far hotter than the return?

If there are flow problems in your boiler, the problem could be due to something that’s hindering circulation. And typically, again it’s the pump that’s the culprit. The fault could be anything from a small air lock which acts like a blockage, or an actual blockage in the form of heating sludge.

If either of these are the issue, and the boiler is still in good condition, the repair cost is going to be low.

The air lock can be removed by bleeding the pump via the bleed screw. And as an extra precaution, your boiler engineer will bleed radiators and towel rails too.

And, if central heating sludge is the issue, this can be manually removed from the pump, and then flushed from the central heating system, using specifically designed chemicals.

Once done, heating inhibitor will be added to the system to break down large sludge particles. The idea being, when a magnetic system filter is fitted, these particles are then small enough for the filter to catch them, removing them from the heating system so this problem doesn’t happen again.

Other Glow Worm boiler issues

For more boiler issues regarding the Glow Worm then please visit the Glow Worm Boiler Problems And Cures page.