Glow-Worm Boiler F.85 Fault Code
The Glow-Worm fault code F.85 relates a problem with the flow or return thermistors and here we have Q & A or require an engineer for an emergency boiler repair.
The Glow-Worm Boiler Fault Code F.85 Error Common Q & A
Just below, we have a list of common questions relating to the F.85 fault code on the Glow-Worm Boiler.
- What does the F.85 fault code mean on my Glow-Worm boiler?
- Is it likely that I’ll need a new boiler if my NTC thermistors are faulty?
- Can I reset my boiler to fix the problem, and clear the fault code?
- What are the NTC thermistors on my Glow-Worm boiler?
- Why are NTC thermistors needed in my boiler?
- If I need new temperature sensors on my boiler, how much are they going to cost?
- What causes a large differential in temperature readings, between the two thermistors?
- How will my boiler repair company know if there’s a problem with the pump?
- How does heating sludge affect the pump?
- If my pump has an air lock, can this be fixed?
- What does an incorrect speed setting have to do with the F.85 fault code?
- My boiler engineer thinks that the pump on my Glow-Worm boiler is faulty, how do they know?
- If I need a new pump to clear the F.85 fault code on my Glow-Worm boiler, how much will it cost?
What does the F.85 fault code mean on my Glow-Worm boiler
If your Glow-Worm boiler is displaying the F.85 fault code on the display panel, the printed circuit board (PCB) has recognised a problem with the flow or return thermistors.
Is it likely that I’ll need a new boiler if my NTC thermistors are faulty
It’s rare that the F.85 fault code on Glow-Worm boilers is going to lead to you needing a new appliance. Instead, the components can usually be fixed, or replaced.
Can I reset my boiler to fix the problem, and clear the fault code
Resetting your boiler isn’t going to achieve anything. The reset function on your Glow-Worm boiler is designed to be used when the appliance has been fixed.
If you do manage to trick the boiler into thinking the problem has been solved with a reset, there’s a chance that internal components could be getting damaged.
And, it’s only Gas Safe boiler repair engineers that can work on gas appliances. So, if the F.85 fault code is being displayed, call a Gas Safe boiler company to come and find the problem. Once fixed, they’ll reset the boiler on your behalf.
What are the NTC thermistors on my Glow-Worm boiler
The NTC thermistors on your Glow-Worm boiler, are temperature sensors. All sensors like this, send their readings to the PCB. It’s this central control board that determines whether the boiler is working within in tolerance. In this case it isn’t, so it will lock out and show a fault code on the display panel.
The flow thermistor is a sensor that’s fitted on the flow pipe. That’s the pipe that allows hot water to escape the heat exchanger, and to be pumped around the heating system, or to taps and showers.
This water releases heat as it travels and when it returns to the boiler, it comes back via the return pipe. The return pipe sensor is monitoring temperature of water that’s coming back to the boiler, after travelling around the heating system.
Why are NTC thermistors needed in my boiler
By monitoring various pressures and temperatures, the PCB can determine if there’s a problem with the boiler. It can then lock out the appliance, so no further damage is caused to any internal components.
In the case of the NTC thermistors on your boiler, the PCB is looking out for oddities such as:
- A large differential in temperature between the flow and return thermistors
- The same reading from both pipes
- A temperature reading that’s too high or low, on the flow or return NTC thermistors
If I need new temperature sensors on my boiler, how much are they going to cost
The cost of replacement NTC thermistors, will depend on the exact boiler you have. Typically, you should expect to pay under £150 including parts and labour.
To make sure this cash isn’t wasted, your boiler engineer will be able to check that the thermistors are faulty, and it’s not a problem with the heating system, rather than the sensors.
Using a multi-meter, they’ll be able to check the sensors, as well as the connections and wiring to them, for power. If no power is present, it’s likely this is the cause of the problem.
If there is power, your boiler repair company will need to investigate further.
What causes a large differential in temperature readings, between the two thermistors
The PCB is going to expect to see a slight differential in temperature between the flow and return. After all, heat is lost as water travels through your heating system. However, when the maximum differential is reached, that’s a sign that there’s a problem.
There are a few things that your boiler engineer will look out for, but the most widespread problem is a pump issue.
How will my boiler repair company know if there’s a problem with the pump
They’ll need to take apart your boiler, then start running a diagnostic on the pump.
There are a few things that could be causing a temperature differential issue, and all are related to the pump’s ability to circulate water. The problems they’re looking for include:
- Heating sludge build-up
- Air locks
- Incorrect speed setting
- Faulty pump
How does heating sludge affect the pump
Heating sludge is rust, that comes off the inside of towel rails, radiators and pipework. Once the rust breaks off, it lodges itself in important components in the heating system, including the pump.
Your boiler engineer will need to take apart the pump to check whether this is the problem.
If it is, they don’t just need to clean out the pump, they need to clean out the whole heating system. They can do this by running a hot flush, using chemicals specifically designed for cleaning heating systems.
To stop this happening again, they’ll then dose the system with inhibitor (to break down any future rust) and fit a magnetic system filter (to catch the sludge).
If my pump has an air lock, can this be fixed
An air lock acts much like a blockage. It can restrict flow and therefore, mean that the temperature differential between the flow and return thermistors is too large.
Luckily, air locks are easy to remove, using the bleed screw on the pump. However, you shouldn’t work on a gas appliance if you aren’t Gas Safe registered.
If your pump has been noisy, making banging sounds, it’s likely that an air lock is to blame. Speak to Gas Safe heating engineer, and they’ll come and bleed your pump to fix the problem and reset your boiler to clear the F.85 fault code.
What does an incorrect speed setting have to do with the F.85 fault code
Whereby the F.85 fault code isn’t a faulty thermistor; the fault could be with anything that affects water circulation in your heating system.
Most quality heating pumps offer the ability to adjust their speed setting. In a large heating system, you’d use the fastest speed setting. In a small property, you’d use the lowest speed setting.
If your pump has been pre-set to the wrong speed setting (too slow), water in the heating system has too much time to cool, before it reaches the boiler to be re-heated. This is enough to cause a large differential in temperature readings between the flow and return thermistors.
Your heating engineer will check the heating pump and adjust the speed setting where necessary.
My boiler engineer thinks that the pump on my Glow-Worm boiler is faulty, how do they know
Sometimes we come across pumps that are faulty, others are just beyond being cost-effective to repair.
our boiler engineer will need to test the pump for power, using a multi-meter. If the unit has failed, then it’s going to need replacing.
And, the same goes for if the seals, shaft and bearings need replacing. Although many manufacturers do sell service kits to fix these parts, the cost is going to be close to that of a replacement. And, it doesn’t make sense spending that kind of money on and old pump.
If I need a new pump to clear the F.85 fault code on my Glow-Worm boiler, how much will it cost
Pumps on most boilers are relatively generic. One of the best brands producing pumps, is Grundfos. You’ll be able to get a new boiler pump (such as the Grundfos UPS2), fitted for around £250; that includes parts and labour.
On average, the job will take around one working day. That includes time to diagnose the problem, obtain parts, and fit the replacement pump.
Other Glow Worm boiler issues
For more boiler issues regarding the Glow Worm then please visit the Glow Worm Boiler Problems And Cures page.