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Ideal Boiler F5 Fault Code


The Ideal fault code F5 return thermistor Q & A or require an engineer for an emergency boiler repair.


The Ideal Boiler Fault Code F5 Error Common Q & A


Just below, we have a list of common questions relating to the F5 fault code on the Ideal boiler.

Ideal Boiler F5 Fault Code
  • What does the F5 fault code mean on my Ideal boiler?
  • What is the return thermistor on my Ideal boiler?
  • What causes the F5 code to be displayed on my Ideal boiler?
  • Can I fix the F5 fault code on a DIY basis?
  • How will my boiler engineer know if the return NTC thermistor is faulty?
  • A loose connection has caused the fault, why would this happen?
  • The thermistor is faulty, how much is a replacement likely to cost?
  • Where could other electrical problems stem from?
  • How can a temperature differential cause the F5 fault code to be displayed on my Ideal boiler?
  • What causes a temperature differential in a boiler?
  • Can faults related to temperature differentials easily be fixed?
  • How much does it cost to fit a new pump on an Ideal boiler?
  • How is the F5 fault code related to the PCB?
  • What can I do to check the PCB isn’t faulty?
  • Can a faulty PCB be repaired?

What does the F5 fault code mean on my Ideal boiler


If your boiler has started displaying the F5 fault code, there’s a problem with the return thermistor, or something related to the thermistor’s operation.

What is the return thermistor on my Ideal boiler


Your boiler will have a flow and return temperature sensor.

The flow thermistor monitors the temperature of water leaving the boiler. The return thermistor monitors the temperature of water returning to the boiler after it’s travelled around radiators and towel rails.

The readings it gives are in resistance (ohms). This reading is passed on to the printed circuit board (PCB), which checks that this reading is within working tolerance.

When the temperature is too high, too low, or no reading is present, the boiler will lock out and show a fault code; in this case, the F5 fault code.

What causes the F5 code to be displayed on my Ideal boiler


The cause of the F5 fault code isn’t limited to one thing.

If could be a:

  • • Faulty NTC return thermistor
  • • Damaged connection to the thermistor
  • • Differential in temperature that’s too high between the flow/return thermistors
  • • Faulty PCB

Can I fix the F5 fault code on a DIY basis


There is no quick and easy fix for the Ideal boiler F5 fault code. You’ll need to call a Gas Safe engineer to find and fix the problem.

Once the problem is fixed, they’ll reset your boiler. The reset function isn’t designed to fix your boiler. If you reset your boiler without the problem being fixed first, it could be dangerous.

How will my boiler engineer know if the return NTC thermistor is faulty


Although visual checks may highlight the issue, the boiler engineer will need to test each connection, section of wiring and the thermistor to determine if it has power.

They’ll do this using electrical testing equipment such as a multi-meter.

A loose connection has caused the fault, why would this happen


If your boiler engineer has diagnosed the fault as being a loose connection, this can be re-secured to fix the F5 fault code.

Most loose connections work loose due to internal boiler vibrations. It’s natural for boilers to vibrate a little.

However, if you’ve noticed your boiler vibrating more than usual recently, it’s likely another faulty part is excessively vibrating and contributing to the problem.

If your boiler has been noisy, get your engineer to inspect the heating pump and fan. If these are air locked or have partially seized shafts/bearings, this could be causing excessive vibrations.

The thermistor is faulty, how much is a replacement likely to cost


You should expect to pay around £100-150 to have the return thermistor replaced. This includes parts and labour, as well as electrical tests to ensure this is the only problem with your boiler.

These additional electrical problems need to be highlighted before going ahead with repairs.

Where could other electrical problems stem from


If vibrations haven’t caused issues or damage, it’s likely that the boiler has been leaking water. If this is the case, you’ll have noticed the pressure gauge on the front of the boiler consistently dropping.

Typically, we find that boiler leaks come from the heat exchanger or pump. When they leak, they can drip onto a range of electrical components.

If these components then malfunction, they’ll need to be replaced. Some parts can be replaced for £50-100. However, there are expensive components such as the fan, gas valve and pump that can cost over £200 to fix.

That’s why we always suggest getting your engineer to check over more than just the thermistor.

You don’t want to be investing lots of cash fixing multiple components if your boiler is extremely old, and there’s other components that need replacing.

How can a temperature differential cause the F5 fault code to be displayed on my Ideal boiler


Your boiler’s NTC thermistors are designed to monitor current temperature, and temperature fluctuations.

There is a maximum temperature differential allowed by the boiler, until it deems there to be a problem. Typically, this is around 35 degrees.

Once this differential is exceeded, the boiler will lock out and display the F5 fault code.

What causes a temperature differential in a boiler


Boiler faults related to temperature differentials aren’t easy to fault find. The problem could be something incredibly simple like an air lock in the pump.

However, the pump could be partially blocked with heating sludge. This comes from the inside of towel rails, pipes and radiators. Basically, it’s rust.

If this blocks up the heating pump, the pump won’t be able to operate at full power. It might even work intermittently and by the time heating water reaches the return thermistor, it’s cooled down to a point where the temperature differential from the flow exceeds the maximum allowed by the PCB.

Can faults related to temperature differentials easily be fixed


This depends on what is causing the problem. If it’s a pump with an air lock, that can easily be bled of air.

Likewise, if the pump is full of debris, this can be cleaned out.

However, the fault could easily be caused by a pump that’s blocked and even after being cleaned doesn’t function correctly. This could be due to a worn shaft or collapsing bearings.

If this is the case, the pump will need to be replaced to fix the problem.

How much does it cost to fit a new pump on an Ideal boiler


An average boiler pump replacement will cost around £250. The exact price will depend on the cost of the pump that is compatible with your boiler. Larger pumps cost more money.

How is the F5 fault code related to the PCB


Every fault code on your ideal boiler, is related to the PCB.

Without the PCB, the boiler wouldn’t work.

The PCB’s job is to control all electronic boiler components. That means telling the gas valve when to open, the burner to light and the pump to kick in.

If the PCB is faulty, the fault codes it is displaying might be misleading. For instance, the F5 fault code shows as being a NTC thermistor return sensor fault.

But, if the PCB isn’t operating correctly this might not be the case; it might be translating the thermistor’s signals incorrectly.

What can I do to check the PCB isn’t faulty


A boiler engineer can test almost any electronic component in your boiler using a multi-meter.

By testing with a multi-meter, they’ll be able to determine if the PCB and it’s connections/wiring are faulty.

You can also let the boiler engineer know if you’ve noticed anything strange recently. This might include things like the boiler:

  • Cycling on and off before radiators are up to temperature
  • Firing up even when it’s not been told too
  • Not responding to heating control demands

This will help them to focus on the most likely culprits when checking your boiler.

Can a faulty PCB be repaired


Unless the damage is minor, such as loose connections, it’s likely the engineer will suggest a replacement.

This is one of the most expensive parts on a boiler to replace. You should expect to pay somewhere around £500 for a new PCB.

Therefore, we always suggest checking other important components. If they need replacing too, you’d be much better off investing in a new boiler that’s going to be covered on warranty.

Other Ideal boiler issues


For more boiler issues regarding the Idea Boiler then please visit the Ideal Boiler Problems And Cures page.