Vaillant Boiler F84 Fault Code
The Vaillant fault code F84 boiler temperature difference on the NTC thermistors that is permanent problem Q & A or require an engineer for an emergency boiler repair.
The Vaillant Boiler Fault Code F84 Error Common Q&A
Vaillant Boiler F84 Fault Code
- What does the F84 fault code mean on my Vaillant boiler?
- What are the NTC thermistors on my Vaillant boiler?
- What are the most common sources of faults with NTC thermistors?
- What causes wiring or connections to become faulty?
- Why does my boiler lockout and show a F84 fault code, and will a reset fix it?
- How will the boiler engineer know if the NTC thermistors are faulty?
- What’s the cost to replace NTC thermistors, and how long will it take?
- Can’t the NTC thermistor be re-conditioned?
- How would incorrect installation of NTC thermistors cause the F84 fault code to be displayed?
- What is the fix for incorrect installation?
- How can the PCB cause the F84 fault code to be displayed?
- What causes boiler leaks, and how would they affect the PCB?
- Will fixing the leak clear the F84 fault code from my boiler?
- Is limescale a problem that could cause the F84 fault code to be displayed?
What does the F84 fault code mean on my Vaillant boiler
According to Vaillant, the F84 fault code will be displayed due to a temperature difference on the NTC thermistors that is permanent.
What are the NTC thermistors on my Vaillant boiler
The NTC thermistors monitor the temperature of water in your heating system. One is connected to the flow, and one to the return.
The flow thermistor will monitor the temperature of water leaving the boiler. If this isn’t at sufficient temperature, the PCB (printed circuit board) tells the gas valve to open, provide more fuel, and therefore heat.
The return thermistor monitors the temperature of water returning to the boiler. This will be cooler than the flow temperature, as radiators and pipework absorb some of the heat.
The thermistors are not like normal temperature gauges. They measure in resistance (ohms). At 20C, they should be reading 12k ohms.
What are the most common sources of faults with NTC thermistors
In the case of the F84 fault code, there are specific thermistor faults to look for:
- Faulty wiring or connections
- Faulty NTC thermistors (replacement needed)
- Incorrect installation of NTC thermistors
- PCB detection fault
- Limescale build-up
What causes wiring or connections to become faulty
Age of boiler components has a lot to do with faults.
Besides wiring that’s become faulty over time, vibrations are a common cause of faults. Many moving parts in a boiler (such as the pump and fan) cause small vibrations.
Over years, these small vibrations can help to loosen connections. As soon as these connections are not secure, the signals they provide to the PCB become intermittent.
So, the circuit board may receive signals sometimes, and others not. More importantly, the signals may be incorrect.
This can lead to intermittent boiler operation. So, you might notice your boiler acting strangely including:
- Hot water in taps, showers or baths not getting up to temperature
- Boiler shuts off before radiators are up to temperature
- The boiler cycles on and off constantly and gets neither hot water or radiators up to temperature, before locking out
Why does my boiler lockout and show a F84 fault code, and will a reset fix it
A boiler lockout isn’t limited to a F84 fault code. Your owner’s manual has a list of different fault codes. Some are warnings (i.e. pressure loss), but the majority will lead to a boiler lockout.
Boilers lockout (i.e. they stop working and show a fault code) because they are no longer safe. That safety issue could be related to:
- Causing further damage to internal boiler parts (such as a stuck motor on a fan, causing it to burn out)
- Or, harm to you (fan isn’t turning, so harmful flue gases aren’t being vented from your property)
As this is a safety feature to protect you and your boiler, you shouldn’t keep resetting the boiler to clear the fault.
Call in a boiler engineer to diagnose and fix the problem. Once they’re happy the problem has been resolved, they can reset your Vaillant boiler for you.
How will the boiler engineer know if the NTC thermistors are faulty
Vaillant state that the thermistors on their boilers need to be reading 12k ohms in resistance at 20C. The boiler engineer will need to put this to the test using a multi meter. This monitors electrical presence.
If the reading is too far either side of the 20C at 12k ohms that it should be, it’s likely the NTC thermistors need to be replaced.
What’s the cost to replace NTC thermistors, and how long will it take
Parts and labour will be upwards of £150. Each thermistor will cost around £30. The labour cost will entail an initial diagnostic, sourcing parts, replacing the thermistors and then retesting resistance.
The cost of replacement will vary based on:
- The exact boiler model you own and therefore, the cost of parts
- Where in the country you are and therefore, the hourly cost of labour
Can’t the NTC thermistor be re-conditioned
It makes no sense to spend time trying to recondition a faulty part, when the part itself isn’t the expensive part of the repair bill.
If you want your boiler to last, replacing the NTC thermistors rather than reconditioning them is a much better idea.
How would incorrect installation of NTC thermistors cause the F84 fault code to be displayed
It’s worth noting this is seriously uncommon, but Vaillant do list it as a potential cause for the F84 fault code.
The PCB can monitor resistance readings from the flow, and the return thermistor. What it won’t be able to determine, is if they have been fitted on the correct pipes.
So, if the flow (hot water out) has been fitted on the return side (cooler water coming back to the boiler), it’s going to be giving the wrong readings the boiler. Instead of providing readings for the hot water, it’s providing readings for the cooler water.
The boiler recognises this as a problem, and will lock out and display the F84 fault code.
Somebody has fitted the NTC thermistors on the wrong side. This could have been a manufacturing issue (i.e. if your boiler is brand new, this would’ve been done at the factory), or if the NTC thermistors have recently been replaced; your engineer is to blame.
What is the fix for incorrect installation
The NTC thermistors need to be switched onto the correct pipes. However, these are small and delicate electrical components.
It’s important to check the resistance readings (i.e. 12k ohms at 20C) to ensure they’ve not been damaged during the two installations.
How can the PCB cause the F84 fault code to be displayed
Without a PCB, your Vaillant boiler wouldn’t work.
Whenever there’s any electrical fault, the PCB needs to be tested.
In this case, it’s communication to the NTC thermistors could be causing the issue. So, even if the NTC thermistors have been determined as the fault, the PCB still needs to be tested.
This is particularly the case if you’ve had fault codes relating to a drop or low boiler pressure, and if you’ve noticed water beneath your boiler.
This signifies a boiler leak and water damage to electrical components.
What causes boiler leaks, and how would they affect the PCB
Boiler leaks can come from a vast range of internal parts. But typically, they’ll come from the heat exchanger or the pump.
Central heating pumps are prone to blowing seals. Especially if they are old and are set on too high a flow rate. The back pressure blows the seals and causes a leak.
As we’re talking about thermistors where limescale can be a problem, it’s likely the heat exchanger is the culprit.
The heat exchanger is prone to limescale build up. This build up comes from minerals in water. Once it’s built up, it creates a hot and weak spot. This burns hotter and will eventually crack the heat exchanger causing a leak.
The problem is, even if it doesn’t drip directly onto electrical components, it can still make its way to them.
The internal heat of the boiler will produce condensation, and make its way into electrical parts like the PCB, causing them to malfunction.
Will fixing the leak clear the F84 fault code from my boiler
This fault code isn’t related to a loss of pressure (or water), so fixing the leak won’t clear the F84 fault code.
However, if you do have a leak, it’s likely been a big contributing factor the F84 fault code being displayed in the first place. The leak should be fixed first, before spending money on new electrical components.
Is limescale a problem that could cause the F84 fault code to be displayed
Limescale builds up on the NTC thermistors, just like the heat exchanger. This causes an inconsistent reading and could be the root cause of the permanent temperature difference.
If the thermistors are replaced, this will clear the fault.
But as we’ve mentioned above, other important components like the heat exchanger need to be checked. Replacing thermistors and the heat exchanger is going to be expensive. So, if you have an old boiler, it might be worth considering a replacement.
Other Vaillant Fault Codes And Error
If your looking for the list of error codes relating to the Vaillant boiler then please read the article Vaillant Boiler Fault Codes And Cures