Vaillant Warranty

All Vaillant boilers come with a warranty from 2 to 10 years depending what boiler was installed at the time. They will also have had to be serviced every year by a gas safe registered engineer.

There is a benchmark that is filled out at the time of installation and this will then be registered with gas safe and with Vaillant, certficates can either be emailed or sent through the post. At the back of the installation manual is a service recored which you need to have your boiler serviced every year and the service trcord filled out to keep up the guarantee.

Vaillant boilers in my opinion are the best on the market but like everything they are ptone to breakdown at times and here is a comprehensive list of what the errors mean and when it’s time yo give us a call to fix tghe problem. Ypu are able to do certain things as a homeowner like unfreeze your condensate pipe etc but anything that invloves ttkjing the cover off you nyst have a gas safe engineer to work on your boiler.


This is the error code that comes up if you have low pressure in your heating system. On most Vaillant boilers there is a digital display as well as a pressure gauge that tells you what the water pressure is in the system. When the water pressure is in the grey that means it’s fine but if it goes into the red then this is too high and could be a sign there is a problem with your boiler

If you have low pressure in your heating system it can be just a case of topping up with water and this is done via the filling loop. This is either two white levers which are labelled open and closed. You just move a quarter turn and this will fill the heating system with water or there will be a silver loop with a black lever bridging from yout cold water mains pipe over to the central heating pipe. When you move the black lever quater turn you will hear the water filling the heating system, when the pressure gauge is either 1.5 bar or half way in the grey area on the pressure gauge turn the black lever back and this will stop the water filling the heating system anymore.

F22 other possible problems

If you find that your having to fill up the heating system serveral times a week this could mean you have a leak in the system which needs to be fixed. First port of call would be to go around all your radiators and check there are no leaks around the radiator valves or any brown or wet stains on the ceiling.

The leak could also be coming out of the back of your boiler through the copper pipe that pertrudes out of the wall. This normally means your PRV or expansion vessell is faulty.

If you can’t find the leak around your heating system and you have looked around all the radiators, back of the boiler then it’s probably under your floorboards downstairs. Rather than rip yp that expensive floor you’ve laid there is another solution to the problem. Fernox do a leak sealer whcih seks up the leak but does involve draining your central heating system. This can be donw by your towel radiator and doesn’t mean you have to worry about draining all the radiators.

Gas Safe engineer

If it turns out to be the expansion vessel ot PRV (pressure relief valve) then you will need us to call out and fix the problem.


This is an error code that crops up if there is a big temperature difference between flow and return pipes. When your boiler heats up the central heating pipes the flow pipe goes to your radiators and the return pipe comes back to the boiler. If this temperature difference is too great then you get the F23 error message.

Possible causes

Central heating pump could be faulty that circulates the water around your heating system. Over time through wear and tear they can lose their power and need to be replaced. The impeller can also get stuck inside which needs to be released so that the pump circulates the water properly.

Sludge in the heating system can also throw up the F23 fault as the pump can’t circulate the water around the system causing the boiler to overheat.

An air lock in the heating system can also throw up this fault. If you’ve had a radiator removed or the system has been drained if all the radiators arn’t bled properly then this can a pssible cause.

There is a thermistor that is attached to the central heating pipe which lets the boiler know if the pipes are getting too hot and to switch the boiler off as a saferty measure. These again can not work properlyover a period of time and need to be replaced.


This type of error code can be for a number of issues and is very much like the F23 fault.

Central heating pump seized or not circulating water around your heating system

sludge or blockage preventing the pump from circulating water

An air lock due to work being carried out on your system and radiators needs bleeding

It could also be to the wiring between the pump and the pcb


The F25 normally means that your flow temperature has gone up too fast. This is when the temperature goes up 5 degrees per second and the boiler goes into lockout mode.

This could be due to a blocked heat exchanger which may need to be flushed out. If you get your boiler serviced every year by a gas safe engineer this can prevent this from happening.

It could also be down to your central heating pump not circulating the water around tour system properly.

Your central heating flow and return sensors could be faulty and need replacing.

It could also be down to extreme temperatures in your flue gases which now need to be looked at by a gas safe registered engineer. This can be done by ourselves, we will take has flue gas analyser readings to determine your boiler is working properly. This could be down to the gas valve and may need replacing.


This error will pop up if the stepper motor on the gas valve is not working poperly. On the PCB board there is a wire that runs from X25 which is the white plug in terminal that runs ro the yellow paerf of the gas valve.

These wires can become perished or damaged causing the gas valve to not work properly.


This error is thrown up to tell you there is a problem with the flame when the gas valve opens.

A possible issue could be faulty flame sensors, these monitor a falme present and to keep the gas valve open.

The spark electrodes ignite the boiler and they can become perished over time. These ignite the boiler when the gas comes through and basically starts the boiler running.

Moisture ot damp can get into the electric side of things and this can throw up the F27 fault

The PCB which is basicaly the mother board can also be faulty so when the boiker treis to fire up. If the wrong electrical currents are mot being sent to the gas valve, the fan the pump this can also bring up the fault and will need to be replaced

We have had it where the fan comes on for a split second amnd then goes off immediately making the flame shut down.


The Vaillant boiler will attempt to ignite 3 times. If the error displays adter 3 attemps then the boiler will go into lockout and dispay the above fault code.

Before you call us out to this fault make sure there are no gas works going on in your street. It’s more than probable that there is a problem with your gas valve.

Possible wiring issue from the gas valve back to the pcb board

Blocked ot frozen condensate pipe normally happens in winter. When the boiler burns the gases it leaves a liquid from the boiler thar runs down a white pipe into either an internal drain or ourside drain. It’s better to run them into an internal drain as this stops them freezing in winter. This is not alwats possible due to the location of the boiler say in a bedroom so the pipes neeeds to br ran ourside. If the pipe is not lagged the pipe is more prone to being frozen in winter and will need to be defrosted with a kettle. Pour the hot water along the pipe down towards the drain and you should be able to hear the ice melting inside the pipe as you do it. Once you have defrosted the ice then you should be able to reset your boiler and fire it up again. This make take a few attempts before firing up again.

PCB fault could cause this error not sending ther right signal to the gas valve it should have the right voltage otherwise it won’t open and close properly. The best way to diagnose this is to do a voltage check at the gas valve when the boiler starts it’s starting procedure.


So when your boiler goes through the ignition process then your boiler will go into lockout and thriw up the F29 fault.

One of the possible causes could be blocked soot in the burner so when your boiler is trying to ignite if there is no gas getting through the burner because it’s blocked then the boiler won’t start. The burner needs to be cleaned which would normally be done on a boiler service.

It could be a problem with your gas pressure going into the burners. We can do a check at the gas valve to check it’s opening and closing properly. There is also a govener at the gas meter which regulates how much gas goes to your appliances, these can also go faulty over time but can only be adjusted by your gas supplier or a new one changed.

It could also be a problem with your electrodes. With usage all the time they become worn and will need to be changed. They will need to be changed which isn’t a massive job.

Your flue could be blocked which gets rid of the products of combustion when your boiler is in use, Leaves can get stuck in the flue system or anything that is obstructing the flue will need to be removed.

Yuo could also have a blocked condensate pipe. If you follow the white pipe that runs from underneath your boiler this is what gets rid of the liquid tghat is produced after buring the gases inside. The water is acidic and needs to run into a drain either internally or externally and be lagged. These can get frozen in winter or if they run internally they have been known to get clogged with food from the kitchen sink.


This is when there is a probelm with the airways of the flue system. Several reasons for this and I will go through them now.

The main reason for this fault is normally the fan, this is what gets rid of ther products of combustion into the atmosphere. The fan helps the fumes out into the atmosphere and over time the bearings will be worn down and not operate the blades to spin. This will need to be replaced as the boiler will not operate if the fan is not spinning.

The fault could also be at the PCB not dending the signal for the fan to operate. A voltage check at the fan will be able to tell you if the problem is the PCB.

The flue could be blocked or not installed correctly but these are more uncommon with the F32 fault.


This is the most common error on Vaillant boilers that we get called out to and most of the time it’s the water pressure sensor that needs to be replaced.

The pressure sensor is attached to the flow pipe and monitors the water pressure in the system to let you know if the boiler needs topping up or not. Over time these can get full of dirt from the central heating system and are not able to monior the water pressure of your central heating system. I used to clean these out and put back in and although the customer would be sorted within a few weeks I woulkd get them on the phone again to tell me the F75 fault had returned so I just replace them now.

It can also be a pump fault but i find this is rarely the case and I think the sensors retail for around £50 were the vaillant pumps can be £300 so I always keep a supply in the van.

Most of the vaillant pumps are made by grundfoss amd are pretty straightforward to replace but does involve draining the boiler to change the part as does replacing the water pressure sensor. The boiler needs to be isolated by the flow and return pipes with an allen key NOT a flathead scrwdriver as this damages the valves.

Always make sure you get a reputable company tp work on your boiler, the amount of times we have turned up on a job and there’s parts scattered everywhere because the diagnostics have not been right is quite a lot and this is expense you don’t need.

If you pay £30 boiler cover a month to British Gas I would think long and hard because you could be putting that money in seperate account and most combi boilers last 10 years before problems start arising which is why most boiler manufacturers only do a mximum of 10 years.

Power Flush

In extreme cases if your central heating system is full of sludge you may need a power flush. We always recommend installing a magnetic filter to your system as this can prevent sluidge getting into your boiler pump and heat exchanger.