Explaining the F75 Fault
This is Vaillants explanation of the F75 fault direct from the manual.
|F75||Fault, no pressure change detection when starting pump||Water pressure sensor and/or pump detective, air in heating installation, too low pressure in boiler; connect external expansion vessel to return|
So the basic fault is that the boiler isn’t detecting a difference in pressure between the boiler being idle and the pump running. This generally falls down to a fault with the pressure sensor or the pump.
Pressure sensor fault the causes and the remedies
The pressure sensor has had 3 revisions since the first ecotec boiler launched in 2005. The very first sensor had extremely high failure rates these were found quite quickly and eventually Vaillant rolled out a replacement of the sensor. The problem is caused by the sensing orifice getting blocked. When this gets blocked it stops the sensor from reporting a change in pressure when the pump runs and tells the circuit board to stop operation and produce the F75 code.
It can block up over time from carbon deposits in the heating system, it can also block up if the system was not properly cleaned and chemically treated when the boiler was installed. Sometimes the filter in the left water-block (the part where the sensor is fitted) can build up with sludge and then block the sensor this would need stripping out and cleaning before a new sensor is fitted to prevent further problems with the pressure sensor.
The sensor is a reasonably inexpensive part so it is always best to replace instead of trying to clean out.
Pump fault the causes and remedies
The original Ecotec came with a pump produced by Wilo, for some reason these had a high failure rate, with them leaking splitting or completely disintegrating inside. Vaillant then had a new pump produced by Grundfos this replaced all ecotec models and is the only replacement spare you can buy today. The new Grundfos replacement is a lot more reliable and doesn’t fail nearly as often.
Most of the time you can tell whether problem is with pressure sensor or the pump by listening to the boiler when it is trying to run. If you reset the boiler using the lockout button (the cross flame symbol) have the boiler call for heat, either run a hot water tap or turn your heating on. If you hear a low rumble of the pump running for 2-5 seconds every 20 seconds or so then 9/10 the fault is only with the pressure sensor, If you don’t hear the pump running at all and it eventually locks out with the fault code then It will be the pump. You will need a gas safe registered engineer to assess this and replace the necessary parts.
Rarely the fault can be cured by topping up the system pressure, sometimes this can be done to get the boiler operational for a short time till it can be fixed.
How much will it cost to repair
If the fault is the pressure sensor which is most common then you will be looking at roughly £30-£40 for the part and between £50-£150 labour if the filter needs removing and cleaning/replacing then this could cost more for labour. Use us and you will get the diagnosis, a new pressure sensor and new filter fitted for £119 plus the price of the parts (usually £30-50 with the filter)
If you need a new pump fitted this will usually cost roughly £140-£160 for the part and anywhere between £100-£200 Labour. We will charge for the full diagnosis of the fault, and fitting of new pump, £119 Labour plus the price of the part.
Our prices are fixed for the labour so even if more faults are found you will not pay more than £119. Lots of companies will advertise online that you will get the boiler repaired for £59 £69 etc. What they don’t tell you is that their fee only covers the initial fault diagnosis up to 1 hour, so if you need a part fitted you would generally be looking at upwards of £400. Plus we don’t charge VAT so theres no 20% on top our fixed price is exactly that.
If you do get the F75 fault on your boiler it is best for you to contact a gas safe registered engineer to have a look at it (preferably somebody that knows a lot about this fault). It would be illegal for you to try to carry out the work yourself and more importantly could be quite dangerous.