Vaillant Ecotec boiler F75 Fault – cause and cure ( 7 causes + 5 step cure = Fault 75 )
* F75 Meaning = No pressure rise was detected on turning the pump
(approximately 0.2 bar increment needed)

•(This document is written only for a competent and registered Gas Safe engineer to help fixing the F75 fault. This applies throughout all Ecotec combination and system boiler models such as plus , pro, VUW , combination{ 824, 830, 837}, system { 612, 615, 618, 624, 630, 637 } ) The Author will not be liable to any damages resulting in following this information.

• If you have F75 error on Vaillant EcoTEC boiler display , reset the boiler to clear the fault. ( press the button with the flame / rabbit / cross on it)

• If it continues to show the F75 Error after multiple attempts to ignite, it may be due to one or more causes as listed below. ( the most likely cause at number [1 ]and the most unlikely but possible cause at number [7] )
Cause Cure – if find defective > replace or Repair

Find the cause >investigate the part>if find defective > replace or Repair
[1] Too little water in the system or air in the heating system due to defective automatic air release valve • check – water pressure > release air> adjustable bypass>expansion vessel

[2] Blocked Pressure sensor
•check pressure sensor>check built in filter
•check the analogue and digital water pressure readings for any discrepancy >if so it is likely that the sensor is blocked

[3] Faulty pump
•check power supply to the pump> it is spinning ?
•If pump has the power but not spinning

[4]Blocked component on the circulatory system
•connector hose / inbuilt filter or heat exchanger of the boiler > drain down and check the components

[5] Faulty Pressure sensor
•If it was found that no blockage on sensor or other components > pump working>no voltage increment on X2 Black and Green from 1.6 volt to about 1.9 volts when the pump is on

[6] Faulty PCB
•no power to the pump during the ignition sequence while pump supposed to be powered ( When Pump sign is displayed on the display on EcoTEC+ Models )

[7] Faulty wiring •connection > no continuity on cables between the connectors

To fix the problem, the actual cause need to be identified and eliminated.

Lets apply common sense and “method of elimination” to fix the boiler as fast as we can with minimum cost to last for as longer as we possibly can.

Step 1: Establish the primary water pressure to be between 1 to 1.8 bar ( Standing pressure )
• There are two pressure gauges in the boiler – Analogue ( with a dial and needle pointer) & digital ( this reading is shown on the boiler display screen)
• If the pressure is less than 1 bar, increase it up to 1 bar. ( 1 to 1.8 bar standing pressure is also acceptable) Reset the boiler and see if the boiler start to work normally or If not..
• Check if there is any discrepancy( difference) between the two pressure gauge readings. Considerable discrepancy means instrumental fault >> one of the gauge is indicating the wrong pressure, practically 99% of the instances the digital pressure gauge is blocked indicating the wrong pressure stuck on too high or two low. If the pressure gauges readings are the same, then we need to find what is the working pressure.

Step 2: Establish the increment in primary water pressure when the pump is on
ie : ( Working pressure – standing pressure) in bar

• Find the pressure on the boiler screen using the buttons . for older boilers press the minus button ( “_ “ button)
• new models follow the “bar” sign and press the relevant buttons under the display panel. when the boiler is on demand the pump sign is shown on the screen, the pressure should increase by about 0.2 bar ( example 1.2 bar standing pressure will go up to 1.4 bar when the pump comes on).
• If the pressure does not increase as such it the cause may be as listed from 2 to 7.
• When the pressure sensor correctly sense the pressure difference, the output voltage of the sensor changes from 1.6 volts ( standing pressure) to about 1.9 volts DC. ( working pressure)
• We can carry out the following test using a multi meter on DC volts. Check the voltage between the black and green wires on the PCB X2 connector when the boiler is on stand by to be about 1.6 volts DC, if it’s not then the pressure sensor need to be replaced given that the water pressure inside the boiler is between 1 to 2bar.
• Now turn the demand on ( Heating on ) and monitor the boiler ignition sequence. when the pump comes on this voltage between the black and green cables ( PCB- X2) should increase to about 1.9 volts DC. if the voltage does not increase as such then the pump is not working properly, or it may be the cause 2, 3, 4 or 5, 6 or 7 (On the older versions of Vaillant Ecotec boilers pumps , the pumps screw can be open to see if the spindle is rotating or not – be careful for the water spillage.. or very hot water may splash on you. On the newer models with a rated modulating pump you can also see the spindle rotation)

Step 3 Investigating the cause 3 and if found defective replace the pump

• Eliminate cause 3 ( faulty pump):- Check if the pump is spinning ( feel the vibration on the pump case )or check if the spindle is turning , if the pump is receiving the correct voltage at the wright time but the spindle is not turning, it is a faulty pump.
• Use a voltage detector pen on the cable from the PCB to the pump and confirm that there is power supply to the pump. Also you can follow the MI to check the operation of the pump. { Call the manufacture to see if they offer further help on this- 01773596615 / 08446933133}
• If all the tests confirm that the pump is faulty, it is time to drain the boiler and change the pump. While the boiler is drained, remove the pressure sensor and check if its blocked. Also check the two hoses ( old versions of Vaillant had these) for blockage, if so replace them, also check and clear the filter near the sensor. Some engineers and even the Vaillant technical help line advice to squeeze the black hoses on the flow & return pipes to the main heat exchanger to find out if its blocked, but I don’t recommend it , because the debris can go into the main heat exchanger and completely block it. Always use Common Sense! Or it’s a boiler replacement. ( Main heat exchanger is too expensive hence repair not cost effective)
• When replacing the pump and make sure the PRV is fitted right inside, and push and hold when tighten the PRV pipe. Sometimes when you tighten the compression nut below, PRV comes out from the housing clip.
• Carry out the step 5 so that the other components ore thoroughly checked and unblocked while the boiler is dry. ( boiler is drained I mean.. not dry.. because the water fills inside this poorly design case like a water tank!)
• Once the Cause 3 is eliminated and if the boiler still not working it is a blocked heat exchanger, it need to be unblocked or replaced. ( Main heat exchanger is too expensive hence repair not cost effective)

Step 4- eliminate causes without Draining the boiler

• Eliminate cause 6( Faulty PCB):- Check the connectors , establish the pump is powered by the PCB during the ignition sequence execution on heat demand. If the pump is getting no voltage at this specific time , the PCB is faulty and need replacing. (Before replacing the PCB eliminate all the other 6 causes and confirm that there is nothing else wrong but the PCB.) Once the correct operation of the PCB is established and the cause eliminated move on to check if the pump is faulty. If the pump is operating correctly and there is also a pressure increment but the boiler is not firing it may be faulty PCB or the connection.
• Eliminate cause 7 ( faulty wiring / connection):- Check the connectors and continuity of the cables between PCB and the pressure sensor , remove the connector from the sensor and reengage.

Step 5- Drain down the boiler and eliminate cause [2] [3] [4] or [5 ].

• Switch the power off to the boiler, turn the flow and return isolation valves off and
drain the boiler. ( If you haven’t done it yet)
• Remove the pressure sensor and check for blockage. If its blocked replace it.
• Check the flow and return rubber hose for blockage – replace or unblock.
• Check the return filter and unblock if necessary.
• If the pipes are heavily slugged, a full de-sludge is necessary.
• Check the expansion vessel charge air pressure to be about one bar. ( although very
rare, lack of pressure on the expansion vessel can lead to F75 Fault)

Part No: ( check against the appliance GC number or call Vaillant to confirm )

1. digital pressure sensor – part no 0020059717
2. automatic de-aerator – part no 104521
3. pump up to 30 kw – part no 178983
4. 37 KW boiler Pump – part no 0020025042
5. analogue pressure gauge – part no 180982
6. connection rubber hose pipe – part no 180946
7. Inbuilt Primary filter housing – part no 179031 ( connector piece)
8. Inbuilt Primary filter – part no 179030
9. Ecotec Plus PCB form 09/07 – part no 0020052093 ( for+ 24 31 & 37 KW and Pro 24 and 28kw boilers)

Relevant Videos on you tube :
1. : how to pressurise Ecotec pro
2. : how to pressurise Ecotec plus
3. : F75 after multiple attempts – it was a defective pump
4. : you cannot hear the pump running when the pcb relay clicks to start the pump indicating defective pump
5. :
6.… : you can hear a noisy pump so the PCB is ok, it may be pump is worn out not generating enough water pressure or the a blockage on sensor or other component in the circularity system
7. : boiler pressure too high and it may be caused by the faulty expansion vessel. Unable to sense the pressure increment and goes to F75
8. : boiler sensor, hose and inbuilt filter blocked
9. : faulty PCB it was found that the pump is not getting the power
10. : normal operation of the boiler
11. : : faulty expansion vessel makes sensor unable to detect the pressure increase – video 1
12. : faulty expansion vessel makes sensor unable to detect the pressure increase video 2
13. : blocked inbuilt filter
Relevant Photos on twitter:
1. = blocked pressure sensor
3. = F75
4. = faulty sensor
5. = rubber hose
6. = inbuilt filter blocked

Please send me any suggestions to improve the above document
Best Of luck!
Nish Malwenna – Director UniC UK