Vaillant ecotec Plus 831 F75 repair Rotherham

blocked pressure sensor ecotec

This was a little repair I did last week on a Vaillant ECOTEC Plus 831 Combination boiler in Rotherham, which had an intermittent F.75 fault. This fault happened mainly when the heating was on.

When the Customer called he was certain that the expansion vessel was the cause of the fault, and thought this would probably need to be replaced as the pressure in the boiler went really high when the heating was on. I was able to turn up at relatively short notice and I was able to repair this boiler with one visit with the parts I carry.

If you are looking for a bit more information regarding this fault then check out my Vaillant ecotec F75 fault code page.

The expansion vessel had a bit of water coming up when you pressed the Schrader valve, but this doesn’t necessarily mean that the expansion vessel is broken. Sometimes when an expansion vessel has been flat for a period of time you get a built of moisture inside. To determine if you need to replace the vessel you just need to see if you can pump it up and if it holds its pressure.

In this case it did hold its pressure hence it did not need to be replaced at this time. The expansion vessels on the ecotec boilers are quite reliable in general and don’t need to be replaced all that often.

The main cause of the F75 error code on this particular boiler turned out to be down to the pressure sensor. This sensors on the ecotec and boilers with this type of design, check if there is a minimum amount of pressure in the system and then look for a small rise in pressure when the pump runs. The boiler will not fire up if the pump does not run or if the pump is deemed not to be working at all.

The pressure sensor was almost completely blocked but was working just enough to register a small pressure rise when the pump ran. Although it was almost at the point where the boiler was about to stop working completely.

The reason the F.75 fault code presented itself more when the heating was on was because the expansion vessel was flat and was causing the pressure in the system to be excessively high when the heating was on. This made it more difficult for the pump to increase the pressure when the pressure was already high anyway

Leaking condensate plastic corrugated pipe inside ECOTEC Plus

leaking condensate pipe Vaillant ECOTEC
Although this wasn’t the cause of the main fault the corrugated plastic condensate pipe had also perished and had been leaking for some time as can be seen by the corrosion in the photos. Someone had done a temporary bodge, I mean repair but there was no guarantee this would hold so I made the recommendation to replace it. The condensate trap is also a part I carry as a stock item for repairs such as this.

The condensation produced by condensing boilers is corrosive and if a condensate leak is left long enough it can destroy a boiler.

Corrosion cause by leaking condensate pipe inside ECOTEC Plus

corrosion caused by leaking condensate pipe

Some of the issues with this boiler were down to poor maintenance. The expansion vessel being flat is something that should be checked on a proper service. The inner rubber tube loses its pressure over time and needs to recharged periodically. The leaking condensate pipe would also have been picked up on a service as well. Thats why its not a bad idea to get your boiler checked over periodically by an “Engineer” who knows the issues with these Vaillant ECOTEC boilers.

I usually make a mark on the pressure gauge where the pressure should be as to stop customers from putting too much pressure in their boiler. These pressure gauges are a bit ambiguous without any additional markings.

Mark on the pressure gauge where the pressure should be

mark on the pressure gauge where the pressure should be

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Areas where I can repair your Vaillant ecotec plus combi boiler

areas that are covered

I cover a large part of South Yorkshire including most of Sheffield, Rotherham and Barnsley. I also cover a small part of Doncaster.


I cover most of Sheffield including central Sheffield, Chapeltown, Ecclesfield, Grenoside, High Green, Oughtibridge, Thurgoland, Wortley, Aston, Aughton, Swallownest, Todwick, Hillsborough and Stannington.


I cover the whole area of Rotherham including central Rotherham, Brinsworth, Catcliffe, Canklow, Masbrough, Treeton, Whiston, Greasbrough, Kimberworth, Thorpe Hesley, Rawmarsh, Wentworth, Bolton-on-Dearne, Goldthorpe, Thurnscoe, Wath-on-Dearne, Mexborough, Swinton, Kilnhurst, Hooton Roberts, Ravenfield, Thrybergh, Bramley, Hellaby, Maltby, Thurcroft, Wickersley and Braithwell.


I cover most of Barnsley including central Barnsley, Birdwell, Stairfoot, Worsborough, Kendray, Ardsley, Athersley, Monk Bretton, Lundwood, Royston, Brierley, Cudworth, Cortonwood, Great Houghton, Shafton, South Hiendley, Brampton, Darfield, Wombwell, Hemingfield, Elsecar, Hoyland, Jump, Barugh Green, Cawthorne, Darton, Dodworth, Gawber, Mapplewell, Pogmoor, Staincross, Wilthorpe, Worsbrough, Thurnscoe and Tankersley.


I cover a small part of Doncaster including central Adwick, Conisborough, Edlington, Warmsworth and Mexborough.

1 comment

  1. Good advice on making a mark on the pressure gauge if only to encourage my engineer from refraining from pressuring up to 2 or 3 times more than needed (he says it will drop over time). It makes me mad and as a householder I would never do this myself! I do wonder whether him doing this has lead to the current leak on our ecotec plus 831.

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