Vaillant ecotec plus 837 F.75 repair

vaillant blocked pressure sensor

This was a repair I did this week in Barnsley on a Vaillant ecotec plus 837. The boiler was losing pressure and working intermittently displaying the F.75 fault code. I was able to repair this boiler with one visit. If you are looking for a bit more information regarding this fault then check out my Vaillant ecotec F75 fault code page.

If you have a Vaillant ecotec combi or system boiler there is a good chance you may come across this fault code or even the F.22 fault code(low pressure). The way these boilers were designed includes a pressure sensor which serves two functions. The first is to determine if there is enough water in the boiler, and the second is to determine if the pump is running before the boiler fires up. When the pump runs it creates a small rise in pressure and the sensor reads this and the printed circuit board interprets this signal.

So if there is an issue with the pump or the sensor then the boiler will not fire up. As clever and safe as this design is, the pressure sensor is prone to issues. The first issue is blockages, if there is any debris in the system especially fine debris then it is likely to make its way into the sensor and block it. If the sensor is blocked it may not detect the small rise in pressure created by the pump. This turned out to be the issue in this case.

Inside vaillant ecotec plus 837 with F75 fault

inside vaillant ecotec plus 837On this occasion the sensor was causing the issue and not the pump. On early versions of the ecotec they were fitted with a pump which was prone to failure and later upgraded to a different manufacturer.

The other issue with this boiler was that it was losing pressure. This is a common problem with combi boilers its not an exclusive problem with Vaillant boilers. On all combi boilers and system boilers there is an Expansion vessel which does the job of taking up the extra pressure in the system when the heating is on. The expansion vessel has a rubber tube inside which goes flat eventually, usually after 2-3 years or so.

When the inner tube goes flat the boiler usually starts to lose pressure every time you put the heating on. This is because the pressure builds up above 3 bar at which point the pressure relief valve opens and lets all the pressure out. So you find yourself having to top the pressure up every time you put the heating on.

Get your expansion vessel checked on a boiler service

If you are getting your boiler serviced to any decent standard then the expansion vessel is something that should be checked, and it is always something I check on a service. In the installation manual it actually advises you to check the pressure in the expansion vessel every 3 years but I find that this interval is far too long.

I think the expansion vessel is something that most “plumbers” or “engineers” or whatever you want to call them tend to overlook. It is seemingly a trivial component but it does an important job. I often see expansion vessel get unnecessarily replaced when all they is to be recharged with air. Aside from the loss of pressure issue I find that excess pressure in the system as a result of a flat expansion vessel causes other issues like leaks. The extra pressure puts strain on the seals in the boiler and can cause leaks.

If you are getting your boiler serviced regularly I would ask your “engineer”(assuming its not myself) to check the pressure in the expansion vessel and add some pressure if necessary. It is my opinion that they should be doing this anyway as a bear minimum but most don’t.

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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.