Vaillant ecotec 937 F81/F91 fault
If you happen to have a Vaillant ecoTEC Plus 937, there is a very good chance you may experience the F81 and F91 fault codes. You will not experience these faults or more specifically the F81 on any other standard ecoTEC Plus or Pro models. This fault is only specific to the Vaillant models with a hot water storage tank. These fault codes are usually accompanied by a lack of hot water. Central heating should not be affected unless you have other issues.
If you look up the code in the manual it says “APC loadpump fault connected to F.91”, then goes on to list several possible causes. This fault can be attributed to several things, possibly even more than one fault at the same time. The plate heat exchanger responsible for the hot water, can have blockages on either the primary or secondary side. Limescale can be an issue as far as I know, although this is not a problem in the area I live in and its not generally something I have to deal with. Blockages on the other side are very common though.
The good news is that this problem can be fixed and you probably do not need a new boiler! Having issues with the plate heat exchanger should be welcomed as good news as this is easier and cheaper to deal with. However in a lot(most) of cases I have come across it is usually a problem with the storage tank pump. This is a secondary pump which circulates the clean drinking water from the storage tank around the plate heat exchanger. Failure of this pump is a very common problem on these boilers.
Unfortunatley these pumps are of a special design and very expensive. They are located on the bottom right hand corner of the boiler and can be difficult to replace. In cases where the installation guide has not been followed, the pumps can be potentially impossible to replace without removing the boiler off the wall. At the bottom of the page there are some images I have taken from one of these pump replacement jobs.
As an end user experiencing these faults, your only option is to call someone competent and Gas Safe registered to fix the problem. Replacing parts on boilers based on guesswork is almost always an expensive endeavour for someone, so try to find someone who understands your boiler.
It is important to note that this is NOT intended to help other engineers and it is most definitely NOT intended as a DIY self help guide for homeowners trying to fix their own boilers.
If you are an engineer seeking information on how to fix this problem and this has been helpful in some way then thats great. However this page is for information purposes only and cannot be relied upon by someone attempting to carry out any repair – regardless of their qualifications. Read my disclaimer before attempting to use any information on this website.
I am based in Wath-upon Dearne, Rotherham, South Yorkshire. I cover the whole of Rotherham and most of South Yorkshire, including Sheffield, Barnsley and Doncaster.
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