Vaillant Eco Tec Plus 624, 637 – fault, error code F.76

Vaillant fault code F.76 is quite a worrying one. It means that main heat exchanger overheat protection had been activated. Vaillant Eco Tec range has a thermal fuse on top of the main heat exchanger that once activated breaks the circuit to prevent boiler from firing. That fuse is can not be reset and is not replaceable as once it is activated (at 140-160 degrees model dependable) main heat exchanger is deemed damaged beyond repair and must be replaced.

Before you go into panic mode though please be aware that this fault code may indicate a much less serious fault with wiring from the senor into the PCB. You will need a Gas Safe Registered heating engineer to confirmĀ  what had caused your Vaillant to show fault code F.76. Thermal fuse blowing and damaged heat exchangers are rare and in most cases this code points to a wiring issue. Unfortunately on my last repair with F.76 fault code the boiler main heat exchanger developed a leak combined with blocked pressure sensor. Sensor did not communicate to the boiler PCB that there was no pressure in the system and kept the burner on although there was no water in the heat exchanger. This in turn activated thermal fuse to prevent a catastrophic meltdown. Picture below shows a partially melted thermal fuse connector.


This is how the main heat exchanger looked inside.


Client decided to replace this 8 years old Vaillant 637 with a new one. Since all connections, flue and brackets were exactly the same the job was relatively straightforward and the boiler got replaced, system flushed and commissioned the same day. It is worth to note that the faulty boiler had never been serviced. New boiler came with 7 years warranty provided it is serviced annually. New boiler installed below.




  • David Todd

    I had the dreaded F76 fault code appear yesterday (on a 4 year old Vaillant ecotec plus 630). I was able to relight boiler on 2 occasions by pressing the button on the front panel and the Boiler stayed on for approx 1 hour on each occasion before shutting done again , however it will no longer re light.

    Engineer can’t come out for another week , so doing some research on here suggests either the heat exchanger ( big bill) or wiring fault between thermal fuse and pcb ( small bill). A visual inspection of the relevant wiringand checking connection looks. Does the fact that I can no longer get the boiler to relight point to an irreparable thermal fuse and therefore replacement heat exchanger being needed ?

    • David,

      I do not share any information regarding boiler repairs with general public. If you PM me with your details and address I can try to see if there is anyone I know in your area that can help.

      The fact that you were able to relight after F.76 points to wiring failure. If fuse had gone you would not be able to relight at all. In most cases it is in fact wiring issue.

      Get someone who is good with Vaillants to have a look at it or PM me and I will try to point you in the right direction.

      • David Todd

        Hi Szymon , thank you for the response. I have arranged for a local qualified gas engineer with Vailliant experience to come out and assess on Wednesday.I will update the outcome on here.