Our Vaillant boiler has shut down with a F76 fault code. Googling says that is boiler overheating fault. I am trying to find out whether this damages the boiler or is there some kind of a built in protection device. Also what can lead to this fault.
Vaillant EcoTec 624 Gas Boiler shutting down with overheating code F76
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A modern boiler will have safety features that protect you and the boiler if something has failed or is not within spec's. Is the system pressure reading 1 bar ?
Doug B
Shy Tot
If your boiler is overheating it would suggest it is at least firing up for a short period of time ? If so I doubt it’s the system pressure to me it suggests the water isn’t getting out of the boiler quick enough, there’s can be many reasons for this, the pump, an airlock, blockage, magnetic cleaner needs cleaning, system is old & needs flushing are among just a few.
Oh & don’t rule out the overheat sensor being faulty but if the flow pipe from the boiler gets very hot but not so future away from the boiler I’d definitely be looking at the pump.
Lastly overheating can damage the boiler section that’s why it’s protected with a sensor & you are getting the fault code.
The previous posts have probably covered most of the potential causes all of which can cause an overheat fault. Worth checking that the radiator valves have not been turned off. Trv valves can be turned to the off position by the customer and then stick in the closed position when turned back on . Has the boilers flow or return been turned off for any reason and then not been turned back on-this could have been done months ago when the heating was not being used. Outside of a few user checks then you will need a gas safe engineer to investigate further. How old is your boiler/ how old is your system / what size are your pipes to the radiators and what about the cirks from ghe boilers etc etc
As above, the boiler has lots of inbuilt protection primarily for you but also for itself.
Turn it off and back on again and stand by it. Try and describe what happens and how long it takes between you switching it back on and the code reappearing. Hold onto the the flow and return pipes and see if either or both become warm.
Googling suggests it is a conventional boiler with a hot water tank. You thus have three modes: hot water only, heating only or both. See if there is any correlation between the fault and the mode selected.
Has anyone touched it or opened it between it working OK and the fault appearing?
Turn it off and back on again and stand by it. Try and describe what happens and how long it takes between you switching it back on and the code reappearing. Hold onto the the flow and return pipes and see if either or both become warm.
Googling suggests it is a conventional boiler with a hot water tank. You thus have three modes: hot water only, heating only or both. See if there is any correlation between the fault and the mode selected.
Has anyone touched it or opened it between it working OK and the fault appearing?
No disrespect but if you hold a 22mm pipe when as the boiler goes into overheat mode you can easily end up with a severe burn or scalding as the latent heat is pumped out of the heat exchanger. Even on a conventional system boiler the heat exchanger only contains a small amount of water and depending on the actual fault the water within can quickly get to 80+ degrees-no need to ask me how I know let’s just say it hurts like hell..
I was kind of hoping he would hold onto the pipes from the moment it was switched on (i.e. when both are known to be cold). That will show if one or both become warm, how quickly, and if the pipe becomes uncomfortable, then remove the hand.
You are right that sticking your hand onto something whose temperature is unknown is a poor way of proceeding. In that case maybe a free thermal camera app for a phone might help with diagnosis.
Thank you for all of your suggestions of the root of possible problems.
First indication was no hot water early evening for showers. No one has touched the boiler prior to entering the fault mode. I am getting a gas safe engineer in, but wanted to rule out a costly boiler replacement i.e. are there built in protection components to protect the boiler if it overheats The boiler will not reset for hot water or rads the fault code comes up virtually instantly (F76), thus no heating is taking place. I have spoken with a friend who is gas safe registered (but is too far away to help) and he has often had to replace a water inlet filter in Vaillant boilers as they become clogged. Boiler is around six or seven years old but we live in a very hard water area and can easily believe that scale in filter(s) or sensors could be the problem.
First indication was no hot water early evening for showers. No one has touched the boiler prior to entering the fault mode. I am getting a gas safe engineer in, but wanted to rule out a costly boiler replacement i.e. are there built in protection components to protect the boiler if it overheats The boiler will not reset for hot water or rads the fault code comes up virtually instantly (F76), thus no heating is taking place. I have spoken with a friend who is gas safe registered (but is too far away to help) and he has often had to replace a water inlet filter in Vaillant boilers as they become clogged. Boiler is around six or seven years old but we live in a very hard water area and can easily believe that scale in filter(s) or sensors could be the problem.
Years ago I had a system boiler that was not circulating — a common but bad practice was to remove the thermostat phial from the cast iron heat exchanger and let it run for a few minutes - I had my hand on the flow expecting it to get hot
it didn’t so I did the same with the return 
omg as I got hold of it I could smell melting paint but too late - a week off work with severe burns but it did confirm my diagnosis of a blocked flow but I never made that mistake again- thank goodness nowadays for infared thermometers 
I’m assuming the boiler has a limescale filter fitted so it won’t hurt to get it changed as they all have various life expectancy. As with a sludge filter these also vary in design and not all are effective. As for you fault if it’s shutting down and displaying the fault code that quickly it sounds like a total blockage so the boiler is protecting itself from damage and preventing you from potentially being scalded. In normal circumstances I would not expect a customer to change a 5/6 year old boiler due to a single fault. B. Gas used to have a policy that if they didn’t fix the original fault they could not charge you so if your concerned you might be ripped off then give them a call or just go online and they will explain the charges..
So usually a new heat exchanger but not always ( imho) but vailiant won’t tell you that you can actually get the thermal fuse on eBay - not suggesting you go this route but compared to the overall cost of a new boiler then it’s probably worth a try ( if this is your issue) . Seems like a poor design from vailiant if this is the case but then again I always used to run the boiler without turning the gas on for a few mins - I don’t recall having to run a test programme either following a drain down and b .gas would soon run a training day if they were suddenly replacing heat exchangers on vailiant boilers on a massive scale but as usual I stand corrected should I be wrong about this..
… disclaimer
I’m not confirming the thermal link shown in the screenshot is the correct part for the boiler in question but just demonstrating that they are available.
… disclaimer
I’m not confirming the thermal link shown in the screenshot is the correct part for the boiler in question but just demonstrating that they are available.
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