Vaillant 418 going a bit wild

[paleo]

Hi,

I have a Vaillant 418 boiler in a vented system with a hot water cylinder. There are two motorised valves. One is governed by a timer. If within the set time the water temperature falls below a set limit, the valve is activated by a thermostat in the cylinder. The other valve is governed by a room thermostat. My central heating installer said that once the valve is activated by the relevant signal, the valve will open which will cause the boiler to fire and the pump to start. There is also a manual bypass valve. I have six radiators, three of which are quite large, all with TRVs. The biggest radiator is in the living room where also the room thermostat is kept. Its TRV is open as wide as it can be.

Sometimes the boiler will switch itself on without any obvious reason: the hot water timer is in the off position and the room thermostat is not calling for heat. The pump starts running but the motorised valves remain shut, so the water is pushed through the bypass circuit. The flow temperature rises rapidly to 85°, even though I have set it for 72°, and things get somewhat noisy. It then stops firing and shows the S7 code (pump overrun) while the situation cools down. It will do this from time to time throughout the day.

As far as I can see, this serves no useful purpose, uses expensive gas and possibly overheats the boiler somewhat.

Does anyone know if this is a feature of the Vaillant which does perform a useful function? If not, how can I persuade the machine to stop doing this please?

 

[ianmcd]

Is this a new problem or has it always done this ?

 

[Mottie]

Hi,

I have a Vaillant 418 boiler in a vented system with a hot water cylinder. There are two motorised valves. One is governed by a timer. If within the set time the water temperature falls below a set limit, the valve is activated by a thermostat in the cylinder. The other valve is governed by a room thermostat. My central heating installer said that once the valve is activated by the relevant signal, the valve will open which will cause the boiler to fire and the pump to start. There is also a manual bypass valve. I have six radiators, three of which are quite large, all with TRVs. The biggest radiator is in the living room where also the room thermostat is kept. Its TRV is open as wide as it can be.

Sometimes the boiler will switch itself on without any obvious reason: the hot water timer is in the off position and the room thermostat is not calling for heat. The pump starts running but the motorised valves remain shut, so the water is pushed through the bypass circuit. The flow temperature rises rapidly to 85°, even though I have set it for 72°, and things get somewhat noisy. It then stops firing and shows the S7 code (pump overrun) while the situation cools down. It will do this from time to time throughout the day.

As far as I can see, this serves no useful purpose, uses expensive gas and possibly overheats the boiler somewhat.

Does anyone know if this is a feature of the Vaillant which does perform a useful function? If not, how can I persuade the machine to stop doing this please?

I had the same problem when my Vaillant 418 was fitted. The 'Vaillant approved' Gas Safe installer had forgotten to take the 24v link out. You only leave that in when you are using the Vaillant controls. Anything else and it has to be removed.

 

[paleo]

Is this a new problem or has it always done this ?

Hi,

Good question. I became aware of it last year and spoke to the central heating man. He said he needed to come by with his electrician. I had another chat with him about 3 weeks ago when he said the same. So far no electrician. The system has played up from time to time so I find it difficult to remember what happened and when. It was installed about ten years ago and I am not longer in touch with the guy who installed it. The electrics look a tad messy to me.

 

[paleo]

I had the same problem when my Vaillant 418 was fitted. The 'Vaillant approved' Gas Safe installer had forgotten to take the 24v link out. You only leave that in when you are using the Vaillant controls. Anything else and it has to be removed.

Thanks. I do not know what the 24v link is or how to remove it but I can certainly mention it to the central heating man. I'll have a quick google...

 

[Mottie]

Thanks. I do not know what the 24v link is or how to remove it but I can certainly mention it to the central heating man. I'll have a quick google...

Looks like this on the circuit board. Just remove it if you have non-Vaillant controls.

 

[ianmcd]

Hi,

Good question. I became aware of it last year and spoke to the central heating man. He said he needed to come by with his electrician. I had another chat with him about 3 weeks ago when he said the same. So far no electrician. The system has played up from time to time so I find it difficult to remember what happened and when. It was installed about ten years ago and I am not longer in touch with the guy who installed it. The electrics look a tad messy to me.

reason I asked is if it has done this from install will be the low voltage link left in, when this carry on started they never really told installers about the link, not defending them, but the information about that link was very sparse and many never knew it had to be removed

 

[paleo]

Thanks. I do not know what the 24v link is or how to remove it but I can certainly mention it to the central heating man. I'll have a quick google...

Hi Mottie, just found your post of 2020 where you show the positition of the 24v link. I'll run that past the central heating man. I'm a bit reluctant to open up the boiler. Thanks for your help.

reason I asked is if it has done this from install will be the low voltage link left in, when this carry on started they never really told installers about the link, not defending them, but the information about that link was very sparse and many never knew it had to be removed

Hi, that makes a lot of sense. I achieved communication with the CH man who said he'll speak to his electrician tomorrow with the view of coming by early next week. The wiring underneath the boiler needs tidying anyway. I am reluctant to remove the 24v link myself; I don't know where the circuit board is and I'd be a bit worried about taking things apart.

 

[ianmcd]

Hi Mottie, just found your post of 2020 where you show the positition of the 24v link. I'll run that past the central heating man. I'm a bit reluctant to open up the boiler. Thanks for your help.

Hi, that makes a lot of sense. I achieved communication with the CH man who said he'll speak to his electrician tomorrow with the view of coming by early next week. The wiring underneath the boiler needs tidying anyway. I am reluctant to remove the 24v link myself; I don't know where the circuit board is and I'd be a bit worried about taking things apart.

To let you understand what the 24v link is, your boiler can use 230v switching from the external controls, or it can use 24v switching, whichever one you use when installing the other link has to be removed or the boiler always thinks an external control is calling for the boiler to come on, it is a simple job to remove the link , you may have read posts on here saying about having to be a registered gas engineer to remove the cover , in your case you dont as you are only removing the decorative case and not the combustion cover, but if you are not comfortable with that then that is fine leave it till your installer can get to you

 

[paleo]

To let you understand what the 24v link is, your boiler can use 230v switching from the external controls, or it can use 24v switching, whichever one you use when installing the other link has to be removed or the boiler always thinks an external control is calling for the boiler to come on, it is a simple job to remove the link , you may have read posts on here saying about having to be a registered gas engineer to remove the cover , in your case you dont as you are only removing the decorative case and not the combustion cover, but if you are not comfortable with that then that is fine leave it till your installer can get to you

Hi, thanks for that. Is it basically pulling out a wee plug? If the electrician can't make it again I might have a go. Obviously switch of the electricity first and then start unscrewing obvious screws. I have the installation manual somewhere. Deconfusing the boiler would be a very good idea; I have the feeling it overthinks the whole "I am a boiler" situation sometimes.

 

[Harry Bloomfield]

Hi, thanks for that. Is it basically pulling out a wee plug?

Almost - it's just a wire link. Undo the two terminal screws, and remove it.

 

[Mottie]

reason I asked is if it has done this from install will be the low voltage link left in, when this carry on started they never really told installers about the link, not defending them, but the information about that link was very sparse and many never knew it had to be removed

When I had my Vaillant Ecotec Plus 418 18kw fitted, I had similar problems from day one. While I was waiting for the engineer to come back, I phoned up Vaillant. Before I could even finish explaining they asked me if I had Vaillant controls. I said no and straight away I was told that Vaillant controls work on 24 volts and if you use any other controls, you have to remove the 24v link from the circuit board. I said it was a Vaillant installer that installed it but they said it’s the most common fault on new boiler instals. I removed the link myself and cancelled the installer. Been fine since.

 

[ianmcd]

Hi, thanks for that. Is it basically pulling out a wee plug? If the electrician can't make it again I might have a go. Obviously switch of the electricity first and then start unscrewing obvious screws. I have the installation manual somewhere. Deconfusing the boiler would be a very good idea; I have the feeling it overthinks the whole "I am a boiler" situation sometimes.

yes you can remove the plug , but better if you just remove the wire and leave the plug in place in case you decide to go for different controls in the future, it is a ten min job

 

[picasso]

After 10 years a stuck microswitch in one of the 2 ports would be more likely the problem.

 

[paleo]

yes you can remove the plug , but better if you just remove the wire and leave the plug in place in case you decide to go for different controls in the future, it is a ten min job

Thanks. If all goes according to plan, the electrician and the heating engineer will come by tomorrow. I'll run that past them,