Vaillant Ecotec Plus 418 - Heating Issues (Constant Anti Cycling?)

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Hi All,

Looking for some advice please! Not sure if this could be a DIY Home fix or if I need to call an engineer.

I have a Vaillant Ecotec Plus 418, which appears fine on the hot water side and is able to maintain temperature however when I call for heating, initially it seems to go for a while and then cuts out. The rads are warm at best, an the heating symbol is flashing. It will fire for a minute if that, and then cycle again. This appears to be constant at the moment and its bloody freezing!

Any advice on what to do would be great..

Photos to follow.

Cheers! :)
 
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Photos:

1 - normal operation
2 - flame cut out and flashing radiator
 

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the pump is certainly quite noisy and has been for a while, though it has been working okay
 
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Did you ever get anywhere with this? I have a 7 year old Ecotec Plus 428. Vented traditional system (tank in loft etc). 10 rads.

Hot water is fine. CH in the mornings it runs for 30 minutes before entering this anti cycle mode where it fires up for a minute then shuts down and goes into the pump overun / delay and starts again 5 minutes later. it does this continuously not adding much more heat until the timer ends 30-60 minutes later

After 90 minutes of nothing (stat is not demanding heat in this window) it fires up again normally for a period. I am currently trying to note all the timings down to get a better grasp on it.

Edit: Boiler runs for around 50 minutes but then stops before reaching target heat and enters cycling mode (5 mins pump over run and then repeats). Its like it needs to cool down before it can run fully. I might try and adjust our timers but should have to do this :(

I have done reading on the Valliant 400 series issues from the excellent blog:Vaillant cycling problem: Info from owners with the problem

This boiler was purchased mid 2015 so I wonder if it has the bad PCB logic being referred to.
 
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What happens if you set the thermostat temperature to at least 5 degrees above the current room temperature?
 
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The heat exchanger has high internal resistance. The flow through the rads has to be sufficient and correct else the boiler will cycle like yours is doing. While I have not had this issue with my instals ( not blowing my own trumpet I can assure you), it is not unknown to have to fit a low loss header to make this boiler behave.

If you look at the flow and return temperatures. You will see the flow rise is quite rapid and return catches up lot slowly resulting in burner shutting down. With low loss header that does not happen. This problem I believe does not take place with a combi boiler but seen it with a system boiler

Through the cylinder, the water wizzes round and return sensor sees temperature rise, electronics are happy that there is circulation

Above is based on what I have learnt etc but may be wide and off the mark. I am sure those that work with Vaillants will have better insight and correct me if my understanding is a dud
 
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You will see the flow rise is quite rapid and return catches up lot slowly resulting in burner shutting down. With low loss header that does not happen. This problem I believe does not take place with a combi boiler but seen it with a system boiler

You're mostly right.

The return is not expected to "catch up" with the flow temperature. Don't think about the water leaving the boiler, going all the way round the system and coming back before the return "catches up" I had the same thinking for years and never understood it 100%.

The way to think about it is look at the return temperature first. If the return is at 25 degrees, the water then moves from the return through the heat exchanger and then should be no more that 45 degrees at the flow, regardless of how long the boiler is on for the flow should be about 20 degrees or less, over the return temperature. If its exceeding this, ie 25 degrees on return 55 degrees on the flow, then this can only physically happen if the water is moving too slow around the heat exchanger. And that is what the boiler is monitoring to prevent it damaging itself.

The reason you don't see it on combis and most system boilers, is that they come factory fitted with the correct size pump, and an appropriately sized and adjusted auto bypass valve to suit the boiler.

With heat only appliances you rely on the installer specifying, fitting and setting the correct component externally, which often does not happen.

The likes of the 438, in 99.99% of cases is far far too big for the property its fitted in, and your limited on corrective action, considering the average UK house needs around 7-8kw on the coldest day of winter. The 438s are almost never piped right either (ie 28mm primary pipework)

The 418 in the original post, working fine on hot water and not heating, would sound like theres a restriction on the heating circuit, or possibly the pump is getting weak and unable to move the water volume required for heating the radiators, which will be a more restrictive path than the cylinder typically.


As far as the modified PCB goes. Being a heat only boiler, it doesn't have any way of detecting that there is water in the system and therefor no way of preventing dry fire without the use of its temperature sensors.

It fires at around 60-70% of its full rate as virtually all zero governor boilers on radial heat exchangers do to get a successful air/gas ignition mixture, it then ramps up to a high rate, so it can monitor the temperature rise on the flow and the temperature difference between the flow and return, this is how it checks for water content, and to check the flow rate is ok, if its not it will either shut off, or lock itself onto minimum rate to prevent damage. The PCB upgrade essentially just reduced the time spent at 100% output to reduce this happening.

However in every single instance, its either a restriction in flow, or a massively over sized boiler (which will also have a restriction in flow)


Solutions as said, a low loss header can go some way to curing the issue, as the flow rate through the boiler is then kept constant at all times, however that often doesn't address the issue if the boiler is far too big for the property as the water will still heat very quickly overall leading to short cycles.

Sometimes its much simpler, often theres no auto-bypass fitted, or its fitted but not set up right (which would keep the flow higher through the boiler in a similar manner to a low loss header thus keeping the temperature difference between the flow and return stable)

If its only just become an issue recently, then blockages, pump faults are more likley.
 
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Or to be radical and disagree with a much respected colleague in @ScottishGasMan , do not fit a vaillant system boiler without checking the flow and returns of your existing system, they can not handle poor circulation , other boilers can , doesnt mean they are better
 
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Great post @ScottishGasMan. I'd add to what you said about the benefit of a LLH in that you have a primary and secondary pump. The primary shunt pump only has to overcome the head required for the boiler (which in the 438s case is considerable). This means smaller domestic pumps can be used, rather than a single larger semi commercial pump.

The other thing I'd add is that poor circulation would normally result in an s53. Whereas short cycling could be an oversized boiler or too low a flow temp.

OP, make sure d.17 (Heating flow/return regulation change over) is set to 0, the default setting. Just a tip from previous experience on this boiler.
 
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Great post @ScottishGasMan. I'd add to what you said about the benefit of a LLH in that you have a primary and secondary pump. The primary shunt pump only has to overcome the head required for the boiler (which in the 438s case is considerable). This means smaller domestic pumps can be used, rather than a single larger semi commercial pump.

The other thing I'd add is that poor circulation would normally result in an s53. Whereas short cycling could be an oversized boiler or too low a flow temp.

OP, make sure d.17 (Heating flow/return regulation change over) is set to 0, the default setting. Just a tip from previous experience on this boiler.

Yes, with the bigger boilers its definetly worth considering LLH or CCTs as you say the primary pump will use up a high portion of its available head just getting flow round the heat exchanger, and 2x domestic pumps is often cheaper than 1 commercial one.

The other thing to note is D.0 Range rating the appliance, its often worth turning it down to what the system actually needs, it wont prevent the high ignition fire, but will lower the output of the appliance in normal running to something more suitable. You can also increase the anti cycle time and lower the flow temperature slightly from 75 to 65 or 70, which keeps it off for a longer period between start ups, gives the system a chance to cool a bit and more chance of boiler not going over the target temperature and cycling when it starts back up.
 
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