Vaillant Hot Water Short Cycling

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Hello

Posted another thread about annoying noises from hot water system. Think I've now narrowed it down to expansion. And repeated expansion as boiler short cycles, pipes cool, reheat, expand, repeat.

Vaillant 418 boiler. Working fine on CH request, previously throwing S.53 states but is not running d. 0 at 10 rather than originally 18.

With not water only requested, boiler fires and runs OK for around 20 minutes. After this, it goes into short cycling, fires for about 20 seconds, reading on front of boiler reaches 75 or 80 C and then it shuts off for around 2 minutes and then just repeats the cycle until time request expires.

If running both hot water and heating together, system seems OK. Hot water isn't reaching tank thermostat temperature, request for heat is constant.

Anything I can start to look at to diagnose this please?
 
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I have this exact same problem and have been chasing it for a year now. After 20 minutes or so of heating HW, the boiler temp will overshoot 75 and switch off, leaving the pump running. Boiler temp will drop and after a few minutes it will fire up again and overshoot 75 again well within 2 minutes. The boiler makes no attempt to modulate (I think it's designed not to modulate within the first 2 minutes of firing). At this point it's caught in this vicious circle until the timer says the HW period is over.
 
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Sounds identical!

Really hope someone here can help us. Have you tried anything thats improved it?
 

CBW

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It’s a process of elimination with these, assuming it’s a combi boiler. Could be any of the following, since you mention s.53:

Weak pump, blocked plate, blocked main heat exchanger, blocked internal rubber hoses if they’re installed. Your best bet is to employ someone who knows these boilers or Vaillant fixed price repair.

Thanks.
 
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Your best bet is to employ someone who knows these boilers or Vaillant fixed price repair.

Might Vaillant's fixed price repair, only cover boiler fault repairs?

If this heats a water cylinder, is the cylinder 'stat maybe set too high?
 
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It’s a process of elimination with these, assuming it’s a combi boiler. Could be any of the following, since you mention s.53:

Weak pump, blocked plate, blocked main heat exchanger, blocked internal rubber hoses if they’re installed. Your best bet is to employ someone who knows these boilers or Vaillant fixed price repair.

Thanks.

Not a combi here, conventional system.
S. 53 has gone away with dropping d. 0 to 10. Down from 18. Was only on heating side also, no s. 53 on hot water
 

CBW

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Ok @miniwill. What setup do you have? - 2port or 3 port valve? Do you actually get hot water, from selecting hot water only?
 
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Cylinder stat at 55. Don't want to go lower than that!

Nor should you need to, but..

My boiler is set to output at 75C when heating the cylinder, but cylinder temperature has to be limited to 60C, other wise it ends up cycling. It a Vaillant system, which uses an actual sensor on the cylinder, rather than a stat.

Maybe you stat is not clicking off accurately? Try as a test turning it down a bit until it clicks, once up to temperature.
 
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It sounds like the cylinder stat.

Does the port of the 3 port leading into the cylinder quickly get hot, once you make a call for hot water?

Does the return pipe from the cylinder also quickly get hot?
 

CBW

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Is it possible you have a dripping hot water outlet, that’s losing the heat from the cylinder?
 
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It sounds like the cylinder stat.

Does the port of the 3 port leading into the cylinder quickly get hot, once you make a call for hot water?

Does the return pipe from the cylinder also quickly get hot?

I'll check the pipes. Stat is brand new a couple of weeks a go, problem was there with the old one too.
 

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