Vaillant 418 modulation - a known fault?

[mogget]

Hi Folks.

I have a Vaillant 418 with a troublesome fault. Occasionally it will fail to modulate properly and get stuck at minimum output, resulting in cold showers (not enough time allotted by timer to heat a full cylinder at 5kW). Pressing the reset button reverts it to normal operation, and the fault will recur after hours to weeks. Three different PCBs have been OK for months to years, then the same fault reappears,

Is getting stuck at minimum output a known fault with the Vaillant 418?

Thanks

Mogget.

 

[picasso]

The next time is gets stuck on low press the I button and see if it reads "s 53".

 

[mogget]

OK will do, many thanks.

 

[mogget]

Looking into it, that might make sense, I will now try to recreate the fault

 

[mogget]

Like clockwork. S.53 caused by the introduction of a slug of cold water when the DHW circuit opens on a cold cylinder and the rad circuit closes soon after. (Glad I haven't been paying too much for the PCB's )

I have a Grundfos 15-60 pump on setting 3, any lower and the boiler will over-temperature and short cycle.

I'm guessing I have a system design issue - anyone willing to save me a lot of thinking by giving me any hints?

Thanks

Mogget.

 

[picasso]

Press the I and + button and the display will show D0, press the I button again and the display should read 18, press the - button until the display reads 15 and see if that cures the problem.

 

[mogget]

I tried that, no luck, will try 12. Should be enough for me as I don't heat the whole house but the full 18kw is required if the whole house is to be heated.

Will also check the pump is not clogged (shouldn't be as the water is squeaky clean and full of inhibitor) or faulty.

Bl**dy Vaillant 400s!

I'll get back to you with the results.

 

[planemad]

I have just purchased this boiler - hope I don't have probs

 

[picasso]

At a rough guess I would say 99% of the 400s "problems" are caused by installers not realizing how restrictive the heat exchangers are and under sizing the pump.

 

[mogget]

Well, I went to fully open the dhw balancing valve, and it started making soime awful noises internally. So I've just cut it out, soldered a piece of copper tube in and put a new gate valve in a more practical position.

Once I've got the air out of the system I'll try it with the gate fully open, check the pump and report back.

I was hoping for a nice relaxing Saturday evening in, but I'd rather get warm again!

Planemad - apart from this S.53 proplem, which I'm sure will be fixed one way or another, it's been a very reliable boiler. I was just venting my frustration - why can't it go wrong on a weekday?!? Of all the times it could pick! ( at my heating system)

Picasso - I've had a 15/60 on speed 3 for years, but the fault is becoming more frequent, so I am suspecting something else is wrong, here's hoping the heat exchanger isn't crudded up.

I should add that I've recently changed it from HW priority to a standard S-plan+ so that's probably not helped matters.

 

[bell824]

alec????
or alecs mate?????

 

[mogget]

Who's Alec?

Anyway have checked the pump, it is fine, clean as a whistle and as I don't get the problem with any number of rads open (from one upwards) I think it would rule out a problem with the heat exchanger. This leaves either an airlock / restriction in the DHW circuit, or poor software design in the boiler. I suspect the latter.

Using a manual bypass instead of the auto bypass would probably fix the fault, but is hardly ideal.

 

[mogget]

OK, I've looked up Alec, I assure you I am not him, I am female for a start...

Bodge for tonight: leave the DHW gate valve 2 turns open so the ABV lifts and mixes the hotter water into the DHW return. Not ideal but it works now.

As we all know though, bodges for tonight turn into bodges for next year...

Mogget.