Vaillant 418 Modulaing problem

[AndyAA]

We had a Vaillant 418 boiler installed 4 years ago along with a VRC400 weather compensator and unistor hot water storage tank, the system is run vented. After much time spent twiddling with various settings I finally got the system to work reasonably well with a minimum of problems with micofireing, over-shoot etc. There is however a problem which has been there from new but did not occur very often. Now it is occurring much more often and needs to be resolved.
The problem appears to be in the PID control loop which regulates the gas valve and the fan speed when the boiler is modulating, especially at low levels of heat output. This control loop does not appear to have any parameters which can be adjusted by the user. It can be seen most easily by monitoring the flow temperature (D40). When it is controlling correctly this is very stable but when the fault occurs it oscillates slowly up and down from its correct value by up to +/- 3 Deg. over a period of about 15 seconds, whilst the return (D41) temperature remains stable. When this happens the control parameters start to exceed their limits and the boiler cuts out and reverts to anti-cycling mode. If this carries on the VRC400 will never achieve its set temperature and the house remains up to 2 degrees down. Altering the flow through the boiler either by turning the pump down or opening the bypass does little to improve it. No fault codes show up and the parameters (apart from D40) remain largely unchanged whether the fault is there or not.
Any ideas anyone?

 

[scoobydo123]

could it be the NTC's contact with the pipes or going a bit loopy from time to time or rpm of the fan not being detected properly. how did you get round microfiring? There is a revised PCB with different logic available

 

[AndyAA]

Good idea about the NTC's, I will try it.
I did consider changing the main PCB but it's a bit expensive. Maybe the later PCBs have a newer version of the firmware which contains the perameters for the PID loop and may also improve the microfiring problem?
I managed to greatly reduce the occurrence of microfiring by setting (D0) down to 12 and setting the anticycling factor (D2) to 30. It makes the house a bit slower to heat up from cold but that is no problem.

 

[Dan Robinson]

A7sing about with the bypass and turning the pump down is the last thing you should be doing.

If anything you need a bigger pump.

 

[scoobydo123]

With regards to new PCB not sure if it would have the revised logic if purchased off the shelf. Certainly it helps with the microfiring as some of the logic was changed in relation to start up parameters. Maybe one of the Vaillant experts will confirm how you can ID if a replacment PCB has the revised firmware/loadware on board?

 

[Dan Robinson]

The existence of am upgraded board is something of a forum mystery..

 

[scoobydo123]

I have one and it works

 

[Agile]

But how do you really know that it is any different from the standard one?

 

[scoobydo123]

You can witness it in the start up routine quite easily during DHW regeneration previously the flow temp could overshoot and microfiring would occur (depending on if you heated the entire cylinder in one hit). Revised logic no such overshoot.

Also microfiring gone if low flow temps requested

 

[Bunnyman]

It does exist an it does solve the problem that sometimes exists with the firing for to long on full at startup an causing s53

 

[Dan Robinson]

Still think a bigger pump might be better