Viessmann Vitotronick 300-K cascade surprise

jbjb

It was colder than it should have been in church this morning .

External controls calling for 63C flow setpoint, boilers delivering 49C. Oops.

Down to the boiler room. 3-boiler Viessmann 200W 49kW cascade with Vitotronic 300-K cascade controller. Boiler 3 (lead boiler, as it happens) is locked out with fault code 'Fd'.

The 300-K is seeing 63C demand on the external 0-10V input and seeing only 51C at the LLH.

Other two boilers are healthy, but running at 41% output. Why? If the cascade is aiming for 63C and delivering 51C, why is the controller not at least attempting to achieve this by running the two healthy boilers at something more than 41%?

I reset the faulty boilerand it immediately fires up and joins the cascade. All of them ramp up to 60+ % and we get to setpoint in about 15 minutes and then ramp down.

Viessman gurus... is there a hidden coding setting somewhere that I'm missing or is that just how the 300-K [sort of] works? The fault had been there for 2 weeks and you'd kinda hope that the 300-K might have picked a different lead boiler by now too!

GGross

It should change the lead boiler at set intervals, but there is programming that can alter those settings, it is possible that you have a dedicated lead boiler that never changes, there is an option to program that way.

I personally have never seen this exact circumstance play out... In my experience if one boiler goes down the other boilers will pick up the load just as you would expect

What is attached to the external 0-10v input?

How long have these boilers been installed?

What is the actual model number of the 200 series boilers?

I believe that you may have some programming that needs to be done. I apologize I may not be able to type out detailed instructions on how to do that, there are several viessmann cascade controls through the years, most with the 300 designation have similar programming options but actually navigating the menus is different on each one

Something else to consider, FD fault can come up as a nuisance error, but it can also be due to a brown out or unstable power supply, in some rare cases there can be combustion or ignition control issues that cause that error. If you get that same error again it needs to be looked at, you can also access a fault history (last 10 faults only) through the boilers onboard control, if you see a combination of EE and EB with the FD code, I would suggest getting someone on with tech service to figure out what is going on.