Viessmann boiler short cycling

Valdas

Hi,

I have noticed my boiler Viessmann Vitodens 100-W(B1HC-26) makes a series of short cycling before starting to heat. Boiler is controlled by Baxi uSense room thermostat connected via Opentherm. I installed some monitoring equipment to get the better understanding of the problem. Bellow is the illustration of the problem:



I will explain the graph from the top to bottom:

• Green bar indicates heating pump is on
  
• Red bar indicates flame is on
  
• Black line indicates max relative modulation level (%)
  
• Orange line indicates DWH temperature (Celsius) (not relevant in this problem )
  
• Red line indicates boiler water temperature (Celsius)
  
• Grey line indicates setpoint temperature (Celsius)
  

The graph reveals that it took 10 times for the boiler to fire up until it started to run. Water temperature inside the boiler reaches ~40 degrees very quickly after boiler fires up and then it switches off.

I would appreciate any ideas of how to solve the short cycling.

Jamie Hall

Is this primary/secondary? And if so, by heating pump do you mean the main circulating pump, or the boiler circulating pump?

Valdas

That's pump inside the boiler. There is no other pump in the system.

Jamie Hall

OK -- but it's not circulating. Is there a valve closed against it somewhere?

JUGHNE

To me it sounds like a simple burner ignition problem
And the first thing I would check is the flame sensor.

Valdas

No, valves are open. I think it must be circulating otherwise boiler would short cycle forever. After 10th spike temperature inside the boiler dropped as quick as it went up so heat was carried over to the system.

Valdas

The boiler is new, flame sensor should be fine. This Viessmann boiler ignites with 70% of it's capacity and only then starts to lower down the flame - that's the reason of the spike. I don't understand why the boiler turn off the flame.

HVACNUT

How new is new?

9 short cycles opening on high temp limit before the 10th times a charm.

Check the wiring to the circulator. Splices, molex connections, grounds, etc.

Seems like the circ is not moving water, until it does.

Was it properly purged of air?
Is there a working air eliminator?
I'm not familiar with the boiler. Is there a factory air eliminator inside the cabinet that's closed?

Cooler1989

I had similar problem. I even got do do some calculation on my own when no one from official service was able to help. It seems that internal pump is to weak to push out initial 70% power of the burner and above that. There is only one sensor of CH water in the outlet. The reason of burner stop is simple - it thinks it achived goal of set temperature. Little does he now that it is only for small amount of water currently flowing through boiler.
Bad news is I could not find solution on the marker. I had to create my own device to connect and fix this issue. Simple by concept: it only detects when the burner starts and increases set point temperature for about 30 seconds. Additional changes in setting in boiler so that it never hits more then 70% in CH and it works like charm. Shame that I had to spend more then one heating season to figure it out and fix. This should work out of the box.
If someone needs some help with this matter you can contact me.

heathead

How is the flow rate thought the whole hydronic system. Does this have a low loss header? The boiler pump will not be big enough to be the only pump in the system in 90 percent of North American systems. Is the internal pump in the boiler trying to be the only pump in the system?

JeroenS

Hi, I have about the same problem with a Vitodens 100, B1HC-26. The boiler fires up, and stays at 20% power. The Low loss header gets warm (55 degrees) on the top (where the input from the boiler is). Return stays at about 27. Emptied the dirt remover (in the return pipe), but nothing came out. Any tips where to look further?