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Discuss Viessmann boiler short cycling in the Central Heating Forum area at PlumbersForums.net

V

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:
Viessmann cycling.png

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.
 
ShaunCorbs

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Remove the baxi I sense does it work / stay on for longer ?
 
OP
V

Valdas

Can you be a little more specific?
House is based on floor heating, system is all new. Circulation pump is the one that's inside the boiler. The problem I have is that boiler switches on and switches off frequently without making any heat. For example temperature inside the boiler is 25 degrees(Celsius). The boiler switches on and the temperature starts to rise up rapidly. It reaches 40 degrees in ~5 seconds and the boiler switches off. The temperature inside the boiler drops to 25 degrees again. After several minutes boiler switches on again and switches off very quickly. It takes 3-10 cycles for boiler to start working.
Post automatically merged:

Remove the baxi I sense does it work / stay on for longer ?
Once the boiler starts working it keeps heating for a quite long time, about 2 hours. It short cycles until it gets working. I haven't tried without the Baxi thermostat as monitoring equipment will not work without it.
 
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J

John.g

Re the "relative modulation level".. is 0 = minimum (modulated) boiler output and 100 = maximum boiler output or vica versa?.
 
D

doitmyself

View attachment 42946

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 black line shows the relative modulation level, not the max level.

0% is the minimum the boiler will modulate down to, e.g. 8kW; and 100% the maximum it will produce, e.g 32kW. This is Opentherm parameter 17, which is a Read parameter, i.e it is just telling you what the current modulation level is.

The max relative modulation (parameter 14) is only used in multiple boiler installations and sets the maximum output of a specific boiler. In single boiler applications it is always 100%, the default.

ShaunCorbs suggestion of disconnecting the uSense thermostat (and connecting the boiler Opentherm terminals togethr or substituting a volt free one) is a good idea as it will tell you if the problem is due to the thermostat or the boiler.
 
J

John.g

So in that single boiler application, above, it should read 100% at all times and not change throughout the firing period?.
 
D

doitmyself

So in that single boiler application, above, it should read 100% at all times and not change throughout the firing period?.
I assume you are talking about the Max Relative Modulation (parameter 14). If so, yes it reads 100% all the time It is a Write parameter, i.e the required value is sent from the thermostat to the boiler. The Relative Modulation will change during firing as it is just informing you of the currrent boiler output. An Opentherm thermostat sends the boiler the required CH temperature; it is up to the boiler how this is achieved and controlled.
 
OP
V

Valdas

The black line shows the relative modulation level, not the max level.

0% is the minimum the boiler will modulate down to, e.g. 8kW; and 100% the maximum it will produce, e.g 32kW. This is Opentherm parameter 17, which is a Read parameter, i.e it is just telling you what the current modulation level is.

The max relative modulation (parameter 14) is only used in multiple boiler installations and sets the maximum output of a specific boiler. In single boiler applications it is always 100%, the default.

ShaunCorbs suggestion of disconnecting the uSense thermostat (and connecting the boiler Opentherm terminals togethr or substituting a volt free one) is a good idea as it will tell you if the problem is due to the thermostat or the boiler.
How the red line (boiler water temperature) should look like in a perfect case? Is spike normal at the moment of ignition? As I know Viessmann Vitodens 100-W boiler starts up with 70% of it's capacity at ignition phase and then modules down if necessary. In my case boiler doesn't get over the spike as temperature inside the boiler gets too high comparing to the setpoint. As the graph shows, boiler gets over the spike only when setpoint temperature goes a little bit over 30 degress.
 
J

John.g

I often think its quite a achievement for a (gas) boiler to ignite @ 70% output and modulate down rapidly before the temperature reaches SP+5C, and burner cut out. If one assumes a Hx content of 3 litres and 10 LPM flowrate and a 10C deltaT before SP+5C is reached then it will only take ~ 12 secs, obviously the Hx metal etc must heat up firstly but even allowing for this, the time probably isn't more than say 20 secs?, if it doesn't then IMO the off or anti cycle time has to be increased to allow a greater deltaT but in your case above, it can hardly go lower than a return temp of 25C so increasing the boiler SP is one possible solution and also, if possible, increase the boiler minimum flowrate.
Also, I see above that the temperature rises from 25c to 40c in 5 secs, theoretically, even with no boiler flowrate this would require a boiler output of ~ 37 kw? so maybe problem with flow temp sensor?.
 
Last edited:

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