Viessmann boiler short cycling

[Klump99]

I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.

 

[John.g]

It makes sense to me to kick the SP up initially to get the boiler to stabilize and then reduce it, you said you had removed the weather compensation sensor so on what basis is the boiler now controlling its SP? the curve you gave looks like the boiler is still looking at a outside temperature from somewhere.
Can I assume it doesn't/wasn't doing this (initially increasing the SP to 50C) when running on the outside temperature compensation and have you the curve for this?.

Edit: I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?.

 

[doitmyself]

I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?

Yes, the curve supplied by the OP is for the weather compensator.

Theree is no such thing as a boiler curve. The boiler is told by the thermostat, via Opentherm, what the required SP is. How this is calculated is known only to the stat manufacturers. I would assume that it will depend on the difference between actual and required room temperatures and take into account the rate of temperature drop.

To quote the Opentherm Protocol:

The slave [boiler] does not need to know how the master [thermostat} has calculated the control setpoint, e.g.
whether it used room control or OTC [outdoor temperature control], it only needs to control to the value. Likewise, the master does not need to know how the slave is controlling the supply.

 

[John.g]

Thanks, that clears it up for me, I had thought of the room stat as just a on/off switch without communication. It is interesting how it starts up at 50C., its a pity that the OTC doesn't seem to do likewise but I suppose that is only a sensor.

I have seen a relations Vokera Vision 20S with a cruder version of this called SARA, if the room stat contacts stay closed for longer than 20 minutes then the boiler SP temperature is increased as long as the "manual" boiler SP is set between certain limits.

 

[Valdas]

There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks. Without OTC thermostat sets 50C SP when it's time to start heating and boiler overcomes the spike without switching off.
[automerge]1586982704[/automerge]

I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.

Was the boiler short cycling because of rapidly rising temperature during the ignition phase? What has changed after replacing flow rate sensor?

 

[doitmyself]

There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks.

I've been working on the 24 hr log you sent me. I have converted it to an Excel file to make analysis easier.

Would it be possible to send me another 24 hr log, without the outdoor sensor, connected, so I can see exactly what happens?

Heres a chart showing how the boiler runs over the 24 hours. The oudoor temperature uses the vertical scale on the left, All other lines use the left hand scale where 100 is either 100C or 100%. The thing I do not understand is why the boiler is switching up and down between modulations which are 20% apart when on heating, but more refined when on hot water. It's as if TPI (from the usense?) was controlling the boiler temperature. The actual room temperature stays very constant at 22C so I haven't bothered to show it.

 

[Valdas]

I have attached the log file containing data for full day when the outdoor sensor was disconnected. I think Baxi uSense thermostat doesn't include TPI software.

 

[John.g]

I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.

I was wondering why you mentioned this but I now know (I think). The boiler has a flow switch which will shut down the boiler if the heating flow rate falls below 200l/h (3.3LPM) but with no alarm. I am assuming that this is a heat boiler only and that its not a combi and that they are not referring to the DHW flow? , but they do refer to the "heating" flow rate.

 

[MCornish]

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:
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 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.

If the temperature rises too quickly then cuts burner out it sounds like the plate heat exchanger is blocked in turn locking out as it’s overheating. Had this recently on the same boiler.

 

[doitmyself]

If the temperature rises too quickly then cuts burner out it sounds like the plate heat exchanger is blocked in turn locking out as it’s overheating. Had this recently on the same boiler.

The OP hasn't replied for five months, so his problem has probably been solved. In any case, he has a system boiler, not a combi, so there is no plate hex.

 

[MikeJhn]

Just read through this thread and would like to say thanks to all of you for persevering with this guys problem, shame he did not thank you personally, but there may be extenuating circumstance's.