Vaillant eBus oddity

[DH2]

Hi all.

My system is:
Vaillant ecoTEC 430 open vent boiler
Vaillant VRT360 programmable room thermostat
Vaillant VR65 control centre
Vaillant VR10 NTC sensor (tucked under insulation on a normal DHW tank)

It mostly all works super, but I have noticed that sometimes, when DHW is demanding heat, the flow temperature set via eBus is 80c and yet other times it is 65c (65c being the rad flow temp I have set on the boiler).
When it is 65c, this causes the boiler to cycle because at that flow temperature, the heat is not disspated into the hot water via the indirect coil as quickly/effectively as at 80c (i.e. the differential between flow temperature and the tank temperature is significantly less), and even when the boiler is modulated as low as it can go, the heat input is still greater than the heat being dissipated in the hot water. So, lots of very short cycles - the boiler starts up and shuts down almost immediately.
At 80c flow temperature, the system is amazingly good at heating the hot water, as it can modulate down and maintain that flow temperature without causing any boiler cycling, and will heat the tank in short order, I suspect this is quite efficient too.

If I reset the whole system, by power-cycling it, it usually causes it to revert to the 80c flow temp for a few days, before it then seems to go back to being pot luck whether I get 80c or 65c.

I am left wondering whether this is a fault somewhere in my eBus system, but I don't know how to narrow it down to whether it is the VR65 not requesting the correct flow temp, or whether that is done at the boiler? Or whether it is determined by the VRT360?

Any ideas/advice gratefully received (before I succumb to trying Vaillant technical support, who in my experience, aren't particularly technical, or supportive!).

Thanks in advance,
DH2

 

[gas4you]

What you've got is the system working normally.

The VR65 boosts the flow temp accordingly to heat the HW cylinder up as quickly and efficiently as possible.

Th CH flow temp is kept down to help keep the boiler in condensing mode, thus saving you gas by keeping the boiler running efficiently

 

[DH2]

What you've got is the system working normally.

The VR65 boosts the flow temp accordingly to heat the HW cylinder up as quickly and efficiently as possible.

Th CH flow temp is kept down to help keep the boiler in condensing mode, thus saving you gas by keeping the boiler running efficiently

Yes, when it is doing that, I agree with you.
But what I am saying is that, randomly it only uses 65c flow temp for the water heating, which causes the boiler to cycle alot, which cannot be good/correct.

DH2

 

[gas4you]

The VR65 will set the boiler flow 'randomly' depending on how hot the residual water is in the cylinder.

If it just needs topping up then it will use a lower flow temp, but not below that set for the HW, if the tank is cold then it will set a higher flow temp to re-heat the cylinder faster.

The VR10 is not really designed for use as you are using it. It should be in a dry pocket, as with the Unistor etc.

 

[David937]

65c being the rad flow temp I have set on the boiler).

Someone on here who knows a lot about Vaillant products told me that the boiler CH knob should be set to max with the VRT360 as it works out it's own flow temperatures.

 

[simond]

That is definitely the case with weather compensated Vaillant ebus controllers.

I would be surprised if the VRT360 does it.

 

[David937]

I too was surprised but when told I set it at to maximum as DM told me to do so.

 

[DH2]

The VR65 will set the boiler flow 'randomly' depending on how hot the residual water is in the cylinder.
If it just needs topping up then it will use a lower flow temp, but not below that set for the HW, if the tank is cold then it will set a higher flow temp to re-heat the cylinder faster.

Ah, not so random then perhaps. I will try to accumulate some data to backup your theory.

The VR10 is not really designed for use as you are using it. It should be in a dry pocket, as with the Unistor etc.

Yes, I appreciate this, but in my experience, it works better than a traditional mechanical cylinder thermostat.

Someone on here who knows a lot about Vaillant products told me that the boiler CH knob should be set to max with the VRT360 as it works out it's own flow temperatures.

I suspect this is the case if you are using the VRT360 in analogue mode. I am not using this mode because it didn't work particularly well, when I tried it (i.e. the boiler waking me up in the middle of night because the VRT had requested a flow temperature of 35c - not particularly useful).

DH2