Probably not. Looking at Vaillant documentation they seem to include something like a VR66 Control Centre in their example configurations. This is mains powered and has an eBUS connection, so I suspect the eBUS power supply is in there.
This makes some sense. In the good old days you would have a Boiler and a Room Thermostat connected by eBUS, so it was logical then to have the Boiler supply the eBUS power. Now eBUS may be connected to Boilers, Heatpumps, Gateways, etc. in a single bus so it is not obvious where the power supply should sit. The eBUS power supply is pretty simple, 24vDC current limited to 100mA. You can have more than one power supply on the bus, but their combined current output should not exceed 100mA, otherwise you risk blowing up the Tx transistor that is shorting the bus when sending a zero. So simplest to just put it in the control centre.
For reference I have a Glowworm Ultracom HXI that uses eBUS. Glowworm is part of the Vaillant group and the Ultracom is just a re-badged Saunier Duval, yet another member of the Vaillant group. As delivered the eBUS connections have 24vDC on them. If you connect up the eBUS Room Thermostat. they automatically start chatting and work as expected.
Following up on calculating heat power using flow rate and delta T (and putting aside the complexity of defrost for the moment). I was wondering what people thought about trying to account for the residual heat supplied as the water cools between cycles. I notice that it is common to zero this period, but although the compressor is not running, the water is still being circulated and some heat is being supplied as the water cools.
I am not 100% happy with this, but I did abs(delta T) as the Vaillant-measured Flow Temp drops below the return (presumably because when the compressor is not running it ends up cooling the water). My feeling is that there may be a more accurate way of doing this by considering the mass of water in the system and the temperature drops over time, but I am not sure.
I am not sure whether this will make a material difference, but I am curious !
Thanks. I saw the discussion on the Vaillant thread just after I posted! I will look into this. My calculations based on this negative delta T are clearly not going to be right.
However, by my way of thinking, there is heat gain between cycles. Another way of thinking about it is that if there are 150 litres of water in the radiators and the heat drops by 3 degrees, that is about 0.5 kWh of heat released into the house during the time the compressor is switched off. Does that seem right or am I missing something?
Between cycles the heat goes from the water into the house, that’s true. But your heat meter does not register this as it can only see heat added or lost between its two temperature sensors. Unless your outdoor unit is connected by terribly insulated pipes, flow and return temperatures are going to be equal between cycles.
Yes, I realise that it could be difficult to measure this heat that is being transferred to the house, but I am wondering whether it could actually be worth accounting for in order to get accurate COP measurements.
By my rough calculations and referring to the chart I posted above, it seems there was about 3 kWh transferred during the heating cycle and 0.5 kWh transferred when the compressor is off, as the water is cooling. This seems big, so my first guess is that I am calculating wrong or not thinking about it correctly!
That heat was all measured. The heat meter measures heat going from the heat pump into the water. The heat being released between the cycles is heat going from the water into the house that was previously put into the water when the compressor is on.
The easy fix is to have a Home Assistant template that triggers when the compressor was off for a couple of minutes but there ist still flow to compute the delta, storing it in an input number. Then you have a template sensor that gives you a corrected flow temperature based on the measured minus the delta. This gives you an approximately correct temperature with the offset being updated after every cycle (the offset drifts a bit with outdoor temperature, this is why you can’t just set it once).
That’s a nice solution to dynamically calibrate, based on knowledge that the offset is truly close to zero when the compressor is off. I am a bit more comfortable with software than hardware!
However, I just looked at three successive cycles when the outside temperature was stable at 11.5 celcius and I am now worried that there is something else going on causing instability The deltas in the compressor-off phase are 0.4K, 0.94K and 0.32K. They seem to be consistent for each cycle, but that seems to be a bit change from one cycle to the next.
There was no sun (I don’t think the sun has been out here in the last two weeks!!). I will monitor and see whether this was a blip and whether it is normally more stable. I was doing some reconfiguration, and so don’t currently have a lot of history.
EDIT: actually, using the limited history I have, I can see that the 0.94K was an outlier. No idea why, but other periods average around the -0.3K level.
This is really weird and I have no idea what caused it. I guess the only thing to do is keep looking at it and see whether it happens again and if there are any discernable patterns.
Does anyone know as to what sensor is the trigger which triggers defrost cycles ? The lowest temp on my system seems to be sensor.ebusd_hmu_evaporationtemp_temps2
Not sure whether that’s a single sensor value that would tell us. Theres a status code scode topic that can take on a “defrost” value. Apart from that I’m guessing there’s some internal logic - possibly you could look at all the compressor-related topics to make out a pattern.
I’m also curious on what triggers that defrost cycle, like André explained, I think also it depends on the behavior of one of the sensor related an other one that tells the unit evaporator is frozen and needs a defrost
Hi all. Apologies for the question, but I wondered if anyone could point me in the direction of information bearing on how to link up an ebusd connected to an Arotherm+ with a HA Green? I should add that although I have a degree in Electronic Engineering it was a… LONG time ago, and I’ve done no coding or indeed anything technical since! But, I’m intrigued to see what I can learn in this area. Anyway, any guidance appreciated!
!
- that’s a common hardware issue with Vaillant and 
The deltas in the compressor-off phase are 0.4K, 0.94K and 0.32K. They seem to be consistent for each cycle, but that seems to be a bit change from one cycle to the next.
