I may have mixed my versions up I think. I think I have 351.06.07 outside and 351.06.06 inside. We likely have the same.
Yes, and from what I understood you may need to replace internal and external boards.
It’s a pretty big deal IMO, if all of the older units are cycling twice as much as they were designed to cycle due to this. Can’t see Vaillant owning up to this very easily though!
I’ll increase the polling rate on the compressor ebus data and collect some datapoints - will report back as soon as I have sth to show.
It’s surely madness that they haven’t allowed OTA software updates? 
Is there a changelog anywhere of versions? Or is that too optimistic?
Sorry for not keeping up, but does this problem affect units left on default settings? or is it just when you start tinkering the problems start?
If it’s every device running old cold, couldn’t we raised warranty calls?
Or will they say “where are you getting your info? you’re not allowed to run ebus readers”. 
I switched back to 60°min this morning to see the difference in cycles and compressor utilization.
We can see nicely how the 100°min cycles are longer. The initial compressor peak as it brings the flow temperature to the target level is exactly the same (10 minutes). Integral goes down to around -100 or -60 according to setting. For -100°min, the overshoot of the integral after the cycle is also higher - the flow temperature goes about 1°C higher for the 100°min so it takes longer to decay below the target value and hence the overshoot is larger.
100°min led to one cycle being around 130 minutes, of that with around 35 minutes of active compressor time.
At 60°min one cycle is around 90 minutes with 25 minutes of active compressor time.
COP of an on-cycle at -100°min was 5.45 (30.0° flow, 26.4° return at 7.4 °C)
COP of an on-cycle at -60°min was 5.43 (29.4° flow, 25.7° return at 7.0 °C)
Performance measurements were done using the Vaillant reported flow rate but my own calibrated PT1000 probes on the flow/return pipes. Those numbers are pretty much spot on for the spec for my 10 kW unit (between 5 and 6 for 35° flow at 7°).
All in all a decent increase in cycle time at I would say zero impact on COP. I might actually try to also activate the silent mode because this initial spike in compressor use is not really necessary - a cycle would be even longer if flow temperature took some time to get to its target.
Thanks for putting in the time to create that, very interesting to see all the numbers and that the compressor isn’t peaking any more after longer wait (50rps right?). I didn’t expect much difference is COP, as Vaillant seems to be able to cycle efficiently, but any improvement is good and adds up over time. Also, it’s better for the compressor to cycle less anyway. No comparison with buggy firmware of course, but cycles will clearly be even shorter than your -60degmin cycles!
It’s interesting that you cycles only go from 25->35min. This is because your flow rate continues to fall so the delta gets bigger and bigger. In my case (UFH in passive slab and highly insulated house) my actual flow temp flat-lines after 20min or so, so I’d expect a bigger difference between -60degmin and -100degmin in my case.
For what it’s worth I think Vaillant flow rate measurement is accurate, as is the electrical meter (although it excludes additional pumps). It’s the Vailant temp sensors that have accuracy issues from what I’ve read/seen. So good approach!.
BTW. With 09.02 firmware is it still using fixed flow-rate (1200l/min when compressor on and 845l/min with compressor off for 7kW) and varying the delta-t, or has any flow rate modulation been introduced?
Yes exactly, the peak is at 50rps - and also stays there when I activate full silent mode so maybe the startup phase is excluded by the setting (or 50 rps is still within the reduced power envelope).
Regarding the cycle time: My unit is simply oversized (thanks to the installer insisting on their DIN-compliant heat survey instead of trusting my measurements) - 7 kW would have been more than enough and the 10 kW unit does not modulate to <4kW output. I’ve seen flatlining flow temperatures and somewhat steady-state operation around -10°C. As I have normal radiators with some self-built fans I don’t have the thermal mass to not notice the temperature oscillations with cycles but they’re typically < 1°C.
It’s good to know the Vaillant flow rate measurement is accurate - I was a bit worried my measurements were off because the energy efficiency in terms of required heating power is so much better than my previous 30-year old gas heater.
At the 09.02 firmware, flow rate is constant and just determined by the “Restförderhöhe” setting (no idea what that is in english - it just changes the maximum pressure of the pump to overcome losses in the pipe system). I’m seeing around 1400 when the compressor is on and 1100 when it’s off - minimum allowed flow rate is 1000 l/h for the 10 kW units).
@Andre_K @Zarch @dfeist Really helpful to see the German thread and the implications that the old boards are pretty poor for oversized HP versus demand. For passivhaus, that is all the time! My boards are 0351 06 05 and 0360 02 02 reported as 1 & 2 via the UniTower display unit (not sure which is which). I shall enquire with Vaillant I think.
I don’t know how the % reduction works on the 10kW (which is already a compressor-limited version of 12kW). On mine though (7kW) 50rps is already 58% reduction.
That is what I’ve seen anyway. In actual fact my heat-meter is sensitive to dirt and reports -5% after some time, but as soon as it’s cleaned it matches vaillant number almost bang-on. Vailant flow/return temps don’t match heat meter very well though (although vailant sensors may be in outdoor unit rather than internal).
So same as earlier firmware. This confims that this is almost certainly by design and not a bug. (other heat-pumps target delta-t of 5k and adjust flow rate to achieve this)
The Vaillant sensors are in the outdoor unit (see my posted screenshot here) and also quite susceptible to outside temperature variations. Quite crappy, but interestingly always wrong in the direction that favors Vaillant’s COP estimates 
.
Is there a good reason to target 5 instead of, say, 3? I get that you want a relatively low deltaT but is there a specific reason for 5K?
I’m not targetting 5k, rather this is how (from what I understand) other ASHP’s works. I assume because 5k is more efficient (?). Vaillant arotherm doesn’t use this same strategy though, and given you see same behaviour on latest firmwae, can only assume it’s by design and not a bug.
The Vaillant units will get to DT5 when it’s really cold and you need more heat output. It’s just mass flow calcs.
heat transfer (W) = specific heat (J/kg.K) x flow rate (kg/s) x DT (K)
So 5019W = 4.2 (water) x 0.239 l/s x DT5
ie, my 5kW model with 860 lpm flow (860 / 60 / 60 = 0.238 litres/sec)
So if flow and SHC are fixed, the only thing that can modulate is DT.
Which is what the Arotherm does.
Hence you will see narrower DT when it is warmer and you need less heat output.
4.2 x 0.239 x DT3 = 3011W
Some other heat pumps fix their DT at 5 and then adjust the flow rate.
You can play with the calcs to get heat outputs.
6930W = 4.2 x 0.33 x DT5
Bottom line, either of DT or flow rate has to modulate to adjust the heat output.
I suppose you could do both, but the controller might have a hard time deciding what’s going on there.
My question was more along the lines of why 5K is often used as a reference - maybe associated to heat transfer between the refrigerant and the water? A cooler inflow will make heat transfer easier between the hot and cold reservoir. But this is probably getting off-topic for this thread. With my system I will currently never see a DT5 since that would amount to 5.8 kW at the minimum flow rate of 1000 l/h - which is beyond my heating needs.
Hi Mick.
I can’t help but wonder if the 5K isn’t a bit of “that’s how we’ve always done it”?
The ASHP controller will decide what energy is needed (either from weather-compensated LWT control and/or roomstat).
The water/glycol circulation flow may be fixed (if non-PWM) or variable (if PWM).
You are 100% correct that this may mean high flow/low DT or low flow/high DT (from Q=m.Cp.DT).
The main consideration is that a lower DT (say 3degC compared with 5degC) will result in increased emitter output (because the average radiator temp is higher, so dT to room is higher), but at the cost of increased circulation pump energy.
@Andre_K poses a good question, which I suspect that you have side-stepped slightly. Circulation rate (hence DT) doesn’t set the duty - it’s merely the transfer mechanism to get the necessary heat from the ASHP to the house.
Please feel free to re-educate me!
Sarah
Thanks for clarifying my point. I guess my question boils down to this:
Neglecting water pump energy use and at the same flow and outside temperature, will DT5 @ 1000 l/h yield a better or worse COP than DT2.5 @ 2000 l/h?
(Let’s assume I tweak my emitters so they yield the same heating power in both scenarios)
In my opinion, Andre, absolutely no difference (given the assumptions you are applying). Interesting to see how Mick responds… 
.
Sarah
It’s going to depend on the specific ASHP potentially, I’m not sure you can make any generic assumptions regarding this. Some models and/or some refigerants may work better at 5K. What we do know is that the arotherm+ i) doesn’t aim for specific delta-t ii) it operates efficiently in this mode.
Good points, Daniel.
My comments above assumed 1) an element of LWT control, and 2) the use of a PHE for refrigerant heat transfer (with attendant close temperature approach, which varies little with water DT).
You’re both right. The end result of the calculation (heat output) is the same regardless how you manipulate the DT, flow rate and SHC.
BUT and this is the big but as Daniel alludes to. The Arotherm works by fixing the flow rate and modulates by adjusting the DT. Narrow DT when it needs low output and opens the DT up when it needs more output. There isn’t really anything to tweak unless you start dropping flow rate.
Personally, I would not get obsessed with DT5 at the radiators. The outdoor unit will still do DT2 when low output.
I got obsessed with DT5 at the rads and ended up closing down my rads so much that I dropped my system volume and ruined my COP performance.
Aim to open all the rads as much as possible so you have as much system volume as possible, but at the same time, the rads need to be closed down just enough to ensure resistance so that all the rads in the circuit get some heat. In my case, fully opening up every rad meant my index rads (the ones furthest away) didn’t get much heat.
Once you’ve got heat to every rad, you can tweak the lockshield slightly to adjust room temp.
So it’s a balance!! (no pun intended).
My radiators are fortunately quite nicely adjusted for now (that took time!). As you suggested, they’re alle fully open. I don’t have any lockshield valves but I adjusted the flow rates in the basement where all radiator pipes connect to a UFH manifold with individual flow control. An uncommon setup, I know, but that’s how I bought the house and it makes adjusting and seeing individual radiator flows easy. Now I’m just trying to see whether there is still room for improvement.
I would word that a bit differently. In my understanding, you cannot modulate DT in itself - the delta is a response of the heating system and depends on the power emitted by the radiators at the given flow temperature. The only thing under control of the heat pump is the flow temperature and with increasing flow temperature the DT will automatically increase as more heat is emitted by the radiators.
Yes, heat output is the same, I think the (more academic) question is whether the condenser temperature is also the same for both scenarios? Heat transfer between condenser and water depends on the delta between the two and the flow rate so that should also shift accordingly for the low/high return temperature scenarios. This sounds like a nice exercise for some coupled heat transfer numerical calculations …
