I tried the quiet mode, but it does not prevent the initial ramp as the peak value is below the quiet mode cutoff. For my 10 kW unit, minimum electrical power is around 1 kW and at the start of a cycle I’m seeing 1.5 kW, which is even less than the 60% reduced power of the quiet mode (1.8 kW).
Hi,
Like @Andre_K explained it, it’s the Energy integral who switch off and on the compressor.
I lowered my hysteresis to 5K and saw no change. The only way to have long cycle is to get the heat out in the house so you’re return is at the lowest temp possible and you’re flow temp is spot on the desired flow temp so energy integral is not “feeded”. the flow rate can help a lot for that , set it to the max so you’re flow temp can be lowered a lot. “the higher the flow rate, the lowest the flow temp”
My 5 kW flow rate is on 75% and I get a 2°-2.5° DT, it’s now a cycle of more than 4 days long Emoncms - app view
This depends on your system. If you have low-volume radiator based system then hysteresis can be the primary mechanism and energy integral not even come into the picture.
Why would you lower hysteresis, this may result in shorter cycles. Was room temp fluctuating too much?
I do the same. I have NR mode on 24/7 which limits compressor to 58% (70rps).
With the 7kW unit it starts the cycle at 50rps always, then reduces to 30rps if demand is low. 50rps is around 1kW, whereas 30rps is just below 700W. So there is a peak, at start of cycle but it’s not very high and still below my NR setting
Let me rephrase this: The EnergyIntegral approach ensures that the time-average flow temperature adheres to the setpoint. If the hysteresis kicks in, there’s no such guarantee, hence the hysteresis is an inferior secondary control mechanism.
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@Ulric and @Andre_K and all the rest: Tnx for all the info… I am digesting and trying to make sense out of it. A lot of things to read into and experiment with. Tnx for the pointers 
Sorry, but what is NR mode ???
Noise Reduction / silent mode which limits the max compressor speed.
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Thank you Sarah for reply, I have analysed further.
I am wondering whether the EBUS definition is correct. It can’t be the EEV outlettemperature I would say. In below graph, the compressor is ramping up, flow and returntemperatures up, high pressure up, low pressure down. But EEV outlettemperature up? How can that be… Am I the only one seeing these strange EEV outlettemperatures? maybe it is the temperature of the bearing of the compressor. According to the manual there would be a temperature gauge at the evaparator inlet, not so much directly after the EEV outlet. I have a arotherm plus, not a split unit, not sure whether that might matter for use of the codes. If I would have such a bad eev outlet sensors, it is hard to imagine that the whole thing actually works.
What is meant by Vaillant with EEVOutletTemperature? I think their heatpumps have suction temperaure sensor just before the compressor? Have you got refrigerant cycle schematic for your model with sensors designation? By rule they should have termistors just before EEV, after EEV on the evaporator coil and on the suction line of the compressor. Which one of them is this reading for?
These are screenshots from three different Vaillant heatpumps from HeatpumpMonitor.org:
On every one of them you can see a sudden dive of COP just after ramping down the compressor after reaching flow water setpoint temperature. Do you have the same anomaly on your Vaillant heatpumps and can somebody log the EEV position on a cycle when this anomaly occure?
This is from the last hour of Heat Geek`s Vaillant:
Hi everyone,
I am Philipp. This is a great thread and I have already learnt a lot here. I would like to join in and hope to get some help or different perspectives, because I am stuck.
Some questions first before I bore you with the details:
What could be the cause of our bad COP, despite always low flow T and long runtimes of the ASHP?
What could be the reason for a low dT (~1-1.5K) and does this matter for efficiency?
Should the flow rate for the Aerotherms always be the max. (i.e. 850l/h for 5kW, 1205l/h for 7kW unit)?
Could the runtimes actually be too long, and/or the flow T too low?
Could the Vaillant data or internal T measurements just be completely wrong?
Could just be something wrong with the ASHP, despite just having had the annual service?
We have a Vaillant aerotherm plus 7kW installed in March 2023. I didn’t know of the heat pump monitor unfortunately, otherwise I would have chosen an installer that builds this in and guaranteed me a min. SCOP.
Reading the Vaillant data, the SCOP for the first year has only been 2.4.
Until recently, in September 2024, after some insisting, the installer came back and re-piped the then 4-pipe-buffer tank (40L Vaillant) to be a volumiser (in and out). Since then, the COP has been around 3. With the data from the Vaillant app and some extrapolation, I predict the SCOP would also be around 3 now, maximum 3.3.
So a great improvement, looking at the previous SCOP but still not at all satisfying compared to what is possible.
What is bugging me is that I really don’t understand, why this is. I do understand that the ASHP is very likely oversized, but not exactly how that explains the bad COP. That’s why I have not completely lost hope yet to improve the SCOP further.
The heat pump is certainly not short-cycling and using a very low heat curve, the flow T are also on the low end. The unit runs quite long, at low flow T and low compressor speed, actually. Only that it then just doesn’t produce that much heat. The dT is quite low, usually 1-2K only.
Heat curve: 0.4-0.5 (currently 0.5)
Room T mode: inactive (pure weather compensation)
Building Flow rate before buffer-change: 1205l/h / build. pump setting: Auto
Building Flow rate after buffer-change: 990l/h / build. pump setting: 50% / some valve was turned down throttling flow.
I am currently experimenting with the flow rate and pump settings.
My installer says our house would need 650-750l/h flow and I would then get better dT and better efficiency. We tried that by throttling the radiator lock-shield valves and got the flow way down there (pump set to 50%, i.e. min. possible setting). Had that for a few hours. dT I think went closer to 3K. But COP was worse in that time. UrbanPlumber via Youtube comment told me that I shouldn’t do that, so I opened all valves and set pump back to Auto. Tried it again later at around 860l/h with little to no effect to COP.
Currently, Building Flow rate is back to 1205l/h / build. pump setting: Auto, pump running at 61% during heating. COP still not good.
I wonder, if the ASHP just runs best at the max. flow rate. Assuming that somehow heat transfer in the house is better at lower flow rates (e.g. better dT?) , then perhaps the 990l/h have been sort of a compromise between those different optima, i.e. going slower makes it worse for the ASHP, going faster makes it worse for the house. Does that make sense?
If the flow rate is so important, then I wonder how the units can work in any house efficiently that isn’t exactly right for the heat pump size?
When the compressor is shut down after heating, the unit keeps the building circuit pump on at a much lower flow rate. I haven’t seen that behaviour on any other Vaillant aerotherm data I have been browsing through on heatpumpmonitor. Has anyone else noticed this behaviour of their aerotherm+?
Does anyone else notice that the ASHP does a rather loud hissing sound?
I assume that is the refrigerant rapidly expanding, but it does seem really loud to me.
House:
1950s Bungalow in Dorset, semi-detached, around 85sqm, 3 Bedrooms, living room is an extension built in 2008. Cavity wall insulation, loft insulated.
Previous gas usage with room T around 19-21C around 10,000kWh per year.
Degree days rule-of-thumb I think gave me a heat loss of around 3.5kW. Installer survey was 5.5kW.
Heating usually set to 20C. Currently timed with setback of 17.5C at night and during working hours. The house never cools down below 19C anyway during those times.
Heating:
Vaillant aerotherm plus 7kW
2m outside pipes to room with the cylinder (Mixergy) and up into the loft to the volumiser. 28mm primary pipes between ASHP, cylinder and volumiser. From the volumiser 22m plastic pipes over the joists in the loft, down as 15mm copper pipes to each radiator. All new pipework, specifically installed for the ASHP. All new radiators as well!
The installer piped the volumiser on the flow side, whereas all schematics put it on the return. I assume it doen’t really matter where that is?
Days in operation: 625 days
Compressor hours: 5144 hours
Compressor starts: 4110
Average runtime per start (5144 hours/ 4110): 1.25 h/start
Building pump hours: 8651 hours
Building pump starts: 782
4-port Valve hours: 11 hours
4-port Valve operations: 153
Fan operating hours: 5314 hours
Fan starts: 4296
EEV steps (El. Expansion valve steps): 43431
No. switch ops: 15
Thanks all!
Philipp
Hi Philipp,
a warm welcome to the forum.
I suggest you look through this post and the whole thread. Vaillant’s temperature sensors for flow/return are sometimes faulty so in order to establish that you really have a COP problem you need to ensure that the sensors are OK.
This is normal behavior that I also see on my unit.
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Hi Ivailo,
thank you for the response. Please find the schematic below, though in german. There is no temperature sensor indicated just after the EEV, only at the evaporator entry. The ebus decoding files connect the message codes to names. Somebody must have given the name EEVoutlettemperature. I wonder whether that is correct, that is why I am asking.
If we follow the logic this should be the temperature for Sensor 28 - Temperatursensor am Verdampfer. Watching the logs this is most likely to be the readings of Sensor 6 - they are slightly above outdoor temperature - so we have 2-3 degrees useful superheat and follow the rise of high pressure curve…I can not clearly see if there is change in EEV possition. Can you find separate readings for Sensor 28 - most likely should be Evaporator temperature?
It can depend what Room Temp Mod mode you’re in and whether it’s Energy Integral that’s stopped the cycle or whether it’s Room Temp Mod Expanded to that stopped it.
I’ve been playing with Expanded and found this behaviour.
The pump will continue to overrun between cycles that have been stopped by Energy Integral (red circles), but the pump will stop if the cycle has been halted by the temperature (blue cirlces).
In this case, the setback of 19C kicked in, so the room temp was way over target temp.
Here below is Room Temp Mod: Active (and i’m assuming Inactive does the same, i need to test), the pump overrun continues, even during setback, like this.
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That’s very interesting to see in detail. During those “heating: compressor shutdown” phases, I think there must be some kind of heat transfer though, right? The flow T is higher than room T and it circulates through the emitters, so it will lose some heat into the house. I assume, the calculation however only uses dT and since flow and return T are the same it under-reports those gains? Perhaps it’s like you don’t feel gravity when you’re falling 
Also, the Vaillant app often shows energy yield during times with no energy consumption, and I thought the app was taking these pump-only phases into account.
While the compressor is off, there’s no heat being produced by the heatpump.
Previously generated heat may continue to transfer from the heating circuit into the rooms.
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To confirm after doing some more testing of Room Temp Mod modes
-
Inactive: Circulation Pump will overrun non-stop between cycles caused by Energy Integral
-
Active: Circulation Pump will overrun non-stop between cycles caused by Energy Integral
-
Expanded: Circulation Pump will overrun non-stop between cycles caused by Energy Integral but Pump DOES NOT run between cycles when indoor target temperature is met. Circulation pump does not fire up again until start of next cycle (ie, room temp is 0.1875C lower than target)
Only other trigger I can think of to turn the circulation pump off and on would be the outside temp threshold?
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