I figured the sensor wires would be low voltage 
After sitting overnight flow was 16.2° while return is 16.0° (I think that the ~1 K vertical temperature gradient in the compartment caused that result).
After a few of pulses of heating on, flow is 19.8° and return 20.5°; 0.7 K delta but now in a pessimistic direction 
. I’d rather have a conservative COP value than an optimistic one.
I might order a new sensor and perhaps pick the two that read closest to fit to the pump.
Makes sense - you can just put all four sensors next to each other, let them quilibrate and measure resistance with a multimeter and pick the ones that are closest together
They are, but an accidental short at low voltage can still damage the electronics. I’ve damaged plenty of things by working “live” in the past and while I’m still doing it for cheap DIY electronics projects (only on DC low voltage < 12V) I would not risk it on something as expensive as a heat pump.
On my 5KW Arotherm, the return temp sensor is reading 0.6 deg C higher than the flow temp when the compressor has stopped and the pump is running at the reduced speed.
When the compressor is running at 25% modulation, the maximum “Cooling capacity (Heat output?)” I see in the Live monitor is 1.2KW and the DT between flow and return reported in the wall unit confirms the same around 1.2 deg DT.
I’d expect to see the minimum rated output around 2.2KW and a similar 2.2 DT between flow and return temps.
The Vaillant app reports a current daily heating only COP around 3.7 (0.3 curve, 12 deg outdoor, 21 deg indoor) Always a lower COP than expected for a low curve and oversized emitters.
After finding and reading this thread, it appears the app may be underreporting due to the temp sensors.
An additional behaviour i noticed - after the first 5 mins or so of a cycle and the system water is up to temperature and the compressor modulates to its minimum, there is a period where the output drops lower, 0.4KW (which is the same as the electrical consumption, a 1-1 COP). It rises over a minute or so to stabilise around the 1.2KW for the remainder of the cycle until Energy Integral switches off.
I dont have MID meters. Would anyone with a 5KW Arotherm and metering be willing to check their Vaillant wall unit Live monitor “Cooling capacity” when stablised at 25% modulation and compare with their metered output.
Any thoughts or comments are welcome. I’m trying to understand the low COP, also if the erratic stabilising behaviour at minimum modulation is normal?
Currently waiting for a 0020269357 to be delivered. In the meantime I swapped the sensors around again as I the installer had noticed the low COP. The “energy efficiency” in the app has gone from below 2 to above 5!
Anyone had/know of an Arotherm Plus where the COP was being reported low?
My daily COP is currently 2. My energy use is ~2kW. It’s not possible my COP is 2 as it’s not possible my heat loss is 4kW at -1C (heat loss is at least 6kW). So yes.
Also a COP of 3 would tally more correctly with the charts (Czechia ones posted elsewhere) and given I have a one zone fully open system with only volumizer on return, I don’t see how my COP could be less than average expected.
Both your posts and previous confirm that sensors mismatch lead to under or over-reporting.
Since my previous post, an engineer from Vaillant has now replaced both sensors.
Before: Return sensor temp was displaying +0.6 higher than Flow (Pessimistic COP)
Now: Return sensor temp is displaying -0.3 lower than Flow (Optimistic COP)
Therefore a DT increase of +0.9 deg compared to how it was originally.
Before I was seeing a COP of around 2.6 @3.3 deg outdoor. After the sensor replacements and refitting at the same 3.3 deg outdoor temp, the COP reported jumped from around 2.6 to 4.0.
Assuming F&R temps should exactly equalise when the HP is off, pump circulating for a period and water temp stabilised?
Without MID metering, true and accurate COP etc is unknown or unproven but figures now make more sense in my case.
I asked the engineer if Vaillant had any known information about poor sensor calibration or re-fitting and if any rectification procedures, he replied no. He said it’s not something they commonly need to replace and seemed a slight reluctance. It was just by luck that he managed to find sensors in the van (despite a 5 hour return journey and being the second engineer to visit, the previous didnt have sensors in his van). 1 sensor was in a sealed packet with a clamp. The other didn’t seem to come with a clamp, looked like he had to prize off the old clamp to reuse. It’s a pity they don’t come as a calibrated or matched pair.
No resistance tests or heat clamp tests were carried out before or after, so the level of tolerance mismatch under or over is left to chance and of no interest to Vaillant to check. Left to the customer to identify.
No pipe cleaning was carried out and no thermal paste applied. Clamp fit seems not that tight and easy to rotate round the pipe.
I asked if he could cover the damaged and open insulation gaps around the sensors and pipe. Fortunately he had some Armacell tape in the van to do so.
The engineer said the same sensors are used in boilers. These issues may be less apparent with wider DT boilers. However, with narrow DT heat pumps that rely on small DT to modulate and also report sensible yield/ output figures, the mismatches are more of an issue.
A consolation from what i can see, kWh electrical consumption running costs don’t appear to be noticeably affected. Not sure if and how general running and operation may or may not be affected.
It seems my return flow sensor may well be reading incorrectly, or even both
Someone burst my COP bubble this morning and said my COP is highly likely incorrect, and far too high.
The below screenshot is for November 19th, lowest temperature was 3c.
This is for a 170m2 house, mostly built in the seventies, Arotherm 7kW Plus, 0.45 WC, Active room modulation set at 20.5c, no set back.
I have Shelly power monitoring, that reports 18.96 kWh for the day, I also have some Shelly temperature probes attached (paste, tin foil and stainless steel tie wraps all under insulation) to the flow and return pipes. These sensors are about 5 meters from the heat pump, flow currently shows 30.4c whilst Valliant shows 31c, but where can I find the return temperature in the Vaillant system, as I can’t seem to find that??
I’ve not pulled the covers off and checked the sensors, may well wait for the weather to improve.
PS. I did intend on getting OEM monitoring installed, but my installer wasn’t interested.
See Mick Walls page: Vaillant aroTHERM | DT5 | Mass Flow Rate Triangle
Where the following explains how to access:
You don’t need any fancy equipment to read the current flow and return values from your heat pump.
Flow, Return and whole host of other useful metrics can be viewed via the VWZ AI Heat Pump Controller (white box).
Press both upper buttons at the same time to get to the menus.
The default code to access the installer menu is 17
Once in there, go to Test menu and find:
Flow Temperature
Return Temperature
Below is a screen grab from the VWZ AI user manual showing some of the other metrics you can see in that test menu.
Thanks, must admit I didn’t look in the test menu.
Heat pumps been off for about 20 minutes, current power consumption is 18w
They look pretty close, need to remember the Shelly sensors are about five meters from the ASHP.
The Shelly sensors are not calibrated, but neither are the Vaillant ones.
Going by the above does it look likely the COP will be massively over?
Well, it’s tough being as badly calibrated as the Vaillant sensors. Any DS18B20 is typically closer to the truth than those particular NTC10K.
You’ve got a baseline 0.6K delta, which means you overestimate COP. The amount depends on your actual deltaT when the compressor is on and ideltaT/(deltaT-0.6K) will give you the “wrongness factor”. For example, 1.3 would indicate 30% overestimation.
Coincidentally you have +0.6 DT difference in your Vaillant sensors. I had this but the opposite way round, mine was narrowing DT by 0.6 and underreporting compared to you widening DT and over reporting. Yes knocks the figures off a sizeable amount to not make sense.
Your Shelly sensors tighter tolerance at 0.1 deg is more impressive.
What has struck me is that Heat Geek base their “guaranteed COP” off these readings. Pretty easy to exceed a guaranteed COP when the flow temperature is permanently half a degree or more warmer than the return. It’s funny how the error almost always flatters the performance of the heat pump.
@Andre_K, do you know if this is the same sensor? Zooming the image shows 10K written on the black plastic part https://www.screwfix.com/p/vaillant-0020137356-ch-flow-ch-return-ntc-sensor/365tg
I ask as I don’t think there are any 0020269357 in the UK right now. Two websites show them in stock – both are the same company (
) – and my order from over a week ago hasn’t arrived. Everywhere else shows them as out of stock.
Good point, well made!
My new sensor has arrived (I couldn’t see much value in ordering a pair) and happily it’s closer to one of the original sensors than the other original is:
Orig sensor 1: 12.62 k Ω
Orig sensor 2: 13.11 k Ω
New sensor: 12.46 k Ω
All at 19.2°C room temperature on the HTC-1 I keep in here. I’m waiting for some ice cubes to melt in a cup to try to get more controlled readings.
The sensor I ordered is this one 0020269357 Vaillant Sensor | The HeatXchange
After five minutes in a cup of water and ice (IR thermometer says 0.7°C):
Orig sensor 1: 31.3 k Ω
Orig sensor 2: 32.4 k Ω
New sensor: 31.5 k Ω
Anyone fancy testing theirs and comparing notes?
I’m afraid I don’t have my notes anymore, but I also only matched for relative accuracy between sensors, not absolute value. The values are very close to theoretical expectations, but the one “off” NTC also isn’t far and we’re not talking about massive differences here anyways.
Now that you have a matched pair remember to use lots of thermal grease and additional foam insulation around them when mounting. Looking forward to the results :).
Tbh I’ve just bunged them back on and taped the ripped insulation back down (nice build quality there Vaillant 
). It’s a metal to metal contact (the best conductors of heat) and the mass of the sensor is tiny compared to the pipe and water within, so I think there’s no issue with the sensors sensing the temperature of the pipes as long as the clip is fitted properly.
I’ve done this a few times and I’m pleased to see that the flow and return delta is now only 0.1 K, sometimes even 0.0 
.
Notably I left the pump doing nothing for a few hours after putting the new sensor in (it’s very mild in the UK today) and a delta of around 0.7 K developed, flow higher than return. Definitely need the circulation pump to be running with the compressor off and ideally the condenser at ambient to get a good read of the delta.
The heat pump is finally reporting a COP in the 3s . From installation it was 4-5; after I swapped the sensors around it was in the 2s or occasionally below 2!
If anyone wishes to test their sensors and discovers that my sensor no. 2 above is a better match for one of theirs than what they have already I’ll happily post it.
That’s a great idea. I’ll also measure my old ones at a defined temperature and offer them up.
Is there a best-practice guide for measuring the resistance?
My flow sensor reads ~0.6C higher than the return, so looking to measure and swap soon.
Ideally have the sensor tip in a circulating water bath with a calibrated probe inside to measure the reference temperature. Without a calibrated reference, it’s going to be more tricky. A well-stirred ice water bath will be close to 0°C but it’s still tough to have independent people reproduce this. Maybe we need a defined community setup along with the Vaillant Temperature Probe Swapping Bazaar …
