Vaillant inbuilt monitoring vs MID meters

I’m visiting this particular system at the moment, and there’s no visible sign of sensors on the internal pipework. I assume they’re buried somewhere in the external unit?

Also, wondering if the Vaillant attempts to maintain a particular dT? Would an under-reporting return sensor (so over-inflated dT) cause the unit to behave differently? I’m seeing persistent cycling here, and rarely any steady state running.

The sensors are in the outdoor unit:

The Vaillant does not try for a certain dT but rather aims for a specific flow temperature. If the current flow temperature is above the target value, an energy integral is counted up and the compressor turns off when it reaches zero. As soon as flow temperature drops below target, the integral decreases and when a target integral value (default -60°min) is met the compressor turns on again. So a flow temperature error would lead to potentially erratic regulation.

1 Like

But an inaccurate observed dT would still result in an inaccurate reported COP, wouldn’t it?

yes, which is what various people see. Some over and some under. My dT is under-reported and so is my COP.

Thanks for confirming.

Such a shame that Vaillant is saving money at the cost of their customer satisfaction and brand reputation to save literally a euro worth of better sensors out of a multi-thousand euro sale price.

They could easily have saved more elsewhere. (For example the removable all-metal front of the uniTOWER.)

Unfortunately, good sensors don’t sell devices, as opposed to shiny metal fronts. I’m sure some beancounter sat down to calculate that replacing the sensors for the few people who care is cheaper than having good sensors in the first place. My pump significantly overestimates COP so if I trusted those COP figures I would tell everyone how efficiently my Vaillant heat pump works! Finally the aroTherm plus are marketed to achieve high flow temperatures for old houses; whicht comes also with high dT. At 10K dT noone even cares about a 1K error, much less a 0.5K error. It’s us nerds with the super optimized flow temperatures that see and care about those inaccuracies :wink:.

1 Like

It is possible that the sensors are “good” and it’s just the installation of them is poor and inconsistant.

Sure, even the best sensor needs proper installation and calibration. For me, the sensors don’t even equilibrate after 8 hours of the compressor being off. That really points towards a bad sensor quality or bad calibration. At that point they should show the same temperature even dangling in free air.

Where from? Do you have a link? Is that a photo of them?

I ordered them from here (site in German only it appears) where you can customize the sensor to your liking.

Rohranlegefühler mit Edelstahlhülse (

I have two PT1000, 4-wire, waterproof (for calibration in a water bath) version with 1/10 DIN accuracy which costs around 25€ apiece (depending on cable length and insulation type). They come with proper fastening clamps and I put thermal grease (Arctic Mx5) under them. They are properly thermally isolated by the Rockwool insulation around the pipes.

I read them with two Adafruit MAX31865 boards I bought off Amazon):

Adafruit PT1000 RTD Temperature Sensor Amplifier - MAX31865 : ID 3648 : Adafruit Industries, Unique & fun DIY electronics and kits

The sensor boards are connected via SPI to a NodeMCU ESP32 where I am doing some custom processing and averaging and then push the data via MQTT to emoncms (happy to share code/schematics if desired).

The sensors were extremely close in measurements out of the box (<0.05K difference over the whole range from 0 to 70°C). I still put them in an ice bath, put my sous vide in there and gradually raised the temperature in 5°C steps from 20°C to 70°C. Based on the datapoints from the temperature plateaus I calculated a 2nd-order correction function to match up both sensors over the whole range, i.e. only a relative correction since I don’t have an accurate calibrated temperature probe as ground truth.


@glyn.hudson and I have been involved in a remedial project locally, a poorly performing LG high temp heatpump has been replaced with a Vaillant arotherm, the good news is that the COP has improved from 2.2 with the LG to 4.8 so far with the Vaillant :grin:

It’s been interesting comparing the Vaillant app figures to the class 1 electric and class 2 heat meter that we provided to the installers. Here’s a screenshot from the app:

and comparison with electric and heat meter values:

Vaillant app Class 1 electric & Class 2 heat meter Error
Date Electric Space Heat DHW Combined COP Electric Combined COP Electric Heat
29/03/24 30.4 175.1 7.9 183 6.0 30.1 144.8 4.8 1.12% 26%
30/03/24 21.9 100.1 16.9 117 5.3 21.9 94.4 4.3 -0.15% 24%
31/03/24 14.4 91.1 1.8 92.9 6.5 14.0 73.3 5.3 3.20% 27%
01/04/24 19 107.7 5.7 113.4 6.0 19.0 90.9 4.8 0.18% 25%
02/04/24 13.3 77.4 6.3 83.7 6.3 13.4 67.0 5.0 -0.46% 25%

The electric values are pretty good but looks like a temperature offset error is pushing up those heat values again. This has to be a relatively easy one for Vaillant to fix.

@glyn.hudson got some good pictures of the temperature sensors I think on his parents system, they look like they are in pockets and so should be good but perhaps they are just not calibrated pairs??


Here’s what I think are the temp sensors, they look like they are in pockets with two red cables attached:

It looks like one of these Vaillant sensors, the same as they use for boilers (part No. 2415622). Can anyone confirm that this is the exact part? Since it’s an OEM part I can’t find any spec data:

Here’s the flow sensor:


It’s hard to see how pocket sensors could produce errors as big as I’m seeing…

30 day COP of 6.55 doesn’t seem believable, and the trend doesn’t match the outdoor temperature like others do. MyVaillant is overestimating heat by about a third, but not consistently.

Could there be air or something messing with the flow sensor? Is there a setting within Vaillant to tell it about glycol?


I guess it’s possible that sensors in pockets could be very poorly calibrated.

Can you compare the flow sensor reading with the flow reading from your heat meter? The Vaillant flow meter looks like a Vortex for meter which are usually pretty good in my experience.

Not that I’m aware of. Have you got glycol?

No heat meter on this system (my father’s).

Yes. If the system expects pure water, then it would assume a higher specific heat capacity, and therefore over-estimate the heat delivered?

Looks just like my system.The inconsistent variation with temperature comes from two points in my opinion:

  • Fixed dT error: The temperature measurement offset error is fixed and does not vary with flow temperature as opposed to the true dT which increases when flow temperature increases (Vaillant uses fixed flow rates).
  • Compressor cycling: Between cycles, when only electric energy from pumps/standby is accumulated there is no heat energy error. Say you have a true COP of 4. During a cycle you utilize 1 kWh electrical energy and get 4 kWh heat. Vaillant overestimates by say 25% and displays 5 kWh heat and a COP of 5. Between cycles only the pumps are active and for sake of this example you accumulate 0.3 kWh electrical just from the pumps. Vaillant includes this in its COP figures and will display 5/1.3=3.8 as COP. So at outdoor temperatures when the pump is not cycling, you will see more of the expected temperature dependence of COP, whereas this levels out when outdoor temperature forces the pump to cycle so that pure pump power becomes a significant fraction of overall power used. For my system at warm outdoor temperatures, this leads to a COP reduction from 6.5 just looking at the compressor cycle to 4.8 including the pump power between cycles.

This doesn’t appear to be systematic enough to be malicious in any shape or form - just marginally viable components without enough of a disclaimer on the instructions.

A boggo uncalibrated NTC is “good enough” for setting a flow temperature (any error you just move the heat curve for that specific unit) and “good enough” for deciding when to start and stop the compressor.

You can try to estimate COP with these and the result is “good enough” for checking that there’s not something catastrophically wrong but not enough to hit the last 20%.

20% price point 80% function?

L0 monitoring = Vaillant estimate
L1 monitoring = eBus with zero offset in the calc (run heat pump circ for long enough with compressor off to establish zero offset then use this in yield calcs)
L2 monitoring = eBus elec/flowrate and direct-immersion onewire sensors
L3 monitoring = MID elec/heat?

1 Like