Just How Inaccurate is Vaillant's COP?
I posted around 9 months ago about COP being 2.1. Since then I've had the installer and manufacturer (Vaillant) out quite a few times to fix various issues. They've told me to ignore COP as Vaillant is very inaccurate and the only way to really know is to (pay them) to install an external piece of kit.
Basically, I want to know how accurate all that is or whether I'm getting fobbed off?
I have near 12 months of data now, however, there does seem to have been some technical issues with the Vaillant heat pump. These were only addressed in June, so I don't know how relevant the data before is. And, over the past year there have been quite a few setting changes.
Currently only using DHW and only for the sinks and occasional bath (not for the shower), so not as efficient as heating. This doesn't paint a full picture, but I want to get ready for the next cold spell so I know what to look out for.
Post-fix COP for July was 1.69 and for August so far is 1.57.
I've now cross-checked the app's electricity consumption against my Octopus smart meter twice. They match within a few percent. So the electricity side isn't the problem.
The setup:
Vaillant aroTHERM plus 5kW (VWL 55/6, R290)
sensoCOMFORT VRC 720f/3 controller + VWZ AI interface
1930s end-terrace, 3-bedroom, Edinburgh
Heat loss: 4.78 kW at -3.5C design temp (86 m², 56 W/m²)
Filled cavity walls (U=0.45), double glazed, 100mm loft insulation, solid floor insulated
Radiators: 2 replaced, 4 added, 6 retained (all rooms meet 100%+ demand at 50C flow)
Heat curve 0.70, design flow 50C, max 55C
Vaillant 200L pre-plumbed cylinder
DHW target 48C, cylinder charging offset 7K
Backup heater off (bivalence -30C)
Quoted SCOP at 50C: 3.41
The timeline:
Sep-Dec 2025: Heating COP 2.29, DHW COP 1.36. Already well below the 3.41 SCOP and what others here report.
Jan-May 2026: DHW COP slid further to 1.09-1.23. Environment energy ratio in the data was near zero — the compressor was basically acting as a resistive heater.
Jun 1: Vaillant engineer visited, tested heating at COP 1.88. Said nothing was wrong.
Jun 5-15: COP got worse after the visit — DHW dropped to 0.37-0.93.
Jun 16: Second engineer visit. Energy counter was reset, a new device serial appeared in the CSV data.
Jun 17 onwards: DHW COP jumped to ~2.1-2.2 on normal days. Environment energy ratio came back to normal levels.
Jul-Aug 2026: DHW COP averaging 2.15, best normal day 2.90. Heating test in late June gave COP 2.65-2.80 at 17-21C outdoor.
u/GFoxtrot posted in my original thread with the same 5kW Vaillant in a similar 1930s property — COP of 4.1 combined in November. Even post-fix, I'm nowhere near that, despite a similar heat loss (~6 kW vs my 4.78 kW).
For those with the same unit, how far off are your Vaillant app COP figures from independently measured numbers?
And has anyone else been told the app is 'very inaccurate' — is that a known thing or just a way to deflect?