I thought I would share my experience with evohome and Danfoss RAS-C2 valves (new valves, installed 2022).

Before investing in evohome, I had read about compatibility issues between the HR91 and the RAS-C2, but based on the information on Honeywell's website, was led to believe that they are now fully compatible; at least when the Danfoss adapter is fitted.

Some people reported radiators not getting hot at all, but my problems were not as serious as that. In my case, the system "worked", but the valves are not being opened all the way, even when a zone is under 100% heat demand as indicated on the controller. If I remove the HR91 head, there will be approx 1 turn of the black wheel remaining. Given that my largest radiator is at the far end of the pipework, and requires the lock-shield valve to be fully open, this is problematic. The only reason I investigated further is because although the heating performance of those radiators was fine, I was struggling to get air cleared out of the pipes following a re-fill. Water would gurgle past and bang and clunk, but the air remained trapped. With the valve fully opened manually, the air rushed into the radiator and could be bled off as normal.

I ended up replacing the HR91 with HR92 on the two furthest radiators. On Stroke 1 (full stroke) setting of the HR92, the valve is fully open, as expected. I'm not overly concerned about the first radiators on the loop, as they're permanently restricted by the lock-shield setting.

I would say that the HR91 is NOT compatible with the RAS-C2 valves. It's disappointing that there isn't even a hidden option to enable full stroke on the HR91, as can be done with the HR92. It's a shame, because I chose the HR91 specifically because I find the HR92 ugly; and the way that you access the battery compartment by ripping off the rotating knob is pretty naff for the more "premium" product.