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Thread: HR91 vs HR92 with Danfoss RAS-C2

  1. #1
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    Default HR91 vs HR92 with Danfoss RAS-C2

    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.

  2. #2
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    Default

    As a follow up to this, I've had to modify the pin length on all of my remaining HR91 to get them to operate correctly. Even the radiators which I initially thought were working, were only actually opening at 70%+ heat demand, and even then, not fully. Any lower, and the valve was completely closed.

    I got the idea from https://randomtechnical.blogspot.com...ss-valves.html, but went with a slightly different approach, which I think is less risky. If you unscrew the entire adapter (white and black piece together), it's possible to push the metal cylinder/bar out of the black piece - it's just friction fitted with the larger diameter section keeping it from sliding out. I then just used a metal file to slowly reduce the length of the end which touches the TRV pin.

    It took several remove/file/test iterations to get the performance I wanted, but on two of my radiators, the metal needed to be filed back to be completely flush with the black piece (but don't jump straight to that extreme). Now, I get them completely no-flow when no heat demand, whooshing noises when low single-digit heat demand (i.e. high velocity due to the tiny opening) and fully open at 100%. No idea how accurate the intermediate steps are, especially given that the TRV isn't linear.

    The behaviour I observed when the pin was still slightly too long (i.e. sticking down approx 1mm from the black piece) was the valve would open to a heat demand of say 8%, radiator stays cold, and it would gradually open more until eventually the heat would start coming through.

    Both the HR92 I bought work perfectly on full stroke.

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