They are total tat. I said it a few times on this forum that the Honeywell zone valves are badly made inside and designed to fail. When you open a broken Honeywell valve and find it has a design fault, then they all will be the same. They start slipping inside and you hear a clicking noise. Bloody things are on unvented cylinders.
I had one fail on me last Friday. Valve came with the unvented cylinder, worked once and then wouldnt open again. It was 6pm by now and I just wanted to get home. I could have done without fitting a new motor and repairing a brand new valve!