Hi, I have an old Ferroli Modena combi serving a 1 bed flat. Never any problems. I’ve just installed underfloor heating. It was working fine whilst I had the ufh and a last temporary rad connected up. I’ve just removed the last rad and now I’m having a problem with kettling. The manifold has a mixing valve and pump. There is only 1 zone, very simple, (no zone valves), so I’ve wired the boiler pump and the manifold pump to run continuously whilst the nest thermostat calls for heat to always have flow around the system. Pump also has an overun. Set both the boiler stat and mixing valve to 40 degrees. I’ve fitted an auto bypass under the boiler. The boiler keeps kettling briefly (very loudly) before its internal stat switches off. The ufh is warming up nicely and the gauge reads 35 degrees. The bypass is on its maximum setting, but nothing flows through it (it stays cold even when the kettling starts). I’ve had it on for hours fiddling, and the return feed to the boiler feels cold to the touch, whilst the supply is very hot, so definitely nothing going through the bypass at any point. If I turn the boiler stat down anywhere below 40 degrees, it cycles on for maybe a minute, then switches off completely and doesn’t seem to want to come back on even though the pumps are both still circulating. I’ve sat here waiting about an hour and it still hasnt come back on unless i turn it up above 40. Baffled. Could the auto bypass be faulty? Do old boilers not like being set to less than 40 degrees? Have a feeling the mixing valve must be allowing just enough flow to stop the bypass working, but restricting it just enough to kettle the boiler. I’m considering replacing the auto bypass with a gate valve and balancing it out manually. Any thoughts?