Headscratcher

Thanks. Found the issue. Flow and return piped incorrectly on mixer... see my response to ProDave. thanks for your help Team DIY.

Spot on. I have just removed the mixing valve and the pump to learn that the plumber installed the boiler flow into the boiler return... so annoyed. I have been battling with this issue for ages and was assured on numerous times it had been done correctly. The indicators are on the bottom of the valve so without removing it, t’was hard to see. thanks for your help. I now somehow have to reconfigure the pipes which is going to look super untidy.

I don’t think there is an over temp stat installed

Not a great photo unfortunately.

The pump keeps running. I am completely bamboozled. I think it might be an issue with the blending valve.

Flow and return are plumbed in correctly and UFH works as it should once i have done my routine with the blender valve of lowering the temperature and then gradually increasing it... I have checked the pump direction and flow and return from boiler and all is installed correctly.

I have had a wet system installed in my house. 5 port manifold with IBO pump and blending valve. 2 port valve installed to allow the UFH to run on it's own circulate with one radiator linked to it to deal with excess boiler heat. I have a combi boiler. Problem is: when the thermostat calls for heat everything is initially fine.. 2 port valve opens, UFH pump kicks in and heat flows from boiler through blender valve... all going well after about 2 mins, the flow metres start to show no flow and the temperature in the manifold drops, with no hot water being mixed. If I then reduce the temperature to 30 degrees on blender valve, the water temperature ramps up in the manifold and I can then gradually reset it to 45 degrees and all is okay. Without this self adjustment the water in the UFH stays at around 20. Thoughts?