JSlippy

Thanks for the responses. I'm fairly sure the mixing valve is connected correctly as it functions correctly 95% of the time. The temperature of water going into the manifold changes based on the blending valve setting and is much lower than the boiler temp (about 60 degrees C), so it must be blended somehow. I've also reviewed installation instructions to check this and it looks right. The pump is a Grundfos UPS3. The supplier has advised me to switch from the UFH 1 to UFH 2 setting to solve the problem, I haven't been able to confirm whether this has worked yet. I can't pinpoint exactly when the problem occurs. We only notice it when one of our zones requests heat and it doesn't get any, but I suspect it could be locking earlier than that and simply remaining locked when that zone opens. I'm not sure about the UFH flow temp. It's set to 4 out of 6 and seems to be around 40 degrees. Boiler flow temp is 60 degrees. The installers flushed the system to get rid of the air and I regularly open the air release cap on the manifold as a check, no air comes out. I can't say for sure whether there's any air left in the system somewhere but I'll keep this in mind as a potential issue.

We're having some issues with the UFH system that we've recently had installed and hoping for some advice on what could be going wrong. We have our mixing valve set to 4 out of 6 (on gas central heating) as we've got LVT floors which need to be protected from sudden temperature rises. Occasionally the mixing valve seems to close entirely, stopping flow to all loops. When I inspect the system the return temperature from the manifold is high (about 40 degrees C). To fix it I need to turn the mixing valve up to 6, which suddenly releases flow through the manifold and loops. This causes the manifold return temperate to drop rapidly to about 20 degrees as the cold water in the loops flows through the system. I can then turn the valve back down to 4 and everything works fine. The only explanation to me is that when the return temp from the manifold spikes, the mixing valve closes completely. The mixing valve manufacturer/distributor Essco tell me this shouldn't happen, the valve should only close if flow drops as a scald protection. But I don't know how else to explain the fact that turning the valve to max temp immediately releases everything. I can get our installers in but they are plumbers, not UFH specialists, and in any case I don't see that there is anything wrong with the installation. I'm trying to get support from Thermrite who sold us the package, but they're not being helpful so far. If somebody could suggest what might be happening and what the solution might be it would be greatly appreciated!