UFH Mixing Valve Issue

[JSlippy]

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!

[ProDave]

Are you SURE they connected the mixing valve correctly?

[JohnMo]

Any photos?

What pump are you using?

How many zones, does this occur in a steady state (ones all open) or when zones are opening closing etc

What is the flow temp of the UFH and what is the supplied temperature from the boiler? Looking at the manual for a T4 mixer that should be 41 degrees

[Triassic]

Is there air in the system. Air can course air locks, which stop the flow in some cases. You opening the valve wide suggest trapped air.

[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.

[Jamieh1987]

Very similar symptoms to my system before I realised the blending valve was the wrong way around (note the H and C on the pic). Worked fine for months surprisingly and then suddenly started locking the boiler out and just circulating water already in the system until I opened the valve fully. Probably isn’t your cause if you’re sure it’s been installed correctly but worth double checking. I swapped the valve around and it’s been fine since, although it’s annoying the temperature marking on the valve now faces the wall 😄.

[dpmiller]

you can turn the knob and index round- note the setting, then remove the screw from the centre of the knob. It and the index ring should then pull off, so you can then reposition and re-assemble to suit.