UFH blending valve reduces flow

Hi guys,

I'm having trouble with a newly installed UFH system. There are 2 manifolds, one with 5 loops, the other with 9. Both are made by Herz, model no 1732007. Pump is a Wilo Yonic Para something something. Boiler is Vaillant Elo Block 14.

My trouble is that setting the TMV to its minimum value I get flow as indicated by the red flow meters. Turning the TMV knob above a certain value reduces the flow to 0. Pump is set to constant flow (?), number 3 of 6 on the scale. I tried removing all the thermo actuators and there's no difference. Flow meters are set to wide open, but it seems it only wants to move water around the loops if the TMV is off.

Both groups have the exact same behavior. My uneducated guess is that something is connected incorrectly, but the flow pipe from the boiler is connected to the input of the blending valve body and it does get warm when water gets into the manifold. Problem is the flow is so restricted that it doesn't go into the loops.

Not sure what to do/try to debug. I added pictures of the setup. Please ignore the messy wiring. It's still WIP to organize. The super thin wires are voltage free connections that are used to detect when the zones are calling for heat and send that to the boiler.

Is this a DIY job then?
If not why not get company back?
Is everything flowing in the right direction, heating F&R for example?

It isn't DIY. Bought the system years ago and it was installed by the company. The waranty expired because I was building the house myself over the years. The wiring is made by me, but the electrical part is working fine.

Not sure if it's flowing in the right direction. They may have plumbed it wrong. How would I test that?

If it's less than (from memory) seven years since they installed the system and this is the first opportunity you have had to discover the problems then they may still be liable for fixing the faults; check the position with Consumers' Advice or your lawyer.

In any event, even if you have to pay something for the privilege, my first move would be to get the original installer back. I'd hope that they'll be able to figure out what's wrong faster than someone with eyes-on for the first time.

When boiler fires, one of the pipes coming into bottom of blending valve will warm before the other.
The difference will be exaggerated if UFH is calling for heat too.
The valve will be labeled for flow & return on valve body.

Can you post a photo, to show something like the below.

OK, saw your one now.

The pipe, on the right, IMO, should be the hot from the boiler.

Hi all,

Thanks for the replies. I just want to document what I had to do in order to get usable heat out of the whole thing.

First of all, nothing was broken in the UFH portion. I suspect the pipe from the boiler to the manifolds might have a blockage that's restricting flow but I'm not certain. I'll confirm this summer.

I removed the mixing portion of the manifold and it was all fine. It's just a shuttle valve with a spring. Didn't even need to clean it.

What ended up working was I adjusted the brass piece on the blending valve so that the thermostatic head (the adjustable white thing that has numbers on it) can't fully close the blending valve (by shortening the effective stroke of the valve pin). I heard other models have an allen key adjustment.
As a result, the heat that goes into the manifold rises to the setpoint, then the thermostatic head starts to close the blending valve a bit so I get flow through the pipes, the heat in the manifold drops slightly and the head lets more heat go in. So instead of getting either heat with low flow or large flow with no heat, I get a hysteretic action (a variation of about 5 degrees in temperature over a few 10s of seconds).

In short I got ambient 25C starting from 19C in about 3 or 4 hours. The water in the pipes never exceeds 40C which is fine for my screed floor.

Next summer I'll try to investigate in more detail and I'll post the results (if any) so that other people searching for this topic have something to go on.

Well done, an elegant solution.

Very interesting, one would think that when the UFH is off and cold that the TMV shuttle would be in a position to allow the hot water from the boiler to be boosted by the maniflod pump and return it all back to the boiler, so maybe too much differential pressure across the shuttle to allow it to move, but if so then you would think that the manifold temperature would rise to the boiler temperature. Your findings seem to suggest that the shuttle was in the fully cold position and couldn/t move until your adjustment allowed some hot water through, so maybe this reduced the differential pressure across the shuttle and then allowed it to move?.
Below is a example of a TMV (mira).

I see the Wilo (manifold) pump is set to constant pressure (CP), any idea what its setting is?, some of these pumps flash the setting on start up and then revert to displaying the power in watts (W).
What make is the boiler circ pump and setting, if available??.

Could it be the Pump.
Isn't there an irregular occurrence of the pump spindle and the impeller parting ways.
Or the impeller not spinning at the same revolutions of the spindle.

The only way to check is pull the pump body from off the pump head

The mixed temp is fine at 40C so needs the manifold pump to be OK for this?