WUFH won't go above 20°c at manifold?

[gtxlr]

Morning all,

 

I'm looking for a bit of help with my downstairs WUFH system.

 

It's a new build house with radiators upstairs and WUFH down stairs. It's running a myson manifold with 8 loops using a grunsfos upm3 pump. Heating is via a valiant 3.5kw (I think)heat pump. Control is via 4x myson thermostats downstairs and upstairs is a vaillant sensocomfort . In the utility cupboard is a Vaillant hot water cylinder and a vr71 and vaillant control panel.

 

Now the issue I'm having is the downstairs WUFH manifold never goes above 20°c so when we turn the heating on via the myson manifold it's circulating cold water and this never heats up the flooring. The blending valve is set to 50°c and I can confirm the loops are flowing and have been bled. I'm guessing I need to raise this temperature but unsure how to proceed?

 

I'm hoping it's just a settings issue..

 

Any advice is gratefully received,

 

Chris

[JohnMo]

The issue with mixing valves on the manifold is they always mix return water with incoming warm water. So if for instance you are flowing 30 degree water to the radiators, you may be getting enough return flow mixing to drop the water to 20 deg irrespective of the knob setting.  Issue is most mixers are deigned to see 70 deg water at the inlet and have minimum predefined mixing and the knob just makes that more not less. 

 

The only ones I have seen that are any good are - IVAR low temp mixer, but that still has a predefined mix - or use an ESBE (other makes available) electronic mixer, your Vaillant may (VR71 should) already have the controls built in to control the mixing valve.  The Vaillant VR71 should be be able to let you run two WC curves one for radiators and one for the UFH. To get this function working - delete the existing mixer, install ESBE mixer, add temperature probes to piping at UFH and wire back to VR71 , split system in to two zones within the controller radiator and UFH. The set the two WC curves and then run both zones together 

[gtxlr]

Hi John,

 

Thanks for the reply.

 

The issue I'm having is the WUFH won't go above 20°c not matter what settings I try, it won't send any hot water to the manifold.

 

Hoping it's a settings issue but I wonder if the valve is stuck shut?

 

Any thoughts appreciated,

 

Chris

 

 

[Nickfromwales]

When running, does the flow pipe immediately prior to the UFH setup from the boiler feel as hot as it does going to the rads? 

[gtxlr]

On 23/09/2025 at 22:28, Nickfromwales said:

When running, does the flow pipe immediately prior to the UFH setup from the boiler feel as hot as it does going to the rads? 

I'll check tomorrow, thanks

[gtxlr]

So I Managed to grab a plumber to have a quick peak today. Turns out the flow and return pipes are on backwards so the pump from the cylinder is fighting the pump for the UFH....

 

He's going to come back in a few days to switch them round.

 

Thanks for your help guys

[Wil]

Hah, my builder’s  plumber managed this on our recent UFH install too. Fortunately a quick swap because of his other poor work. You do worry about some trades…

 

At least it’s an easy fix 👍

[gtxlr]

8 hours ago, Wil said:

Hah, my builder’s  plumber managed this on our recent UFH install too. Fortunately a quick swap because of his other poor work. You do worry about some trades…

 

At least it’s an easy fix 👍

It is worrying that these things aren't checked before signing it off.. ah well at least we've got to the bottom of it now.

 

Just gotta live with a cold downstairs for the weekend 🤦