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UFH with Capillary Type Mixing Valve

Discuss UFH with Capillary Type Mixing Valve in the Water Underfloor Heating Advice area at Plumbers Forums


Do people with this type of mixed flow temperature control have any problems with achieving the required manifold flow temperature?

The one below I think is similar to the TIO Mixing unit.

In the unit below, the hot (boiler) water enters from the bottom left, red arrow, up through the "mixing headwork" which is a spring (to open) operated valve with actuator head controlling the opening of the valve via a capillary sensor attached to the flow manifold, the other liquid filled end of the capillary tube then positions the HW valve as required, this HW then mixes with the UFH manifold return temperature (marked System Return) to give the required manifold temperature, the reamining water then returns to the boiler via the bottom right side, marked "Primary Circuit Return"

Someone on another site can only achieve a mixed flow temperature of 35C despite a HW inlet temp of 54C, the readings are, (10 Loop) manifold flow/return temps 35C/32C, manifold flowrate 28.95LPM, ( by calc) 3.95LPM at 54C C is mixing with 25.0LPM at 32C to give a mixed flow of 28.95LPM at 35C and a UFH output of 6.06kw. The manifold temperature control is set to 45C.

It seems to me that the recirculated (System Return) water is uncontrolled so difficult to understand how this system actually works?.

The bypass valve is used to increase the boiler return flowrate & temperature.

Last edited:
That’s an injector type needs high flow temp coming into the manifold eg 65-70dc
Yes - the recommendation above is accurate for your system. This system relies on a higher temperature difference to effectively mix and achieve the desired manifold temperature. Your current issue indicates the water isn't hot enough to meet the desired 45°C setting. The efficiency of the system is also likely affected by how the return water is managed, which could be disrupting the intended mixing process. To improve this, adjust your boiler settings to supply hotter water and inspect the system for any obstructions or setup issues that might hinder proper water flow and mixing. This adjustment should help achieve a more consistent and desired manifold temperature.

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