Underfloor heating not heating

[rb1980]

Hi all . All of a sudden my ufh doesn’t get up to temp . Boiler fires up but between the boiler turning off when it reaches temp ans then the 2 minute off and then off when it reaches temperature , the return from the ufh is so cold , that the flow to the manifold never gets hot enough and hence the cycle goes on . Turned on my radiators to get the water hot and then turned on one loop of the underfloor .its only a small room maybe 8x10ft and 10ft at best from the manifold so it’s not a long run . Flow now stays hot ish but even with the pump on 3 and the flow meter showing 4ltr/ min the flow temp never gets above 45 and the return 35 , mixer valve is set to max . I know 45 is supposed to be aboht right for flow BUT I should be able to have it higher and the return should also comeback hotter which it used to do . Seems even though the flow is showing as ok there is actually some restriction somewhere .
Ideas ? Mixing valve ? Air ? Many thanks

 

[rb1980]

Just as an addition , the flow temp just before the pump is really hot so it’s much higher than the 40 odd that’s getting to the manifold

 

[Madrab]

If you can't get it any hotter and there is correct flow through the circuit and the temp into and through the pump is 60deg + from the Boiler, then it's got to be the mixing valve.

 

[rb1980]

Well up to the pump it’s hot , coming out it’s not if that makes sense..you can feel when the boiler comes on as the pipe before the pump cools a little then gets almost too hot to touch when the boiler fires up . To the right of the pump at the Manifold there is hardly any change this is the setup

 

[rb1980]

The temp at the gauge at the red valve to the right of the pump used to go as hot as can be , now it won’t get above 45 at best if thst