Auto bypass required ?

[skeltonmfc]

Hi Guys

I had UFH added to my extension, my system was a Baxi 30he with CH and unvented cylinder.

The UFH has never really worked correctly first of all I noticed they had install the flow and return the wrong side of the circulation pump which I rectified myself as they have disappeared on me. The UFH now works but I have noticed an issue that if the CH is not running the boiler cuts out if the UFH is calling for heat. If the CH is running then the UFH runs fine, I’ve been searching online for diagrams


Do I need an auto bypass valve on the flow and return after the circulation pump ? At the moment the circulation pump is in the loft and the boiler on the ground floor about 8m away and all my rads have TRV fitted?

 

[John.g]

What temperature is the boiler set to, the UFH manifold (flow) temp and what is the UFH return temperature, if known?.
Also boiler make/model.
Edit: See its a Baxi 30he

 

[ShaunCorbs]

You will find the ufh heat load is less than the boiler min load so it will cycle two options use a buffer or llh/ccts

 

[John.g]

That boiler has a minimum output of 11.9kw so will definitely spend a lot of time cycling with the UFH only on.
A LLH or some form of bypass won't prevent this but will reduce the dT across the boiler, with no by pass and assuming boiler flow temp of 70c with UFH manifold of 45C/35C then the boiler dT is 35C, a boiler flow temp of 65C will give a dT of 30C and 60 will give a dT of 25C, the boiler may not have a dT trip.