So i think i have narrowed it down without however knowing the solution. Buffer creates a loop. Hp provides the required water temp to the buffer but somehow the energy is not diverted to the ufh but returns to the hp, hence the low dt and the reduced temp at the manifolds compared to the outlet reading of the hp. Does this means that there is unbalanced flow between the primary circuit (hp and buffer) and secondary circuit (buffer and manifolds)? The circulating pump in the first circuit is adjustable but the second is not

Posted by: @iraknic

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Does this means that there is unbalanced flow between the primary circuit (hp and buffer) and secondary circuit (buffer and manifolds)? The circulating pump in the first circuit is adjustable but the second is not

Yes, almost certainly.

I dont know if the HP can control the buffer, some can.  If the buffer has two temperature sensors at different heights then its possible in principle to use this information modulate the speed of one or other pump so that on average the rates are the same (which is ideally what you want).  It would be worth looking at the HP manual and the buffer in case it can do this.

Otherwise you are down to adjusting the pump rates.  The HP might adjust the primary pump to suit its needs.  If it does so and unless the HP does feature buffer management, Id be loathe to touch this. That said there is generally no harm in turning the primary up, HPs complain about low flow (because its not enough to take the heat away) more than about high flow.

Im surprised that there is no adjustment on the secondary pump, many have three settings.  

Having the primary a bit faster than the secondary will tend to force the thermocline (barrier between 'hot' and cold' water, assuming its stratified) down the tank, which means that the feed to the emitters should be at the same temperature as the feed from the heat pump, thus minimising the sacrifice in efficiency which would occur if the feet to emitters is lower temp than the feed from the heat pump.  Obviously this will cause a little bit of recycling through the heat pump, but its better so far as I understand these matters than pushing the thermocline to the top which is what happens if the primary is slower than the secondary.

Do you know why the buffer is fitted?  Generally they are a bad idea in heat pump systems, albeit that many installers put them in because its pretty much guaranteed to reduce callouts (by masking problems).

@iraknic how are you monitoring your UFH delta T?

@jamespa the buffer was fitted to increase water in the system due to the existence of zones. @editor
I am measuring the temps at the manifold using an IR thermometer. In my case it seems that the primary water pump is slower than the secondary (after the buffer) one? Can the secondary flow be reduced by manually decreasing ufh flow at the manifolds?

Posted by: @iraknic

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Can the secondary flow be reduced by manually decreasing ufh flow at the manifolds?

Yes.  Its the same as turning down the lockshield on a radiator

i attach the setup of the buffer, highlighting the return lines and the specs of the water pump that is placed after the buffer. I thought of closing a bit of the outlet and return valves of the left side of the buffer (to and from) the manifolds and increase the flow of the primary pump (from hp to the buffer) to see what happens

@iraknic Please note, I don’t have any experience at all (diddly squat) of UFH plumbing but if a circuit has both send and return valves, it is probably only necessary and better to just constrict the flow to the circuit and leave the return without any restriction. I may be talking through the wrong orifice of course! Regards, Toodles.