We get a lot of reports here from people who are having problems with their systems, many of who have a buffer tank.  I think its clear that very best advice, in most cases, would be not to have the buffer tank in the first place or to replumb it as a volumiser, however that generally isn't an option (without compromising warranty) by the time the problem is reported.  The most typical scenario we see here (sadly) is a 50l dumpy tank with absolutely no control that attempts to maintain stratification.

Obviously the suggestions in any particular circumstance depends on the circumstance, but lets presume for the sake of this argument that its reasonably clear that the buffer tank is causing a loss of efficiency, which is commonly the case from what we see here.  Lets also presume that the installer has, one way or another, disconnected so the householder is effectively on their own.  What suggestions can we give?

Obviously one option is 'do nothing and get a professional in' but sadly that is often not realistic.  Can we offer any practical suggestions to someone who will have no specialist tools?

In the circumstance above here is an outline strawman (detail can be added later) and I am inviting people who may have experienced (and maybe solved) the problem to tear it apart and rebuild in the spirit of being constructive

1. Dont panic!

take things slowly and calmly, make notes of any changes you make

2. Assess the problem

get at least 2 digital thermometers, eg from Amazon.  Measure the temperature difference between flow from heat pump and flow to emitters.  Be sure that the probes are well in contact with the pipes and put some insulation over them, also swap the probes over and note all four readings, so we can eliminate probe variability.  Do this when its cold, or at least fairly cold, outside (say 10C or less) and the heat pump is running.

For every degree that the temperature of the flow to emitters is less than the temperature of the flow from the heat pump you are losing about 3% in efficiency.

3.  If the loss above is significant, you can attempt to mitigate it

There are two possible ways to reduce this. 

 (a) you can reduce the deltaT across the emitters by speeding up the pump on the secondary side

 (b) you can reduce mixing by ensuring that the volume flow rate on the primary side is equal to, (or, in practice, slightly more because equal to is impossible), than the volume flow rate on the secondary side, thus pushing the thermocline down the tank.

4.  Here are some outline guidelines on a process

If you have more than one pump on the secondary side care will be needed to keep them in balance.

You will need to work out what adjustments are available to you.  It may be possible to change the primary pump speed either in the installer menu or on the pump itself.  It is very likely possible to change the pump speed on the secondary side.  

At this point you should note whether the secondary pump is controlled by the heat pump or by the room stats (often via a wiring centre).  If it is controlled by the heat pump note if there are two cables or one cable from heat pump to secondary pump.  If the latter then its probably a PWM pump the speed of which is controlled by the heat pump in which case it is likely best to leave well alone.

Unless the above indicates leave well alone you can then try to adjust the pump speeds to improve matters.  Before you start note all current settings so you can put them back.  You should ideally look to maximise the secondary pump (volumetric) speed and adjust the primary to be slightly higher - you can check by minimising the temp diff flow to flow across the buffer.  If you reach the max speed of the primary before it matches the secondary, turn down the secondary until you can match them.  You may need to iterate.

The above will need to be done with all zone valves/TRVs fully open (which is how you should be operating anyway).  It may not be possible with heat pumps (eg Daikin) which attempt to maintain a fixed deltaT flow to return by modulating the pump. 

The result should be a significantly smaller difference between flow temp from heat pump and flow temp to emitters from buffer.  If it is then good, if not then reset to initial conditions.

Comments from all welcome, @ashp-bobba, @pirate-rich and other professionals your comments would be particularly welcome as would @toodles who, if my memory serves me, has done this himself, but all are welcome to make constructive comments.