@ch_18, welcome to the forums. 

On the principle first: converting a 4-port buffer into a 2-pipe volumiser on the return is a logical step if your goal is to reduce mixing losses and improve emitter temperature consistency. 

On your question about port orientation, in a true volumiser configuration (not decoupled) you’re not trying to achieve stratification in the same way as a buffer. So keeping both the emitter return and the heat pump return on the lower ports can be acceptable, provided the flow path through the vessel is continuous and you’re not inadvertently short-circuiting the top section.

In practice, like @judith alluded to, this is the more conventional way of plumbing in a volumiser.

On the pump question, a 7 kW Vaillant’s internal pump is generally capable of handling a typical single open circuit system provided the hydraulic resistance is reasonable. The fact that it’s currently circulating 1200 litres (I assume you mean system volume, not flow rate) is less relevant than the required flow rate and head. What's important here is whether, once you remove the secondary pump and decoupling effect of the buffer, the internal pump can achieve the required design flow at the actual system resistance. If it can't, you'll drown in flow errors.

Posted by: @editor

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 So keeping both the emitter return and the heat pump return on the lower ports can be acceptable, provided the flow path through the vessel is continuous and you’re not inadvertently short-circuiting the top section.

Without doing some horrendous modelling, or fabricating some transparent tanks, I would think that there is quite a significant risk that the upper section is effectively short circuited if both connections are at the bottom, particularly if the erstwhile buffer tank was designed to encourage stratification.

There will be some turbulence of course, and a bit of a convection current (hopefully not much if the tank is well insulated) but not a lot really to stir it up.  So personally I would go in at the top and out at the bottom even if it makes the plumbing more difficult.

@editor Thanks for the reply. Yeah I thought in at the top, out of the bottom would be the best solution for converting to volumiser but was hoping to minimise repiping, just incase it doesn't work for example if the internal heat pump circulation pump can't cope. 

If you look, I have a spare 2 ports on the left hand side of the buffer. One is about 2/3 of the way up. Would this be a better option? Or does it have to go to the top nozzle?

I have 2 circuits, both open loop. One is supplying approx 120m2 of UFH circuits and the other is supplying upstairs (2 floors) of rads? You think the pump would struggle supplying enough flow?

Thanks again