Firstly its not the coil volume that is normally the issue it's the area, the typical 'requirement' being 3sq m.  In principle system volume of the dhw loop could be an issue during defrost, but because the dhw coil is immersed in warm water in practice not and neither is it something UK installers bother about (and we get lots of defrosts).

So I'm not really clear what your installer is worried about.  If he is only worried about system volume then a volumiser will fix it, but it's almost certainly unnecessary.  If he is worried about coil area then why is he talking about volume?

The truth is it will very probably work as is if you are happy to heat it at ft70, but whether you can persuade your installer that is another matter given that he seems to be concerned about the wrong thing!

@jamespa 

The installer has simply dismissed the idea as non-standard. The volume issue was practically raised by me but he refuses to leave his comfort zone…. Midea ASHP requires a minimum of 40lt in the circuit and I seriously doubt the existing cylinder circuit+coils have this water volume.

I think once we feel comfortable with our new ASHP , we will tackle the cylinder issue and try a volumizer. And I will sign another disclaimer taking responsibility that it will work (much as I did for not installing a buffer with the ASHP)

Posted by: @mk4

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The installer has simply dismissed the idea as non-standard. The volume issue was practically raised by me but he refuses to leave his comfort zone…. Midea ASHP requires a minimum of 40lt in the circuit and I seriously doubt the existing cylinder circuit+coils have this water volume.

They wont have that volume, neither will most others, and the installers interpretation of what Midea say is unconventional to say the least (ie I have never heard this interpretation for any heat pump and it makes no logical sense so far as I can see.  It does make logical sense to worry about coil area though).  

The problem with the volumiser is that it means that you heat 50l (or whatever the size) of water unnecessarily to a high temp each time you heat DHW.  This is pure waste and thus undesirable.  This is why people dont put a volumiser in the DHW loop.

The possible problems with a small coil are defrost and cycling.  You are right that, by increasing the flow temperature to 70C you are heating the tank exactly the same as with a boiler.  Its very likely indeed to work particularly if you are only targeting a tank water temp of say 50C.  If you are targetting a very high tank water temp then you may get to the point where the heat loss from coil to tank is less than the minimum output of the heat pump, at which point its may cycle.  This is of course also true of a boiler, which is one reason I think its unlikely.  Of course this could simply be tested and a change made only if its a problem (suck it and see)

Defrost is potentially another story however.  When a defrost occurs the heat pump is reliant on sucking heat from the system to melt the water.  This is the primary reason for specifying a minimum system volume.  If it occurs during a DHW cycle then all of the stored water in the hot water tank itself is effectively engaged, as its thermally connected to the system via the heat exchanger (coil), hence why system volume of the DHW loop isnt generally an issue.  Its in principle possible, if you have only a small coil at the bottom of the tank, that defrost cools the bottom of the tank before the water melts, ie effectively only part of the tank is engaged not the whole tank.  Is this likely?   I would guess not at all, but its in principle possible and its not really practical to do a test. This then is the argument for changing the tank.  Of course if it only frosts for a few days a year you could disable dhw heating via the heat pump and use the immersion instead, if there is one.

An alternative to changing the tank is to add a circulator pump plus an external PHE so that the heat exchange occurs in the external PHE instead of (or in addition to) the coil.  Some advocate this anyway and there are some DHW tanks that are built this way.  Its quite a cost effective route.

In summary, and having thought some more about this, I wouldn't add a volumiser.  You options practically are

1. Leave as is, test to check it heats, take a risk on defrost which can be mitigated by engaging the immersion during frosty conditions, albeit thats a bit of a pain.

2. Add a circulator pump plus plate heat exchanger to 'plate load' the cylinder

3. Replace the cylinder

Hope that helps.