The other point I'd make is around solar export.  With the export tariff levels dropping, it is expected that self-consumption will become far more important than maximum export.  Therefore any means of storing energy, whether that be as electricity or heat, and again for both wet central heating or DHW, is going to save more money than the export unit price.

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.

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)

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!

We use solar panels (not PVs) with a rooftop 160lt solar cylinder for DWH and as backup (for the cloudy days) an in house 100lt cylinder that used to get heated with oil (using an internal coil for heat exchange). 

The coil is too small to operate with the ASHP (its water volume is below the minimum 40lt required by the ASHP) and we were of course told to replace it with a suitable 200lt cylinder, which we’d like to avoid.

The current cylinder resides in a separate dedicated loop (for the time being not connected to the ASHP). Would it work to add a volumizer at the return of this loop to supplement the liters required by the ASHP? Has anyone come across such a set up ever? When operating in DHW mode, the ASHP can deliver the 60 to 70 degrees that the oil boiler was delivering and which were more than enough to heat our current cylinder - so hopefully no worries on that front.

Our installer dismissed the idea as non-standard. So we either do the replacement or operate with the internal electrical resistance our cylinder has. 

Note: in our part of the world, there are no obligatory guidelines regarding the size of the DWH cylinder. And we can get by with solar DWH for circa 8 months a year. 

I guess there is no perfect solution.

Have you looked at the new heatgeek cylinder made by Newark? Its supposed to be the most efficient. (edit, sorry I think you meant Heatgeek by HG)

The issue I have is that numbers for showers etc wildly vary, and atm we have an oil fired boiler which provides instance hot water so it's very hard for me to work out standard usage atm.

If you are considering export and TOU tariffs for import, consider a Mixergy cylinder heated via an immersion (You may need to have it plumbed to the heat pump for MCS, but could leave it unheated or marginally heated via the heat pump).

Although this on the face of it sounds nuts, the advantage is that no matter what size of cylinder is foisted upon you, you only heat the water that you need because these cylinders heat from the top downwards.

I have one and two of us use about 3kW of power overnight to heat our DHW at 7.5p a unit (Octopus Intelligent Go) and although I have an Eddi, I disconnected it because I can export at 15p a unit during the day.

7.5p a unit is equivalent to a COP of about 3.5 compared to daytime prices and although you could argue heating the cylinder via a heatpump at night would give a much higher equivalent COP, you would have to heat the whole 200-250 litre cylinder rather than just the volume you actually need.

You dont say how many are in your house, but the two of use use about 60 litres of hot water a day.

Something more informative about the Sunamp unit we have; the Thermino eP210 is the model that has electrical element heating from solar or grid depending on one’s setup. The unit heats up a solution of ‘salt ‘n’ vinegar’ basically. There are models that can be heated with an immersion heater element or ones that can heat the ‘Phase Change Material’ (PCM) using a high temperature heat pump. Having solar PV panels, we opted to provide solar / grid power to provide the energy required. As to the temperature of the working system, the PCM is heated to approx. 65-70 C. and when you request hot water, the cold water flows through the internal piping and exits the unit at approx. 55 C. I think. I am a bit vague about the actual temperature at exit as I have the Sunamp regulater in line to ensure we are  not at any risk of being scalded. So water is effectively heated on demand rather like it is in a combi boiler.

There is virtually no risk of Legionella having a field day in this system so no additional measures need be taken. For the two of us to have daily showers plus all the washing up needs catered for, we use ~4.5 kWh in winter and ~3.5 kWh in summer. Regards, Toodles.