Caleffi speak in one of their tutorials about the benefits of a 3 pipe buffer.  Someone else may have taken the trouble to understand it.   As always I would ask the question - why do I need a buffer at all.  If the answer isnt satisfactory then I would find another installer.

I note a mixture of rads and UFH both run at the same flow temperature as no mixing is shown.  That means either your floor are going to be very warm or your rads are going to be very cool.  Are these existing emitters or new, whats going on here?

For what its worth I did ASHP in three community centres.  Because they were principally large rooms, I went for A2A and background electric heating in the 'small' areas.  It worked a treat and achieved a factor of 4 reduction in carbon.  

@jamespa When I first saw the subject line, I thought that Sir Arthur Conan Doyle had joined our ranks! Toodles.

@jamespa Sadly the answer to why a buffer tank was, I've been working with Heat pumps for 47 years and we know this works.

The club has 14 showers that will be used quite heavily on a couple of days every week in the summer (it is a sailing club). I hope they will have enough hot water with 2 300 litre DHW tanks. In winter it is little used. They have 8Kw of Pv and the same in batteries but with an older gas boiler fed by an LPG tank both of which need replacing. My suggestion of some solar thermal was rejected out of hand.

The club officer basically called in his favourite Installer to get a quick quote to get a council grant so it won't cost the club much to fit but we will see how much it costs to run! 

It sounds rushed to me but also that it may not be possible to question the direction.  I hope you are lucky!

Since he has been doing it this way for 47 years he should be able to provide some references and so you can speak to customers with a similar installation.

If I were the club officer I would be having a quiet word with the Council about flexibility to use another installer, then do the procurement job properly.  They are unlikely to want to waste their money and the club needs certainty that the system is going to work.  From what you say it hasn't got it.  Id also seriously consider A2A for the clubroom (you presumably need it to heat up for a short time only - so heating the air makes more sense than heating radiators which then heat the air).  600l is 42 l per shower which sounds enough, but if each is used multiple times in a short period it wont be.  This needs nailing down IMHO.

Im not getting the warm and fuzzes that due diligence has been done and just because its not the Club's money is hardly a good reason to skip that stage.

Coincidentally, I had an introductory visit from a very reputable installer the morning and after a very quick look at my present system he suggested that a 3-port buffer may be an option in my case.  He attempted to explain it to me but I was more interested in covering other topics on this first visit and what he explained didn't sink in. I think it may have been something to do with the difference in water volumes or flow rates/pressures in different branches of the system. It's something I will need to understand if the requirement firms up.

Update. The system was fitted and commissioned last week, by Friday the radiator zone was still not coming to temp. On Wednesday we had a meet with the Installers and went through the system. Some adjustments were made (I am not sure what) and it started to warm up. The system relies entirely on thermostats calling for heat.  I tried to question the need for the buffer and the response was quite bullish. They believe all systems need a buffer and they will always fit them and tried to tell me that it is a requirement of Part L!! They insist that open zone thinking is misguided! (@editor maybe you can get Total Renewable Solutions Axminster to explain their their reasoning on buffers in a podcast?) Next I will try to find how the Pv and Bess will be used to offset the electric useage. @jamespa since the coordinator of this project is the Vice Commodore and a building contractor I have be circumspect in my observations, after all what do I know? I haven't  got a heat pump and most of my, er 'knowledge' is from this forum and they are the experts. 

Posted by: @jancold

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They believe all systems need a buffer and they will always fit them and tried to tell me that it is a requirement of Part L!! They insist that open zone thinking is misguided! (@editor maybe you can get Total Renewable Solutions Axminster to explain their their reasoning on buffers in a podcast?) 

I will make a note of Total Renewable Solutions Axminster, but I'm guessing that like all other installers that use buffers they won't come on the episode. But if they're so bullish, maybe I'll get lucky. 

Buffer tanks are not a requirement under Part L. As I understand it, Part L is about the conservation of fuel and power. While it sets standards for energy efficiency and system design, it does not mandate buffer tanks in heat pump systems. In fact, considering how inefficient many buffer tanks are (especially when poorly installed) you could argue that Part L should advocate for their removal rather than their inclusion.

I'm scheduled to record a video on our buffer tank tomorrow which I hope will go live this weekend to show the short fallings of the system in our property.

@jancold thanks for the update!

Posted by: @jancold

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@jamespa since the coordinator of this project is the Vice Commodore and a building contractor I have be circumspect in my observations, after all what do I know? I haven't  got a heat pump and most of my, er 'knowledge' is from this forum and they are the experts. 

Fully understood.  Since I'm just some bod on the internet in no way connected to the project Im not bound by these constraints.

Posted by: @jancold

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I tried to question the need for the buffer and the response was quite bullish. They believe all systems need a buffer and they will always fit them and tried to tell me that it is a requirement of Part L!! They insist that open zone thinking is misguided!

There are (I understand) still people who believe that the world is flat.  I wouldn't rely on them for navigation.  There are (i have heard) people who believe that injecting chlorine might be a good way to treat disease.  I wouldn't rely on them for health advice.

Seriously though I hope my concerns are misplaced and that it all works at a reasonable running cost.

@editor Looking at some of the changes to Part L this at the end of 5.20 this had me perplexed.

NOTE: There is no need to install thermostatic room controls in rooms/zones without heating in new or existing dwellings.  

So I don't need a thermostat in my unheated conservatory,who would have thought!!

@jancold Just make sure those pipes you don’t install don’t leak either! Toodles.

@toodles Sound advice sir! (At present having a consumer unit changed so running his coffee machine from his EV, so smug )