@jamespa

is that better if so I can add the rest

Unfortunately not.  Maybe the website interprets any jpg as an image and renders it as such, reducing resolution in the process.  Perhaps if you add the images to a zip file then upload the zip file this will stop the website doing that.  

I see @iack has replied with an answer on how the heat pumps are connected which is useful, but he hasn't commented on your dhw circulation loop.  The Vaillant system diagram does show dhw circulation but its not clear if this is local or through the whole house, I presume the latter.  Thats key to understanding your DHW conundrum which is essentially nothing to do with the heat pump.  Unless you have a plumbing diagram for the house you will need to work this out by looking at some pipework.  Hopefully there is a plumbing diagram in your pack of documents.

@jamespa Its not letting me add a file as just then loads all the pictures again

Posted by: @paullacey

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@jamespa Its not letting me add a file as just then loads all the pictures again

on a computer compress the images to a zip file then upload the zip file.

We still need info about your DHW plumbing through the house in addition, in particular confirmation or not that there is a whole house DHW circulation loop and whether whoever put it in bothered to insulate the pipework.  Hopefully there is a plumbing diagram somewhere, otherwise some inspection needed.  Vaillant wont be able to help with this as its nothing to do with the heat pump per se, but once its understood they may be able to help with any config tweaks necessary.

@jamespa I think you are right about a hotel type hot water feed.

Hot water seems to return, see the secondary return shown in this section of diagram.

Or maybe just a return from shower mixer valves?

@jamespa I think it might be circulated round the whole house as the plumber did mention something like this when he came out to see why we didn't have hot water.  However I do have trust issues with him as he's supposed to be coming out again today and on phone call to him yesterday I said I had been putting immersion heater on to get hot water for now and he then said that the immersion heater should be left on all time anyway.  This is after he told me that was a backup, when we ran through the basics when we first moved in!

Posted by: @paullacey

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However I do have trust issues with him as he's supposed to be coming out again today and on phone call to him yesterday I said I had been putting immersion heater on to get hot water for now and he then said that the immersion heater should be left on all time anyway.

The immersion should be a backup to the heat pump only, so you are right not to trust him if thats what he says.  When he comes ask him to show you the circulation pipework (preferably at several locations) which should be very well insulated, ideally as well insulated as the tank itself, which has likely got 50mm.  If it isnt very well insulated then it will be continually losing heat to the house, hence the need for the heat pump/immersion to be doing DHW continually and you are effectively heating your house either with the immersion heater (at a COP of 1) or with the heat pump but at very poor COP compared to the 4-5 you should be getting with UFH.  Either way not good.   Also ask him what the design intent was for the DHW, was it to be heated by the heat pump or the immersion.  If the latter then somebody got the design wrong.  Again this is not a heat pump issue its a DHW design issue which would apply whatever the heating source, so don't let him try to bluff that one out by saying he would never have advised a heat pump.

Unfortunately there will inevitably be a (possibly fairly) price to pay for instant DHW from the tap (as opposed to waiting for it to work its way through the pipework), that's unavoidable, but it should be minimised by good design.  Personally its a price I wouldn't be prepared to pay so would not put in a circulation loop, but of course its your choice.  The best might have been to cluster all the facilities that need water near the plant room, thus designing out the problem.  

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As a separate matter the heat pump diagram looks v complicated, in particular it has both a plate heat exchanger and a buffer between heat pump and emitters which seems extreme to say the least, personally I would want neither if at all possible because together they are likely adding 30% or more to your heating bill.  I really cant imagine there is a need for both so it would be good to ask why both were fitted.   I guess this design was done by Vaillant but it might be worth checking.  If you can zip the jpgs and upload them that way I will take a closer look.

@jamespa Hi many thanks for coming back on this.  I let you know what he says as he's supposed to be coming round this afternoon around 4pm.  I'll try and work out how to get the pictures uploaded in better quality as well

@paullacey Hi

I have managed to save the files at a resolution that is legible so dont worry about these.  The 'Vaillant' ones were originally in colour but have been printed in B+W which is a pity, you might want to ask for the originals!  The combination of a buffer and a PHE is definitely worth enquiring about because it results in you having 4 water pumps when you really need only 2 and as I say above possibly a 30% increase in your heating bill.  The PHE is presumably to separate a glycol filled primary from the secondary, glycol to prevent freezing.  Why also the buffer then, the PHE already provides system separation for what reason is further system separation needed?    If the purpose of the buffer was to provide extra system volume then a 2 port volumiser would do this without another pump and performance hit.  It would be interesting to find out what A. Rice, who did the design, was told about this back in 2022!

In addition to performance concerns, if anything ever goes wrong with such a complicated system it will be the devils only job to diagnose.  I suspect most plumbers will run a mile.  

Another feature of the system is that it can only do either DHW or Space Heating not both at once.  This is entirely normal for any system with weather compensation but with your circulation loop (unless it is very well insulated) it is a problem, because inevitably the DHW will need frequent reheat.  Maybe thats why the plumber suggested to leave the immersion on.

First things first though, lets confirm the position with the DHW.  Then we can move onto the space heating!

Overall though, I am sorry to say that this looks like an ill thought through and unnecessarily complex design both from the heating point of view and the heat pump point of view.   That said it wouldn't take a lot to simplify it. and all the relevant stuff is close together in the plant room.

Please remember Im only an ordinary householder who has been studying this stuff for a couple of years and has a background in physics and engineering.  Im not a specialist.

Posted by: @paullacey

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I think it might be circulated round the whole house as the plumber did mention something like this when he came out to see why we didn't have hot water.

That accounts for why you start with an overly-high DHW temperature, and then end up with cool showers.

There's simply too much energy lost in completing the circuit.

Please post any photos you have of the insulation on the DHW circuit.
If this can be upgraded to 25mm wall thickness, including the bends/elbows, then that might be all that's needed.

@paullacey While the others wait for more info do you mind if I ask a couple of questions and see if I can also help? 

Did I read correctly that the DHW issue is effecting one shower or one area?

If so is this the closest or furthest on the run?

Is there a pattern or is this area now always at best warm?

Is the hot water issue also effecting the sink in the same room?

What is the tank temperature at the time you experience the lack of hot water?

If the above is Yes, furthest, alway cold, no to the sink and the tank is 50 then it is likely a supply (pressure) or TRV issue. Sometimes the secondary pump cannot work on all circuits, usually due to balancing or sometimes it can be that the water drawn for a blended outlet can be piped differently, for example rather than take the regulated cold main from the same line as the tank fed cold main it is taken from a closer or direct outlet and this can make a difference of supply pressure making it hard for the TRV to regulate and become overpowered by the cold main.

Just a thought