@davidnolan22 - Screenshot of energy input/output attached.

System was commissioned on 8th April.

Note: value for 16th is significantly higher since for some unknown reason MELCloud didn't bother to give a value for the 15th and just included it in the values for the 16th. This is the same in both MELCloud and MELpumped.com (which I suppose just uses the same data).

Your point on lack of cycling is exactly what I was thinking too. Of course it would be better if I could get more heat transferred out of the system through those larger rads and after a visit from the installer (flush/balancing)/a bit of my own tinkering this summer hopefully I will get the troublesome few rads sorted but it's not like it's causing cycling at the moment so I can't believe it's the main factor negatively affecting efficiency.

@CBrenewable   @ashp-bobba seems to be on the case so I will step out for now to avoid possible confusion, name check me if you want me to step back in/it is not solved.

@cbrenewable Can you look on the side of the pump body, should be an arrow showing direction? please have a look at both pumps if you can, TBH even if they are both the correct way if one passes through the other when only needing DHW or heating or 2 are pumping together you are likely looking at one of the main issues. 

All you needed and nearly all off these system ever need is a simple Y plan circuit with one pump capable of the power needed for the index circuit and your suppler should always be qualified and know what to do to work these things out as its very basic maths and basic heating eng stuff.

@ashp-bobba Thanks. I've checked the arrows and confirm both point in the correct direction.

I note your concern over whether both are needed. Assuming for a minute that they are both needed, in what scenario would you expect both to be on and working?

I've run the DHW and Heating just now to see which come on and when. My findings:

DHW: Only Flow pump active (I didn't notice the mode). No lights on return pump so assuming it's not active.

Heating: Both pumps active. Flow pump operating in Mode II and Return pump operating in Mode III.

I found a copy of the itemised delivery list from one of the the installer's suppliers and have identified the pumps as: https://theufhgroup.co.uk/products/bastion-a-rated-circulation-pump-6-metre-head-inc-cable-zl-3002-25-6

I also found this data sheet that gives some info on modes but I'm not totally sure how to interpret it: https://theufhgroup.co.uk/cdn/shop/files/ZL-3002.pdf?v=184044485459458873

I had a look at the data sheet but wasn't really able to understand why one pump is in II and another is in III mode. Someone else who knows more than I may have more luck.

@cbrenewable Lets get to the modes and setting a bit later, sorry to ask but I need to make sure I understand the circuit that has been made. can you confirm the attached arrows on your picture is how the circuits are configured to flow and confirm the below?

DHW: Only Flow pump active (I didn't notice the mode). No lights on return pump so assuming it's not active. (pump you describe is pump 1 on the annotation and pump 2 does not run?)

To add red circuit is DHW and blue is heating 

@ashp-bobba - yes that all looks correct.

Pump 1 is on the flow and Pump 2 is on the return.

So if I understand it correct while in DHW mode, the main ASHP pump (1) has to go through the DHW circuit in the cylinder and then pull through the non operating heating return pump (this is not correct and will cause a lot of restriction on that line), I have attached an annotation showing the change the pipe at the bottom in red so the water does not need to go through that pump that is off if your contractor feels they need to keep the second pump.

I have also attached another annotation as an option showing the pump No2 to be removed and then the circuit would resemble something of an S Plan, if the 1 circulation pump is not enough to overcome 6m head for the index circuit you can get more powerful pumps. 

ASHP are super sensitive to hydronic balancing to have a return boost pump like this you would need balancing valves and I hate to say in this case if there is no possible way to reach the desired pressure use a Buffer and separate the circuits. To me what you have is a system with a buffer set up and someone just removed the buffer part but now you have uncontrolled flow and pumps in the way of each other set at different rates.

Best system simple Y plan 1 pump index circuit head calculated, balanced circuits and let the Mitsubishi control everything.

Posted by: @ashp-bobba

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To me what you have is a system with a buffer set up and someone just removed the buffer part but now you have uncontrolled flow and pumps in the way of each other set at different rates.

... presumably because the customer (very sensibly) said that they didn't want a buffer!

Why do heat pump installers persist in making things complicated?

@jamespa Hi James, I don't know but I have 2 more in my area to look at next week after Easter. There seems to be this issue sometimes. 

CB, I think perhaps ask them kindly what they think about designing to run on one pump, perhaps get their perspective, I do think you have a mixed bag of smaller issues perhaps causing this lower efficiency. I think the pump conflicts, balancing of the DT on the rads and the bypass are likely causing some of this. Please remember we are diagnosing from pictures on a forum so this is advice for a way to approach the issue. We engineers carry several very expensive commissioning tools that help us test and commission these systems to standards you are looking for. 

I hope my advice at least points you in the right direction.