Can I suggest you read (if you havent already) this 'starters guide' to heat pumps?

I think you may be getting things confused.  Firstly dont worry about the refrigerant circuit, thats for the designers to worry about.  Its the water circuit which goes through your emitters you need to concenterate on.

Posted by: @trebor12345

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HPWF - m3/hr Heat pump water flow.  What is this, I can see it modulates down from 2.06 m3/hr to lower numbers.  I thought it was the rate of flow of refrigerant across the heat exchanger, but it says Heat pump “water flow”

Yes it will be the rate of flow of water to the emitters.  This water also flows across an internal heat exchanger where it absorbs energy from the refrigerant.  The rate may well modulate up and down

Posted by: @trebor12345

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HPH4 - Inverter frequency operation.  The speed that the compressor runs at, and 120hz is the maximum?  This is a modulating compressor?

Yes it is the speed of the compressor and yes it is a modulating compressor.  Dont know if 120Hz is the max, but it will be there or thereabouts.

Posted by: @trebor12345

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HPTi - Heat pump inlet temperature and HPTo - Heat pump outlet temperature.  Is this the temperatures across the heat exchanger on the water side of the system ?

Yes, which should hopefully equal the temperature drop across the emitters unless your buffer is 4 port not 2 port (if so thats disappointing) or there is a long distance/lots of loss between heat pump and emitters

Posted by: @trebor12345

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I see the forum often uses a formular  Power kW = Flow(l/m)/60 x DT x 4.2.  Is there a more details explanation of how I should be using this formular to manage my system?

You dont (or shouldn't) need to.  Once the water pump speed (or speed range) is set by your installer the heat pump will sort it out.  In any case you cant directly control DT, its a function of flow rate and emitter output (your heat pump may modulate the water flow rate to maintain it, or it may not, different heat pump designers make different decisions about this point.  

Posted by: @trebor12345

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The system is set for a Delta T of 5 degrees.  Is this 5 degrees across the heat exchanger on the refrigerant side or water side??

 Water side

Posted by: @trebor12345

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The heat pump produces a temperature of say 45C to the heat exchanger, which turns into a flow temperature of say 30C.  Why does the water side not achieve the 45C?

Where are you getting the figure of 45C and indeed 30C from, this could be a problem but until we know what you are measuring we cant be certain.

Posted by: @trebor12345

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How does this Hitachi system work.  I have a set temperature, and it adjusts the compressor speed and flow rate of refrigerant to maintain the set temperature across the heat exchanger (water or refrigerant), it this how it works?

The primary control loop of pretty much all heat pumps attempts to adjust two things namely

1. The compression ratio of the compressor - to match the refrigerant temperature to the desired flow temperature (defined by the WC curve or as a fixed temperature).  The refrigerant temp will be controlled to a value above above the desired flow temp because heat flows from hot to cold not the other way round  

2. The frequency of the compressor to match the power output of the unit to the demand from the house (which it can work out by observing the return temperature.

There are various secondary control loops but this is the basic way it works

Posted by: @trebor12345

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I run a fixed temperature as the Hitachi weather compensation system is very erratic (outside temperature fluctuations from the sun)

There should not be outside temperature fluctuations from the sun.  The OAT sensor should be installed in the shade or if its attached to the unit protected from the sun.

What flow temperature are you running at when its cold outside?

@jamespa Thank you for the reply.  That clarifies things.  I hadn't read the starters guide, but will do now.

It's a 2 port buffer.

When zero or below, I run a flow of 32 or 33C.

Where are you getting the figure of 45C and indeed 30C from, this could be a problem but until we know what you are measuring we cant be certain. - I made the numbers up to try and understand how the system was working.  I often see a refrigerant temperature of 45C and a flow of 30C with a return of 25C.

Posted by: @trebor12345

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It's a 2 port buffer.

When zero or below, I run a flow of 32 or 33C

OK, thats good and low.  With low flow temperatures like this there is less to be gained by operating weather compensation than if you were operating at higher FTs.  if you can fix the OAT sensor so its in the shade, I still suggest you do try WC again - it really should not be in the sun!  If you cant probably dont worry too much!

Posted by: @trebor12345

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I often see a refrigerant temperature of 45C and a flow of 30C with a return of 25C.

I honestly don't know if 15C difference between refrigerant temp and flow temp is normal, it seems excessive but it does depend on the heat exchanger.  If you aren't getting excessive bills and are getting a reasonable COP 9ratio of heat out to electricity in - which somewhere it will tell you) I wouldnt worry.