Sorry I cant help further.  Knowing when the various components (compressor, fan, EEV, water pump) change state might illuminate the matter, but otherwise its presumably just a sequence devised by some Hitachi engineer.

Thanks for your help.

@trebor12345 @jamespa  I do not know your timing sequences but I will try and venture an opinion here. I think that the unit is doing a two phase defrost. The Hitachi heat pump uses hot gas bypass for defrost. See diagram.

With this method, the hot gas out of the compressor is diverted before the condenser and routed through the evaporator to defrost the ice. It opens a valve to the evaporator and closes the valve in the line from the receiver. This is the first long period of time that you are seeing. The second phase kicks in to clear the defrost residue from the evaporator by using the warm air of the cooling cycle i.e. a normal defrost cycle by cooling the house. It's an  efficient method which reduces heat taken out of the house.

Posted by: @heatgeek

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@trebor12345 @jamespa  I do not know your timing sequences but I will try and venture an opinion here. I think that the unit is doing a two phase defrost. The Hitachi heat pump uses hot gas bypass for defrost. See diagram.

 

With this method, the hot gas out of the compressor is diverted before the condenser and routed through the evaporator to defrost the ice. It opens a valve to the evaporator and closes the valve in the line from the receiver. This is the first long period of time that you are seeing. The second phase kicks in to clear the defrost residue from the evaporator by using the warm air of the cooling cycle i.e. a normal defrost cycle by cooling the house. It's an  efficient method which reduces heat taken out of the house.

The trouble is that I do not know enough about how heatpumps work at the moment, to offer any reply to this. 

@trebor12345   I will give you a quick simple explanation using the same diagram in non-defrost mode. Heat pumps work by extracting energy from the air, even from air below freezing temperature, and using what is known as the compression cycle. Each time air or liquid changes state it either gives off energy (air to liquid) or absorbs energy (liquid to air). Likewise if it is compressed or decompressed.

Therefore starting at the compressor on the bottom right, it compresses the gas which becomes hot at high pressure (gains energy). This passes through the condenser (a plate heat exchanger) where the gas gives up heat to the water flowing through it for space heating. The compressed gas turns to liquid at low temperature which collects in the receiver. This goes into the evaporator (copper coil at the back) where the fan blows air over it and it evaporates into gas, absorbing energy from the air in the process. The compressed gas then goes into the expansion valve (the yellow thing) where the gas is decompressed to a low pressure and gives off heat to become a low pressure, low temperature gas. This goes into the compressor where it is turned into a high temperature, high pressure gas once again. It is this gas that is used for the defrost.

This process garners much more heat from the environment than the electrical energy used for the fan and the compressor and this gives the heat pump its COP (ratio of energy out to energy in).

Posted by: @heatgeek

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@trebor12345   I will give you a quick simple explanation using the same diagram in non-defrost mode. Heat pumps work by extracting energy from the air, even from air below freezing temperature, and using what is known as the compression cycle. Each time air or liquid changes state it either gives off energy (air to liquid) or absorbs energy (liquid to air). Likewise if it is compressed or decompressed.

Thank you, that helped in my understanding.  I believe this Hitachi split system takes water from the buffer tank/underfloor to defrost the evaporator (I only know this via my "Micky Mouse" temperature probes I have fitted to the pipes).  Is my assumption correct? Is there an explanation as to how this works.   

I have been making some adjustments to the heating and would like to clarify a point.

If you are having short cycling issues, is it best to slow the water down over the heat exchanger to extract more of the heat? (having it going over the heat exchanger quickly it will extract less heat).

@trebor12345   Firstly, the brochure description of the Hitachi says that it definitely uses hot gas bypass for defrost as per the process I described. This is an internal process of the heat pump and is very efficient as it does not draw water from the house. After this initial phase, I think that it goes into cooling mode to draw water from the house, as you describe, to help clear the residual ice using the fan to blow it off. This is the only way that I can rationalise the sequences you are describing.

Posted by: @trebor12345

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If you are having short cycling issues, is it best to slow the water down over the heat exchanger to extract more of the heat? (having it going over the heat exchanger quickly it will extract less heat).

If you are talking about the heat exchanger in the heat pump, at any point in time the heat pump is producing a certain amount of heat depending on settings. This gets carried away by the water through the combination of DT and flow. Whether it is high flow/low DT or low flow/high DT, the heat carried out is the same. Heat is power. As an analogy, the power of electricity is given by the voltage multiplied by the current. Therefore 2W = 1V x 2A or 2V x 1A. So, slowing flow will not carry away more heat. It will widen the DT as the power output is constant.

However at high OAT, e.g. 15C, the heat pump can be producing more heat than is needed by the house as it cannot modulate low enough. Under these circumstances, the heat pump will cycle and give heat in bursts. One has to accept this and it is not necessarily inefficient if there are no power surges at each cycle, like old fixed power heat pumps. Modern inverter heat pumps are good in this respect. You also need to get your weather compensation right as many have described in this forum.

Posted by: @heatgeek

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@trebor12345   Firstly, the brochure description of the Hitachi says that it definitely uses hot gas bypass for defrost as per the process I described. This is an internal process of the heat pump and is very efficient as it does not draw water from the house. After this initial phase, I think that it goes into cooling mode to draw water from the house, as you describe, to help clear the residual ice using the fan to blow it off. This is the only way that I can rationalise the sequences you are describing.

Posted by: @trebor12345

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If you are having short cycling issues, is it best to slow the water down over the heat exchanger to extract more of the heat? (having it going over the heat exchanger quickly it will extract less heat).

If you are talking about the heat exchanger in the heat pump, at any point in time the heat pump is producing a certain amount of heat depending on settings. This gets carried away by the water through the combination of DT and flow. Whether it is high flow/low DT or low flow/high DT, the heat carried out is the same. Heat is power. As an analogy, the power of electricity is given by the voltage multiplied by the current. Therefore 2W = 1V x 2A or 2V x 1A. So, slowing flow will not carry away more heat. It will widen the DT as the power output is constant.

Thank you for the clarification on how the de-ice cycle works.

Regarding the question on flow rates across the heat exchanger.  I have this split system (hp outside and unit inside).  So I thinking of the heat exchanger that is in the inside house unit.  I am trying set my underfloor right, so do I have a fast flow over the heat exchanger or slower?  From you ohms law methodology it doesn't matter, is that right?

@trebor12345   By way of further explanation, in cooling mode the compression cycle of the heat pump reverses i.e. the gas flow goes in the opposite direction and the output from the heat exchanger now goes to the compressor. It becomes like a fridge and uses cold gas to cool the water flow which then takes heat out of the house and the HP fan blows out warm air instead of cold. This reversal is controlled by a 4 way valve inside the heat pump.

@trebor12345     The heat exchanger function is the same regardless of whether it is inside or outside. Your heat pump is a 7kW unit, not 3kW as per your description. The 3HP in the model number is the horse power rating of the compressor. To provide 7kW with 5C DT, the flow will be 20 l/min. I presume that this will be the 100% pump speed setting on your unit. I am not familiar with the Hitachi so I don’t know their philosophy of operation. I think it is fixed flow setting. However, you say that you have a buffer tank. Do you have a secondary pump then or is it just a volumizer? How is this configured? This makes a huge difference. You need to describe your system configuration in more detail.

The cycling issue will be dependent also on how over-sized your heat pump is. You have a bungalow to modern standard so I would have thought that your peak UFH heat requirement would not be more than 20W/m2. This would translate into a 4kW HP, yet you have a nominal 7kW with maximum up to 11kW. What is your design load calculation? What are the u values of your walls and ceiling? How was the unit initially set up by your installer?

Posted by: @heatgeek

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@trebor12345     The heat exchanger function is the same regardless of whether it is inside or outside. Your heat pump is a 7kW unit, not 3kW as per your description. The 3HP in the model number is the horse power rating of the compressor. To provide 7kW with 5C DT, the flow will be 20 l/min. I presume that this will be the 100% pump speed setting on your unit. I am not familiar with the Hitachi so I don’t know their philosophy of operation. I think it is fixed flow setting. However, you say that you have a buffer tank. Do you have a secondary pump then or is it just a volumizer? How is this configured? This makes a huge difference. You need to describe your system configuration in more detail.

The cycling issue will be dependent also on how over-sized your heat pump is. You have a bungalow to modern standard so I would have thought that your peak UFH heat requirement would not be more than 20W/m2. This would translate into a 4kW HP, yet you have a nominal 7kW with maximum up to 11kW. What is your design load calculation? What are the u values of your walls and ceiling? How was the unit initially set up by your installer?

Is that 7KW input or output.  If 7KW input that's way oversized. I have attached an image of the name plate on the heat pump, for confirmation.

I have attached a image of the system, which shows the 2 port buffer tank and underfloor pump. You will note all of the actuators have been removed from the manifold.

20W/m2 sounds about right from what I have read on build standards.  I do have a document which is 27 pages long which is called Full SAP Calculations, but it a lot of numbers.

The system was commissioned by Hitachi, I have their report if its of value?