Posted by: @abernyte

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This whole "hot water from the primary is used in the defrost" is confusing me.  Unless it is not general across all makes the Ecodan reverses the flow of hot refrigerant gas to defrost the evaporator after stopping the fan and primary flow pump.

Are you sure.  Where does the defrost energy come from on that case?

Most stop the fan but leave the primary pump going thus extracting the defrost energy from the house.  There are pumps that can start an inline electric heater and I think someone posted about one that has a built in water tank (not sure exactly how it's used).

The energy must come from somewhere.

Well according to Mitsubishi's technician in the video it is the hot refrigerant gas being reversed via a reversing valve and expansion chamber.  No need to steal heat from the primary circuit or any volumiser.  There may well be brands that do but that seems a very low tech solution compared to using the hot refrigerant gas in the system.

Sofaik all others do the same, ie they operate the refrigerant circuit in cooling mode to effect defrost.  They thus cool the house and warm the heat exchanger using heat extracted from the house. 

I think it's highly likely mitsubishi don't switch off the water pump because the water is the source of the energy.   

@jamespa 

My mitshubishi does not turn off any pumps flows continue and takes some heat from the house.  I’m fairly sure this is normal behaviour 

"I think it's highly likely mitsubishi don't switch off the water pump because the water is the source of the energy."   So the official Mitsubishi video is wrong? He clearly says that they use the already compressed hot refrigerant to defrost along with a tiny bit of heat from the HE at the end of the process, not the water from the primary as the heat source. Surely that is where the energy comes from.

On my Ecodan 14kW the primary pump stops for the 2/3 minutes it takes to clear the ice from the evaporator. The entire process from start to finish is over so quickly, and infrequently, that if it wasn't for the vapour cloud and water running off you can easily miss it. 

Now I am really confused! 

@abernyte 

Post a picture of your flow and return, you will see they swap over for a short period during the defrost.  

the video does say in takes a tiny bit of heat from the water.

the picture shows it stole a tiny bit of heat from the house to sort itself out. 

Is that not just indicating that the primary pump has stopped momentarily while the unit defrosts using the hot refrigerant gas?

Posted by: @abernyte

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Is that not just indicating that the primary pump has stopped momentarily while the unit defrosts using the hot refrigerant gas?

No, that wouldn't account for the flow temp being lower than the return temp.  This shows that the water pump continues to operate and as a result the water output from the heat pump is chilled by the fact that it has given up energy to the refrigerant and ultimately to the fins.  Its quite clear from this plot that the Mitsubishi does defrost in exactly the same way as almost all other heat pumps.

My Mitsubishi does exactly the same as David's above, there wouldn't be enough volume of water/heat in the primary circuit alone to defrost the heat pump my run is very short.

@abernyte no, it doesn’t turn the pumps off on mine.

Thank you gentlemen/people (can't be too careful now) perfectly reasoned arguments that fit the observed evidence.  The only fly in this ointment is why the manufacturer does not mention this in it's otherwise clear and technical explanation of defrost. Taking a "tiny bit of heat from the HE at the end of the process" is the only mention.  One would think if they were setting out to make such a video then the simplest route would be to say "we are briefly going to steal some heat from the house or DHW to defrost the HP" But they don't. 

Sigh....split jury, should never have studied law!