Im not sure what you are monitoring but defrost is normally done by

switching off the fan

reversing the direction of refrigerant flow so the heat pump is operating to cool the house

thus using the heat stored in the circulating water to warm and defrost the evaporator

Hourly defrosts are not uncommon.  In summary I cant see anything wrong with what yours is doing - is it causing a problem?

What flow temp are you running at/have you adjusted the weather compensation curve down until its as low as possible consistent with heating the house (difficult with current weather!)

Posted by: @jamespa

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Im not sure what you are monitoring but defrost is normally done by

switching off the fan

reversing the direction of refrigerant flow so the heat pump is operating to cool the house

thus using the heat stored in the circulating water to warm and defrost the evaporator

Hourly defrosts are not uncommon.  In summary I cant see anything wrong with what yours is doing - is it causing a problem?

 

What flow temp are you running at/have you adjusted the weather compensation curve down until its as low as possible consistent with heating the house (difficult with current weather!)

The fan does turn off, water from inside is then pumped in reverse, power is take to heat the water and then the fan and compressor runs.  Takes about 5 minutes, so this seems OK.

I am currently running a fixed flow temperature, which I adjust dependant upon the weather.  The minimum I can run is about 30C. When it really cold I would increase to 33C.  The weather comp on the Hitachi system is not very good.  The temperature probe on the Heat Pump detects the sun and also the increase in temperature from the de-ice cycle and causes the flow temperature to be erratic.

I am OK with a de-ice cycle ever hour, what I don't like is the thermal off 8 minutes prior to the de-ice. This happens every time irrespect of inside/outside/flow temperature.

Posted by: @trebor12345

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The temperature probe on the Heat Pump detects the sun and also the increase in temperature from the de-ice cycle and causes the flow temperature to be erratic

Hitachi is sadly not alone.  The OAT sensor should be in the shade and out of any influence from the heat pump itself.  only some manufacturers do this!

Posted by: @trebor12345

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I am OK with a de-ice cycle ever hour, what I don't like is the thermal off 8 minutes prior to the de-ice. This happens every time irrespect of inside/outside/flow temperature.

Im guessing it might do this to minimise heat wasted by immediately flushing hot refrigerant through the cold part of the circuit.  Thats only a guess however - it does seem a long time but the actual FT decrease in this period doesnt look that large.  The exact logic is probably in the mind of the designer!

The first dip is the thermal off period where the compressor stops for 3 minutes and the refrigerant in the pipe from the heat pump to the internal unit drops a few degrees, the heat pump then starts up and runs for 5 minutes as can be seen by the slight rise in refrigerant temperature.  (as measured by my temperature probe).

The second dip is the de-ice cycle where there is a significant drop in temperature as water is drawn from the buffer tank/underfloor.

Posted by: @trebor12345

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The first dip is the thermal off period where the compressor stops for 3 minutes and the refrigerant in the pipe from the heat pump to the internal unit drops a few degrees, the heat pump then starts up and runs for 5 minutes as can be seen by the slight rise in refrigerant temperature.  (as measured by my temperature probe).

Interesting. What does the water pump do during this.  

I understand the compressor stopping, its will need to do that to switch from heating to cooling mode.  3 mins seems a reasonable time for this.

I don't understand whats happening in the 5 minute period though.  Is it just some remaining refrigerant reaching your temperature probe, or some refrigerant being warmed by the inrush of water from the house (whats the water pump doing during this cycle?)? Basically is it still going through the 'change over' from heating to cooling, or is this in fact the early stage of defrost?  Its not difficult to imagine that the change over does take some time to settle down if indeed it ever does during a defrost.  In their original concept the change from heating to cooling happens only twice per season.

Posted by: @jamespa

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Posted by: @trebor12345

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The first dip is the thermal off period where the compressor stops for 3 minutes and the refrigerant in the pipe from the heat pump to the internal unit drops a few degrees, the heat pump then starts up and runs for 5 minutes as can be seen by the slight rise in refrigerant temperature.  (as measured by my temperature probe).

Interesting. What does the water pump do during this.  

I understand the compressor stopping, its will need to do that to switch from heating to cooling mode.  3 mins seems a reasonable time for this.

I don't understand whats happening in the 5 minute period though.  Is it just some remaining refrigerant reaching your temperature probe, or some refrigerant being warmed by the inrush of water from the house (whats the water pump doing during this cycle?)? Basically is it still going through the 'change over' from heating to cooling, or is this in fact the early stage of defrost?  Its not difficult to imagine that the change over does take some time to settle down if indeed it ever does during a defrost.  In their original concept the change from heating to cooling happens only twice per season.

 

In the 5 minute period the compressor is running, taking electrical power and producing heat.  This, what Hitachi call a "Thermal Off" I might call a Short Cycle.  But it happens with so much regularity, it's almost like it what supposed to happen.

So in summary, heat pump shuts down for 3 minutes, takes no electrical power, starts back up producing heat and then 5 minutes later starts its de-ice.

Posted by: @trebor12345

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So in summary, heat pump shuts down for 3 minutes, takes no electrical power, starts back up producing heat and then 5 minutes later starts its de-ice.

Where does the heat during the 5 min period go, to the evaporator to de-ice or to the heating system (which makes no sense).  Is the water pump running?

Posted by: @trebor12345

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But it happens with so much regularity, it's almost like it what supposed to happen.

I imagine that this is indeed the case, its unlikely to happen at random!

De-ice may have two phases or more(mine does).  In the first phase that I know of the evaporator is heated by the refrigerant, getting its energy from the heating water.  The fan is off.  In the second short phase the fan starts up and blows off whats been melted (and maybe some more is melted during this phase), resulting in an impressive plume of steam.

Posted by: @jamespa

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Posted by: @trebor12345

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So in summary, heat pump shuts down for 3 minutes, takes no electrical power, starts back up producing heat and then 5 minutes later starts its de-ice.

Where does the heat during the 5 min period go, to the evaporator to de-ice or to the heating system (which makes no sense).  Is the water pump running?

Posted by: @trebor12345

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But it happens with so much regularity, it's almost like it what supposed to happen.

I imagine that this is indeed the case, its unlikely to happen at random!

De-ice may have two phases or more(mine does).  In the first phase that I know of the evaporator is heated by the refrigerant, getting its energy from the heating water.  The fan is off.  In the second short phase the fan starts up and blows off whats been melted (and maybe some more is melted during this phase), resulting in an impressive plume of steam.

The heat generated in the 5 minute period is with the pump on which goes to the manifold and floor.

My de-ice cycle is exactly as yours.

Posted by: @trebor12345

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The heat generated in the 5 minute is with the pump on which goes to the manifold and floor.

 

Weird.  This implies that the switch from heating mode to cooling mode is after this 5 minute period, in which case whats the 3 minute previous previous for?  Somewhere there is probably an intelligible description of the hitachi defrost cycle.

Posted by: @jamespa

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Posted by: @trebor12345

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The heat generated in the 5 minute is with the pump on which goes to the manifold and floor.

 

Weird.  This implies that the switch from heating mode to cooling mode is after this 5 minute period, in which case whats the 3 minute previous previous for?  Somewhere there is probably an intelligible description of the hitachi defrost cycle.

I have no idea, to me there is no logic.  There is no information on-line about this Hitachi system, so a bit of a loss.