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Daikin Short Cycle Query

 
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Air Source Heat Pumps (ASHPs)
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 Ubert767
(@ubert767)
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Topic starter 07/11/2025 10:34 pm  

This is a plot from a Shelly EM monitoring the supply to my 6 kw Daikin.

Are the spikes between the DHW reheat episodes indicating "short cycling"?

Thanks

Screenshot 2025 11 07 at 21.13.27


   
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 JamesPa
(@jamespa)
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08/11/2025 9:50 am  

Posted by: @ubert767
↑

Are the spikes between the DHW reheat episodes indicating "short cycling"?

 

I would say yes.  Have you got a buffer/LLH and secondary pump.  How is is set up (thermostat? Weather compensation?).  Are your emitters completely open (no TRVs or TRVs set to max no Zone valves or zone valves permanently on)?

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Ubert767
(@ubert767)
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Topic starter 08/11/2025 10:11 am  

@jamespa

Thanks for your response.

There is no buffer, LLH, secondary pump or indoor thermostat. The system is  running weather compensation and fully open loop and I've calculated the system volume to be about 120 ltrs of UFH, the HP was a retrofit to the system.

Rob



   
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 JamesPa
(@jamespa)
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08/11/2025 10:28 am  

Posted by: @ubert767
↑

@jamespa

Thanks for your response.

There is no buffer, LLH, secondary pump or indoor thermostat. The system is  running weather compensation and fully open loop and I've calculated the system volume to be about 120 ltrs of UFH, the HP was a retrofit to the system.

Rob

Hmm

 

The spikes are very short (so quite possibly very little energy and thus of little significance) but too low to be just the water pump.  It would be useful to look at a couple of them expanded out.  Have you got any flow temp logging?  Do you know whether the compressor is coming on during these spikes?   They might be something else.

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Ubert767
(@ubert767)
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Topic starter 08/11/2025 10:51 am  

Here is an expanded view of this morning's plot. I was wondering if the spikes are the result of the inverter doing its thing? I haven't yet checked on the compressor's state during this period.

Screenshot 2025 11 08 at 10.36.20

The only other data I have are temperatures of various points on the HP outputs/inputs.

Screenshot 2025 11 08 at 10.37.53

 



   
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 JamesPa
(@jamespa)
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08/11/2025 11:09 am  

Posted by: @ubert767
↑

Here is an expanded view of this morning's plot. I was wondering if the spikes are the result of the inverter doing its thing? I haven't yet checked on the compressor's state during this period.

With the expanded view you can see it ticking along at about 300W other than during the DHW reheat and between 4.30 and 5am.  The spikes are very narrow so I think you are right its likely just the inverter doing its thing, something else kicking in periodically or possibly even an artefact not representing whats actually happening.  There is little energy in the spikes and they arent particularly high so Id keep a watch but wouldnt be particularly concerned.


This post was modified 2 months ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Ubert767
(@ubert767)
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Topic starter 08/11/2025 2:41 pm  

@jamespa 

Initially, I didn't know wether the dreaded "short cycling " was regarded as the frequency of the modulation  of the compressor by the inverter  or as the frequency of "cold "compressor starts.

I'm very grateful for you responses.

Rob



   
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 JamesPa
(@jamespa)
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08/11/2025 3:18 pm  

Posted by: @ubert767
↑

@jamespa 

Initially, I didn't know wether the dreaded "short cycling " was regarded as the frequency of the modulation  of the compressor by the inverter  or as the frequency of "cold "compressor starts.

I'm very grateful for you responses.

Rob

Its the heat pump, more specifically the compressor, cycling on/off quickly.   The most important parameter is the on time, anything less than about 10mins would be considered short.  Short cycling reduces efficiency (because heat is wasted, and for other reasons) and increases wear.  Given what the expanded plot shows I now dont think this is cycling at all so nothing to worry about.

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Ubert767
(@ubert767)
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Topic starter 08/11/2025 3:26 pm  

@jamespa 

Thanks again for your reassurance.

Rob



   
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Toodles
 Toodles
(@toodles)
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09/11/2025 3:07 pm  

@ubert767 Just for comparison, here is a plot of my Daikin EDL08 Monobloc system from the last 36 hours or so; the pump is controlled via a Homely smart controller and we heat the whole house to 22.5 deg. C 24/7. You will see that when the OAT drops to ~7 deg. C, the system just ticks over at ~400 - 500 Watts without any cycling. Regards, Toodles.

IMG 1385

 


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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Toodles
 Toodles
(@toodles)
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09/11/2025 4:09 pm  

I might add that we do have an LLH with a secondary pump (Wilo Pico). Having put temperature probes on all four LLH ports, I have been able to set pump speed to match the primary pump in the Daikin which, once the system settles down, runs at a flow rate of 15 litres/min. As this is the flow rate the majority of the time, I have been able to set the pairs of LLH ports to be within 0.2 degrees C most of the time. When the pump starts up, it runs at a flow rate of 28 litres/min. but very quickly settles back to 15 litres. There are also brief excursion down to 7.5 litres occasionally so at some points the LLH must be experiencing some distortion - but minimal I think. Regards, Toodles. 


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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Toodles
 Toodles
(@toodles)
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09/11/2025 4:15 pm  

Herewith a plot of flow rates during the last 36 hour or so. Regards, Toodles.

IMG 1386

 


Toodles, heats his home with cold draughts and cooks food with magnets.


   
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