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Mitsubishi cycling while on chilled water output

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 36 minutes ago
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downfield
 downfield
(@downfield)
Estimable Member Member
Joined: 7 months ago
Posts: 56
Topic starter 29/06/2025 5:39 pm  

We have our Mitsi 14kW Zubadan on light cooling during hot weather.  The flow temp is set to 15C on the controller to avoid condensation problems.

Today I noticed that the unit is cycling every few minutes because it is chilling down to around 12.5C  and then turning off.  The return temp is around 19C and seems fairly consistent.

Not sure if it makes a difference but I have a 100 litre 4 pipe buffer but very little other volume as all Jaga fancoil rads - perhaps this volume is too small for the ASHP?

Anyway, is it possible to reduce the cycling?

IMG 8651
IMG 8652

Mitsubishi Zubadan 14kW with Mixergy 210l DHW in 220m2 barn property. 24 solar panels = 9kWp with GivEnergy 5.0kW Hybrid inverter and 19kWh GivE batteries. Jaga Strada fan-assisted rads throughout. Landvac vacuum glazing/triple glazed windows.


   
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 JamesPa
(@jamespa)
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Joined: 2 years ago
Posts: 2602
29/06/2025 7:10 pm  

Sorry to start with the obvious but are the radiators getting cold and are the fans on.

Heat pumps cycle when cooling for the same reason as they cycle when heating, insufficient load.  Also I wonder how a 4port buffer works when cooling, it's possible that most of the incoming cold water falls to the bottom and returns to the heat pump, while most of the incoming warm water rises to the top and returns to the radiators (which is one reason why I ask whether they are getting cold)

Assuming the rads are working at full fan and getting cold (it's worth checking the flow temp to the rads from the buffer) then it's cycling because the load is less than the min output.  Not much can be done about this other than a volumiser which will increase the cycle time but with about the same on/off ratio.

This post was modified 1 hour ago 2 times 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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downfield
 downfield
(@downfield)
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Joined: 7 months ago
Posts: 56
Topic starter 29/06/2025 7:20 pm  

@jamespa yes the rads are getting cold and the fans are on - but not 15C cold - more like 17C with a return of 19C.

I think the buffer operation point is a good one - that could explain the cycling.

The buffer is of course in a (currently) very hot loft space which won't help matters.

Mitsubishi Zubadan 14kW with Mixergy 210l DHW in 220m2 barn property. 24 solar panels = 9kWp with GivEnergy 5.0kW Hybrid inverter and 19kWh GivE batteries. Jaga Strada fan-assisted rads throughout. Landvac vacuum glazing/triple glazed windows.


   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 2 years ago
Posts: 2602
29/06/2025 7:46 pm  

I think that explains it.  Unless you can somehow increase load or system volume I think you are stuck with the cycling, unless of course others can suggest something.  Bypassing the buffer will probably help but of course that's major surgery.  

This post was modified 26 minutes 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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