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Mitsubishi Ecodan 11kw Defrosting Issue.

 
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Air Source Heat Pumps (ASHPs)
Last Post by meehow 4 months ago
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dgclimatecontrol
 dgclimatecontrol
(@dgclimatecontrol)
Trusted Member Member Professional Installer
Joined: 9 months ago
Posts: 71
09/09/2025 4:35 pm  

@morgan When you remove the cover most people will find it daunting so I would say leave well alone. I've never seen any ME sensor in the wrong location and they must have the strictest manufacturing methods of any brand.   Its also not the installers job to check them unless a problem is diagnosed. Most wouldn't have a clue if they had been fitted correctly from new, apart from perhaps the return air sensor.



   
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Morgan
 Morgan
(@morgan)
Noble Member Member
Joined: 5 years ago
Posts: 698
09/09/2025 4:42 pm  

Posted by: @dgclimatecontrol
↑

@morgan When you remove the cover most people will find it daunting so I would say leave well alone. I've never seen any ME sensor in the wrong location and they must have the strictest manufacturing methods of any brand.   Its also not the installers job to check them unless a problem is diagnosed. Most wouldn't have a clue if they had been fitted correctly from new, apart from perhaps the return air sensor.

@dgclimatecontrol

I have no intention of removing the cover. As you suggest, way too daunting for me. Thanks for the input. Appreciated.

 


Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.

2 ESS Smile G3 10.1 batteries.
ESS Smile G3 5kw inverter.


   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 440
09/09/2025 10:19 pm  

@hcas and @others This is something I picked up over the years, most brands have the same issue, most sensors are in their pockets and ok but, once in a while one can be slightly off and it is easy to visually check them and "wiggle" them, paste them or re-fix them rather than the values slightly off, it can sometimes be hard to work out whats not quite right when you get slightly offset values due to a loose probe. In this case on install and commissioning and during a service we just check every probe, its just to make sure.


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 440
09/09/2025 10:20 pm  

@morgan this can just be done by your sepecilist when they service the unit, infact you could insist on it as part of the service.


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 meehow
(@meehow)
New Member Member
Joined: 6 months ago
Posts: 2
05/11/2025 8:54 pm  

Hello. The Ecodan pump worked well last season, but this season there is a problem: it does not defrost and is covered with a 2 cm layer of frost. It does not start defrosting at all, as can be seen in the program. The outside temperature is around 0-2 degrees Celsius. It has done this several times, always in the morning. Do you know what could be the cause? I checked the sensors immediately after turning off the frosted pump in the morning. Th3 24 and Th7 2, which was also the outside temperature. However, during operation, the two-digit display on the outdoor unit shows H6 and H2 when switched off. I tried to change the pump's operating mode to heater only on the controller, but it does not turn on the heating. Does defrosting have anything to do with the heater?


This post was modified 6 months ago by Mars

   
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 ThunderMink
(@thundermink)
Active Member Member
Joined: 1 year ago
Posts: 9
Topic starter 03/12/2025 12:40 pm  

Ok, so now i have finally sorted out the main problem, TH3!! Erlier i had followed the troubleshooting guide and measured all the thermistors and all was in range so it looked like the problem was elsewhere. I then isolated all the sensors and put thermal paste on them and that led to a good rise in COP, but as usual with heatpumps you wont really see the problem before it gets cold outside 😀 Two weeks ago the outside unit started to freeze up again, checked the outside unit thermistors using FTC5 main controller running information 005 = TH3 and 007 = TH6, TH3 showed 0C while TH6 showed -11C, then measured the pipe with a thermocouple and it was about -10.8C, then i Removed the sensors and did a proper calibration. TH3 and TH6 was removed and all thrown in ice water and sat for an half hour or so. TH6 = 0.2C and TH6 showed 11C, so the problem all along was a shitty thermistor that had drifted by 11 degrees C. Temporary replaced TH3 with unknown beta thermistor of same resistance value and everything works and defrosts completely remove all the ice every time. Will replace TH3 with an original part when i find one to order but as is it is working now. This was quite a learning experience. Hopefully this will help others with defrost problems



   
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 ThunderMink
(@thundermink)
Active Member Member
Joined: 1 year ago
Posts: 9
Topic starter 03/12/2025 12:43 pm  

@meehow Try to check the temperature of TH3 if your unit have it installed. Mine was off by 11 degrees C causing defrost cycle to malfunction and finish its cycle prematurely or not start at all causing same problems as your ashp



   
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 F1p
(@f1p)
Estimable Member Member
Joined: 6 months ago
Posts: 112
03/12/2025 9:59 pm  

@thundermink 

The logic is Outside Air - TH3 >= 10 then defrost



   
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 ThunderMink
(@thundermink)
Active Member Member
Joined: 1 year ago
Posts: 9
Topic starter 04/12/2025 8:47 am  

@f1p Thanks for the info, good to know



   
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 meehow
(@meehow)
New Member Member
Joined: 6 months ago
Posts: 2
12/12/2025 4:34 pm  

@thundermink I managed to find the fault. I did it a long time ago, otherwise I would have frozen 😉 . But you're right, the cause was TH3 showing the wrong readings. I bought a replacement with the same parameters for pennies and it works. Greetings from Poland.



   
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