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Freeze Stat/Defrost setting….Ecodan in freezing conditions

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(@amanda1)
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Several members have suggested I start a new topic re Freeze Stat (Defrosting) settings on the Ecodan. I should state that I am still an ASHP newbie, and an ASHP ‘layman’, and the technical terminology still evades me. This is specifically about the Freeze Stat setting, and not about the pros and cons of heat pumps in general , or about Weather Compensation Mode, or the other settings. Assume that all your other settings are as good as you can get them.

The Freeze Stat setting is possibly overlooked by some ASHP owners because average winter temperatures in the UK rarely fall below zero for very long. With a default setting of 5 degrees C the defrost function of the Mitsubishi Ecodan will start to kick in, meaning that the pump will run frequently to defrost itself. The immersion heater will also be used to heat the house while the pump heats itself. Result =££££££ if it goes on for a long time. So if the outside temperature drops to 5 degrees and you have this setting on, then the pump will cycle more and it will cost you more while it does this. 

The minimum setting for the Freeze Stat is 3 degrees. You can set it to *. But if you set it to * because you need it to be lower than 3 degrees, you are deactivating the Freeze Stat function and you become reliant upon the concentration of antifreeze in the system preventing icing up.

I live in an area of the UK which is famous for its snowfall and arctic temperature in Winter, and the temperatures in December can be as follows:

IMG 4964

Compare with the rest of the UK:

IMG 4967

My new Ecodan has the Freeze Stat setting at **. I am currently checking with the installers the % concentration of glycol in my system. I am trying to avoid having to have the defrost cycle of the pump running 24/7 for most of December and January, which of course guzzles electricity. 

Mitsubishi make big claims for the PUZ WM85VAA ASHP - that it is capable if operating in ambient temperatures of -20C.

My question is this: I wonder if there are many Ecodan owners or ASHP owners out there who have a similar problem - that the ambient temperature is below zero for prolongued periods- and who have tried having the Freeze stat set to *, and if this was a disaster, a success, a work in progress….??? 

Or, if your ASHP has operated on the defrost setting for a protracted period of time, what was the energy consumption per day like in KWh?


   
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(@walkers-heatpump)
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So my understanding (I obviously maybe wrong) is that pretty much all the freeze stat does is turn on the circulation pump so the cold exterior water is pumped through the home to keep it at about 20°. The actual energy usage isn't particularly high, perhaps 100watts or so per hour, but obviously adds up over a long period, likely during overnight setback periods.  

As for defrost I didn't think the Ecodan used an immersion? My understanding was instead it would go in reverse so whack a bit of cold into the house briefly whilst it melts the ice. Defrost shouldn't run constantly, only say once (or twice) an hour for a short period? 

I don't have any glycol in the system so I've not messed with the freeze stat

Melcloud should provide a breakdown of how long it's in each mode


 


   
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(@amanda1)
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@walkers-heatpump the freeze stat temperature setting is the temperature at which the defrost cycle that you describe begins. There is also a Cold Weather Protection setting which kicks in at much lower temperatures than the defrost cycle. My system definitely has an immersion heater ! There is a big switch in the cupboard that houses the cylinder and the FTC6, that says “Immersion Heater”…..and my consumer unit has also got a switch in it that says “immersion heater”.


   
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(@walkers-heatpump)
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@amanda1 that immersion should be the one in the hot water tank I would think, would come on once a week to do the legionella cycle on the hot water.


   
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(@amanda1)
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@walkers-heatpump I only know what the switches say….😂 In my case the Legionella cycle runs every 15 days. Check out the Heat Geek website.


   
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(@scrchngwsl)
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I actually have some data on this from my own system:

Screenshot 2023 06 06 205737

Hopefully you can see this rather ugly graph. What it shows is the flow and return temperature of the water in my radiators (the top 2 lines); the indoor temperature (the basically flat orange line in the middle); and the outdoor temperature (the one at the bottom).

What you'll notice are the following:

  1. The outdoor temperature is typically at or below 0 degrees C for most of the day, but every now and then has these spikes to about 5 degC.
  2. The flow/return temperatures are around 45 and 42.5 degC respectively for most of the day, but every now and then drop down to about 35 degC.
  3. These two things happen at the same time.
  4. The orange indoor temperature line doesn't change at all throughout the day and stays around 22 degC

The reason for this is the defrost cycle kicking in about 15 times in this 24 hour period. I know this because I've counted the spikes on the outdoor temperature (there are 14 I can see) and the troughs on the flow/return temperatures (there are 16 that I can see). So call it 15 defrost cycles per day. They last no more than 2 minutes at a time (you can't see this on the image but I can see it when I hover over the data points and look at the time stamps).

As @walkers-heatpump says, the defrost cycle doesn't use the immersion heater, it simply runs the heat pump in reverse and temporarily extracts heat from the water flowing through your radiators in order to melt the ice that has accumulated on the outdoor unit. As I said in the other thread, I live in the South East and while we've had some prolonged cold spells, they've never been that cold, so this is probably the best data I have. I can say with confidence that with temps hovering slightly below zero, the defrost cycle is imperceptible and makes no difference to the indoor temp (which you can see from the orange line). I don't know what it would be like in the much colder temperatures you experience, however.

ASHP: Mitsubishi Ecodan 8.5kW
PV: 5.2kWp
Battery: 8.2kWh


   
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(@amanda1)
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Topic starter  

@scrchngwsl thank you - that description of the defrost cycle tallies with what the heating engineer told me this afternoon. Your indoor temperature is high!! I obviously have the disadvantage (or is it an advantage) of having had the ASHP installed at the start of May so all this joy is still to come. I know that my system should be able to cope with the cold weather, but whether I will be able to cope with the bills is another matter! However, all the feedback I am getting in this forum suggests that consumption will not be as high as it was with my storage heaters!


   
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(@kev-m)
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Posted by: @walkers-heatpump

So my understanding (I obviously maybe wrong) is that pretty much all the freeze stat does is turn on the circulation pump so the cold exterior water is pumped through the home to keep it at about 20°. The actual energy usage isn't particularly high, perhaps 100watts or so per hour, but obviously adds up over a long period, likely during overnight setback periods.  

As for defrost I didn't think the Ecodan used an immersion? My understanding was instead it would go in reverse so whack a bit of cold into the house briefly whilst it melts the ice. Defrost shouldn't run constantly, only say once (or twice) an hour for a short period? 

I don't have any glycol in the system so I've not messed with the freeze stat

Melcloud should provide a breakdown of how long it's in each mode


 

Explanation here

https://renewableheatinghub.co.uk/forums/rules-terms-of-use/say-hello-and-introduce-yourself/paged/48#post-19959

The only thing I'd add is that the ASHP only kicks in at the configurable 3-20 deg set point and when the flow gets below 15 deg, until then it's just the pumps running (and I agree it's about 100W).  That's from my data; the manual doesn't tell you this.   

 


   
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(@kev-m)
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As @walkers-heatpump says, the defrost cycle doesn't use the immersion heater, it simply runs the heat pump in reverse and temporarily extracts heat from the water flowing through your radiators in order to melt the ice that has accumulated on the outdoor unit. As I said in the other thread, I live in the South East and while we've had some prolonged cold spells, they've never been that cold, so this is probably the best data I have. I can say with confidence that with temps hovering slightly below zero, the defrost cycle is imperceptible and makes no difference to the indoor temp (which you can see from the orange line). I don't know what it would be like in the much colder temperatures you experience, however.

Pedantic maybe but it doesn't do this. It's nothing to do with the radiator flow; it's the hot refrigerant in the ASHP that is reversed to melt the ice in the coils. This explains

Although your room temperature isn't dropping, the flow temperatures are. Your ASHP has to raise the flow back up each time and this uses power.  If you had a power trace you would see a spike.  Luckily (or by design) your ASHP has enough power to maintain your chosen room temp despite the defrosts but it is using more of it.  Below is a similar trace from mine. If I analyse my data I reckon it's using 15-20% more power because of defrosts. (that's defrosts, not Freeze Stat). 

Defrosts

 

   

 


   
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(@scrchngwsl)
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@Kev-M Understood - thanks for the clarification! Not pedantic at all and I'm always happy to be corrected 🙂

I do have a power trace and it does indeed spike, but not that much higher than the "steady state" load. 15-20% more power consumption as a result of the defrost cycle sounds about right to me.

ASHP: Mitsubishi Ecodan 8.5kW
PV: 5.2kWp
Battery: 8.2kWh


   
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 Linz
(@linz)
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Posts: 6
 

This has been plaguing us since last year. Our pump is set at 7° and with living in the north seems to be constantly on through the winter costing unnecessary ££££££'s.  Can anyone help us with changing the minimum setting to 3° please.🙏🙏


   
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(@kev-m)
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Posted by: @linz

This has been plaguing us since last year. Our pump is set at 7° and with living in the north seems to be constantly on through the winter costing unnecessary ££££££'s.  Can anyone help us with changing the minimum setting to 3° please.🙏🙏

@Linz,

can you explain a bit more what the problem is and what you want to do?  You can't change anything to do with defrosting as far as I know. Freeze Stat can be adjusted or disabled but it won't make a big difference to your bills even if you disable it.  Explanation above in this thread or look at p 36 in the FTC manual to see how to change it. It's in the Service Menu, Operation Settings. 

https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_FTC6_PAC-IF071-3B-E_Installation_Manual_BH79D843H02#page-38-39

 


   
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