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  1. #1
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    Advice needed for mini-split



    Hello everybody
    A year ago the uncle of my wife gave me a working 12 000BTU ,R410 inverter mini-split system which someone was going to throw away,because it was replaced with a newer one.Since long ago i had an idea to try to use a mini-split as a air-to-water heat pump and use it to heat 2 rooms in the apartment of my parents.I know that the original air-to-water heatpumps use plate heat exchangers,but i can't afford one because they are very expensive for R410.So i decided to use only the outdoor unit and replace the indoor unit with a copper tube laid down in a 60l water tank so that the refrigerant is heating the water and then a pump circulate that water over fan-coils in the rooms.
    First I must say that the metering device is a cap tube and it's in the outdoor unit.So of refrigeration point of view,the indoor is only a coil with fan,and the replacement was simple-just wound the new coil,brazed 1/4" and 3/8" flare terminals to the coil ends, then silver blazed the coil to the tank's flange and put everything together.Since i can't find practical information how to calculate the required heat exchanging area i decided to get data from commercial plate heat exchanger for 12000BTU or 3.5kW and it turns out that it's about 0.33m2.Eventually i made it from 10m of 3/8" tube which gave about 0.3m2 - close.I'll not go into detail about electrical part of the conversion since it's not in interest.
    Finally i got everything to work in my garage for about a month and a bit more and it's working OK,BUT i have a feeling that it's not working efficiently.Some thoughts and facts-according to the manufacturer the unit's COP is 3.5 at +7C(44.6F) outdoor and +20C(68F) indoor.To get an idea about the real COP with minimum interventions i decided to measure the time required to heat the tank by 4 degrees (say form 32 to 36C) and compare with the energy consumed.I can't remember the real numbers,but it turns out that the energy put to water is about ~2.2 to 2.5 times consumed ,depending on the outdoor temp so the COP is 2.2-2.5 which is way lower than the stated from manufacturer.Need to say that i don't believe blindly to that info since it's cheap chinese crap but...
    Now i wonder if there is a problem at all ,or the lower COP is due to the higher condensing temp (i run at about 40C water temp,compared with the usual 20-25C room air temp).Forgot to mention that the amp draw is about 5.5A which is within specs-the plate says the maximum consumption is 6.8A.
    These mini-splits have only one service valve at the outdoor unit's 3-way valve,so i can only measure one pressure at time -suction in cooling mode and discharge in heating mode.Since i don't want to disassemble the unit but i wanted to measure both pressures,SH and SC i decided to put temp probes at the inlet and the outlet of the evaporator (which is outdoor coil in heating mode).Since the evaporator inlet in that case is right after the cap tube,there is liquid/gas mixture,so i can calculate pressure from the temperature measured,and in the same time i can measure the SH by subtracting two temperatures.So in short the results:

    Ambient- 0C (32F)
    ---------------------
    Evap inlet- -1C (30.2F)
    Evap outlet- 3.65C (38.57F)
    Suction pressure (from evap inlet)- 6.73bar (97.71psig)
    SH- 4.65C (40.37F)

    Condensing pressure- 34bar (493psig)
    Condensing temp- 56C (132.8F)
    SC- 11.5C (52.7F)

    Discharge temp- 64C (147.2F)
    Amp draw- 5.5A

    I know these splits work with flooded evaporator,so the SH should be 2-5deg Celsius around design temps,varying with temperatures.Knowing that and the fact that the volume of my coil tank is smaller then the original+lineset so no additional refrigerant needs to be added,as a non HVAC guy i can't figure is the smaller COP caused only by the higher condensing temp,is it about the charge,or it's just the manufacturer lie?
    Any advices about?:
    1.Can that method for measuring superheat be used or i'm in deception?
    2.Are my pressures and temps right?.I think that suction is higher -only 1C difference from ambient seems low to me,shouldn't that difference be much higher for better heat transfer?.Discharge temp and condensing temps seems right to me,amp draw also.SH-normal,SC-no idea
    3.I know that best method for charging these splits is weighing in but,how a cap tube system like that-modified or without specs is charged via SH,or what should be the target SH?.I've seen SH charging charts in the net ,but i don't know if it apply for this type of mini splits?
    Thanks in advance,and sorry for the long post.I wanted to be as descriptive as possible.I'm not a HVAC technician,so i don't have experience to judge if it's OK or not,but I like these matter,and want to learn something.Sadly only reading books is not enough



  2. #2
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    Re: Advice needed for mini-split

    No ideas?

  3. #3
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    Re: Advice needed for mini-split

    .I'm not a HVAC technician
    says it all!
    Grizzly
    Despite the High Cost of Living it still remains Popular!

  4. #4
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    Re: Advice needed for mini-split

    And because of that,i have no opportunity to learn something and get advises?No one was born with built-in experience and knowledge.Reading forums intensively from some time,i saw many people which are newly certificated and nevertheless asking same or similar questions as mine,but they get answers to their questions.Is the certificate the problem or something else?.I'm working responsibly with refrigerants,without letting them in nature and it's all for personal use,I'm not selling anything to someone.I only want to learn something new and interesting to me,but maybe that's a problem.

  5. #5
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    Brian_UK is offline Moderator I am starting to push the Mods: of RE Site Moderator : and general nice guy
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    Re: Advice needed for mini-split

    Only charge by weight.

    Without knowledge of the design specification of the unit it is hard to pass comment.

    Ambient of 0C may be the limit of the performance scheme.

    Running high condensing temperatures is possibly adding extra load thereby reducing efficiency.

    The modifications to the 'indoor' units controls may have a bearing on the system operation.
    Brian - Newton Abbot, Devon, UK
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    Re: Advice needed for mini-split

    Specification of this (and many like this one) mini-split is very poor-manufacturer states that the optimal performance is achieved at +7C outdoor temp and +20C indoor (these are Dry Bulb temps) along with some maximum amp draw in heating/cooling and maximum/minimum working outdoor/indoor temps.No other data,so nothing really useful.On the other hand after reading many compressor specifications,it turns out that the specified capacity is achieved at standard conditions (ASHRAE):
    Condensing temp- 54.4C
    Evaporation temp- 7.2C
    Return gas temp- 35C
    Liquid temp- 46.1C
    Ambient temp-35C
    The compressor is Toshiba DA108X1C-20FZ3 which has a capacity of 10 900BTu ,not 12 000BTu.....
    So the logical conclusion is that the manufacturer is trying to match the coils with the compressor at these temps but there's no data about pressures and so on.
    What is the common approach to charge that kind of systems if the factory charge is unknown by some reason?.There should be a way to do that,or otherwise the AC turns unusable?

  7. #7
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    Re: Advice needed for mini-split

    To put the question in other way-what's the difference between standart and dutctless mini-split,beside the fact that the metering device in the normal AC is in the indoor unit,and on the mini-split it's in the condenser?.What is the big difference so that the standart split with fixed metering device can be charged by pressure,and the mini's can't?

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    Re: Advice needed for mini-split


  9. #9
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    Re: Advice needed for mini-split

    Quote Originally Posted by Th.Mihov View Post
    To put the question in other way-what's the difference between standart and dutctless mini-split,beside the fact that the metering device in the normal AC is in the indoor unit,and on the mini-split it's in the condenser?.What is the big difference so that the standart split with fixed metering device can be charged by pressure,and the mini's can't?
    The difference now-a-days is that there are no more "standard" splits, like yours they use an inverter drive now. The inverter adjusts the compressor speed to suit the system operation monitored by the on board sensors to maintain conditions laid down by the manufacturer.
    It is therefore almost impossible to charge to a fixed superheat unless you have the system running at the exact speed to match the ambient and coil/airflow temperatures.
    The manufacturers have invested time and money in setting the required amount of refrigerant for their system. It is a waste of your time and money not to use the information provided by the manufacturer.
    Brian - Newton Abbot, Devon, UK
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  10. #10
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    Re: Advice needed for mini-split

    narkom,thanks.It's a useful reading
    Brian_UK,mine is inverter too.But it has a test button (like many others),which forces the machine to work at "full throttle" in cooling mode,so some troubleshooting can be done.In my country although they are 10+ years there are still many non-inverting air conditioners in use,so we are always a few years behind normal countries as usual,but that's another story.
    Assuming it's a non-inverting type,the variability of compressor and fans speeds does not play a role.So can this method be used in this case?
    As i said before,HVAC is not my occupation.It's something that's interesting to me,i would like to learn about it and practice,so i'm not perceive it like a waste of time and money.In it's free time someone like to go fishing,someone like to modify it's car,someone like to do nothing,i like to learn and experiment with new things,and if the end result is acceptable-why not use it?
    First case that comes to my mind as example about using the method which i'm asking for is that in my unit's manual there's no information about additional charge for longer pipework.In my case,the piping is about 1m for high and low side.Assume that i want to take the water tank further from the AC,say about 10m away,how can i know how many refrigerant i must add,when ther's no information?.I'll dump some quantity,but i must verify is it the correct amount,is it less,or more.Here comes the pressures,temps,SC,SH and so on,which will tell me what's going on.That's why am asking.The question is principal,not about my actual case.

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