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