View Full Version : Mini split issue

05-06-2017, 09:39 PM
Good evening.
It's a long history,but in short:i bought a used mini-split system from the "famous" chinese manufacturer Hisense.It's 12000BTU.When i was byuing it was dismounted and i had no tools to connect everything and check.I have only applied power for few seconds to the compressor with closed service valves and it ran-good sign.The outdoor unit's fan was not running when i applied power.I have checked the starting capacitor and it was blown all over the inside of the outdoor unit.I replaced it and ran fine when power is applied.So i bought the mini-split.I have changed the service valve of the thinner tube,because it's thread was broken,then i have connected everything,evacuated to 150 microns,and charged with scale the right amount of refrigerant-1100g of R22.The problem was that the high-side pressure was rising above 30bar and the compressor was overheating and the termal protection was shutting it down.Also the suction pressure was too low-about 2bar at 15-16C ambient.I thought it is some kind of restriction,maybe in the cap tubes or somewhere else in the high side,so i disassembled everything,blown everything with air compressor (i know it is wrong,but i don't have nitrogen,and it's not worth to buy bottle and etc. just to use it one time),checked the four-way valve,checked the check-valve,replaced the capillary tubes,put a filter-drier on the suction side before the accumulator (i know it's best to put it on the liquid line,but it's impossible because of the filter's direction when reversing between cooling and heating),evacuated down to 150 microns and charged.The problem now it is that when in heating mode,thin layer of frost appears on the evaporator (outdoor unit's heat exchanger) and all the way to te compressor including the suction line accumulator just next to the compressor,when the outdoor unit's fan was not running.Here i need to mention that it turned out that the indoor unit (the PCB is there) won't start the outdoor fan,so i'm controlling it manually by applying power to the fan.If i operate the fan for few seconds,the frost disappears and the suction pressure rise up,but the discharge (of course) rise up too and seems too-high to me (about 27bar).So when the outdoor fan was not running i got 3bar suction pressure and 25bar discharge.The indoor unit's fan was at max speed,the indoor temperature was 26C,outdoor 23C,compressor surrent 6.3A,compressor temp 77C,23C subcooling.
Although charged correctly,i decided to add more refrigerant,just to check if the suction pressure rises and the answer is no.I overcharged it a lot-according to the data plate when in heating at 27C indoor and 25C outdoor the compressor current is 7.8A,but when i added refrigerant the current went up to 8.3A,but the suction pressure went only to about 3.8bar.
It's hard for me to formulate my questions,but i'll try:
1.According to my humble knowledge of mini-split systems they work as follow in heating mode:compressor starts,the outdoor unit's fan starts too.Discharge pressure rises to about 19-20bar,then the indoor unit's fan starts.the pressure continues to go up,to about 25bar,and then the outdoor fan stops.When the pressure drops to 19-20bar the fan starts again and that procedure repeats until the desired room temp was reached.Correct or not?
If it is correct,then in my case,when the pressure is high due to high ambient temps (maybe?) the outdoor fan don't need to run,so if that is the normal behavior frost should not be forming right?
2.When should the outdoor fan really run?.More precise-is it necessary to run with these high-ambients,or only when the ambients are low,thus the discharge pressure is lower and drops quickly?
3.Are these numbers normal for these conditions or not?(for me the suction side is low).What should they be?
4.Why when adding refrigerant,the suction pressure rises only slight,not dramatically,when the compressor current "says" overcharge?Restriction again?
5.Except restriction,undercharge(it's not my case),dirty coil (i have cleaned it) and low ambient (it's not now),what else can cause a low suction pressure?
I must say that in mini-split the metering device (in my case cap tubes) is in the outdoor unit with the compressor,heat exchanger and the four-way valve.The indoor unit is just the heat exchanger with the PCB.Here is a schematic of the inside with the check valve and the cap tubes
Thanks for any advices.
P.S-I'm not an refrigeration technician.I bought this ac unit to learn,because it's interesting to me,and i'm curios to make it work :)

05-06-2017, 11:31 PM
Are the capillaries that you changed the sane size as the originals?

Did you evacuate the system via both service ports?

You could have fitted a heat pump filter drier which would allow flow in either direction.

Pop Alexandra
06-06-2017, 10:47 AM
Have the same issue. Please let me know if you find out anything useful.

06-06-2017, 02:24 PM
Yes,i have replaced cap tubes with the correct size.As for the evacuation,originally the unit have only one schrader valve on the wider seevice valve.I added the second one by my self-there was unused tube sticking out of the outdoor exchanger,ofcourse it was soldered at the end.I have cutted it and soldered a schradrr valve,so that i can measure both the discharge and suction pressures at the same time.But yes,i evacuated from both sides.But if there was air/noncondensables,pressures should be high,not low and may be they should fluctuate?

06-06-2017, 04:22 PM
How does the unit perform when in cooling mode?

[Edit] Forgot to ask, what is the length of pipework you have between the indoor and outdoor unit? Is it within specification?

Also, what is the unit model number?

Capillary, right size - good; I assume correct bore not just same outside diameter.

06-06-2017, 08:39 PM
I didn't measure pressures/temps in cooling mode,because it got too late,but i'll measure them tomorrow.Without looking at any numbers,it seems right-it's cooling,no frost anywhere,nice cool air from the indoor unit.
The length of the pipework is about 2.5m.With that short pipes,i don't think that any additional refrigerant should be added.
The model number is KFR-2101GWE.
As for the cappilary -yes,the bore is 1.37mm as far as i can measure it correctly.