multi-city units

[littlebigdad]

I need to recover and weigh in the charge on 3 city multi units tomorrow. Is there a way to pump the systems charge into the condenser to make recovery easier? What would be the best way to do this job. thanks

[Brent Ridley]

I haven't had much experience with the city multi units. I would suggest calling Mitsubishi tech support with any questions regarding these systems. 678-376-2900. You may also be able find information @ www.mylinkdrive.com.

[littlebigdad]

Thats what i figured i would do, just thought i might get a heads up before i get down there. thanks for the reply

[Pete838]

Mitsubishi isn't very helpful unless you've been to their seminar or your company is approved or certified or whatever they call it. On the last few we've done we've removed refrigerant to a recovery cylinder and weighed back in from there. No special technique.

[littlebigdad]

well, i recoverd two units today. the first was 20 pounds over charged the second was 33 pounds under charged pulled a vacuum on that one and it was holding for about an hour, pressurized it to 500 psi with nitrogen till monday. i hope it holds. dont really want to leak check this mess

[manny238]

How did your units get overcharged or undercharged? I just finished installing 3 citymulti units they made me measure my liquid line by length and size so they can provide me with the amount of refrigerant needed. We kinda got it easy though my company hasnt sent any techs to get the start up training. So Mitsubishi did the start up for us on all three units. So far ive had nothing but luck with these units. 3 years ago i installed 2 citymulti units and never had one service call.

[littlebigdad]

we're not the installing company. got called in to fix somebodys problem. Im not sure how many people have been here but we are trying to make it right.

[Cold shot]

Just use a good recovery machine & keep recovery tanks cool.

[Cold shot]

I have installed 3 of the City Multi systems- a great tip is to inventory your refrigerant tubing (lengths) & save the unused pieces from each branch line. Measure those left over pieces & subtract to get a very accurate line length. I have use this method & weighed in charge, have never had any issues on any of the jobs. Oldest system still working installed in 2004! These are great systems only if installed properly, follow good refrigeration practices & they'll be very dependable.

[ga-hvac-tech]

Mini's, including single head, multi-head, and systems designed for many multi-heads... are ALL--VERY sensitive to charge... it is part of how they work. The 'rule of thumb' stuff we did/do with conventional split systems does NOT work.

Cleanliness is important!

Hmmm... I realize we are talking about a large amount of refrigerant... however if many folks have worked on the system... do we have any way to know if it has been contaminated with N2, air, wrong refrigerants, poor vacuum procedures, etc? If it were me, I would sell the owner on NEW refrigerant. If so, standard practices of cleanliness in leak check and triple evac would be my approach.

If you are going to re-use the refrigerant: Pull a vacuum on a CLEAN recovery cylinder. When you start to pull your charge, flush out your hoses, gauge set, cooling coil, etc. Be sure ONLY refrigerant gets into the recovery cylinder. I would err on the side of releasing a 'deminimus amount' of refrigerant... rather than chancing any inert gasses. Pull the charge down to almost zero. \
At this point do your leak check if you are going to.
Then carefully pull a thorough vacuum to specs. I go for at least 250 Microns most of the time. Remember to measure your microns at the unit, not at the gauge set or pump.

CAREFULLY calculate the charge and recharge... being careful your scale is not affected by things like hose weight and bumping.
ACCURACY is the most import thing here...

[Cold shot]

Use a high quality electronic vacuum gauge (not your $100 variety). Gotta trust your tools!

[waregl82]

Been through the City Multi training course. They have a design program where you "build" your system with available components. It will give you exact weigh in charge and liquid and suction line sizes. I like to recover, and weigh in virgin refrigerent.
Also, if you want to troubleshoot a city multi system, you will need the mitsubishi "box" and a laptop, and place your clips on the data circuit between components. The "box" is pricey, I think about $400. The rig will give you the diagnosis.

[Malarres]

If your concerned about a leak best bet it to focus on BC controller flare fittings will leak if a torque wrench is not used. If isolation valves were used at BC check schrader cores. Most checks I find oil present.

[waregl82]

The fact that the units are grossly overcharged/undercharged tells me the previous techs had no idea what they were doing. With the modulating compressors and fans, the old rules go out the window. In the course they specify that the system leak pressurization test must hold for 24 hours. Download all the "myLink" data from the Mitsubishi website. Use the system builder tool and set up the compoonents with the actual lengths of piping. Be accurate and the program will calculate the exact charge. Spend some time reading and understanding the documentation before you start the process. Check pressurization test requirements, and follow the instructions with method. This equipment is high tech and a different mindset is required to service. I changed a compressor on a Mr Slim my own company put in. The compressor was grounded in less than a year, and I know beyond a doubt (almost) that the system was not evacuated properly. The old method (get the vacuum pump started before lunch, then charge after lunch) doesn't work. I sarted pulling the vacuum and left it overnight to get it right. It has to be done with best practices. Good luck.