Mitsubishi City Multi - BC Box LEV3 Stuck Open

[maddiedog]

Hi,

Could anyone tell me the likely effect on the cooling capacity of the units connected to a 16 way BC Box that has the valve LEV3 stuck in the open position? From my observation there is little cooling at some of the indoor units connected to this box and the outdoor unit seems to be idling all the time. System is heat recovery type with twinned outdoor units

[JoeAT50A]

Any Maintenance tool data for us to analyze?

[marc5180]

With LEV3 stuck open you would get a high to low bypass and possibly 1500 overcharge error since the discharge superheat would be low due to floodback.

Look on the SW1 data and look at sensors TH16 and TH15 (either side of LEV3)

[maddiedog]

Thanks for replies. I'm not a Mitsubishi service engineer so haven't got the maintenance tool data for analysis. I have aksed Mitsubishi for a comment as to the likely effect on the connected indoor units and been told it could vary from no cooling to partial cooling. As I understand the operation of LEV 3, a stuck open valve would be bypassing refrigerant from the indoor units but I was hoping Mitsubishi could give an estimate of the likely capacity loss.

[JoeAT50A]

Sound to me contaminants or carbons stuck on the LEV valve seat..
Best is to get BDT engineers to assist you on your issue.

[marc5180]

Can I ask why you would like to know the capacity loss?

Do you have a problem with a particular unit or is it just a theoretical answer you are looking for?

LEV3 is probably the most important LEV in a city multi system since it controls the flow through the subcooler circuit and also helps to control the 3-4 bar diff.

I would have thought the capacity loss would be difficult to estimate due to too many factors that could influence it.

[Thermatech]

In cooling only mode the LEV 3 at the BC box is only controlling the flow of liquid through the subcooler circuit. The BC box constantly monitors the temperature sensors & pressure sensors to calculate subcooler performance & adjusts the LEV3 to match target.
Generally the LEV 3 in this mode will only be open 10% to 30%.
But if the LEV 3 is locked at 100% then you have a major liquid to suction bypass & some liquid will flash off in the suction line making low suction temp.

In this case the outdoor unit is constantly monitoring the suction temp & as it is well below target the outdoor unit ramps down compressor speed to minimum.
With the outdoor unit running the compressor at minimum speed there is little or no cooling at the indoor units because the liquid that does get to the BC box from the outdoor unit is bypassing straight through the LEV3 & back to the outdoor unit on the suction.

You might get some limited cooling if you only operate 1 or 2 indoor units but if you operate any more indoor units you wont get any cooling as the outdoor unit wont ramp up the compressor.

The LEV 3 problem needs to be resolved to get any performance from the system. Provided you have reasonable access to the BC box then swapping out the LEV 3 head is easy & should have the system fully operational within 1 hr.
But if the problem is with the valve body & some contamination locking the needle full open then the refrigerant charge needs to be recovered & a new valve body brazed in.

[maddiedog]

Hi,

Thanks for all the very helpful replies. I have had LEV3 replaced now and am hoping all is well but very useful to have a good explaination of the effects to tell the affected parties. Again many thanks.