Results 1 to 6 of 6
  1. #1
    Join Date
    Jun 2022
    Location
    UK
    Posts
    3
    Rep Power
    0

    Mitsubishi FDC250VS & four FDT60



    Hi everyone,
    I have a question regarding our aircon system and hope the fantastic knowledge on this forum can help.

    We recently purchased a building that has an aircon system comprised of a Mitsubishi FDC250VS outdoor unit and four FDT60DV ceiling cassettes, the whole system is operated by a single RC-E5
    wired controller.

    All four cassettes operate within the same floor space, we find that we don't need two of them working and my question is can individual cassettes be disabled or turned off?

    Each cassette has a power isolator switch but if I turn off one the whole system stops working, a red LED flashes on the controller.

    Many thanks in advance for any help that can be given.



  2. #2
    Join Date
    Apr 2008
    Location
    Southampton, England
    Age
    49
    Posts
    1,045
    Rep Power
    35

    Re: Mitsubishi FDC250VS & four FDT60

    All four need to run together because individual units can't regulate refrigerant, that's done by the outdoor unit. Also outdoor unit is sized to run all four, i.e 25kW outdoors and 24kW (4x6kW) indoors
    When you turn one or more off then the system registers a communication error which is those flashing lights.
    You could change to a smaller outdoor unit if you can get one compatible but the pipework would need resizing so pretty much a new install I'm afraid.
    You could block one (or two at a push) of the outlet vents, might help?

    Cheers,
    Andy.

  3. #3
    Join Date
    Jun 2022
    Location
    UK
    Posts
    3
    Rep Power
    0

    Re: Mitsubishi FDC250VS & four FDT60

    Hi Andy,
    Thanks for taking the time to reply and explain, much appreciated

    My background is in electronics so I was naturally curious to try and find out how these systems worked. I had assumed that the cassettes could control refrigerant but the pipework layout kind of suggested this was not so and you have confirmed that.

    The controller shows one master and three slaves, unfortunately the two cassettes that we don't need push out very cold air (closest to outdoor unit) and the two we do need cool ok but not as good (furthest from outdoor unit).
    Again, if you could feed my curiousity it would be appreciated and forgive my assumptions...

    If the load was only 12kW would the outdoor unit simply not run as much compared to it having a 24kW load?

    I thought that in a system like this there might be some way to "balance" the cassettes "refrigerant wise" to compensate for the pipe run distance?

    Today I disconnected power, signal line and the controller X/Y lines from one cassette (always experimenting) in the assumed belief that if the controller cannot see the cassette then it will just run the other three.
    Silly me! I just get an E14 error message for the "missing" cassette.
    Do you know if there is an initial process where the controller "finds" the cassettes? If one cassette could be deleted from the controller I then assume that the system would run the other three but would the 25kW source/18kW load mismatch be detrimental?

    Sorry for all of the questions!

  4. #4
    Join Date
    Apr 2008
    Location
    Southampton, England
    Age
    49
    Posts
    1,045
    Rep Power
    35

    Re: Mitsubishi FDC250VS & four FDT60

    Quote Originally Posted by pabro View Post
    Hi Andy,
    Thanks for taking the time to reply and explain, much appreciated. Sorry, missed your reply but have a few answers for you

    My background is in electronics so I was naturally curious to try and find out how these systems worked. I had assumed that the cassettes could control refrigerant but the pipework layout kind of suggested this was not so and you have confirmed that.

    The controller shows one master and three slaves, unfortunately the two cassettes that we don't need push out very cold air (closest to outdoor unit) and the two we do need cool ok but not as good (furthest from outdoor unit).
    Again, if you could feed my curiousity it would be appreciated and forgive my assumptions...

    If the load was only 12kW would the outdoor unit simply not run as much compared to it having a 24kW load? Correct. The compressor is inverter controlled so it will slow down (to a limit, 50% I think as a general rule) thereby limiting its duty.

    I thought that in a system like this there might be some way to "balance" the cassettes "refrigerant wise" to compensate for the pipe run distance? Pipes are sized to accommodate required refrigerant flow at maximum duty downstream from the outdoor unit. The branch pipes used to split the pipe run to run to different indoor units should ideally be equally spaced so refrigerant flow is equal to all units. It might be the case that the two further units have a longer pipe run so the two closest units get the lions share just because there is less resistance to flow.
    If you had a VRF system (more £££'s!) where each indoor unit has its own metering valve then this wouldn't be as much of an issue. Because you have a system where there is one metering valve on the outdoor unit you are at the mercy of the pipe install and how equal the resistance to refrigerant flow is to each unit.

    Today I disconnected power, signal line and the controller X/Y lines from one cassette (always experimenting) in the assumed belief that if the controller cannot see the cassette then it will just run the other three.
    Silly me! I just get an E14 error message for the "missing" cassette. E14 is no master indoor unit. When they are installed you set the Dip switches as master unit, slave 1, slave 2, slave 3. It might be by sods law you removed the master one!
    Do you know if there is an initial process where the controller "finds" the cassettes? If one cassette could be deleted from the controller I then assume that the system would run the other three but would the 25kW source/18kW load mismatch be detrimental? On power up the outdoor will check to see what's connected first. Power up again and the process repeats, it doesn't store the infomation. I'm not sure if there is a limit on duty connected, some makes do for sure but for MHI I can't be certain. Assuming that isn't an issue and you've removed one unit from the system electrically, it still has refrigerant flowing to it because the pipes are still connected. In cooling it would freeze up and start dripping water, in heating there is a possibility for a high pressure fault because in both cases the fan isn't spinning.
    Sorry for all of the questions!
    Hope that makes some sense. In essence it's probable that two units cool better because the refrigerant flows easier to them (longer runs and more bends to the others = more resistance). Perhaps the branch pipes are spaced too far apart between the furthest units?
    To take a unit off requires altering pipe sizes to suit and that assumes the outdoor can run at a lower duty to suit.
    Some units you set the ceiling height and this ups the fan speed for a higher ceiling. I had a look but couldn't see that option on those units, only found an old manual so could be wrong there. Otherwise like I say, if you can block of a couple of vents on the closet units might help. Blocking off only one vent seems to be the general advise but I'd imagine you'd get away with two.
    Last edited by Tayters; 24-06-2022 at 12:17 AM.

  5. #5
    Join Date
    Jun 2022
    Location
    UK
    Posts
    3
    Rep Power
    0

    Re: Mitsubishi FDC250VS & four FDT60

    Thanks for taking the time to give such a detailed reply, most informative and I have learned a lot!

    Of course in my naivety I have not considered that it isn't a matter of just isolating a ceiling unit electrically, as you have pointed out it is still connected to the pipework and refrigerant would continue to flow through, so there is no advantage in doing this.

    I have investigated the pipe runs and the two coldest running units are about 6 metres from the outdoor unit with very direct pipe runs and the others are a further 15 metres with a more convoluted pipe run so you are 100% correct.

    I will end my experiments with aircon at this point!

    Thanks again for your help.

  6. #6
    Join Date
    Apr 2008
    Location
    Southampton, England
    Age
    49
    Posts
    1,045
    Rep Power
    35

    Re: Mitsubishi FDC250VS & four FDT60

    No problem, glad it helped!

    There is a bit of liberty with the positioning of the branch pipes but it certainly seems yours are a step too far unfortunately. I guess the main run should have gone midway then branched off and double back to the nearest units (the units already cold) as well as onwards to the two furthest units. It could be repiped in principle, perhaps worth considering if you have the funds but best get the runs checked first against manufacturers install recommendations.

    If you need a recommendation let me know where you're based and depending where you are could point you in the right direction.
    Otherwise make sure the filters are clear on the units and only other thing I can think of to cause the issue is of you have lost some refrigerant but need to weigh it out to confirm ideally, otherwise run full pelt and check discharge temp of outdoor unit on controller, should be below 100°C. Give you something to fiddle with anyhow. :-)

    Cheers,
    Andy.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •