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MLZ-KP18NA

Here is my situation.

The Mitsubishi ceiling cassette  MLZ-KP18NA is installed in my living room ceiling. The unit is connected to a 4 ton Mitsubishi system outside. Above the ceiling is the attic which gets extremely hot on a sunny 90° day. I bought a digital thermometer with two j couple sensors so I could measure return and supply on the ceiling unit. On a cloudy 70° day return air is 72° and supply air is 45°all Fahrenheit of course. What happens is on a sunny 90° day return air is 74° and supply air is 64°. The result is the unit cannot keep up with the heat load and the room continues to get warmer and warmer.

The installer has checked refrigerant levels and pressures and whatever else should be right and says everything is fine with the system. Of course everything is not fine with the system for me as I can't cool the living area on a hot day.There is very little insulation around the ceiling unit in the attic just a few batts of insulation.

Am I correct to assume that the attic temperature has a lot to do with my problem? 

I'm thinking of just installing a wall mount unit and taking the loss on the ceiling unit.

Thanks for any assistance.

Bill




Comments

  • Jamie Hall
    Jamie Hall Member Posts: 16,312
    What insulation -- if any -- is on the ceiling between the attic and the living space? If none, what you have is the poor air conditioner bravely fighting a very good and effective radiant ceiling heating system...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • billrdickson
    billrdickson Member Posts: 4
    There are two levels of bat insulation and I jhad mass save come out and seal up the house. The installer did a manual j load calculation and the unit is sized correctly for the heat load. My question is. Is the high temperature in the attic affecting the performance of the ceiling unit. The Delta t across the unit drops significantly as the attic heats up.

    Thanks for your input
  • ratio
    ratio Member Posts: 2,821
    If the ΔT drops, the system isn't keeping up. If the ΔT stays the same (or similar) but the discharge temp rides up, the head isn't keeping up. Everything is going to be wanting more cooling when it's warmer out, you'll need to look at the whole to find the culprit.
  • mattmia2
    mattmia2 Member Posts: 3,427
    You need to know the wet bulb temp of the return air to compare 2 different sets of operating conditions. That energy could be gong to condense the moisture in the air.
  • billrdickson
    billrdickson Member Posts: 4

    I've been through a lot with the installer as far as measuring everything in the system including wet bulb. 

    I was just trying to find out if anyone else had issues with this cassette system being installed in the ceiling with an attic that gets extremely hot in the summer.

    Again thanks for the responses.

    Bill


  • ratio
    ratio Member Posts: 2,821
    If it's really just stuck in a hole cut into an unconditioned attic space, I can certainly see that influencing things. Any chance of getting an insulated box built above it?
  • billrdickson
    billrdickson Member Posts: 4
    Yes that's what I was thinking. I'll build an insulated box around it in the attic and see if it makes a difference.

    Thanks. I'll post what I find out.
  • mattmia2
    mattmia2 Member Posts: 3,427
    It should be underneath the insulation and it should be sealed to the ceiling and the unit itself should be sealed so it isn't drawing in attic air.