Where is the coldness going?

[MushCreek]

I've been having trouble with one of my Mitsubishi Hyper Heat minis. I replaced the indoor coil a couple years ago due to a slow leak. It's been pretty good, but I noticed that it's not cooling well again. I used a 'recharge' kit with a 28 ounce can to top it up a bit, as the temperature at the service valve was high. I messed up bad with the recharge kit, because they don't tell you that it's going to leak refrigerant way too cold for gloves when you go to screw the tank on. Next time, I'll be ready with pliers.

Go ahead and scold me for trying to DIY this. The unit is obsolete (thanks, Mitsubishi) so I have nothing to lose. I'm getting some HVAC tools, as I want to install a unit in my barn, and I'm tired of dealing with the hack HVAC techs around here. I've ordered a vacuum pump and gauge set. Seriously, the last 'licensed' HVAC tech I had out here knew less about installing and charging a mini-split than I did.

At any rate, I got some refrigerant in, and it's fairly cold at the service valve, although not quite as cold as my other unit. The good unit runs 46 F., the bad unit is running about 50 F. Likewise, if I check the fittings at the back of the indoor unit, they are also about 50 F. Here's where the mystery comes in. The air blowing out is about 66 F. The 'good' unit blows about 50 F. Where is the cooling going? Is it possible the coil indoors is plugged somehow? I can't figure out how the refrigerant lines can be so cold, yet the unit isn't cooling well.

 

[Bretny]

Evaporator dirty? Or condenser dirty?

 

[MushCreek]

On the outside of them? No, they're very clean. I turned up the fan, and it's blowing colder air than before. Now I need to find out why the fan is so slow in automatic mode. The identical unit in the BR goes on demand, so when it's hot, the fan runs faster to cool the room down. The one I'm working on only does that if I push the thermostat way down (or override the automatic fan speed).

 

[metlmunchr]

So, if you manually increased the fan speed and the air temp off the evap got colder, that would indicate the compressor speed is a function of fan speed. Since you can turn the temp setpoint down and the unit responds but just not at the temperature where it should, that would indicate to me that there's a problem with the thermostat's calibration rather than some problem with the refrigeration cycle itself.

Some googling shows https://www.greenbuildingadvisor.com/question/temperature-adjustment-offset-on-mitsubishi-minisplit and some of the responses there give a procedure for getting the remote and the unit's temperature back in sync. This was a discussion of a unit from around 2016 so maybe it's applicable to yours.

You could still have a problem with the refrigerant charge, but it doesn't sound to me like that's your main problem.

 

[wawaw]

The refrigerant isnt moving, sounds like valve problem.

 

[wawaw]

Sounds like your refrigerant isnt moving. Valve problem not opening.

 

[metlmunchr]

Sounds like your refrigerant isnt moving. Valve problem not opening.

What valve isn't opening? The only valve in a mini split with a scroll compressor is the reversing valve. It doesn't open and close. It shifts to reverse the flow of refrigerant from heating to cooling.

Any solenoid valve in a refrigerant cycle that moves from open to closed would cause the unit to either pump down and shut off on low pressure or it would cause the unit to kick off on high pressure, depending on the location of the valve. Either action would occur within seconds of the valve closing.

 

[Bretny]

Theres no TXV?