Mitsubishi Mxz-4b36na Service Manual

[Varinia Swicegood]

Ifthat doesn't work, you can move the power head of the EEV (called LEV by Mitsubishi). Photo 7 on page 88 of the manual shows the location, they're smaller black caps, with a harness that plugs in to the control board, that fit over a stainless bulb on the copper lines that lead to the ports that go to the indoor heads. (You'll need to take the service door off to get to them, but you don't have to strip it down.) Pop the one off of the port that connects to the indoor unit & replace it with the one that corresponds to the new wiring configuration.

It does not. Setting everything to minimum (61 on the wall units, 67 on the air handler) results in the air handler basically not doing anything at all and the wall units cooling adequately (but maybe less than they should be).

The installer actually is a Diamond Elite contractor. I'm now convinced that it means absolutely nothing other than the fact that they are able to move a lot of Mitsubishi product. If you haven't seen it, you should take a look at one of my previous posts on the disaster that the original install was here: -cautionary-tale-on-hvac-contractors

I don't suspect a leak in this case because system performance has been consistently bad with everything running from day one. Even after they pumped down the system, relocated the branch box, outdoor unit and linesets the performance didn't change at all.

There are power limit considerations depending on the combination of indoor/outdoor/branch boxes. Have a look at this tech note and make sure you not running afoul of any limits. Some applications require an extra power supply.

Measuring pressure in the various states makes sense to me. Measuring compressor amps would also be a clue - the compressor not running at high power when the SVZ isn't working well could be a control problem.

With the air handler and a single wall unit running the air handler somewhat works, but the supply temps on the air handler indicate that it's running at a greatly diminished capacity (currently around 65F) compared to what it can do running solo.

Looking at the service manuals, it seems like things need to be pretty far out of whack to trigger an actual error code. For example -- for code 1501 "Refrigerant shortage trouble" discharge superheat would have to be >144F, which seems insanely high based on the research I've done, but I am definitely not an HVAC tech.

Also, if your SVZ is in a hot attic and with insufficiently sealed/insulated ductwork you could be losing a ton (literally) of cooling, particularly in the afternoon when attics, such mine, can hit +120f.

Total system charge was recorded as 21.2lb. When I plugged in my system to Mitsubishi Diamond System Builder with estimated lineset lengths, I received 20.8lb as the correct total. It's likely my estimates for lineset length as causing the discrepancy there.

My SVZ air hander is in what is currently unconditioned attic space, but I'm not certain that's not the root cause of the issue. No matter how hot the attic is, the air handler performs well when run on it's own.

All of my ductwork is wrapped in R8 fiberglass wrap or is R8 flex duct and everything is correctly sealed. Supply temperatures are good even on the hottest days when running the air handler by itself.

I heave a feeling that something is not configured or wired properly. I would check the install manual and compare to the actual setup to make sure the switches are all set correctly and the control wires are connected to the correct ports.

Multi splits can operate in strange ways when low on refrigerant. I had one that was a bit low that would run fine with two heads one, but would provide no heat or cooling with a single head. Since it is a newish install, I doubt it would be a refrigerant issue unless the installers really messed something up.

As best I can tell all the wiring and addressing is correct -- every zone operates correctly when operated independently with consistent delta temps of around 27F for the wall/floor units and around 18F for the air handler (as measured at supply/return grilles, not the air handler itself).

I wouldn't put it past the installers really messing something up with the refrigerant. The outdoor unit, branch box and several linesets were all relocated at one point during the install (after the system had already been charged).

On hot days I notice that my 18,000 Btu SVZ air handler would perform perfectly when the other 4 other head units are off but if I turn on another head unit the performance immediately drops off to the point the SVZ is not able to keep up on hot summer days.

For example this afternoon it was 85 F outside and sunny. Running by itself the SVZ air handler cools nicely with duct outlet registers reading 49F. But when I turn on the kitchen MLZ cassette the temperature of the SVZ air handler ducts registers quickly rises to 60F+ and no longer keeps up with the room heat load.

Evaporator temperature, which controls latent (water) removal. I'm going to guess absolute values (vs delta), so 6C is colder, which takes more compressor output to achieve. It may have been backing off the compressor once the old target temp (9C) was reached. A static pressure/airflow issue would cause 9C to be achieved at too low compressor output.

Just wanted to say thank you for this post. I suffered with the same issue last summer after having the system installed and after making the suggested changes, the air handler was able to keep up with today's 90 degree day. Really appreciated.

Back in July the installer finally came back out for yet another visit. Fortunately this was actually a hot day and we were immediately able to reproduce the issue with the air handler supply temperatures with all zones running.

They ended up spending a couple hours on the phone with Mitsubishi, concluded that the system definitely was undercharged and ultimately ended up adding an additional 2lb (!!) of refrigerant to the system over the course of about an hour. They did also change the air handler static pressure and Target ETm, though I ended up setting the static pressure back to the more appropriate 0.5" default.

We had some cold weather earlier in November (down to about 18F overnight) and we saw no problems at all with performance, though I do admit certain rooms in the house were a bit less comfortable than using the (now disabled) boiler and baseboard convectors.

In general my understanding is that these system will tolerate being slightly overcharged, but will not tolerate being undercharged at all. This was compounded on my system by the fact that the air handler also has the longest lineset. Out of curiosity, does your air handler also have the longest lineset?

Now that it is performing well, just work on balancing. if one thermostat is running rooms that are warm and rooms that are too cool, weaken the flow to the warmer rooms so the cooler rooms have time to warm up

I would bet that the installers did not purge or use a nitrogen charge when soldering. Perhaps they leaked lubrication from a poorly sealed joint until the unit died.

Now that I know how to start the test mode I can kick both air handlers on simultaneously and check performance. The device has internal temperature sensors that will indicate coil temperature during the test. I can use this with the chart to check refrigerant. I have to dig out my gauges and see what suction pressure looks like. I don't have a recovery unit so I have to rely on these inferences which should be fairly reliable.

As you point out my biggest fear is that my line set and valving is getting hung up with debris. This would be catastrophic and require disassembly and replacement of control parts. So keeping my fingers crossed.

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