Can cross-wiring in a Mitsubishi multi-split system cause evaporator coil failure?
I’m seeking expert opinions on whether a cross-wiring installation error in a Mitsubishi multi-split system could lead to evaporator coil failure. Here are the key details:
1. System: Mitsubishi multi-split with three indoor units connected to one outdoor unit via a branch distribution box in the attic of a shop/studio located in the Pacific Northwest in climate zone 4C.
Indoor units:
Studio: MSZ-FS18NA
Exercise room: MSZ-FS09NA
Office: MSZ-FS12NA
Outdoor unit: MXZ-SM36NAMHZ
Branch box: PAC-MKA32BC
2. Installation: Completed in March 2023 as part of new construction.
3. Construction final inspection: May 2024
4. Problem: In September 2024, due to poor room cooling and a flashing error code 14, a technician was called and he determined the system was leaking refrigerant. The system was shut off until repairs could be made. In October 2024, a technician determined that the exercise room’s evaporator coil (MSZ-FS09NA) was leaking. The system only had 10 lbs of R410a instead of the required 18 lbs.
5. Evaporator repair: The evaporator coil was replaced in January 2025, and the refrigerant was refilled. The system was restarted after being shut off since September 2024 when the leak was diagnosed. The leak repair took about 4 months, which the installing contractor attributed to delays in obtaining the replacement coil from Mitsubishi.
6. Observed issues: In February 2025, I observed the following:
Office unit (MSZ-FS12NA) was outputting air at 141 deg F despite a 63 deg F setpoint, with the room at 76 deg F.
Exercise room unit (MSZ-FS09NA) was outputting air at 57 deg F despite a 66 deg F setpoint, with room at 57 deg F.
7. Wiring error discovered in February 2025: I emailed my observations above to the installing contractor. Their technicians came out and discovered that the exercise room and office indoor units had been cross-wired since the initial installation in 2023. The exercise room unit was controlling the office unit’s output and vice versa. The technicians corrected the S1/S2/S3 wiring connections and the system appears to be operating normally for the last week.
Questions:
1. Could the cross-wiring error between the MSZ-FS09NA (exercise room) and MSZ-FS12NA (office) units contribute to or cause the evaporator coil leak in the exercise room unit?
2. How might the combination of cross-wiring and operating with low refrigerant levels impact the system over time, particularly considering the different capacities of the indoor units involved?
3. What specific damage could result from potential freeze/thaw cycles during cooling mode and overheating during heating mode in this multi-split configuration?
4. In your professional opinion, is it reasonable to attribute the coil failure in the MSZ-FS09NA unit to the installation error, considering the system specifications and observed issues?
Any insights would be greatly appreciated!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I do not see any way the miss wired controls could make holes in the coils allowing refringent to leak.
Do you have open cell spray foam insulation in your home?
Walta
My understanding is that the cross wiring can cause unnecessary freeze/thaw cycles in a coil which can lead to metal fatigue and eventually a hole. For example, in the summer in cooling mode the office gets quite warm because it is on the second floor and has a lot of south facing window area. The office indoor unit commands cold refrigerant but that goes to the smaller sized downstairs exercise room evaporator coil due to the cross wiring mistake on the branch box. The exercise room fan is not turned on because that room isn't calling for cold air. This causes the evaporator coil in the exercise room to freeze up quickly. Water expands when it turns to ice and that can cause flexing of the coil and fins every time it goes through a freeze/thaw cycle.
In regards to the question about insulation, there is no open cell foam insulation in the structure.
The way I see it the outdoor and indoor coils are made pretty much the same. The outdoor coils of a HP are covered in ice/ frost thick enough to block the air flow and then defrosted about 2-6 hours or so without any damage after decades of use.
I think you would have a hard time proving the electrical problem directly lead to the leaky coil.
Every refrigeration system will spring a leak someday we never know when it will happen.
I am guessing this one is no longer under warranty and you will have to pay the bill for the coil. Sorry
Walta
I can't see how cross wiring would pinhole a coil. Those usually happen from either manufacturing defect or refrigerant oil that has turned acidic due to moisture contamination.
I didn't actually see the leaking coil so I don't know if it was pinhole or a crack in the tubing. If the leak was caused by fatigue due to freeze/thaw cycles I would expect a crack rather than a pinhole.
Copper freezes at 1984°F what happens to unconfined water on the surface of too copper is irrelevant to the copper.
Walta
@snow rider 3d I have seen miswiring on multi-split systems, and most of them have caused abnormal temperature control issues, like what you describe. However, whether miswiring directly causes a coil leak depends on other factors, such as whether the system has been running low on refrigerant for a long time. When the refrigerant is low, the evaporator coil can over-freeze, then melt, causing continuous expansion and contraction of the copper tubes, weakening their structure over time. If the leak occurs in an area that is subject to repeated freeze/thaw cycles, it can be concluded that this is a contributing factor to the problem.
I don't know how long the system was operating with low refrigerant. It had 10 lbs of R410a when the leak was detected. It should have had 18 lbs. That part of the building has been unoccupied most of the time since it was built so the leak may have been there for awhile.
Freeze/thaw cycles that cause expansion/contraction of the coils will lead to fatigue failure, what is usually called "brittle fracture". This will manifest as cracking, and usually will be most pronounced at areas of high mechanical stress, which I would expect to be at locations where the tubing is making a bend (such as when it loops back into the fin area repeatedly at either end of the coil assembly). You wouldn't get pinholes from brittle fracture.
I think Akos may have the answer. I have seen pinholes develop in systems where the compressor failed and the dryer wasn't replaced when the new compressor was installed. What can happen is the compressor will fail, and contaminate the system with burned enamel (that's what we suspect anyway), or moisture sneaks in during the repair (such as a rushed job where they didn't evacuate the system completely), then you have corrosive lubricants that can rot things out and cause pinholes.
If the compressor is OK, then I'd suspect moisture contamination, or possibly a manufacturing defect. I would check if the dryer was replaced (it's a metal cylinder that gets brazed into the liquid line, usually near the compressor). If the dryer has not been replaced, I would have it replaced at the next repair that involves any refrigerant work or brazing on the refrigerant lines.
Bill
Bill, thanks for the detailed explanation about brittle fracture due to fatigue. As I mentioned to Akos, I don't know whether the evap coil leak was due to a pinhole, a manufacturing defect, or a brittle fracture from fatigue. The only parts replaced were the evaporator coil and the refrigerant, so no compressor or dryer replacement was deemed necessary by the technician. I am concerned that improper operation due to the cross wiring mistake may have caused premature wear and tear on the compressor in addition to the evap coil but at this date the installer believes the compressor and dryers are ok.
I typically require that the dryer be replaced ANY TIME the system is evacuated and recharged. The dryer is a cheap part (small ones are maybe $20-30 or so), but they do usually have to be installed by brazing. This is relatively cheap insurance though. Any time you have to reclaim refrigerant (please be sure your tech doesn't "vent" the old refrigerant!) from a system due to leaks and/or part failures, there is a chance of contamination.
I'm not sure if you'd see extra compressor wear or not here. The multisplit systems typically flow refrigerant in a loop through all the units anyway, so I'm not sure how much difference it would make if the wrong head was calling for cold or heat. I haven't ever seen that exact problem in the field before.
Bill
Bill the way I am reading the OP’s question and maybe wrongly is “My indoor coil had a leak and the controls were miss wired, will I get the manufacture or the installer to cover the cost of this out of warranty repair?”
I say it is not happening seems like you are giving him false hope.
Bill, please explain how this "brittle fracture" happens to only affect indoor coils subjected to water freezing and yet outdoor coils survive this happed all the time and seem to be made in the same way as the indoor coils.
Walta
Brittle fracture would be the same regardless of location. It's a common failure mode for metals that are subjected to constant cyclic motion, such as bending a paper clip wire back and forth until it gradually hardens enough to break. I did not mean to imply it only happens in one particular area, I had only suggested it as a possiblity based on what was described (freeze/thaw cycling).
I think the most likely scenario is there was some kind of manufacturing defect with the problematic coil, and that would be an issue for the manufacturer's warranty. If the installer was at fault in any way, it would be related to the evacuation and refrigerant charge and possibility for contaminants to be introduced into the system. If the refrigerant issue was due to a failure of some other part of the system (such as to deal with a low refrigerant condition caused by leaks in the problematic coil), then I think that would still be a claim against the manufacturer's warranty, since the problem began with that problematic coil.
I don't see how the miswiring would cause a coil to fail. I also agree with you that the indoor and outdoor coils are essentially the same, the only differences being maybe the overall size and shape of the coil, and one or the other may use smaller or larger copper tubing. From a materials and general construction sense, there isn't really any difference between the two.
Bill
Here are some pictures of my office indoor unit on Feb 11 that illustrate the improper operation of a cross-wired system. The indoor unit is outputting air at 141 deg F causing the room temperature to be approximately 76 deg F even though the remote control setpoint was 63 deg F. The hot refrigerant was being sent to the office due to the cross-wire mistake and the exercise room calling for heat. The exercise room setpoint was 66 deg F and it's room temperature was approximately 57 deg F so heat was being commanded but it was all going to the office which was significantly overheated. This type of operation stresses the system and can prematurely cause component failures.
141*F is not hot enough to cause failure of the coil materials. If you were seeing temperature-related failures, I'd expect those to show up in the electronics and possibly plastic materials of the unit, not the coil and refrigeration tubing. As an example, if I go solder a copper wire, I usually set my soldering iron around 600-700*F. This is hot enough to melt and flow the solder, but it does absolutely nothing to the copper in the wire I'm soldering -- it's not even hot enough to cause discoloration of the copper wire.
Bill
Hi Bill,
Thanks for your reply. I'm not trying to say that 141 degF is too hot, just that the system is essentially running in an open loop mode rather than as a normal closed loop system using feedback from temperature sensors to turn off the heat when the setpoint temp is reached. During cross wired operation the system may work harder than it would normally and that can stress components. I believe the evap coil leak may have been caused by excessive freeze/thaw cycling last summer when the system was in cooling mode and it may haven't gotten worse as the refrigerant leaked out of the system. For most of the summer, no one was in the rooms affected by the cross wiring error and it was September before I noticed the system wasn't working properly and there was a flashing error code. I don't have any data from last summer/fall so I showed some recent data after the leak was repaired but before the cross wiring error was fixed.
Regards,
Bob
The system should be able to run open loop continuously without problems. These systems are built to be able to run long operating cycles, so running for hours on end shouldn't be a problem. I doubt that's the issue here.
I should also note that in my now nearing 30 years working with very large cooling systems in commercial facilities (I work mainly with datacenters and telecom switching sites), I have *never* seen a failure of a coil due to brittle fracture or icing. The worst I've seen from freeze/thaw cycling is maybe some bent fins, which isn't much of a problem. I have seen compressors fail from the wrong lubricant being used, and I've seen problems from system contamination. I've seen things that look like manufacturing defects too, usually related to vibration (I've seen clamps on lines near compressors gradually wear through the copper tubing, for example).
I don't think the cross wiring issue caused a system failure here. I think your most likely scenario is that the coil had a manufacturing defect, probably an issue with the tubing used to make the coil.
Bill
None of your assumptions play out. The cross wiring contributed to lack of comfort (inability to maintain set temp), but would not cause the mechanical issues you experienced.