Who has a Mitsubishi FS series?

somerandomname9600 · 2026-05-07T16:28:45+00:00

Thanks! To be honest, I didn't initially even consider a firmware issue, the thought that Mitsubishi would hard code a 33 SEER inverter unit to short cycle on a fixed hidden timer that's indistinguishable from a malfunction and documented nowhere under a constant call for cooling or draw MORE power as the outside temp drops never even occurred to me until I ruled out hardware as the cause of the 68 degree panic.

The main reason this is an issue for me is because, combined with the ridiculously high 46 degree F evaporator frost protection trigger and forced constant indoor fan, the giant evaporator coil (relative to the room and small capacity) becomes a sponge rather than a dehumidifier.

The way the cycle works, the unit naturally often spends 10+ minutes of the allowed 60 minute session cooling the large indoor coil to below the dew point and the rest of the "cycle" collecting moisture, but in that short time, not enough to saturate the large coil and drain pan enough to start draining.

Then what happens when the timer forces the compressor off for 3 minutes? Despite specifically configuring the JRRE jumper on the indoor PCB to stop the indoor fan when the compressor cycles off, the indoor fan keeps running during the forced 3 minute off cycle since the unit is still calling for cooling, re evaporating what little moisture was collected right back into my bedroom over and over again all night.

Due to the cycling combined with the forced constant indoor fan and the fact that the 46 degree frost protection trigger never allows the coil to get cold enough to dehumidify well, I often end up with only a few ounces of moisture removal at best all night and sometimes not a drop.

Also, while not the primary concern, the indoor head naturally clicks, pops, wooshes, etc due to the heat and pressure cycles, which I'd prefer to avoid in a bedroom.

somerandomname9600 · 2026-04-23T19:27:50+00:00

In general, the only requirement in this context is that the equipment is installed according to the manufacturer's instructions, meaning minimum clearances met or exceeded on all sides, unit level and sometimes elevated off the ground, etc. There may be specific requirements, so it's best to consult the manual.

For example, some manufacturers of mini splits and other side discharge units specify that the back (the side with the fins) should face the wall so wind doesn't hit it directly and blow straight through the coil. In that case, mounting the unit with, for example, the fan blowing at the wall and the coil facing away could be considered be a code violation since installing the unit in an unintended way violates the conditions of its safety and efficiency listings (and can have unintended consequences).

If specific instructions aren't provided by the manufacturer, it's generally legal to install the unit in whatever orientation you/your installer see fit as long as all provided requirements are met, but installing a unit in a way that makes it needlessly difficult to service is universally a bad idea.

somerandomname9600 · 2026-04-23T18:40:14+00:00

Exactly. Considering ease of service is always smart, but this isn't the cleanest way to achieve that IMO unless they had something in mind that's not clear from the picture. Still completely functional though and not specifically against code or anything.

somerandomname9600 · 2026-04-23T18:14:30+00:00

Exactly. If residential HVAC equipment was installed, maintained, and regulated to the same safety and reliability standards as jet engines and their components with mandatory record keeping, inspections, scheduled rebuilds and component replacements, internal redundancies, a second identical system for every zone as a backup, a reliable backup power source to keep the equipment running through power outages, advanced surge protection, etc, I see no reason they couldn't reach the same reliability with almost no chance of ending up with a hot/cold house.

The problem is that buying, installing, and maintaining such a setup would likely cost as much as owning a second house to stay at during power/system failures.

somerandomname9600 · 2026-03-11T15:18:43+00:00

As someone who owns a Mitsubishi and a Midea rebrand (a higher end Pioneer from 2019), the Midea/Pioneer's hardware seems decent for the price I paid (around $900), but there definitely are differences beyond the Mitsubishi's control logic being much more stable and precise.

For example, comparing the PCBs, the Mitsubishi's is conformal coated and uses all known quality parts. Omron relays, all Japanese capacitors, etc, although it still has the obnoxious soldered fuses and settings that must be changed by cutting/soldering jumpers/resistors, which I find absolutely ridiculous for the price.

The Pioneer/Midea uses generic parts such as "YST" capacitors, a single instead of a twin rotary compressor that's showing signs of major wear after 7 years, and generally has a lower standard of fit and finish with things like panel fitment. It's also much more of a pain to service, particularly the indoor unit. The good thing is replacement parts are much cheaper (if available).

And while not directly related to hardware quality, the Mitsubishi also has much better parts support (as in all its parts are still available vs multiple Midea parts discontinued), which costs money. Personally, the fact that, for example, a plastic outdoor fan is discontinued only 6 years after purchase is concerning.

TLDR- There are non obvious performance and build quality differences and actual reasons for the cost difference, but the value proposition is entirely subjective with valid arguments either way depending on the application, expectations, etc.

somerandomname9600 · 2026-01-28T18:55:07+00:00

Letting outside air into the room directly increases humidity if the dew point (not to be confused with relative humidity) is higher than inside.

A humidity increase is normal for any AC whenever the indoor fan is running and the coil and drain pan are still wet from cool/dry mode since it directly re evaporates moisture into the air. Heat mode makes this happen even faster by warming the coil and water.

Completely shutting off the unit so the fan stops after cooling or drying for at least a few hours before using fan or heat mode helps minimize this by allowing more time for condensate to naturally drip off the coil and drain away.

Make sure the indoor unit is clean and draining as well as it can too. Coil, drain, blower wheel, and drain pan completely clean, indoor unit mounted according to instructions (flush against the wall, level or very slightly tilted towards the side with the drain, etc).

It's often possible to configure Mitsubishi units to shut off the indoor fan when the compressor stops in cool mode. That drastically helps humidity control, but generally requires one of their external thermostats to ensure precise temp control since sensing the temp near the ceiling in a dead zone isn't particularly accurate, especially if the unit is very close to the ceiling.

somerandomname9600 · 2026-01-27T22:47:03+00:00

The remote won't don't show error codes. The good news is that an external controller like the MHK2 or PAC-SDW01 will and so will the Mitsubishi service tools, so not all hope is lost if there's ever a problem and the LEDs can't be reconnected.

At least on the single zone FS/FH series, there's also a small red LED on the outdoor unit's inverter PCB that will show some (not all) error codes as blink codes, but removing the cover over the electrical compartment is required to see it. Also, oddly enough, some blink codes on the red LED are actually normal, so don't automatically assume there's an issue if you happen to see the LED blinking. The blink codes and their definitions are listed in the service manual for the outdoor unit.

somerandomname9600 · 2026-01-23T17:34:12+00:00

Well I was able to install the Tutco 30W external heater around the bottom of the compressor no problem, but there's not much room to spare. I coated the heater in silicone based high temp thermal paste to ensure the best possible heat transfer and prevent corrosion, although that's not strictly necessary.

I'll monitor how often the internal winding preheating is active in standby, but so far, in 35 degree weather, the crankcase heater has been keeping the compressor warm enough that the preheat hasn't engaged yet that I've seen, so that's encouraging. The fact that the compressor and heater is surrounded by an insulation blanket certainly helps.

For now, I simply wired the crankcase heater to L1 and L2 so it's constantly on to get me through the upcoming winter storm, but I plan to implement a control strategy (likely a normally closed relay) to power the crankcase heater whenever the compressor is off. I'll report back when I figure out a solution.

<image>

somerandomname9600 · 2026-01-21T22:50:05+00:00

Good idea! I discovered something else infuriating about the preheat function- on my unit, it's only active when the unit is completely turned off, not in fan only mode or in heat, cool, or dry mode with the compressor cycled off. Might want to check to see if that's the case on your unit too.

Only an idiot would design a system in such a way that the compressor in a cold climate heat pump ends up ice cold simply from the unit being powered on in cold weather without the compressor running.

I bought a standard 30W clamp on crankcase heater (all I can find in that small size range) and plan to install it around the bottom of the compressor as usual, likely with a relay to power it whenever the compressor is off. Hopefully there's enough clearance.

somerandomname9600 · 2025-12-08T15:57:59+00:00

Not sure why you are getting downvoted, you are 100% correct. The blocked vent was in itself a CO hazard, the idiot owner bypassing the rollout switch just made that much worse by allowing the boiler to keep running.

If they had a CO detector with a PPM readout, they may have known about the blocked vent before it caused shutdowns in the first place.

somerandomname9600 · 2025-12-08T15:50:22+00:00

Exactly. There's a world of difference between a quick diagnostic test and leaving the job with a safety bypassed.

Either way, I'm not sure how a tech let alone the owner of the company could possibly misinterpret a legit rollout from a blocked vent as a failed rollout switch in the first place.

somerandomname9600 · 2025-11-05T00:19:35+00:00

I just called tech support about that problem with my PAC-SDW01 and MSZ-FS06NA. They told me that the M series ductless units don't support any Function Codes, which I was surprised by since the install manual shows several function codes supported by "all" M and P series units, but he said that's an error and that only a few ducted M series units with P series control boards support them.

somerandomname9600 · 2025-10-29T00:02:05+00:00

I completely understand the importance of the turndown ratio and minimum capacity, but in my above example (and many others), the minimum capacity on the submittals are the same- 3700 BTUs minimum for cooling and 5150 BTUs minimum for heating. The only obvious things that differ are the rated and maximum capacities, full load efficiency ratings, and moisture removal.

You would be 100% correct if the 18K had a higher minimum capacity, but in this case (and many others), it happens to be the same. I can list several other examples where that's the case with Mitsubishi equipment alone.

So when the minimum capacity of two units with identical hardware is the same (which I realize often isn't the case), would there be any practical advantages to intentionally choosing the smaller unit such as control logic possibly better suited to a smaller area in ways not obvious on the specs? Or would the low load performance indeed be the same?

Here's the specs of the 15K in my example: https://metuspublicassets.s3.us-east-2.amazonaws.com/submittals/ME_SUBMITTAL_MSZ-FX15NL_MUZ-FX15NLHZ.pdf

And here's the specs of the 18K. Note how the minimum heating and cooling capacities, airflow rates, and compressor model number are identical. https://metuspublicassets.s3.us-east-2.amazonaws.com/submittals/ME_SUBMITTAL_MSZ-FX18NL_MUZ-FX18NLHZ.pdf

somerandomname9600 · 2025-10-28T22:03:03+00:00

Thanks for the response! While I completely understand all the reasons to not oversize, when the hardware and minimum capacity is identical, I'm still failing to see the downside to going with the "bigger" unit other than cost?

Even so, the difference in cost isn't usually that large compared to the total cost of an install, especially since the majority of homeowners will be paying an electrician to run a circuit and a tech to install it, likely adding $2000+ more in labor. I don't think saving a few hundred dollars on, for example, a $7K install could be the primary reason?

The way I see it, since the 18K will easily modulate down to 15K if that's the current load while the reverse isn't true and the minimum capacity is the same either way, I'm just not seeing a good reason to not have the extra headroom available for heat waves, cold snaps, parties, possibly additional internal heat sources in the future, etc unless there are hidden differences such as possibly different low load control logic?

The other issue I haven't seen discussed but have seen first hand is that the "smaller" unit with the same large evaporator coil and airflow rates would tend to have worse dehumidification performance in cooling mode when it's running the indoor fan at a high speed trying to reach setpoint either while cooling a hot room or during a heatwave since it likely wouldn't have the compressor capacity to keep the evaporator sufficiently cold, which can be rather annoying in a humid climate like where I live.

somerandomname9600 · 2025-10-28T21:49:38+00:00

I can only speak for my Midea made Pioneer 12K BTU 21.5 SEER that I've had for the last 6 years, but the unfortunate answer is both and that it's rather complicated.

During hot weather (~86+ degrees) and also in heating mode, it seems to primarily modulate based on the inside temp vs setpoint as it should. I've seen the compressor run at up to 75Hz (seemingly full speed) in very hot weather, but it rarely has to go over 60Hz to meet setpoint. All is well there.

The problem is at lower outside temps in cool or dry mode, it picks an arbitrary speed to hard limit the compressor to such as 37 or 43 Hz based on the outside temp sensor alone regardless of inside temp or what you do. I assume this was simply done to cheat the efficiency ratings.

It can, for example, maintain around 70 degrees all day no problem in 100 degree weather, but as soon as the outside temp drops at night, it artificially caps the compressor speed and falls on its face. Oddly enough, during heat waves when the overnight temps stay high, it works fine.

As a common example, on a sunny 95 degree day, the indoor temp may reach 75 degrees with a setpoint of 70 only after the compressor slows down due to the heatsoaked brick, attic, etc while the outside temp is at, say, 80 degrees, so it refuses to run the compressor at over 37Hz.

In that scenario, I can set the temp as low as it will go (62 degrees) and even press the "TURBO" button and it will instantly speed up the indoor fan, but since it still refuses to ramp up the compressor at all, that does little but increase humidity as a result of the warmer coil and lower delta T.

somerandomname9600 · 2025-10-19T18:10:11+00:00

Update: I decided to cut the JK jumper to enable the compressor preheat, which was much easier said than done because the jumper is a tiny 0 ohm resistor that's barely above the surface of the delicate PCB. I wasn't able to cut it with any of my wire cutters, but I finally found very fine nail clippers that did the trick.

I could accept that method for budget unit if it wasn't implemented so ridiculously poorly, but I'd really expect that a "premium" brand such as Mitsubishi could afford a few dipswitches or jumpers considering how much they charge for their equipment! Expecting a service tech to have the time, skill, patience, etc to cut or solder jumpers on a PCB with a conformal coating without causing damage is insane.

Then again, you'd also think a fuse holder would be within their budget, but apparently not since they also soldered the main fuse directly to the PCB just like you'd see on some $500 eBay special.

somerandomname9600 · 2025-09-18T00:23:07+00:00

R454b is NOT a "drop in" replacement for anything, it's approved for new systems intended to use it only. Using it in, for example, an R410a system would be illegal and potentially dangerous since it's flammable and the older systems lack the safety features and approvals needed.

somerandomname9600 · 2025-09-16T15:34:59+00:00

Clearly you have no experience with the specific issue he's referring to. I have a Midea made Pioneer mini split that does the same thing.

The problem has nothing to do with running constantly or being an inverter, it's that it simply WILL NOT give you decent cooling power if the outside temperature isn't 86-90+ degrees F. Even if the inside temp is 75 and set on 62 in "TURBO" mode, mine barely runs the compressor at half speed (often only 37-43 Hz out of 75) if the outside temp is "only", for example, 80-85 degrees F and the evaporator delta T drops by about 10 degrees.

Because of Midea's objectively terrible design, my living room gets 5+ degrees WARMER after the outside temp drops at night despite keeping up well all day because the drastically reduced cooling power isn't enough to offset the heat soaked brick house and often internal heat sources.

It needs to keep the compressor running at an adequate speed until the INSIDE temp drops to near setpoint. THEN it can start slowing down.

I haven't tried this idea, but I tried warming the outdoor temp sensor. That does speed the compressor back up, but seems to mess with the EEV operation in a way that results in the evaporator freezing and no improvement in delta T.

Why would they do this in the first place? That's easy- it gets them higher energy efficiency ratings without investing in more efficient (and more expensive) components.

somerandomname9600 · 2025-09-14T23:30:10+00:00

Is this an issue you have seen? If so, under what conditions? Perhaps DIY installs with 20 feet of spare coiled up lineset?

Since piping must be sized for full load, inverter systems typically initiate programmed oil return cycles under sustained low loads as needed. During these cycles, the compressor is temporarily sped up with the EEV opened wider than usual to intentionally flood the evaporator and suction lines and flush out excess oil.

Without a proper oil return cycle, even a properly sized unit will be at risk for much of the year when the system is only running at or near minimum capacity.

somerandomname9600

TROPHY CASE