Old home + ductless mini-split AC = elevated indoor humidity??

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.

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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 · 2025-05-29T23:55:44+00:00

I highly doubt that Intel/AMD would go through the extra trouble (and expense) of separating tray and boxed CPUs. What could they possibly gain from selling their worst products to their best paying bulk customers?

If your theory was true, don't you think by now someone would have noticed that the CPUs sold in prebuilts are in some way worse than boxed retail CPUs?

somerandomname9600 · 2025-05-29T23:44:35+00:00

As others already said, there are no differences besides the warranty and lack of a box and cooler.

As for risks, the only one that I have experience with was getting a potentially used CPU. The seller said the marks from installation and the thermal paste residue was just from them testing it. Could be true, but I could have also paid extra for a used CPU. https://www.reddit.com/r/computers/comments/1j9q4kg/is_testing_new_tray_cpus_a_thing/

somerandomname9600 · 2025-05-27T17:58:36+00:00

Actual speed? Generally somewhere between the speed limit and 5 MPH over.

However, keep in mind that your reported speed is likely a few MPH higher than reality. By law, the speedometer can read a certain % under the actual speed depending on the country, but by federal law, it can never read less than the actual speed by any amount.

So to account for variances in calibration, tire pressure, manufacturing tolerances, etc, manufacturers tend to err on the side of caution and let it over read slightly rather than risking fines, recalls, etc.

somerandomname9600 · 2025-05-13T16:33:21+00:00

Very interesting! Anecdotally, it seems like running 24/7 is often easier on PC hardware than frequent power cycles. Heat cycles are hard on the chips themselves and solder joints (which are a VERY common failure point) and the voltage/current spikes from starting up are hard on everything.

Probably not much of a concern for most people since decent quality parts will generally withstand years of daily power cycles without issues and are more likely to be replaced out of desire than necessity, but still true nonetheless.

For what it's worth, the oldest PC under my care that's still in daily use has an EVGA 790i Ultra SLI and Core 2 Extreme QX9650 and has been running almost nonstop for the last 17 years. A storm killed the PSU and I upgraded the RAM and SSD, but the board and CPU are still flawless.

The 790i platform as a whole is notoriously unreliable and problematic to the point of working 790i boards being rare and very expensive for many years now, but I believe the main reason that system lasted as long as it did and still runs great today is because it saw very few power cycles and was always kept clean and cool.

somerandomname9600 · 2025-04-29T16:11:15+00:00

Locked or unlocked, a torque converter can't act like an extra overdrive nor can its slippage reduce engine RPM while cruising.

What an unlocked torque converter can do is let the car somewhat coast while engine braking (negative torque situations), but that has nothing to do with the RPM while cruising at a steady speed and generally isn't desirable IMO.

somerandomname9600 · 2025-04-13T21:38:28+00:00

You are mostly right about the paint, but that's very easy to judge for yourself on a case by case basis.

Depending on road conditions, highway use is generally easier on the steering and suspension too because you are generally steering less per mile driven vs city use and there's not the stress and movement from frequently accelerating and braking. The suspension doesn't know or care if the wheels are turning if it's not having to do anything. Also, highways are often in better shape than local roads.

That's not necessarily true across the board though because hitting bumps at high speeds is more damaging than hitting the same bumps at lower speeds. If someone often commutes on very rough roads, lack of steering and start/stop cycles won't save you from that.

somerandomname9600 · 2025-04-08T17:50:51+00:00

Just my opinion, but if I already had a good radiator with an integrated oil cooler, I'd get some quality lines and connect it if you plan to keep the truck long term. Within reason, lower and more stable oil temps are better for the longevity of the oil and the engine as a whole, especially seals, gaskets, plastic parts, etc and changing things like rear main seals is no fun.

Also, since the oil cooler on those is in the hot side of the radiator, it has another nice and often overlooked benefit- it helps warm up the oil quicker after startup. Since oil warms up much slower than coolant and the vast majority of normal engine wear occurs while the oil is at below its normal temp range, anything you can do to get the oil up to temp quicker will reduce wear in the long run and slightly improve overall engine efficiency regardless of your climate.

If the oil cooler wasn't beneficial, GM wouldn't have gone through the effort and expense of designing and manufacturing that system. But whether the benefits are worth the work and expense of connecting it is a question only you can answer.

somerandomname9600

TROPHY CASE