Do you sometimes get the feeling that everything is junk? Me too. Of course, there's junk, and then there's JUNK. One applies different standards to a $1 toy from China and a high-end product costing five figures. It's the latter I'm going to look at today. Join me in figuring out how you'd design it better. We can make this a continuing series too. Want to contribute? Write me.
Okay, let's dig in. Just to help out the search engines a bit, let's call this a Mitsubishi heat-pump review . Which it kinda sorta is. With an engineering slant of course.
Quick background: We recently had a couple of heat-pump systems installed. These are sometimes called “mini-splits”, and consist of an outdoor compressor/fan, and one or more indoor heat exchanger/fan units. They are similar to split air conditioner systems, but can heat as well as cool.
I'll start off with the most objective issue. The evening after the first system was installed, we turned on our induction cooktop to start making dinner, and instead of the usual, almost silent operation, a loud buzz emanated from the device. “Ugh”, I thought. “Now what?” I'd installed it a few months earlier, so at least it was still warrantied. After a bit of moaning & groaning, the thought of a possible connection to the new heat-pump entered my mind. Sure enough, turning off the heat-pump restored the cooktop to near-silence.
Here's what's going on:
00:02-00:32 Noise from cooktop with heat-pump ON
00:33-00:43 Noise from cooktop with heat-pump OFF
I theorized that some mixing was going on between the frequencies used to drive the induction cooktop and those used to run the various motors in the heat-pump, with the cooktop and/or pot as the speaker. To probe things out, I ran FFTs of my AC line with some baseline loads, then with the two heat-pump systems. I think you'll notice some noise pollution:
With the MUZ-series system running, showing a dithered(?) spike a bit over 24kHz.
With the MXZ-series system running, showing noise over the entire audio spectrum.
They're not quantitative measurements – just the scope probe resting against the AC wiring.
Since Mitsubishi is unwilling to accept any responsibility, what are my options? How do you apportion blame between the heat-pump and stovetop manufacturers?
The heat-pump is the aggressor. Even if it meets applicable standards, I'd say it's at fault. An analogy: If, as a ham, I cause interference to someone, it's my responsibility to fix it, even if all my gear is in perfect condition and fully meets all standards. Okay, maybe that's not a perfect analogy, but it's close.
I could slap big AC line filters on the heat-pump connections. They'd need to be in the 40A range, and most filters don't have much attenuation in the AF region anyway. I've looked at a few on Digikey, and maybe series-connecting a couple of ca. $400 filters might help. Or maybe not.
I have a big blue 15µF line-rated capacitor I shall try across the line soon, but I don't have high expectations. The source impedance of the noise is probably too low.
Then again, the cap will exhibit 10Ω at 1kHz, so it might do something after all (I wonder what the source impedance curve of the pole transformer looks like). I'm slightly disturbed that the 60Hz current through the cap will be a continuous 1.4A. I hope the electric meter truly reads only real power.
Do you have any other ideas?
Okay, on to some more subjective, functional, and “UI” issues with these systems.
Is it HW or SW?
HW or SW? Sometimes the source of abehaviour is obvious, other times, not. Here are some problems that leftme scratching my head:
The MXZ system supports three indoorheat exchangers. Understandably, you can't run heating on one unit atthe same time as cooling on another, though, you can use the fan-only mode simultaneously with cooling. But not with heating.
Thesystems feature a “Drying” mode that aims to reduce humidity without(much) cooling. Stop here for a moment and think how you mightaccomplish this given the technology at hand in the heat-pump system.I'll wait.
I'd do it by alternating between heating and coolingmodes. Water would condense out as normal when cooling, yet bymaintaining a constant temperature, the relative humidity woulddrop even faster than with a plain air conditioner, along with theobvious advantage of…constant temperature. Sometimes it's humid, yet nothot enough to warrant any cooling.
As far as I can tellhowever, Mitsubishi accomplishes drying by simply running in coolingmode, but at reduced capacity. It's not of much value, especially asthere's no humidistat to control the process. It's either on or off.That makes it essentially unusable when you're away from home. Whatcould have been a really useful function (especially for us in humid climes) is ruined by poor software/system design.
Howlow does your thermostat go? Probably down to at least 10°C (50°F);perhaps even lower. So, why would Mitsubishi think to limit the minimumsetting (heating or cooling) to 16°C? It makes no sense. But that's notall. In °F mode, the minimum setting is 59°, that is, 15°C. Thanks alot. But, that's still not all. The remotes have a Smart Set buttonthat behaves like a preset. Press it, adjust temperature and a coupleother things, then when you press it again, the settings revert to theirprevious state. Press the button anytime to recall your preciouspreset. What's the connection? For some reason, you can set thetemperature to 10°C in your preset. Not 11. Not 12, nor 13, 14, or 15. Iguess this is their idea of a “setback”/unoccupied temperature. But WHYhave this distinction and limitation? It's crazy.
I'm temptedto reverse-engineer the remote-control signals so I can build my own andset it to whatever temperature I damn well please.
Mostly, mykludge workaround when I want a low temperature (but not 10°, and notOFF) is to set the fan to low speed (at temperature = 16°). This has theeffect of heating the immediate vicinity of the exchanger, with therest of the area staying cooler. I guess the low-fan workaround might beuseful in drying mode too.
I understand the system has to dosome things “in the background”, like occasionally defrosting theoutdoor unit when in heat mode by presumably running in cool mode for afew minutes. Unfortunately, this means that very cold air spills out ofthe exchanger during this cycle, so, do hope you're not sitting anywherenearby. Funny thing is, this problem could be partly mitigated bysimply closing the vertical direction vanes, but hey, maybe this wouldcause other problems. What do I know?
Theseheat-pump systems are quite efficient (I particularly like that heatmode has an efficiency of 350% or so 🙂 One contributing factor isthe use of BLDC motors throughout the system (the cause of my AC noisewoes I suppose), but there's another kind of noise problem this creates:acoustic of course.
The compressor fan speed is continuouslyvariable (strictly speaking, it probably isn't, but I don't notice anyspeed steps). That’s great for efficiency, but there are times when thespeed just seems to hunt around, and that's extremely annoying. I'llgladly pay $1/year more for electricity due to the efficiency losscaused by fixed fan speeds…say 16 of them. Some speeds also seem to hitresonant frequencies, so, maybe avoid those. And some speeds seem tohave a warbly, “FM” character to them. DDS or some other frequencygeneration issues? Avoid those too. And sometimes there are noticeablehigh pitches from the PWM drives. Really? Splurge an extra dollar onparts and drive those puppies at inaudible frequencies.
Thenthere's some miscellaneous weird behaviour from the heat exchangers.Fans often come on for a couple of seconds, or even less than a second,then abruptly shut down. I don't know why. Bad temperature sensing? Badsynchronization to compressor housekeeping?
Units sometimes havea hard time sustaining high fan speed if two are running. This may berelated to the previous point, or a separate issue.
Units regularly blow room temperature or only mildly warm air for extended periods, even under easy conditions, like outside temperature between –5° and +5°C. Maybe it's a feature .
Theabove three issues occur in heating mode, but I've also seen extendedcycling in drying mode, with blowers repeatedly running for less than aminute before shutting down for a rest break. I don't know enough aboutheat-pump design to even guess what these problems might be.
Okay, next, on to some usability issues.
As with so many consumer items, the manuals are just awful. Here's a choice example:
Swingairflow (change of airflow) makes you feel cooler than stationaryairflow. The set temperature and the airflow direction are automaticallychanged by the microprocessor. It is possible to perform coolingoperation with keeping comfort. As a result energy can be saved.
The perfect marriage of: bad writing, and, lack of any real information (and, useless information: “microprocessor”).
Other functions are also not described in enough detail to understand. For example, one fan speed setting is labelled Auto . What exactly does that mean? Or, there's Absence Detection , which “changes the operation to energy-saving operation when nobody is in the room ” (with a PIR (passive infrared) sensor I think – more on it later). What does that mean exactly?
The two types of remote control, with their sliding covers fully removed. Only the one on the right supports programmability.
Indoorunits with the PIR sensor use the fancier remote on the right, others,the simpler remote on the left. That's right. Despite the high-end aspirationsof these systems, Mitsubishi displays its customer hostility bywithholding the essentially free programmability feature from some ofthem. Hmm, reminds me of some T&M gear.
The simpler remote does not support fan-only operation. Is this a limitation of the exchanger, or an artificial restriction?
The buttons don't auto-repeat…not even the temperature-setting ones (the oft-used six-“position” Fan button would benefit from repeat too; yes, five presses to step down by one speed). Oh…time-setting buttons do repeat. Hooray.
Both remotes do support a limited programmability feature, the Timer .This is a one-shot, delayed turn-on or -off. It's a mildly usefulfeature, and another way to do a workaround for a nighttime setback, aslong as you remember to enable it each night. I don't generally use itthough.
The full programmability of the larger remote is good,but not great (n.b., odd that they duplicated the time & temperaturebuttons). Of course, temperature settings can only go down to 16°C, not10°. Only temperature can be set, not mode or fan speed. And it isedge- or event-triggered. That is, when I enable programmed mode,nothing happens until the next change of state, whereas all otherprogrammable thermostats I've used immediately adopt the currentprogrammed setting when enabled.
Perhaps the most interesting feature of the programmable units is what Mitsubishi calls the i-see sensor, the PIR sensor I mentioned above.
Judgingby the marketing verbiage, this is an 8-pixel vertical array that scansthe room to map out an 8 × 94 grid. As mentioned, the unit performssome unspecified power-saving operation when the room is unoccupied.When in the room, your position is monitored, and the blower can be setto either: blow air in your direction, or, avoid blowing air directly at you. That's pretty neat…if it works.
Apartfrom sensing body heat (and supposedly differentiating and ignoringpets), it also senses surface heat, blowing, for example, more warm airat a cold outer wall than a warmer inner wall.
I imagine these features would work best in a smaller room. I've found no value to them in the larger rooms here.
Anda tiny implementation detail makes me even less likely to want to useit. The turret scans the room by rotating a small amount about onceevery second. Even though the motor is pretty quiet, it invariably getson my nerves. The turret is perfectly capable of slewing smoothly, asdemonstrated during power-up/down.
Aswith many modern consumer goods, there's some pretty advancedengineering embodied in these heat-pumps. But bad design decisions, evensmall ones, tend to carry far more weight than what they got right.Sure, a system full of BLDC drives, sensors, and advanced plumbing tooka lot of effort to design. But all it takes to ruin it all are a fewstupid decisions, like a 16° limit, or noise where it could be easilyavoided, or of course, spewing out line noise in total disregard ofother AC loads.
And while there may be a fair bit of bad andmarginal hardware design out there, let's face it: software runs ourworld now. Apart from the AC noise problem, all the described issueshere could have been avoided with more thoughtful software design.
Why& how do you think this happens? Uncaring management? Lack ofsystems overview? Time pressures? What are your thoughts andexperiences?
—Michael Dunn is an editor at EDN with several decades of electronic design experience in various areas.
“You may wish to consider a large transformer with the cap on the heatpump side. The reasoning is that most line transformers probably will not overly well at 24Khz proving that added source impedance with which to combine with the cap to attenuate the no
“Another EMC problem. There are two worlds in EMC. In one, people just want to meet the emissions or immunity standards so they can sell product. Then there are those to look into problems that occur even when a product meets standards.nnThe heat in the
“Understanding always helps. Is the noise differential mode or common mode.nYou wrote, ” just the scope probe resting against the AC wiring.” Try making an inductive pick up with a split ferrite bead. Go around the AC line one wire at a time and around
“Michael,nTry making a loop “sniffer” probe with a paper clip.nnSignal and Noise Measurement Techniques Using Magnetic Field Probes by Doug Smithnhttp://emcesd.com/pdf/emc99-w.pdfn”
“Well, I have two three evaporator Panasonic cooling only. I think Mitsubishi is the same company but the “higher end”. My remotes are a bit less complex.nI have a real problem with a stupid timer. Actually a stupid programmer that wrote the timer co
“Assuming that the grounding for the heat pump is per the manufacturer's recommendation (a loose ground will generally be a bad thing), you can see this discussion from ARRL RFI which discusses the same issue: http://www.arrl.org/forum/topics/view/46
“Thanks for writing this review. I've always wanted one of them, but always dissuaded by the cost.nnA friend, HVAC contractor, advised window units are about as good, and MUCH cheaper. Never mind the split systems probably need professional installation,
“Have you checked the wiring from your electrical panel? If the load wires are running parallel you may be able to add some separation. You may be able to twist at least one of the lines.”
“Because the consumer is willing to accept it. Only when the people complain loud enough and VOTE WITH THEIR WALLETS will 'they' listen. “
“BLDC motors suggest power supplies. Is it possible the power supply outputs aren't filtered quite well enough?nnAs was pointed out, software runs our lives. Complaints to the manufacturer may eventually result in helpful firmware updates (but probably n
“There was no way to anticipate the feedback you experienced as it was also based on the line length. Steven Tabor”
“Try looking at the AC line at the heat-pump with a scope (carefully!) You will probably see a repeating glitch every cycle at the AC line rate or at the heat-pump motor rate. These spikes can be really wicked. From this time-voltage display you can eng
“Say, Michael, is this the “house from hell” with the goofy AC wiring you wrote about not that long ago? I assume you fixed all the dangerous stuff already.”
“Did you look at a wider range of spectrum – it looks like the scan is fairly low frequency. Also, did you look in the time domain. It is a common technique to vary the timing of the drivers around so that the noise is spread across the spectrum. This prod
“RE: Having a second feed or transformer for your heat pumps. nThat may not be a far out as you think! Where I come from in Michigan, a dedicated feed and meter is installed, along with a RF receiver, to regulate the water heater load. Talk to your uti
“So, that piece of marvelous engineering is still singing as well? You'll never get ANY sleep with these appliances!”
“I'd be quite certain that the sharp 24kHz spike is from PWM switching of the inverter outputs to the motor. For household products, designers often want to switch just above the human auditory range.nnIf so, you would need to get at the motor leads to d
“Get a gas stove. It cooks better.”
“nFor customers with large loads, our local utility imposes power quality standards which I believe are based on harmonic distortion. Variable frequency drive pumps for irrigation are a notorious problem. You may want to request an assessment from your
“Econo Cool sounds like they raise the temperature setting while creating a dynamic airflow by moving vents. (Think of a simple oscillating fan.) The energy savings are via the higher temperature setting. (I have a Costco purchased tower fan which oscil
“While the root cause is the 'noise' of the heat pump, and filters there are at least bulky …nHow about trying to silence the induction heater? An AC filter there could be smaller. nAnd if you are willing to tear down the heater :nCan you run it wit
“Some of your UI problems may be due to lack of training of the local HVAC contractors. A few years ago, my aunt had two Mitsu systems (Mr. Slim types, similar to your newer models), installed as part of an airport noise management project, and I download
“Split systems are in wide use in Japan, as are induction ranges. I do not think your problem is so obvious. I installed my own split system for my LAB and just had to get the AC guy to do the final hookup ($200). So people should not think it is a difficu
“If you try your cap… try it near the cooktop, the R+L between source and sink migth help…..”
“Michael,nnWhat brand and model is your induction Cooktop?nnThanks,nEvelynnhttp://www.magneticcooky.com/”
“http://songlongmedia.com/tai-nghe-bluetooth-plantronics-explorer-120.html”
“I'd be surprised if the noise is common-mode. Wouldn't there have to be quite a large ground current to generate such noise in the AF band. Guess I could try to measure the ground current. Also, CM noise seems a lot less likely to interfere with other app
“Panasonic & Mitsubishi are definitely different companies. I don't recall seeing any Pana heat pumps when I was researching, though Fujitsu is apparently well regarded, and I came close to buying theirs. There are quite a few HP mfrs, many from companies
“Yeah, a mag probe might be more sensitive in this situation.”
“I may get brave enough to probe the lines directly (using two SE passive probes for differential measurement). Then it would be easy to see how the diff noise compared to CM noise.”
“Thanks for the link. It's talking about RFI though u2013u00a0the original poster having interference up to 10m (30MHz)! How did *that* pass EMC testing? For now, I'm sticking to the assumption that I have to get rid of the AF noise. If I suppress RFI to
“There's easily a 2:1 cost range in heat pump systems. I've even seem some small ones at retail (Frigidaire IIRC), though I think they still did require tricky coolant line work, so…yeah.nnCheck the SEER specs. You'll find a pretty wide range of effici
“The heat-pump wiring is well separated from others. And unfortunately short. A bit of extra resistance might have simplified filtration :-^/ nnAWG 8 u2013u00a0already twisted inside the jacket :-)”
“Given that such an inductor would be hundreds of dollars, I'm hoping it doesn't come down to that. But it *would* give me more control over the filter characteristics compared to buying a module.”
“I asked Mitsubishi to update the remotes' firmware to make them less ridiculous. Not sure how far up the ladder *that* request got :-}”
“Yeah, that's the place.nnI think all the outright downright dangerous stuff is fixed now, thanks 🙂 It's certainly about a million times better than before. nnActually, you remind me there's still some outdoor lines that need TLC. “
“Oh no, it was returned ASAP. Just for fun, I used something like what's in your profile picture to zap all the electronics before returning it 😉 Wouldn't want anyone else to get stuck with it.”
“The cap has pretty good specs actually:nhttps://en.tdk.eu/inf/20/20/db/fc_2009/X2_B32921_928.pdfne.g., 10mu03a9 at SRF of 200kHznnThe 24kHz spike does appear dithered.nnInteresting u2013u00a0some impressive (& expensive) looking power fee
“Now that I've used it a while, I much prefer induction. Feels safer too ;-)n”
“Re econocool, yes, but I'd still prefer some real information, like how much the temperature is raised. I'm sure it's useful in some circumstances, and the exact same effect can be accomplished through regular settings. The larger remote's NaturalFlow ran
“The large heat pump and the cooktop are both 40A (230V) circuits. Though putting a filter on the cooktop means I only need one, not two…assuming I don't mind that noise on my AC lines.nnI'm not so ambitious (nor warranty-voiding) as to tear into the u
“Interesting thoughts, though I'm not aware of any way for the exchangers to affect the remotes (they're IR for starters). I *could* take a closer look at the installation guides…nnYeah, the contractor didn't impress me much. And I think the 3-zone uni
“Here are a couple more articles I think are relevant to the topic at hand, though in different ways :-)nnhttp://www.edn.com/electronics-blogs/benchtalk/4437568/Bringing-up-PIC–The-agonynnhttp://www.edn.com/electronics-blogs/measure-of-thin
“Ikea, the only model they sell. Made by Whirlpool.”
“True… Triad has a nice giant 10kVA toroid(!) for a mere $1300 :-} Even more effective: Get the utility to install a second pole transformer, and give the heat pumps their own electrical panel. Hmm.”
“I forgot to mention in the article that the wideband AF electrical noise generated by the MXZ unit actually makes the circuit breaker “sing”!!! Believe it or not.nnBreakers generally have several mechanisms for opening the circuit, one of which is mag
“Exactly. Standards only test what they test. As I discovered when I tried to report a dangerous (I thought) level of 10kHz audio spewing out of my dishwasher. No standards cover that, so no one cared.”
“Try reversing the phase L1 and L2 on either the heat pump(s) or the induction cooktop. “
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