Mitsubishi Mini-split Overshoot — time to cut jumpers?
I have a 1.5 ton Mitsubishi mini-split serving a living room of 500-600 sq ft. Since installation, it has never been able to regulate the room temp close to the setpoint. The problem is that the temp at the air intake close to the ceiling is not representative of the room at living level and is adversely affected by outdoor temps during the day particularly in the cooling cycle.
An MHK2 thermostat was installed two years ago. It changed matters so that now, in cooling mode, the unit drops the room temp about 2 deg F below the setpoint. Even though the room is below the setpoint, the unit cycles on from time to time. The offset between the setpoint and room temp varies day to night and with the outdoor temp. This mini-split is easily the worse heat pump I have ever lived with and the only one that can’t regulate the room temp.
I have found Mitsu Application Note 3048 — How to turn off the indoor unit fan when set point is met. On my unit, this is done by cutting two jumper wires. A local HVAC company spent one hour on the phone with Trane tech support to confirm that cutting the jumpers (and which two) will shut off the indoor fan when the setpoint is reached. That is a recommended solution to poor temperature regulation that I’ve found on this forum and others.
Is cutting the jumpers the right solution to my problem? I ask this because I can see that the onboard control is still active and trying to control room temp based on the unrepresentative intake air temp. Will stopping the fan take the onboard control out of the loop?
I know that the onboard control is still active because I placed temp sensors at the air intake, the cold air outlet and next to the MHK2 to monitor what is going on. As the room warms up, the air intake temp (top of the unit) reaches the thermostat setpoint first (due to stratification and the outside influence), which triggers the unit to come on even though the room temp is still 2F below the setpoint.
Thanks for any help you can offer.
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I have found that mini splits in general do not do well regulating to an accurate setpoint when left to their own devices.
i manually adjust the fan do get it to do what I want.
You know the pattern you are seeing.
I turn the fan down to low at night, and then depending one what the weather is going to be, turn it up in the morning.
If it is actually cool at night I may shut it off completely
Let me add the resolution to my mini-split control problem. I now have this running satisfactorily. I run a ceiling fan continuously in the center of the room to mix the air vertically so that the air temp at the intake to the unit is representative of the room. The setpoint has to be +4 deg F above the temperature I want! Some of this is a temp delta from 5' above the floor to the intake point (9' off the floor) plus it appears that the internal temp sensor is not in the intake air stream but located off to the side where it is heated by the equipment running inside the indoor unit. The ceiling fan keeps that differential constant. Then, I covered up the IR sensor with aluminum foil except for one small corner so the remote works. This sensor looks at an exterior wall (3 of 4 room walls are exterior) that is >50% windows and out across the porch into the courtyard. It apparently sensed the outdoor temp as if it was the temp on the interior surface of the wall causing the setpoint to drop by 2 to 3 F during the hot afternoon hours. Now, I get about 1/2 deg of setpoint drop in the afternoons, but that is about OK. Overall, I managed to re-engineer the room and negate the unit’s IR sensor to the point that it holds the room a comfortable temp. The thermostat is used only to set the setpoint. What a piece of HVAC crap.
The Mitsubishi mini-split may be a fine piece of equipment when installed in the theoretical room that it was designed for. But that's not my (pretty typical) home. Maybe these work fine in European or Asian markets where building methods and standards are a lot different, but not in the U.S. My installer (RTFT here in Tucson) did me no favor recommending this, installed it too close to the ceiling, failed to understand (or tell me) that the old system's return air duct had to be insulated and roofed/drywalled off to eliminate the outdoor influence, and has spent the last year avoiding my calls. Trane did nothing to help me either. They did recommend 5 "top" installers in Tucson who had received their highest level of training. Number 1 had already been out and said they had never seen a mini-split on a thermostat. Three more weren't interested in the job. Number 5 sent a tech out, who spent an hour on the phone with Trane tech support. The tech told me that they were "traned" to handle the mechanicals including the compressor and refrigerant, but had no ability to diagnose control system problems. BTW, Trane confirmed that you can convert a Mitsubishi mini-split to work on thermostat control by shutting down the indoor unit fan (see Mitsubishi Electric Application Note: 3048). It is done by programming on some units, by cutting jumpers on other units including mine. After Trane identified the jumpers to cut, the tech's mgmt refused to do it since they didn't want the responsibility. So much for "traning".
Sorry for the rant. I hope this information helps someone else.
Is the problematic head the only head attached to the compressor?
IE
mini split = one head and one compressor.
multi split = several heads connected to the same compressor.
Walta
Yes. Mini-split = one indoor unit + one compressor.
Deleted
18,000 BTUs of equipment seems like a lot for only 600 square feet.
Is a hard wired remote sensor like this one available to fit your model?
https://www.ecomfort.com/Mitsubishi-PAC-SE41TS-E/p30827.html
I would be surprised if Mitsubishi was using the remote controls IR receiver to sense the rooms temperature. If you have and documentation relating to the IR sensor, I would like to read it.
Walta
It is a 1-1/2 ton unit for 500+ sq ft with 85 percent of the wall area being exterior. The same installer did a Manual J for the predecessor (roof top) unit 15 years ago and came up with 1.5 tons cooling. Maybe it should have been 1 ton for the mini-split. Right or wrong, that is what I have and why.
I do not know about the wired remote sensor. I believe the unit senses intake air temp using a “room air sensor” on a circuit board off to the right side of the unit inside the case, which is why it runs several deg F above the intake air temp. Would the wired sensor replace the onboard sensor for that purpose?
I have the wireless remote temperature sensor for the MHK2 thermostat and have used it in the recent past (but not now). When under thermostat control, and sensing the temp at either the thermostat or the wireless sensor, the on-board control system continues to run in the effort to fine-tune the temp of the intake air -- UNLESS you shut off the fan when the setpoint is reached by programming or cutting jumpers (per App Note 3048). As best I can tell, it still uses the intake air temp in that effort.
I decided not to cut jumpers once I determined that I could run it with on-board control by stabilizing the sensed intake air temp and adjusting the setpoint +4F above the desired room temp. It now runs with the thermostat providing the set point but the air temp sensed at the indoor unit.
If I cover the IR remote receiver with aluminum foil, the unit will regulate to a very constant 79F room = 83F setpoint, with the ceiling ran running to stabilize the room-to-intake differential. Pull the foil off and it drops the room temp by 2 to 3 F every afternoon as it looks out into the courtyard under the Tucson sun. I have assumed this is the onboard control system using the IR radiant energy that it senses to make a setpoint adjustment. With just a corner of the sensor area left uncovered, I see a regular drop every afternoon of around 0.5 F, which is not bad and does improve the comfort level (even though I am not in the room to enjoy it). This unit does NOT have the I-see sensor. (In the main part of the house, the two remote sensors do this for the Honeywell VisionPro thermostat that controls a rooftop Trane HP but it is a small adjustment. None of them look outside through a window.)
If it is actually something else going on with the IR receiver, I’d very much like to know. Thanks for your comments, Walta!
Mitsubishi and Fujitsu will cool to -2 of set temp and heat to +1… or thereabouts. This is the nature of the beast. They are not precise. Set up the target temp to your liking and let the machine do its thing
I did not do your manual J but at 400 square feet per ton is 30% above the local lazy rule of thumb of 600 SQF per ton that almost inevitably oversizes a system by 30% or so. Seems like you could almost leave the window open and still have the unit cycle off.
I suggested the remote sensor because I personally do not believe the onboard sensor can accurately sample the room temp and the remote sensor would allow one to find good sample.
Does it seem your unit cycles on and off several times per hour? With your variable speed compressor on hottest days, it should speed up or slow down and never turn off.
Consider if your unit is oversized it would almost always be running at its slowest possible speed. Note at slow speed getting oil to flow back to the compressor is an issue after so many minutes of slow run time the computer must run for a few minutes at max speed flushing the oil back to the compressor. An oil return cycle running could explain your overshooting the set point from time to time.
Walta
Thanks for your reply. I am finishing the drywall closure of the old return air duct above the indoor unit and will repeat my tests after that before moving forward. It's already filled with fiberglass batt, so closing it up should not make a lot of difference. I like the idea of a wired room sensor at living level if one is available for my unit. That will rationalize things.
IIRC, the original Manual J said 1 ton cooling load then 1/2 ton was added for a rooftop mounted unit. The mini-split outdoor unit is on a pad and never exposed to direct afternoon sun, so could probably have been a 1 ton unit. I detect no short cycling.
In fact, the unit is clearly moving refrigerant through all the time and modulating it. On a recent afternoon, the outlet air delivered into the room between 2 and 3 pm ranged from high of 68F down to low 60F when outdoors was reaching 105F and the indoor room temp was steady near 79F. There are 2 or 3 modulation cycles within a typical hour in the afternoon. Sound right?
At this point I am glad I did not cut the jumpers and decided instead to try to re-engineer the room first so that the unit could work under on-board control. I am outraged at my former installer for walking away from this, at Trane for giving no help, and even the company that sent a tech to confirm which jumpers to cut and then refused to do it. Maybe I should be glad for their caution, because it led to the current situation which I think is better than a conventional on/off thermostat controlled heat pump.
After 7-8 weeks of living with the mini-split, I decided to install a wired remote controller. Now, in the heat of the afternoon, the room is a steady 79 F within +/- 0.2 F. Impressive. At/after sunset as it cools off outside, the room temp falls steadily below the setpoint. It bottoms out at about 76 the next morning and stays below the 79F setpoint until around 2 pm. The unit continues to output chilled air during these hours that varies between the low 50s and low 70s. Having finally found the specs on the outdoor unit, it is 18,000 btu/hr maximum capacity and 5,800 btu/hr minimum.
The room is 442 sq ft. Using Walta's 1 ton per 600 sq ft rule of thumb, the cooling requirement is 0.74 ton = 8,840 Btu/hr. At 1.5 tons, my unit is x2 oversized and its minimum capacity is very high compared to the cooling requirement (actually 2/3rds). It simply can't turn down far enough to not drive the room temp well below the setpoint.
So what do I do? Option 1 is live with it and turn the cooling on / off depending on time of day. Option 2 is to cut the jumper so that the indoor fan turns off when the setpoint is reached. The unit can still circulate the oil but (I assume) it will not cool the room with the fan off. Is there any downside to doing that? How inefficient is this unit running 24/7 at its minimum capacity most of the time?
Trane has 1-ton and 0.75 ton NTXWMT units that turn down to 3,800 btu/hr minimum. There are similar 1-ton and 0.75 ton NTXWST units that can turn down to 1,500 btu/hr. Is there any chance I could be enough better off that it would justify replacing my 4-year old unit with one that is properly sized and, possibly, with a low minimum capacity?
I have the Mitsubishi SVZ-KP36NA whole house indoor unit and I’m experiencing pretty much the same problem. In order to have a final temperature of 74 degrees I have to set the controller, RedLink MRCH2, to 77 degrees. Once the set temp is reached it has to return back to 77 before the compressor will energize again. Where I live, the humidity will get to an uncomfortable point so by the time it climbs back to 77 it is very uncomfortable in the house. For about a month I have checked the operation and from about midnight once it has reached the set point to 7am the next morning only the fan in the indoor unit is running according to my power company’s usage chart. Then with an ohm meter on one of the leads to the outdoor unit and would check it regularly during the day and sometimes it may be noon or after before the unit would come back on.
I contacted Tech Support where I purchased the unit as to how I could change something where there wouldn’t be such a temp swing and how to have the fan cycle with the thermostat/controller.
Was instructed how to go in Advance Menu Options and make these changes but that hasn’t changed anything.
Anything new on your end?
I installed a wired controller for my unit, which replaces the temp sensor inside the indoor head. Now, my unit knows the true temperature of the room. My unit accurately holds the room temp I've ask for when it is hot enough outside, but I've discovered it cannot turn down far enough to follow the load when things cool off. So, it chronically over cools the room. I am working that problem now.
Without this, the units regulate based on the temp at the internal sensor, which will differ from the temp in the room at living level. If that temp difference is stable during the day and over time, then no problem ... you offset the setpoint from the temp that you really want. In my case, the differential varies by up to 3 F during the day. It becomes 2 to 3 F in the afternoon when the sun is beating down on the roof top about 18" away from the air intake on the indoor head. Set an 81F setpoint to get 79F in the room. When the sun goes down or there is cloud cover, the difference shrinks and it collapsed overnight to nearly nothing. Since the setpoint was left at 81F, the room would warm up. This is now all gone with the wired controller and its temp sensor. Good luck.
Consider whether a wired controller would help your problem. I don't have an Advanced Menu Option on my unit so can't help there.
I'm going to get back with Tech support about a wired controller.
Thank you for your response. So far not really happy with this set-up. :(
Could you share which wired controller you installed on your unit and t'stat?
there are different ones for different models. My controller is PAC-SDW01RC-1 Simple Ductless Wired Remote Controller. If you go to https://mylinkdrive.com/ and punch in your model number (use lower case letters), you will eventually find a webpage that lists compatible controls.
Another option is the relocate kit for the intake/return air temperate. It lets you mount a sensor on the wall (no buttons or screen just a sensor with better placement). There are a couple options. (you'll have to recombine the links for some reason they're being chopped).
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\M_SUBMITAL_PAC-SE41TS-E.pdf
https://mylinkdrive.com/USA/Controls/PAC_USSEN002_FM_1?product&categoryName=Controls
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10009\M_Submittal_M21EAA307.pdf
https://mylinkdrive.com/USA/Controls/M21EAA307?product&categoryName=Controls
Thank you for your help, I will check them out.
I purchased and installed the Mitsubishi M-Series SVZ-KP36NA multiple position ducted system along with the RedLink Wireless Remote Controller Kit MHK2 in July 2024. I was completely dissatisfied with the 3 degree temperature swing that seemed to be the normal thing with the MHK2. In order to have a 74 degree room temperature I had to set it on 77 degrees and then once it reached 74 degrees it wouldn’t come back on until the house temp has risen back up to the 77 degrees and by now everything in the house has risen up.
After going back and forth with joshdurston | Sep 17, 2024 04:49pm | #14, I determined that the controller for my situation was the Thermostat Interface PAC-US445CN-1 so I checked with the Tech Rep at the supplier where I purchased the unit and he agreed and sent me the following: SB_PAC-US444CN-1_Thermostat_Interface_201801.pdf (mitsubishitechinfo.ca) , and the Mitsubishi Application Note: 3062.
I installed a 208/230 transformer to the bottom of the “control box”, drilled a hole, inserted a rubber grommet for the power leads from the transformer to be attached to Terminals S1 & S2.
I purchased a Non-Programmable Honeywell Thermostat and connected it to the Interface and now my system operates as a conventional HVAC system. When doing the initial setup options of the t’stat I chose “Conventional Forced Air Heating” and “Standard Efficiency Gas Forced Air”. When t’stat calls for cooling the unit comes on and runs a reasonable length of time and when the set point is reached the whole system shuts down.
This works for me and my goals but maybe not for everyone and I’m happy with the results.
(Not sure how to post this other than reply to joshdurston )
Lamars I see thermostat interface kit has inputs for W1 & W2, Y1 & Y2
Your thermostat can run 2-stage both heating and cooling with some programing and 2 more wires. That would give you a low heat and cool operation that would reduce the cycling.
It is a shame that the poor control system is forcing you to abandon the variable speed feature you paid so much extra to get.
It is nice to hear that the thermostat interface kit is working as intended.
Walta
That’s correct but that’s the reason that I selected “Conventional Forced Air Heat” over the “Heat Pump” selection while setting up the thermostat so that I would only have the single stage, the cycling is pretty normal.
You would think that with the cost for the MHK2 and as highly programable that it is you could bypass the intended use and get the temp swing closer, which was my main conplaint.
I’m not that concerned about the intended cost savings with the system just wanted it to be a little closer to a conventional heat pump system, which if I had of investigate this a little further, I probably wouldn’t have gone with this one.
I think enabling the two stage controls would be advantageous as you would have longer run times, less cycling, better comfort and lower energy usage while using the thermostat the equipment you already have.
Walta
On the MHK2 thermostat, I was very disappointed as well. It will transmit the setpoint to the unit and its measured temp does have SOME influence -- I put the MHK2 in the freezer and within a few minutes the mini-split shut off. HOWEVER, its temperature clearly does NOT replace the on-board sensor for purposes of setting the variable coolant flow rate. The wired controller DOES replace the on-board sensor.
Mitsubishi, Trane and my installer served me very poorly, but I have yet to find an installer here in Tucson who has any experience with controls (wireless or wired). They just slap these things up and tell people that if a mini-split gets within 3 deg of the setpoint then it is doing fine. Trane tech support told me that specifically. BS.
Well I'm glad you got to a point where it's working to your satisfaction. I'm a tinkerer and love to try all the permutations, but maybe you're a set it and forget type of person. If you like to tinker, I would try some of the alternate dip switch settings.
I see you set it to high fan speed (SW1-3ON/SW1-4OFF) you may want to try it on "custom auto" which is a variable fan speed with a more aggressive response than auto.
You might want to try it with SW2-6 (fixed/variable capacity staging) both on/off even with a single stage connected. With it off it will vary the capacity using it's return air sensor and run time, it will be "less responsive" but will cycle less and likely run at a higher COP since it's allowed to modulate more.
I'd also experiment with SW1-1 Thermo-OFF time delay(how the unit reacts after the call is over ON = hard stop, OFF allows it to cycle to maintain equilibrium) The recommendation is to leave it on so defrost and oil recovery can happen whenever. The unit will try and maintain the temperature with a 1degF offset for up to 2 hrs after a heat/cool call.
I doubt all these settings will make you happy since they make the control a little more fuzzy, but some of them may have some benefit and be transparent to you, even it's it's only a quieter modulating fan.