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joanneh311

Heat pump operation in cold weather

Joanne
3 months ago

We just had two Mitsubishi heat pump systems installed last year. One outdoor unit supports 2 minisplits and one air handler with a backup hot water coil in it from the oil fired boiler.

The other outdoor unit supports 4 minisplits, dining room, living room, and two unused bedrooms which are turned off.


The area with the air handler seems to be warmer, being controlled by a wall mounted thermostat that should automatically kick in the hot water coil if I understand it correctly. The tstat is sort of all over the place, and actual temps are often way off what it’s set for, but it does heat (and cool) sufficiently.


The unit with the four minisplits doesn’t put out enough heat, even with only two of them in use. With the remote set at 70, the room will often be 61-62 degrees. Since this system uses remotes, the boiler doesn’t automatically come on unless the separate wall tstat is set higher than the HP units have heated the room. I had been keeping them turned down quite low as I didn’t want to burn oil if I didn’t have to.


So. It appears that I need to set the boiler tstat at the temperature I want, (generally around 65) and make do with whatever amount of heat the HP will provide to bring the temp up at least partially before the boiler kicks on. Is this correct?


I emailed our HVAC guy to get some guidance, but wanted to understand how this should work.

I am in CT, where it’s been mid-40s during the day and about in the 30s at night. Sometimes a bit lower.

thanks for any info.


Comments (75)

  • Joanne
    Original Author
    3 months ago

    The system is not the hyper heat model. MXZ-4C36NA2U1 410A outdoor AC / Heat pump unit. A spec I saw for ours indicates that it should function at outdoor temps down to 5 degrees.

  • PRO
    Charles Ross Homes
    3 months ago

    "There's more heat to transfer in 50's degree air, thus you get better heat indoors." This one's good for partial credit. While the enthalpy of air increases with increasing temperature, the home's heating load (which is proportional to the indoor/outdoor temperature difference) decreases as outdoor temperature increases. Any improvement in indoor temperature would be a product of both.

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  • PRO
    Austin Air Companie
    3 months ago

    "There's more heat to transfer in 50's degree air, thus you get better heat indoors." This one's good for partial credit.


    I am not looking for credit Charles. From you or anyone else. I am a Texas licensed HVAC contractor with 30 years+ in HVAC and 15+ years running my own HVAC service and repair company. I know you'd rather refer to me as a freon breather or some other denigrated term. Just as much as I loathe the new construction industry for the foolishness they propigate at my expense as well as others.


    Charles if you're only here for credit and clicks -- I am not going to ablige you with that agenda.


    ---------------

    The system is not the hyper heat model. MXZ-4C36NA2U1 410A outdoor AC / Heat pump unit. A spec I saw for ours indicates that it should function at outdoor temps down to 5 degrees.


    Joanne good to know. The ability to get down to 5F degees should provide you I believe with better operation than what you're getting. If your contractor added refrigerant to the machine do you know the way in which they did so?


    Mini splits refrigerant charge is critically charged. Meaning a few ounces above or below the required amount can have serious implications as to how good or bad it operates. To make matters worse heat pumps charged outside of measuring the amount are charged in cooling mode. In winter when temperature is less than in summer makes this an often fool hardy adventure.


    The most accurate way when dealing with mini split heat pumps is to remove the whole charge. Weigh it and compare that with data plate information and then recharge it via the weigh in method. That takes more time, costs more.. but that is the price of accuracy when dealing with critically charged machines.

  • PRO
    Charles Ross Homes
    3 months ago
    last modified: 3 months ago

    Ray,

    If you want to explain the thermodynamics underlying the performance of the OP's heat pump, you should at least tell the entire story--not just part of it. I don't consider that to be unreasonable. The decrease in heating load with increasing temperature is significant, but it wasn't even mentioned.

  • PRO
    Austin Air Companie
    2 months ago

    Charles, there's trade schools for that. I don't make training videos, or teach these kinds of subjects.

  • Joanne
    Original Author
    2 months ago

    Update: The tech came out again with the owner of the company and they had Mitsubishi tech support on the phone. They went through everything, including indoor settings, outdoor error codes, and any possible control wiring issues where they pass through the basement.


    The indoor units have a ”one-flow” or ”two-flow” setting that allows either the top two smaller vanes only, directed upward, or the top and also a larger vane below them that directs the air toward the floor. I had both vanes operating because I thought that with the cooler air near the floor, directing heat downward made sense.


    They suggested that the warm air blowing down toward the area where the heat sensor is might be causing short cycling. They mentioned that they have heard of this happening before. So we set to one-flow and watched it for a day. We got a lot more heat.

    Too much heat. I had been all set to give the contractor the good news on Monday but it was not to be.


    With two adjacent rooms set to 68 and 64 overnight, the temps at 7 a.m. were 77 in both rooms according to separate thermometers. It was in the single digits outside. We turned the remotes down a bit, and the rooms are still running anywhere from 6 to 8 degrees warmer than the settings by late afternoon.


    This makes me wonder if the temp sensing is not working correctly at all. While too much heat in winter is better than not enough, something’s not right.

  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    Joanne, that's interesting.

    It sounds like an issue with sensing temperature. Mini splits are a bit weird about that and many of them use brand specific thermostats. Or proprietary controls, meaning that there is no option to use another thermostat unless the manufacturer has that option to provide you a different control.

    The return temp of traditional systems use return air temp to cycle the system (when the thermostat is placed properly within the return air stream or where the return air filter is located), so the head of a mini split is problematic in this area because the return is right at the spot in which supply air is being delivered. (Find out what the temp is of the return air, being drawn back toward the head on the wall. That should provide you some clues.)

    Some come with remote and I believe ability to cycle using the temp on the remote, but I've heard thru the grape vine that even the remotes sensing ability isn't all that when compared with traditional HVAC systems control options.

    Are you using a different thermostat to determine the temps in those rooms?

  • Joanne
    Original Author
    2 months ago
    last modified: 2 months ago

    The units have individual remotes, no temp sensors in them.

    We have small thermostats edit: thermometers. in the rooms to see what the actual temp is because of the inconsistent operation, which also happened to a lesser degree when using it for cooling. We had to fiddle with the setting in summer too. We have several kinds of thermometers, both digital and old style glass bulb, so they are pretty accurate. Braces and belt. They are placed at about waist level and out of direct sun.

    The DR has an old Honeywell T87 heat-only for the boiler which also registers the same as the portable ones.

    I saw the guys using a tester on the air coming out, which ranged from about 107 to 110 degrees, the lower temp was when both heads were calling for heat at the same time. I didn’t notice if they checked the air at the return, as I didn’t hover. Outdoor temp at the time was in the low 20s.

    I don’t know if it’s relevant, but the two heads are back-to-back against the same wall between the two rooms.

    Appreciate all of the input and suggestions from everyone.

  • klem1
    2 months ago

    On behalf of my Jr college professor friends ,I want to thank OP and other participants for this study material. Customer service ,human relations and the application of common sense are increasingly becoming obscured. In this case three individuals holding themselves out as a technician ,business owner and manufacture's technical expert have been unable to make new equipment preform so homeowner has turned to online advisors for a free solution.

  • PRO
    Austin Air Companie
    2 months ago

    The units have individual remotes, no temp sensors in them.

    We have small thermostats edit: thermometers. in the rooms to see what the actual temp is because of the inconsistent operation,


    Joanne,

    Where does the unit display temperature reading? If only on the remote, it would have to be on the remote itself. If the temp the unit senses is on the unit itself it should display it? Does it do that? I've seen a few with temps displayed on the unit itself.


    But there is no conformity in this among the mini split manufactures. Designs run the gamut with mini splits which can be a major source of problems.


    I've seen a few with remote only option, which depending on a myriad of factors (within the home) and where you place the remote? These are spot cooling / heating appliances. If the temp on the remote is used to cycle that temperature on the remote could be influenced by air all around it.


    To "maybe help you" understand this better: Originally HVAC was called "central heating and cooling". Because the system was located in the center of the home (structure). The center of the home was where the filtered return was placed. This created a natural air flow back to the center of the house. (return air, once this reached set point temp the unit shuts off)


    If the thermostat wasn't in the return air? The supply temp as you say could be 110F running in heat mode. If temp sensing is in that stream of air in some fashion? You're likely to get a myriad of short cycle issues.


    The supply air of a "central HVAC system" was typically meant to be placed around the outer areas of the structure. The thermostat to control the operation was placed at the center of the structure, in which there was no supply air anywhere around it or exterior issues to influence the thermostat in any way.


    This is why design flaws are a tragic mistake. Usually these kinds of issues only come into play in more extreme climates. Where temperature extremes cause a wide variety of issues. Those that live in more mild climates never experience half the trouble (in my opinon) as those in more extreme climates (hot or cold or combination of both). That at face value doesn't mean you never have trouble in a more mild climate. HVAC industry was built on complaints of too hot or too cold either one.


    So getting your system to operate confortably is probably going to be a difficult challenge from what I've read from you this far.

  • Joanne
    Original Author
    2 months ago

    Neither the remote nor the unit have dispays for room temp. The unit only has two green lights that indicate power and something else (calling for heat? not sure)

    The separate thermometers we are using display the temp and humidity, as well as the high and low temps over a period of time. Not sure how the time works on this.


    The remote has no temp display, only the temp, fan speed, time, and mode (heat, cool, dehumidify, etc)



    Thanks for the explanation. I understand completely what you are saying about the return air temp and supply air being kept separated. I worked in the service department of a Plumbing/HVAC company for 7 years, ( I scheduled 12 trucks and did all customer service functions) though my understanding of HP controls is limited. If it seems like I ask a lot of questions here that are elementary, it’s because I don’t like to assume I know the answers.


    The installer suggested we turn on the two upstairs units. Something about excess heat being automatically directed to those additional indoor coils.


    Yesterday we set the bedrooms to 61 with fan on auto. this morning the two rooms were both 72 degrees, as was the unheated hallway outside the open doors.

    The two downstairs units were set to 63 and 65, with actual room temps 66 and 68, respectively. So 3 degrees off.


    The units seem to be running continuously, with very warm air coming out.

    Right now the remote in the LR is set to 61 and the room is 70.


    I’m sweltering at 68, and a bit too warm at 66.


    Reached out to the installer this morning with this information. I’m starting to run out of patience, though he seems to be willing to spend the time to figure out what the problem is, so I’m giving him the benefit of the doubt. Maybe it’s an unusual problem.


  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    Yesterday we set the bedrooms to 61 with fan on auto. this morning the two rooms were both 72 degrees, as was the unheated hallway outside the open doors.

    Joanne,

    yeah if there's no sensing temperature somewhere to initiate an off cycle whether it be in the remote or at the unit... it needs that to cycle off.

    If in the remote and remote is way across the room I could see you getting wide variances in that kind of design. But yeah there's the problem with this style of unit. Lack of control and just settle for what temp you get?

    Then people wonder why I don't recommend mini splits for a whole house heating / cooling solution. This is part of it, among many other reasons.

    They're typically only fine in my opinion for occasional use room as a spot cooling / heating appliance. Like a craft room, he / she shed, work shop etc. You turn them on while using the room, then shut them off when not using it.

    With 11F off shoot above what you're trying to get, that's off to far. It makes your set points useless.

  • Elmer J Fudd
    2 months ago

    A slightly misleading geographical comparison, CR Homes. Minisplits have become widely used in Asia, Australia, and Europe for a few reasons - the first is that ducted HVAC in residential

    structures is virtually unknown so that adding AC to an existing install isn't something do-able. A second reason is that the advent of AC came long after the construction of many of the buildings for which minisplits have been installed. Put another way, it's most amenable to retrofitting existing buildings, many of which (abroad) are masonry, and/or apartments in larger buildings, etc. such that other approaches are really not available.

  • PRO
    Charles Ross Homes
    2 months ago

    Elmer,

    To be sure, residential ducted HVAC systems are more common in the U.S. than many other countries. That said, Ray is suggesting that ductless mini splits are inherently difficult to control. That has not been my experience. Indeed, if it were true, ductless mini splits wouldn't be utilized to the extent they are elsewhere in the world.

  • mike_home
    2 months ago

    ...it's most amenable to retrofitting existing buildings, many of which (abroad) are masonry, and/or apartments...

    If you take a drive through the many of the residential neighboorhoods of Brooklyn, Queens, Staten Island, and the Bronx in New York you will see thousands of houses built prior to the 1960s. Many single and multiple family homes have converted from window ACs to mini splits for cooling.

  • PRO
    Austin Air Companie
    2 months ago

    Ductless systems have been used throughout Asia, Australia, and Europe for decades with good success.


    Now compare those structures to the ones being retrofitted in America. I think you'll find the problem really quick. (everytime? no. No two structures are alike here. )


    It appears some mini split models are troublesome in the control dept. Just start rereading this thread Charles, the proof is staring you in the face. The contractors who installed it have now already checked on it a few times already.


    To call this anything but a design flaw from what I've conversed with Joanne about over the past several days is mind numbingly obvious.


    I've dug into manfacturer literature on this machine, they offer nothing in the way of how to control it from what I have found thus far. (Spent over an hour looking yesterday) Trying to sort thru all the mystical conconcocted variations of machines the manufacturer includes along with this model. (this model is a so called revision to a previous model that has been discontinued. Yet the design flaw escaped them?)


    But hey, pick and choose what words I've used here to publicly berate me with. That's very helpful?

  • PRO
    Charles Ross Homes
    2 months ago

    Mini-split heat pumps aren't as popular in the U.S. that they are elsewhere. I'm betting that installation and servicing of mini-split heat pumps isn't the particular HVAC contractor's core business nor core competency. Multi-head mini-split systems are more complicated than single units. It's more likely the issue the OP's problem is the product of improper installation by an inexperienced technician than an inherent control issue with mini split heat pumps.

  • mike_home
    2 months ago

    My HVAC guy suggested doing a hard restart on the systems, turning off the breakers for 15-20 minutes and then turning them back on.

    He’s stumped and has reached out to Mitsubishi tech support.

    In this case three individuals holding themselves out as a technician, business owner and manufacture's technical expert have been unable to make new equipment perform...

    Don't blame the equipment if it is installed incorrectly and the installer does not know how to identify the cause of the problem.

  • Joanne
    Original Author
    2 months ago

    The contractor stated that he has installed many of these systems, except that he has not used the floor mounted heads very often. He uses a computer program to determine the sizing.

    I think the wall mounted units are hideous, and requested these specifically.

    Yesterday, both rooms stayed within a degree of the setting. Overnight, they overshot the setting by 3-4 degrees.

    He has ordered a couple of the MHK2 controllers. If I’m not mistaken, the only thing we will learn after they’re installed is whether the temp sensor in the unit has been malfunctioning? I guess we’ll see.


  • klem1
    2 months ago

    Call it design flaw,call it poor installation,call it what you like but there is no excuse for units not preforming after all this time. "maybe it's an unusual problem" is no excuse,unusual problems and mfgr flaws are part of the business competent contractors are prepared to confront and solve. To what standard of performance will contractor be held to? I question whether units will preform efficiently and have longevity when they are finally limping along and homeowner has to get on with other things in life. If the truth was known I'd give even odds "mfgr support on the phone" was actually drinking buddy George or contractor's wife.

  • PRO
    Austin Air Companie
    2 months ago

    He has ordered a couple of the MHK2 controllers. If I’m not mistaken, the only thing we will learn after they’re installed is whether the temp sensor in the unit has been malfunctioning? I guess we’ll see.


    Joanne,


    Features:

    • Includes MRCH2 thermostat, MIFH2 wireless receiver, and MRC2 cable
    • Backlit display
    • Can sense temperature at indoor unit (default) or at controller
    • Fahrenheit °F / Celsius °C
    • No batteries required
    • Single zone compatible
    • Multi zone compatible

    It can sense temperature at the indoor unit or at the controller (thermostat).


    So this will give you some additional options on how to control the on / off cycle.


    The con if you will is finding a place to put the controller (thermostat) that isn't going to be influenced negatively by any air around it.


    (unless there is a temp sensor failing within the indoor head, if not a design flaw to any degree this should have already been determined with some technical test provided by manufacturer. You shouldn't have to jump thru this many hoops to prove it. Sorry, that screams design flaw to me.)


    How hard would it be for the manufacturer to show the set point temp on the indoor head to illustrate the cut out temperature setting where the unit will off cycle? I guess they don't want to make it easy for you to demonstrate the design flaw of their own unit. Model change and discontinuation mode to keep the gravy train going. It's alot of work to keep abreast of these units / changes and so on. They change quicker than most techs change their underwear. I don't consider that a selling point.


    If the controller / or the indoor head setting for temperature controll doesn't do the job to your satisfaction, Then the illustration of return air I provided earlier should tell the story of why air flows (supply) as well as return are both equally important in HVAC design. Not to mention why ductwork can sometimes be the better solution. Design however is just as important when it comes to ductwork.





  • PRO
    Austin Air Companie
    2 months ago

    Hi Joanne,


    So I spent some more time exploring and this picture below is about as clear as I think this can be made as to the problem you have. I made some changes to this picture added in design problems pointing to these issues in heat mode. (center of the attached picture - click it to enlarge it.)




    The room in heat mode: according to Mitsubishi documents with a set point of 73F and outdoor temp of 5F there is going to be over shoot of 7F based on what is shown in the above picture 60 min later.


    The center of the picture shows a room with color shaded denominated temps in that room clearly showing hot / cool / cold spots. In a room that was chosen by MFG. So depending on a host of various factors ( how drafty the structure is / how cold it is outside etc)


    Based on these findings I think it's going to be quite the challenge. The problem as I see it is the "throw" of getting heat across a room without drawing that warm air back into the unit to the point in which the unit cycles off.


    I believe this is why the doc shows 60 min later with a set point of 73 and a room that shows an 80F dark orange. In the picture we don't know what temp Orange / Yellow is? It's what ever you get. (over shoot is built into it. Or in laymen's terms Design Flaw)


    To the cooling side the room is a light colored blue thru out the room. We don't know what temp blue is?


    --------

    The new thermostat controller. This new thermostat has the ability to add Honeywell redlink 3.0 temp sensor. Like if you don't trust the sensor in the unit itself (the head floor mount unit) find a cool spot to mount the redlink sensor in the air stream of air being returned to the floor mounted head.


    Maybe that will make it a bit more accurate? I think within 5F of your set point is going to be a challenge though based on everything I seen / discussed in this thread already.


    Of course let us know how you make out... so we keep things real here. (not fake)


    Have a good weekend y'all.

  • PRO
    Charles Ross Homes
    2 months ago

    Would an HVAC manufacturer advertise a system with an overshoot of 7F? With apologies to Mr. Spock, "That would be illogical, Ray." The artist's rendering--and it's only that- is to illustrate that air flow leaving the indoor unit in heating mode is divided and directed both up and down to provide better mixing in the room. Those temperatures will indeed be higher than setpoint if the temperature leaving the indoor unit is higher than room temperature, which is the case in heating mode. Measure the temperature at a supply grill a ducted, forced air system. In heating mode it will be higher than setpoint by a lot more than 7F. Is that overshoot?

    Mini-splits, just like your oft-lauded Bosch inverted-driven heat pump, are designed to work at steady state, adjusting their operation to match the heating or cooling load. It would be inappropriate to assess the performance of a single system in a transient state and then declare mini-splits to have inherent temperature control issues.

    Troubleshooting is an up close and in-person job, right? Unfortunately, the tech assigned to this case appears to be over his/her head. Hopefully the OP will get in-person assistance from someone more qualified. In any case, let's wait for the facts before dismissing mini-splits as having inherent temperature control problems.

  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    Would an HVAC manufacturer advertise a system with an overshoot of 7F?

    The picture I got was from Mitsubishi literature Charles.

    It doesn't state an over shoot of 7F, this was concluded by referencing the color coded chart of the room in question to a set point on the document of the conditions compared to the color coding scheme they provided in the picture. We know the top of the scale dark orange color to be 80F. We know that dark blue color to be 41F. If corner of the room is green in color what temperature does that represent? I honestly do not know other than green is far away from yellow, yellow orange and dark orange that represents higher temperature according to the color coded temp chart the picture uses.

    I am only using this picture (one they provided me... they meaning Mitsubishi) to illustrate the problem.

    If the set point is 73F and the dark orange color represents 80F is that not simple math? This is what the picture shows does it not?

    This particular floor mount unit also shows the ability to draw heated air back into it to heat that air up even more as a quicker way to heat, but yet they offer no explanation what this does to limit short cycling at the head itself due to this over heated air?

    In a traditional set up with ducted returns, you can have temperature differences across rooms due to a whole host of different factors. But the subject matter for that (ducted heating and cooling) is entirely different than the issues faced with a "heating and cooling head" that is merely controlling the comfort level in a single room.

    We're in a thread discussing control issues for a mini split? The service people are changing the controls for one that can use the head or the thermostat itself as a means to cycle the unit.

    But yet I'm not allowed to suggest there is a control issue for a mini split that the thermostat is in the midst of being replaced? (The thread is right here Charles "scroll up and re-read it")

  • PRO
    Charles Ross Homes
    2 months ago

    You've confused work done by the advertising company's art department for work done by a manufacturer's technical group, Ray. The marketing materials you shared are artist's pictures--not a thermograph of temperatures. An IR thermograph would only show wall temperatures--not room air temperatures (it infers surface temperatures by measuring radiant energy.) And thermographs are color photographs--usually with some reference temperatures indicated. The purpose of the graphic is only to make the point that there is little temperature variation in the room after an hour of heating. That's also the reason why there is no temperature indicated for the blue shown in air conditioning.

    You've stated your dislike for mini-split heat pumps in multiple threads, It appears that you're looking for data to support your opinion. However, the performance of a single system isn't a representative sample to support a general claim about mini-splits. Multiple-head systems like the OP's are more complicated than single head systems and it appears beyond the capability of the folks working to remedy the issue.

  • klem1
    2 months ago

    I don't have anything op can use to improve their system performance but I do have some thoughts related to the playground squabble over design. "IF" the head is designed to direct air flow upwards and downwards simultaneously , that is flawed. "IF" flow is upwards in cooling mode and downwards in heat mode , that could be acceptable. "IF" head is designed to recirculate air across coils BEFORE air has made a trip through room ,that is flawed. Electronics and gadgetry can change mechanical behavior but not physics.

    I expect most m-splits' performance and economy could be improved with a little ducting but I'm afraid consumers would reject the appearance. It was common knowledge in the heyday of window shakers that mounting unit nearer ceiling than sitting on window stool noticeably improved performance. To my satisfaction where appearance took backseat to performance I proved a bit of duct/deflection helped a window shaker so I think it would a ms as well. Again,that doesn't solve op's predicament but does point out what I believe to be poor design in Ray's picture.

  • PRO
    Charles Ross Homes
    2 months ago

    " but does point out what I believe to be poor design in Ray's picture."

    Au contraire, monsieur Klem (I figure if Ray spells "air" as "aire," he'd appreciate a French accent to my post.) In my engineering opinion, the Mitsubishi design is interesting. It might be gimmicky or not. The advertising graphic suggests it's effective, and I'll take that at face value. Let's consider the design rationale.

    Stratification of air in homes is well known to HVAC designers: Warm air is less dense and thus more buoyant than cooler air; it rises as it is heated. It's not uncommon for temperatures at ceiling height to be a couple of degrees higher than at floor level. When designing conventional ducted systems, returns are located to promote mixing and minimize temperature and relative humidity differences. Best practice is to locate returns high on the wall or ceiling for operation in air conditioning mode and low for operation in heating mode. If you're fortunate enough to have both, you can switch them seasonally. While that works for ducted systems, a different approach is clearly needed for a ductless system like the OP"s. With regard to mixing--and more importantly, occupant comfort-- directing warm air both up and down in heating mode makes sense, as does directing cool air only up in air conditioning mode. This is consistent with your "window shakers" providing more occupant comfort when installed near the ceiling.

    The folks at Mitsubishi have been designing and manufacturing ductless mini-split heat pumps since 1959. I'm betting their engineers know what they're doing and that they don't confuse marketing materials and technical literature.

  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    It might be gimmicky or not. The advertising graphic suggests it's effective, and I'll take that at face value.

    If you're happy with a 7F over shoot from your set point. If you're setting it for 73F with an expectation of 80F.

    This is what the whole argument is about -- The OP has experienced as much as 11F over shoot from set point. Sorry Charles, I still call it a design flaw.

    The OP has also experienced too little heat as well. Worse than a gimmick. It doesn't work.

    With the addition of a new stat with control from that stat maybe things will be better for this model. I think there's still going to be a variance but we have these kinds of trade offs with traditional HVAC ducted systems when thermostats are located on the far side of a home as an example.

    It's not uncommon to have 3-5F temp difference across the structure. In extreme cases worse than that and again 9 times out of 10 it's due to design of the house / design of hvac within that house among other factors.

  • Joanne
    Original Author
    2 months ago

    What’s weird is that some days it works pretty well, within 1 degree or so. I was hoping it was working now, but on other days they’re all over the place, either too high or too low.


    Still waitng for the separate thermostats to come in.

  • Joanne
    Original Author
    2 months ago

    What’s weird is that some days is has been working pretty well, within 1 degree or so of the setting. Other days they’re all over the place, either high or low.

  • klem1
    2 months ago

    "The folks at Mitsubishi have been designing and manufacturing ductless mini-split heat pumps since 1959. I'm betting their engineers know what they're doing and that they don't confuse marketing materials and technical literature."

    That is the position most consumers take on all products but those who keep fingers on the pulse will say the trust isn't always deserved. Stepping away from OP's Mitsubishi for a moment, let's look at the two never ending threads about dust settling in homes. The overwhelming consensus in those threads is that quility,longivity and functionality of laundry appliances are in a death spiral. How would you account for appliance desighn engineers not only failing to improve but taking value downhill? Is there any reason to believe ac equipment engineers are more dedicated to consumers than their brethren at GE and Maytag? I think not and the root cause is demand for price with little or no demand for quality and accountability. It's the new way of American life. I attended a party this weekend to watch AFC and NFC games at the home of a couple who can easily afford the best. The host mentioned her driveway gate opener battery was failing and she is nervous about leaving gate open until battery can be replaced. Her fil recommended a vendor known to be the cheapest battery source around. I made no comments but I know the vendor and his off brands don't have a good reputation with high volume purchasers. Point being if my Grandchildren lived there I would want the best available when it comes to security ,esp since his son and dil can easily afford it.

  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    What’s weird is that some days is has been working pretty well, within 1 degree or so of the setting.

    Joanne,

    When it's behaving what is the outdoor temp like? Just trying to see if there's some correlation due to outdoor ambient conditions.

    These are mostly communicating machines and I suspect the over shoot function may be set that way due to outdoor ambient. As the picture shows 5F out door ambient / set point 73F and within an hour the room is 80F.

    If this is all by design, the outdoor ambient temp maybe yet another factor to consider. Because this temperature varies you may not have an answer to this question right now.

  • Joanne
    Original Author
    2 months ago

    Outdoor temps have been in the 30s to 40s daytime and 20s at night, but they still wig out at either temp, both day and night. it’s intermittent and unpredictable.

    I’ll watch the outdoor temps more closely to document when the problem occurs more accurately.

  • PRO
    Charles Ross Homes
    2 months ago

    There's a control issue here--might be due to a sizing error, an installation error or a defect in one or more components. These systems get beta tested by the manufacturer so an inherent design flaw isn't as likely as the other causes. The OP needs more technical capability than the installing contractor has on staff. That's a fairly common experience related on houzz.com threads.

  • PRO
    Austin Air Companie
    2 months ago

    I’ll watch the outdoor temps more closely to document when the problem occurs more accurately.


    Joanne, ok. I don't know if this is a cause of anything or not. But Inverter heat pumps algorithm likely make choices of operation due to outdoor ambient temps which is achieved thru outdoor temp sensor of the unit in question and with communicating aspects of these units possibly the indoor temp as well.


    The colder it is outside may cause it to ramp higher & put out more heat and or over shoot the set point some.


    I found out some additional advantages of installing the MHK2 controller. (click to enlarge)


    MHK2 controller has the ability to use indoor unit sensor, the MHK2 controller, Redlink Indoor air sensor OR the average of MHK2 and Redlink Sensor.


    In addition to all that: an improved indoor unit function code list that is expanded to 28 indoor unit codes. (probably fault codes)


    If there is some random fault that is causing some issues it sounds like the MHK2 controller would be the next step to provide the necessary clues to finding what it is.

  • klem1
    2 months ago

    Sophisticated controllers are above my pay grade but if I might,I'd like to suggest Joanne write down all codes that are displayed and relay them to Ray. I expect codes to cast some telling light on this drama.

  • dadoes
    2 months ago

    I'm surely stoopid, as is anyone and everyone who isn't a trained and experienced HVAC repairer/iinstaller ... but I don't think the 73°F vs. 80°F in the photo above is referencing an engineered intentional setpoint overshoot. It may be referencing the differential of the heated outflow air vs. the room temperature intake air.

  • PRO
    Austin Air Companie
    2 months ago

    I don't think the 73°F vs. 80°F in the photo above is referencing an engineered intentional setpoint overshoot. It may be referencing the differential of the heated outflow air vs. the room temperature intake air.


    Typical heat pump generated supply temp is usually in the range of 100F to maybe 120F depending on a number of factors. (partly due to outdoor ambient temps)


    So with those supply temp numbers you can see from the picture why some areas of the picture are elevated in temp. We only know what temperature is due to color shading, there is nothing to pin it down as to what the actual temperature is? (unless we put thermometers in various parts of the room right?)


    That's the whole point here there is nothing specific. Use a graded color chart to guess?


    So taking an average supply temp I provide you above please provide the formula so we all know here?


    ------- Is this what you're saying below? ----------


    A 7 degree differential (delta) from supply to return? That is going to produce an 80F room in an hour with an outdoor ambient temp of 5F? Uh no, that will never work or never produce a room with 80F heat in the deep throws of old man winter.

  • PRO
    Charles Ross Homes
    2 months ago

    "A 7 degree differential (delta) from supply to return? That is going to produce an 80F room in an hour with an outdoor ambient temp of 5F? Uh no, that will never work or never produce a room with 80F heat in the deep throws of old man winter." Absolute statements like "never" will absolutely get you in trouble, Ray. Like this one.

    The required heating load is also a function of the size of the conditioned area; the area of wall, floor and ceiling surfaces with unconditioned areas on the other side; the type of insulation and it's R-value; the amount of glazing; solar heat gain; air infiltration, etc. --none of which we know. In the SIPs home I built a couple of years ago, you could heat a room on the coldest day with a high wattage light bulb. A 7F delta-T might require occupants to disrobe.

    The marketing folks at Mitsubishi weren't trying to make any other point than the ability of their system to produce a uniform room temperature in a reasonable timeframe in heating mode at the lower end of outdoor operating temperature range. That's all. You're reading it as some sort of technical literature. It's not.

    Your comment about delta-T suggests some confusion with how mini-split heat pumps work. Let's take a moment for an "in-service." A ductless mini-split heat pump is an inverter-driven (variable speed) heat pump--like the Bosch heat pump in your home, but typically smaller capacity (that's the "mini" part) and without the ducts (that's the "ductless" part.) A mini-split's point of difference from a conventional (single-stage, fixed-speed) heat pump is that the heating or cooling output is modulated to match the load-- just like your Bosch system. As a room heats up (in heating mode) or cools down (in air conditioning mode) the load gets reduced. As the room temperature approaches the desired (setpoint) temperature, the mini-split's compressor speed and fan speed are reduced to respond to the decreased load (if the fan is set for fixed speed, then only the compressor speed will modulate.) The temperature differential (delta-T) will be reduced.

    Mini-splits are designed to operate as close to steady state operation as possible to avoid the energy penalties of starting and stopping the compressor. At steady-state operation, delta-T is much lower than would be the case with a ducted, fixed-speed heat pump operating in on/off mode.

  • dadoes
    2 months ago

    Charles, I purposely didn't mention the possible inverter variable-speed aspect to see if anyone else would do so.

  • PRO
    Charles Ross Homes
    2 months ago

    No worries. You're not in the stoopid category.

  • PRO
    Austin Air Companie
    2 months ago

    Then why does it over shoot then Charles?


    You forget we are in a thread with proof that the OP is having problems with the temp put out by the heat pump and is Over Shooting the set point temperature. Sometimes as much as 11F past set point (above set point) worse than what the so called "not technical literature" pay no attention to it, but "we provided it anyway" shows.


    Then go on to imply name calling? Real mature Charles.


    After all this time, length of this thread with continued OP involvement describing the problem you're suggesting it just due to the unit being an inverter that is often advertised as being more accurate is just due to the way it operates and pass it off as that?


    I've been in homes the builder has more or less abandoned, so it wouldn't surprise me Charles. What would surprise me if you admitted it.

  • PRO
    Charles Ross Homes
    2 months ago

    The OP related a problem with their system not producing adequate heat in heating mode. That's not an overshoot issue. As of yet, the root cause of the problem hasn't been determined. It could be due to any of the causes noted in my post above.

  • PRO
    Austin Air Companie
    2 months ago

    From the OP:

    The units seem to be running continuously, with very warm air coming out.

    Right now the remote in the LR is set to 61 and the room is 70.

    I’m sweltering at 68, and a bit too warm at 66.


    And this wasn't even the worst. Charles apparently can't be bothered with reading the entire thread?


    Why come to a forum board if you can't / won't read what is there? Enlighten us Charles with your productive comments.

  • PRO
    Charles Ross Homes
    2 months ago

    The OP's four-head system is producing inadequate heat in some rooms on occasion and excessive heat on occasion. It's a system control issue that the folks working the problem don't seem equipped to diagnose You've labelled the problem as a "design flaw" based on a graphic in marketing materials. That's rich.

    It will be interesting to learn what the root cause of the problem is. Unfortunately HVAC "telemedicine" has its limitations. I think that determining and fixing the root cause is going to require importing technical expertise the OP's contractor lacks.

  • Joanne
    Original Author
    2 months ago

    FWIW, our contractor mentioned to my husband that he has a bunch of used heads in his shop that he had to replace with new ones rather than spend more time trying to fix problems. Oi.


    Still waiting for the new thermostats. For the past few days the two units have been mostly running at 2 degrees over setpoint with the occasional too-little heat. Temps outside warmish for this time of year.


    If it was just the 2 degrees higher I could live with it, but when it’s low my oil-fired boiler keeps coming on. I cut our oil-price contract almost in half this year, thinking that the HPs would do the job.

  • PRO
    Charles Ross Homes
    2 months ago

    Your contractor may be setting you up for the "component replacement game." To play the game, the contractor starts replacing system components more or less at random in the hope that the otherwise undiagnosed problem goes away. The contractor "wins" if you pay the bill whether the problem is solved or not.

    A properly-designed cold climate heat pump system shouldn't need back-up heat that often in CT. Ask to see the ACCA manual J calculations and the manual S equipment sizing for your systems. The contractor should have also calculated the balance point for each system which is the temperature below which the heat pump can meet 100% of the heating needs (i.e., when back-up heating is required.). If the contractor can't produce that documentation or there are glaring errors, you're off to a good start diagnosing the problem..

  • PRO
    Austin Air Companie
    2 months ago
    last modified: 2 months ago

    I doubt that Charles, because there's time Joanne has too much heat as well as times when it's not enough.

    Still waiting for the new thermostats. For the past few days the two units have been mostly running at 2 degrees over setpoint with the occasional too-little heat. Temps outside warmish for this time of year.

    To me it points to a sensor related issue. Whether this is by design is hard to say (we're discussing this in a forum board where some read and some don't -- you know who you are).

    Joanne, I know you said previously that they added refrigerant to the system and so if they did that (depending on how, what and why?) As temperature increases outdoors an over charge (even a few ounces) can put some monkey's with wrenches inside the unit. It's a critically charged machine. (laymen's terms it isn't going to work if the refrigerant charge is wrong on a critically charged machine.)

    So other times when it's colder points to a sensor related problem, warmer something else happens -- that may or may not be refrigerant charge related? The way to determine is to remove all refrigerant and weight it, then put back what is needed by weight.

    I mention this because you said they have used heads that they've replaced. If they do not understand refrigerant charge requirements for critically charged machines, they will probably never fix it.

    Sizing related issues? the unit would just short cycle if too big OR too small run all the time with no off cycle and not enough heat when colder out. (This particular unit capable down to 5F)


    If you have a room that requires exactly 17,000 BTU of heat maybe they make heads for 15,000 BTU or 18,000 BTU --- Pick one, either one will be either undersized or over sized. Why the sizing game or the blame thereof isn't quite what you think it is.


    The closer you look at sizing, the more it becomes apparent the unit isn't really "properly sized".

  • PRO
    Charles Ross Homes
    2 months ago

    I listed a variety of potential causes upthread, Ray.

    When troubleshooting, I recommend beginning at, well, the beginning. For HVAC systems that means verifying the size of the system is appropriate to the application. If it's not, then everything else is likely to be continuing education for the contractor at the client's expense. The OP complained about the cost of back-up heating which suggests size may be an issue. If the contractor ran the calcs, there's little effort on their part to provide the OP with a copy.