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Community and Q&A

Minisplit Discharge Temperature and Fan Speed

maine_tyler | Posted in Mechanicals on

It was recently brought to my attention that discharge temp for cold climate mini-splits is quite high, with a rough target delta T between discharge and intake of 35-40 F.

The poster gave me what I’m sure is a very good response for how this works, but I’m needing some time to process. I hope if that poster sees this they will respond again because I’m sure I just need some more talking through.

I still don’t understand how modulation is achieved if an indoor unit with a set coil area maintains a certain delta T (i.e. discharge temp), and fan speed is allowed to be set manually. Does setting a manual fan speed override the thermostat?

My conundrum is this: If CFM is held constant, and delta T is held somewhat constant, then what is modulating in terms of output power – btu/h?

I understand the outdoor compressor modulates so it can maintain a given delta T (or a range) in varying outdoor temperatures, but this doesn’t explain how the indoor unit itself modulates the output. It seems either delta T AND/OR CFM needs to modulate.

I was also told the indoor units themselves can modulate very low, meaning the bottom end of system-wide modulation is dictated by the outdoor unit. Is this not true?

I’m sure I’m missing something obvious. Go easy on the jargon as I’m not a heat pump /refrigeration tech!

Here’s the thread for reference, posts #31 and #32:

Is this post-ASHP electricity use expected?

And another thread that talks about this high output temp:

Output air temperature of mini splits on heat?

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  1. bfw577 | | #1

    The service manual for my Gree Sapphire has extremely detailed data on how the fan and compressor operate. I have also been monitoring my return/supply temps and electricity consumption for 2 years. The fan on my Gree is controlled off the ambient room temperature delta and some software limitations. The modulation is all done by adjusting the compressor frequency. There are limitations such as let's say I turn my unit on high fan to heat 72 and the room is really cold. The unit will run at max capacity and as it gets closer to the setpoint and the inverter modulates down it will automatically drop the fan speed. When it reached minimum capacity its only using like 160 watts with a delta t of 15. Whenever its at minimum it will only run on low fan speed even though its still set to high. Same applies if my unit is at max capacity on high fan with a 125 degree discharge temperature. I can't set my unit to low fan as the software won't allow it. Dropping from 500 cfm at high to 150 cfm at low with those supply temps wouldn't be good for the equipment. My Midea/Carrier unit behaves the same way.

    An electricity monitor will let you see all of this in real time .

    Below is how the auto fan speed is determined from my service manual. I recommend you read section 6.4 as it explains exactly how it determines compressor frequency and fan setting.

    1. Under heating mode: auto speed under heating or auto heating mode:
    a. When Tamb.≤Tpreset+1°C(1.8°F), indoor fan will operate at high speed;
    b. When Tpreset+1°C(1.8°F)<Tamb.<Tpreset+3°C(5.4°F), indoor fan will operate at medium speed;
    c. When Tamb.≥Tpreset+3°C(5.4°F), indoor fan will operate at low speed;
    There should be at least 180s operation time during switchover of each speed.
    2. Under cooling mode: auto speed under cooling or auto cooling mode:
    a. When Tamb.≥Tpreset+2°C(3.6°F), indoor fan will operate at high speed;
    b. When Tpreset<Tamb.<Tpreset+2°C(3.6°F), indoor fan will operate at medium speed;
    c. When Tamb.≤Tpreset, indoor fan will operate at low speed
    There should be at least 210s operation time during switchover of each speed.

  2. bfw577 | | #2

    Here is a pretty good graph of my supply temps and electricity consumption that might help illustrate it better. The 2 blips are 2 defrost cycles. The fan is set to high and has been running at high the entire time. Notice after the defrost cycle the electricity consumption spikes to 1.2 kw briefly to make up from the defrost cycle. You can see the corresponding increase on supply temps but the fan speed has been the same. The compressor simply increased the supply air temperatures briefly but the cfm hasn't changed.

  3. bfw577 | | #3

    Here is my other unit a 12k Midea. On this graph at the beginning this unit is running at minimum capacity of 290 watts. The 2 spikes on the electricity graph in beginning are software driven oil control cycles. Single zones can operate at some extremely low minimum's. My Gree can turn down to a crazy 80 watts in cooling and 180 in heating mode. At those crazy low speeds the compressor is potentially not moving enough refrigerant through the system. Every 15 mins the inverter will surge to around double the wattage to circulate oil through the system. During this time the fan is still on its lowest setting. You can see that correspondimg huge increase on the supply temperatures during the oil control surge.

  4. aunsafe2015 | | #4

    Thanks for the data bfw. Love seeing that stuff!

  5. maine_tyler | | #5

    Thank you bfm.

    How are you measuring supply temps-- Instrument/placement?

    So it sounds like temperature does modulate, but it also sounds like fan speed is always somewhat automated and not able to be overridden manually? Still a bit confusing... what's the point of manual fan mode then?

    "My Gree can turn down to a crazy 80 watts in cooling and 180 in heating mode. "

    So I get that compressors can often modulate extremely low, but how does this correspond to the indoor unit. My assumption is that it would have to significantly lower supply temp AND reduce fan speed. Is that essentially all there is to it? Get the temp and CFM low enough to match the compressor output?

    Part of what I'm wondering is if it's always truly the outdoor unit that determines the bottom-end modulation (at least with 1:1 units)? Or, for example, if a user comes into the room and pushes the 'high fan' mode are they to some extent overriding the low-end modulation of the outdoor unit?

    1. Expert Member
      DCcontrarian | | #6

      The amount of refrigerant moving determines the amount of heat moved, and the speed of the compressor determines the amount of refrigerant moving. There's really no way around that. If you move too little heat the refrigerant won't vaporize when it needs to, which leads to liquid refrigerant entering the compressor which will wreck the compressor. And you just can't move more heat than the refrigerant delivers.

      So the modulation of the compressor determines how much heat comes out of the head. They'd rather keep the temperature delta constant and modulate the blower speed in the head, but if the user insists on a constant fan speed the only thing left to modulate is the temperature delta.

      1. maine_tyler | | #9

        "The amount of refrigerant moving determines the amount of heat moved, and the speed of the compressor determines the amount of refrigerant moving."

        Are you saying the temperature of the refrigerant is a constant?

        I am pretty in the dark about the technicals of heat pumps, but it seems plain logic would dictate that the only way to deliver the minimum btu/h rating of the outdoor compressor to the indoors is to have sufficiently low refrigerant flow (or temp?) and low enough CMF at the indoor unit. If the delta T was kept fairly constant, and/or fan speed was constant, how on earth would the delivered heat ever be reduced? If CFM is too high for a given refrigerant flow, the delta T would be reduced. Is the low fan setting on these units really low enough CFM that pretty high delta T's can still be maintained at bottom-end modulation? I guess I could try out some math...

        And then if that IS true, is the high fan CFM rate high enough that the maximum btu/h rating makes sense?

        Anyways, bfw's answer implies there IS a delta T fluctuation, so maybe greenright can come help sort this out. S/he was referring to different manufacturers I believe.

        1. Expert Member
          DCcontrarian | | #10

          The temperature of the refrigerant in cooling mode is determined by its boiling point so it's going to be pretty constant. The temperature in heating mode is determined by the heat of vaporization so it's also going to be pretty constant. If there are variations in pressure those would vary but my understanding is the pumps used in compressors are constant pressure, variable flow types.

    2. bfw577 | | #7

      I'm using an Acurite weather station sensor with a wired remote sensor. I have also have their online hub and temp/humidity sensors in every room in my house.

      I have both an Emporia Vue and Efergy Engage energy monitors for electricty. I can't recommend the these enough. You can literally see 1 second electricty consumption on your heat pump and chart its modulation in real time on your phone.

      There is different software for every manufacturer on how things are controlled especially the fans. The fans on my units always seem to let you run the fan higher but there is software keeping it from running under certain conditions ac DC and I pointed out. My observation is the fan limit is only noticeable when the unit is near minimum and max capacities. Outside of those zones I can set the fan speed to pretty much anything I want.

      So my unit is running at 50% capacity around 900 watts in med fan speed for an hour and the supply temps have been exactly 105 degrees. I set the fan to high and for the next hour the unit runs at the exact same 900 watts for an hour. My supply temperatures were now 95 degrees for that hour. That is pretty much all there is to it. You can even calculate your rough COP if you wanted to with your delta t and electricity. I have actually found running the fan on high on my units drops the electricity consumption and boosts the efficiency. I see an immediate decrease on the inverter electricty consumption when switching to high fan. Its not a huge amount but its noticeable.

      1. Expert Member
        DCcontrarian | | #8

        Running on high the unit will dehumidify less for the same amount of sensible cooling, so the COP will appear higher. This is a trick manufacturers use to get higher COP ratings.

        1. Expert Member
          DCcontrarian | | #12

          I have to correct myself, I got the causality backwards.

          At a higher fan speed, for the same amount of cooling the coil will be warmer. Less coolant is required and the return temperature is higher, both of which improve the COP. With the warmer coil less dehumidification happens.

          The upshot is the same: if you need significant dehumidification you may not be able to achieve the highest COP quoted by manufacturers.

          1. bfw577 | | #26

            I was referring to heating. I found running the fan on the highest setting significantly boosts the efficiency based on years of my extensive energy monitoring. You can actually see the electricity consumption drop on an energy monitor as soon as you make the switch to high fan. I get about a 5-10% increase in cop by running manually on high fan.

  6. greenright | | #11

    I assume OP had one of my previous posts in mind.

    So this is how it works... now, I do not have access to the source code for the firmware of the outdoor and indoor units, but beyond that here it is for heating:

    1. The thermostat calls for heat. Along with the call the current room temp and set temp are communicated to the outdoor. That is why minisplits do not work well non-native thermostats as those two parameters are not provided and flexible operation is not possible.

    2. The outdoor ramps up the compressor to heat up the internal coil

    3. Once the coil reaches 120f or so (discharge vapor target is 140f) the indoor fan is turned on.

    4. Speed of fan is either high/med/low fixed or is "auto" in which case the difference of room temp from setpoint determines the speed- 3f or more- high speed, 2f or more- med speed, 1f or more - low speed

    5. Compressor is ramped up for 10-15 minutes or until high pressure hits 425-ish psi.

    6. Once 5 is reached based on the heat demand (sensed from return air vs setpoint) the high pressure target is set from 375 to 425 psi and the system tries to maintain that with 1-5f superheat (closer to 1f) and 130-140f vapor discharge temp (resulting in 35 degree delta-t at 68-70f indoor). I am very suspicious that the target high pressure is in very coarse steps and might have only 3 values - 375psi for 1f or more from setpoint, 400psi for 2f or more from setpoint and 415-425 (mitsu/fuji) for 3f or more from setpoint. The compressor speed and ev pulsing here is where the trade secret crown jewels are.

    7. #6 is maintained until setpoint +1f is reached.

    8. Oil return and defrost are executed as needed based on outdoor heat exchanger parameters and/ or time passed.

    How is modulation done? it is all in #6. As the setpoint is approached, the target high pressure is reduced from 425.... down to 375 in a regressive manner to maintain the 140f discharge which as room temp raises becomes easier. This puts less and less load on the compressor resulting in dropping of power consumption. Pulsing the ev trying to maintain the 1-5f superheat is what results in the wavy consumption pattern for the compressor as load "floats". If ev pulsing frequency range is not enough to keep parameters in check the compressor is stepped up or down.... but load generally has a downslope direction as heat demand and target high pressure is reduced over time as set temp is approached.

    There are prolly a number of smaller nuances, but in general Fujitsu and Mitsu operate in the above manner.

    On overcharging vs undercharging - once the system has stabilized (generally accepted after 30 minutes of operation) if charge is right a compressor step up or step down should lead to pressure going up or down and staying constant at that level. If after several minutes of constant compressor speed pressure has been flat but then it starts to progressively curve up - the unit is prolly overcharged. If the pressure is flat for few minutes and compressor speed is unchanged and then pressure progressively starts to dive - the unit is undercharged. This of course assumes constant indoor fan speed. Lowering the speed increases high pressure and raising speed lowers high pressure. When you see constant compressor speed in the cruising stage of the cycle and you see pressure all of sudden curving up or down you need to check that fan speed stayed constant before drawing charge level conclusions. Of course if your fan is hard set to one of the 3 speeds - this is not a consideration- only if fan is on "auto" you need to be aware of its speed changes.

    On charge levels- any hvac tech will blindly repeat the mantra that the minisplit is a critically charged unit and few ounces make a difference and the only way to properly charge is by weight. This is what the manufacturer wants the truth out there to be as it leaves little place for error and interpretation. However, I can tell you that if setpoint is 3 degrees or more off room temp in heat mode and you are running the indoor fan on high and your discharge pressure is 415-425 (Mitsu/ Fuji) and discharge temp is 130-140f and superheat is 1-3f - your unit is properly charged. In cooling mode the target is about 115-125 (Mitsu/ Fuji) for low pressure and high pressure is not monitored nor acted upon.

    Hope this helps.

    1. bfw577 | | #13

      Mitsubishi is not using any pressure sensors to control the compressor. There isn't a single pressure sensor on any of their popular single zone MU-FZ12k, 18k, etc. Its all done through temperature sensors. Gree and Midea/Carrier and others also don't have any pressure sensors and its all done with just 5 or 6 temp sensors. It's a great design as it eliminates a leak potential from a pressure sensor. The temp sensors can be tested a quick with a multimeter and are cheap and can be replaced in seconds. No need to open or touch any refrigerant lines. Just sensors that mount external. Any issue with them will also trigger an error code an some units will still operate with 1 bad sensor.

      This guy has one of the best explanations on how a mini split operates. Its a Mitsubishi he is talking about in operation and in his pictures.

      "There is usually a temp sensor mounted by the manufacturer on both sides of each EEV, even if the sensor is a little distance away. The system will also have other temp sensors as well. The temp sensors are monitoring the compressor discharge temp, the saturated refrigerant temp in both coils, and the temp on the refrigerant lines exiting both the condenser and the evaporator coil. The EEV will not only adjust for an efficient superheat but it will also maintain a steady range of vapor pressure depending on the conditions."

      1. greenright | | #14

        Yes, there are many cheaper models that do not use pressure sensors to control anything. My description is obviously about units that do have pressure sensors. That goes without saying, but thanks for pointing it out.

  7. maine_tyler | | #15

    Well I'm sure these detailed answers somehow answer my question, can someone explain why it shouldn't be much more simple. E.g. see my below calcuation:

    q = CFM x 1.08 x ∆T
    1,600 = 140 x 1.08 x ∆T

    1,600 = 151.2 x ∆T
    10.58 = ∆T

    1,600 is the minimum btu/h rating for the 9k Mitsubishi 1:1 unit.
    140 is the low fan CFM.
    The delta T would need to be 10.58 F at LOW fan setting with minimum rated output. Even lower at higher fan speeds.

    So where have I gone wrong? Let's keep this as simple as possible and keep the bells and whistles at bay unless they're strictly necessary for comprehension.

    1. maine_tyler | | #16

      I should add: thank you for the detailed responses. I will digest them in time. I am trying to keep a simple brain on the rails.

    2. maine_tyler | | #17

      And for fun, max output at 47F of 18,000 with fan-high CFM of 437:

      q = CFM x 1.08 x ∆T
      18,000 = 437 x 1.08 x ∆T
      18,000 = 471.96 x ∆T
      38 = ∆T

      1. bfw577 | | #25

        I commented with more information below but this is pretty much spot on what I see on my 12k Gree. At max capacity supply temps have a delta t around 40+ with an 18k btu output.

        Same goes for minimum modulation. I believe that Mitsubishi you referred to uses 80 watts or something crazy low at minimum. I have seen delta t's around 12 in very cold weather at minimum on my unit.

    3. Expert Member
      DCcontrarian | | #18

      The compressor senses the returning refrigerant temperature and modulates accordingly -- the lower the delta, the slower it runs. If it can't modulate low enough, it shuts off.

      1. maine_tyler | | #19

        Are you aware of what the original question was?

        Someone purported that delta T was kept up around 30+ F.
        I was asking how that can be-- how does the INDOOR unit modulate if that is true?

        I'm trying my dardnest to NOT make this about the outdoor unit, but it doesn't seem to be working.

        1. Expert Member
          DCcontrarian | | #20

          BTU's are going to be equal to temperature delta times flow, both for the air and for the refrigerant. Also, the exit temperature of the air has to be the same (+-) as the return temperature for the refrigerant. If BTU's are fixed, and air flow is fixed, the delta for the air has to be fixed as well. That sets the output temp for the air, which also sets the return temp for the refrigerant. The compressor can modulate two thing -- the send temperature of the refrigerant, and the flow rate of the refrigerant. I believe in practice what it really modulates is the speed of the compressor, which impacts both the flow rate and the temperature.

          If the compressor is at its minimum and it delivering more than the heating demand, the room temperature rises, the thermostat shuts off and the compressor is notified.

          1. maine_tyler | | #21

            That's great. Is it clear that the delta T being referred to is the delta T of the discharge air vs the intake air? And that the question was how could a coil of refrigerant with a fan blowing over it modulate to the min btu/h rating of the outdoor unit if the indoor unit maintained a set--and rather high-- delta T of 30F?

            Perhaps I'm just beating a dead horse, but it seems no one will simply agree with the logic fallacy of that... I'm not sure why.

            So there's one of three things going on:
            1) I'm right and I perhaps simply misinterprested Greenright's statement about delta T.
            2) I'm right and Greenright simply made some sort of misstatement or didn't mean for it to apply to low load situations?
            3) I'm wrong for some number of reason I can't fathom. 1 reason could be that the indoor unit actually does NOT allow for modulation as low as the outdoor unit would allow, which would be a big deal because it would mean the outdoor unit min-output ratings are not what we've been led to believe they are. This is why I'm asking the question.

            Before you respond, re read the original question because I don't think you're getting what it's asking. (Which could be entirely my fault for poor wording / intent)

          2. Expert Member
            DCcontrarian | | #22

            In the original post, you ask, "My conundrum is this: If CFM is held constant, and delta T is held somewhat constant, then what is modulating in terms of output power – btu/h?"

            If those two things are constant the only thing that can be modulated is the duty cycle -- how long the unit runs for.

          3. maine_tyler | | #23


            IF CFM is set, and delta T is set to some high value like 30F--as was suggested-- then the indoor unit would not be capable of modulating as low as the purported modulation of the outdoor unit. As you say, it cycles.

            That's is what I am saying.

            So the answer is:

            Delta T is not set as suggested (per my interpretation of what was suggested),


            No you are wrong Mainetyler, delta T IS set to a high target (higher than 10F say as per the above calc) and thus the indoor unit will cycle BEFORE the system hits the min-mod of the outdoor unit.


            Fan speed modulates lower than is given in the submittal, so delta T can in fact be kept higher (or some other answer like this).

            Sure seems like its option 1, but it has felt like pulling teeth.

            I see my question was a bit confusing when I said "what is modulating in terms of output power -- btu/h?" The '--btu/h' was not a suggestion/question of what was modulating, it was an add on to indicate the commonly used unit of power, power being that which is reduced/increased during modulation. I should have used parenthesis or left that off completely-- (rereading that I see it could definitely be confusing.)--(--)(--) Hopefully all those dashes and parenthesis clears it up. ;0}

    4. bfw577 | | #24

      On my Gree at minimum heating modulation of 180 watts the supply temps are very low compared to mid and higher ranges. At minimum they run around 20 or less. I see delta t's of 30-50 quite often closer to max capacity. Here are some real numbers for my unit. I am logging data with a Bluetooth cfm sensor, temp sensors for delta t and emporia electricity monitor.

      My listed fan speed is 200,350, 470 l/m/h.

      So at minimum modulation of 180 watts my fan is defaulted by the software to low. I confirmed with my cfm monitor I'm at 200 cfm and Delta t is 20 degrees.

      Cfm 200x20×1.08 is 3703 btu/h. Unit is rated 3700 at 47 so dead on.

      3703÷3412/200 watts is a cop of 5.4. Same as the rating.

      Here is the same unit at max. Notice the Delta T has doubled to 42. Cfm is 471 on high.

      471x42×1.08 18,316 (rated 18700)

      I have found that everything matches up identical with its NEEP performance sheet that I attached based on my measurements. The fans also appear to lock on to their exact cfms in their submittal when set manually. I found this with multiple brands and I believe it has to do with locking in the fans at a set speed for the AHRI testing.

      Every mini split I have worked on operates pretty much the exact same way. I have never seen one operate with a fixed supply delta t. The 30-40 number is only seen on units running at or near max capacity. At low speed modulation these units are deliverering low supply temps at low temps. Its as simple as that.

      I don't think people realize how easy it is monitor the real time performance of a single zone mini split with just a simple delta t temp setup and electricity monitor. I can look at my phone and see the btu output and cop of my unit in real time.

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