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Community and Q&A

Heat Pump Operation and Control

daveh4 | Posted in Mechanicals on

I have several questions related to heat pump operation and control via thermostat, which the manufacturers have done an outstanding job of not explaining.

First, my variable speed super efficient minisplit air handler says that I should wire my thermostat as a conventional single stage heating and cooling (1H/1C). Does anyone know how this makes sense? Yes the air handler has a return air temp sensor, but that doesn’t help if it doesn’t know the set temp, and I’m assuming the thermostat just sends a discrete on/off command, not the actual temp it’s set to.

Second, the “Dry” mode feature of their ductless minisplits has almost no information associated with any manufacturer. I’m specifically dealing with a Carrier system (40MBAAQ24XA3) but I’m finding it across the board. I can’t figure out if this is a feature you have to program semi-permanently or if it can be controlled by a Humidistat, and if so HOW. Assuming I have a humidistat capable thermostat, does anyone know how is the control communicated to the air handler? Is this as a 2nd stage cooling mode, aux mode?


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  1. rliebrecht | | #1

    There are a small list of AC controllers that are meant to work with ductless systems: Mysa, Sensibo, Ambi, etc. They operate like remotes, so they can control different modes. Some (Mysa and Ambi) operate like thermostats in that they vary the setpoint automatically to achieve a set temperature at the thermostat, rather than the head. Ambi tries to factor in outside temperature and humidity as well. They are all very patchy - the sensors in an Ambi are often out of whack, and their customer service is offensive. They're early products, so you make some tradeoffs. But on the whole, you can't beat a thermostat designed to manage heat pumps and eliminate the need to manage modes and set points.

  2. Expert Member
    Akos | | #2

    The heat pump and air handler is actually made by Midea. I'm surprised it does not come with a wired thermostat like an actual Midea unit. Wiring it up to single stage thermostat somewhat defeats the whole purpose of a modulating unit.

    I would give Carrier a call and find it if they offer a communicating thermostat for the unit that will give you proper modulating operation. The Midea thermostat that works with their ducted unit is KSACN0401, plugs into the LED module that looks pretty close to the one of the Carrier unit.

    The next best thing would be to use their wireless thermostat in "follow me" mode. In this case it uses the temperature sensor on the remote for controls.

    Dry mode on most mini splits puts the air handler fan into low speed which improves the SHR of the unit at the cost of efficiency. This is best used only during the shoulder season and only if you have actual humidity issues. If there is no sensible cooling, it can still overcool the place. There is no RH based control for this but I guess it can be automated with a 3rd party thermostat.

    1. daveh4 | | #3

      I always thought it was strange that the unit didn’t come with a thermostat, similar to other brands. But since it was equipped with a 24V transformer for 3rd party thermostats I assumed I could use one and achieve the same functionality. If what you say is true then carrier is screwing over all of their customers. I’m supposed to carry around a remote control? The remote is infrared, need line of site. The unit is in the attic. It’s a centrally ducted air handler. How does that work? What about wifi capabilities? No carrier equivalent to kumo cloud??? I don’t want to believe that.

      1. Expert Member
        Akos | | #5

        The IR receiver is typically on a longish length of wire, I think it might even come with an extension.

        You can bring this down to the ceiling a hallway bellow the unit and mount the wireless remote on the wall there. The follow me mode would then control unit based on the temperature in the hallway. There remotes have a pretty good range, you don't need the receiver right above, somewhere within sight is good enough.

        Definitely not an elegant solution considering the price of one of these units. I hope Carrier has a better solution.

        1. daveh4 | | #6

          According to the attached compatibility chart there is no other solution. Their only allowance for wired thermostat is via the 24V transformer and their installation manual says it must be configured as a 1H/1C single stage. My blood pressure is a little hot at the moment.

        2. daveh4 | | #7

          Hi Akos, wanted to circle back to you on this. Apparently Carrier offers a 24V Interface Kit (KSAIC0301230), which is optional for all their minisplits but built-in on the Air Handler 40MBAA unit.

          This kit allows full variable operation with a 3rd party 1 or 2 stage thermostat and even automated dry mode operation with a humidity controlling tstat. Apparently they use some proprietary algorithm looking at return air temp, coil saturation, supply air temp etc to "learn" the proper system response to a cooling or heating command. I'm some what dubious how well this all works without a communicating tstat but at the very least it is a pretty cool concept and if even half as functional as they claim puts Carrier a head above the competition in terms of control. This would allow full variable operation while also control of non-Carrier back-up or supplemental HVAC equipment. Anyways I'm eating my crow on my earlier complaints but happy to see they have in fact come up with a good solution.

          1. Expert Member
            Akos | | #8

            You should be. These interface kits almost all operate the same way which is to slowly ramp compressor speed up, the longer the heat/cool call is the higher the final compressor speed. They even have a dip switch for setting the ramp time. Most thermostat interface kits from other mini split manufacturers operates the same way. It is somewhat modulating but still a lot of on/off.

            A proper communicating thermostat is completely different operation.

            I can't say how much this would effect efficiency or comfort. A communicating thermostat generally keeps the room where the thermostat is mounted within a fraction of a deg F

  3. daveh4 | | #4

    I’m realizing I did not understand that these modulating units have very specialized controllers to utilize their full capabilities. I guess I just assumed that the communication protocols were standard to all thermostats and any 3rd party communicating thermostat would be able to unlock the modulation of minisplits. If this isn’t the case I wonder why? This isnt complicated technology.

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