Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

4 wire control cable, How does the control work? Mini split heat pump.

Hello,

First time poster.

I have searched the web for this information and have come up short.
I am a controls guy and get to integrate lots of cool discrete units.

On a mini split heat pump the indoor units "talk" to the outdoor units via the 4 wire control cable.
The four wires are, ground, AC, AC and Control.

So one wire for the control to turn it on and off and ask for heat or cool.
Does anyone know if this is level based ac, serial coms, or something else.
I could go in and test the wire but I'm not able to access that right now.

I did control one system by mimicking the IR remote and learning all the settings and then sending the IR signals to each unit from a PLC. This worked for 4 years but has too many moving parts. Simple is better.

Any thoughts?

Peter

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 16,589
    Comment went to la la land. Try again here.

    Some of the most recent and sophisticated systems may use a serial bus, similar in concept to the CAN bus used in recent cars.

    However, as a general rule, in our trade one is playing with 24 VAC as the control power, and the controlling device will either be switching a line up to 24 volts or dropping it to ground -- or in many older devices simply closing a circuit between two of the wires without any regard for the voltage on the wires at all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,678
    @psdonald42

    Your correct 2 wires AC power, 1 wire ground, the other wire starts and stops. I believe the 3rd wire in that case is AC 120 volt.

    Some units depending on the brand require an ac circuit to the indoor unit and another ac circuit to the outdoor unit with a communication wire between the units

    Some units the ac power goes to the outdoor unit and then a 4 wire (thwn) or (control tray) (TypeTC cable) is used between the units which it sounds like is the case you are dealing with
  • ratio
    ratio Member Posts: 2,871
    Many (most??) mini splits communicate via a signal imposed on a conductor that is at line voltage to ground. It's not like a standard condenser, energize the control conductor to call for cooling, it's actually bidirectional communication.

    I saw somewhere on the internets where someone had reverse engineered the Mitsubishi physical layer & was working on interpreting the data, but it hadn't been updated in years.

    Some mfgrs have an adapter that will take standard Y, G, etc. calls & communicate them to the condenser to allow a mini split condenser to operate a standard evap coil (some changes to the coil are necessary).

  • Jamie Hall
    Jamie Hall Member Posts: 16,589
    A minor comment, if I might -- if one or more of the wires to which folks are referring is 120 VAC, and the rest are signal or 24 VAC, they cannot be in the same cable or conduit. Nor can the 120 VAC and a 24 VAC line share a ground.

    I believe (someone check the latest code, which I don't have) that 24 VAC hot, ground, and switched signal lines can share a conduit with a "USB" or CAN type serial bus or ethernet bus, and the latter three can have a 5 volt DC reference and ground in the same cable.

    Personally I wouldn't do it. If I were running a high speed digital line, I'd want it separated physically from anything much else, and at least twisted pairs if not shielded and twisted pairs.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • psdonald42
    psdonald42 Member Posts: 3
    Good information people.

    I will attempt to measure and decipher the 4th rail when i get home. The power to the wall unit is i'm pretty sure ground, N, Hot and signal.
    Now the outdoor unit is variable speed and slows and speeds up according to delta from setpoint, and can also be heat or cool, so something sophisticated is going on.

    I can think a a few ways of doing this without coms. How about 24VDC signal, 0 to 2V off. 2 to 12 V cool with 12 being full cool, 12 to 14v off 14 to 24v heat. 24V full heat. Of course it will be nothing like that.

    Any guesses?

    Peter
  • HVACNUT
    HVACNUT Member Posts: 4,084
     You get continuing  swinging AC voltage between 90v and 240v between terminals 1 and 3.
    I don't know if that helps or can be used to control another source. 
  • ratio
    ratio Member Posts: 2,871
    I suspect that all the different brands speak a different dialect, if not language; but here's a start:
    https://github.com/LenShustek/M-NET-Sniffer
    I've installed two units that actually do 'communicate' like a standard split (Y call for cooling), a Carrier 4 ton mini split, and some other smaller one, I forget the brand. Everything else that I've worked with is communicating, referenced to a hot conductor (which puts it straight on Sparky's plate, no matter how much he argues), or via RS485 (VRF/VRV systems, the big uncle of mini splits).
  • pecmsg
    pecmsg Member Posts: 2,213
    edited September 17

    A minor comment, if I might -- if one or more of the wires to which folks are referring is 120 VAC, and the rest are signal or 24 VAC, they cannot be in the same cable or conduit. Nor can the 120 VAC and a 24 VAC line share a ground.

    I believe (someone check the latest code, which I don't have) that 24 VAC hot, ground, and switched signal lines can share a conduit with a "USB" or CAN type serial bus or ethernet bus, and the latter three can have a 5 volt DC reference and ground in the same cable.

    Personally I wouldn't do it. If I were running a high speed digital line, I'd want it separated physically from anything much else, and at least twisted pairs if not shielded and twisted pairs.

    If all the wires are the same size and type of insulation then Yes they can be in the same conduit, chase, Harness.

    Fujitsu's I run 18/4 cable.
  • ratio
    ratio Member Posts: 2,871
    @psdonald42, this's been tickling at me for a while, I finally realized what it was—Mitsubishi has BACnet/Modbus (RS485) & Lon interfaces available, no need to reinvent the wheel, see http://mylinkdrive.com/USA/Controls