Original Thermostat

Original Thermostat

If you’re like me, you tend to sleep with the temperature quite a bit colder than your daytime setting.  My specific setpoints are:
76 °F for the day time, and 68 °F at night for the summer.
71 °F for the day time, and 64 °F at night for the winter.
While it’s a little thing, it’s becomes tedious to do these settings changes every evening / morning, especially knowing there are so many programmable and/or Smart thermostats on the market for the household climate control systems.

However, the RV world is woefully lacking of any type of programmability on the thermostats.  Worse yet, RV AC/Heatpumps have some things stacked against them when looking for aftermarket kit:

  • RV AC/Heatpumps made the weird choice to use 12 Vdc control panels (instead of 24vac) — as if you could run them without 120 Vac power?
  • RVs can often have more ‘heat’ options than our sticks & bricks brethren.
  • RVs also tend to have two speed fans in our AC/Heatpumps

This all combines to make replacing an RV thermostat with anything other than the OEM unit a daunting task.

Being the inventor of CoachProxy, and knowing Tiffin opted for RV-C enabled thermostats on newer coaches… I had initially thought about retrofitting those new thermostats into my coach.  Alas, that is not as easy as it sounds.  Turns out, there is a LOT of changes in the wiring and Spyder Controls system that goes along with those RV-C enabled climate controls.  So, this was off the table.

I also considered building my own climate control system based on an Arduino + some relays… and I bought the stuff to do a test unit, but there’s never enough time and I never got that elusive “round tuit”.

Recently, I started looking at integrating some Z-wave items into the coach ( Aeotec Home Energy Monitor, Multi-sensors, remotes ) and thought, there has to be a Z-wave option out there that will work.  The big caveat is that they must be able to run on battery alone ( No 24vac / Common wire requirement ), OR run on 12vdc natively.  One thermostat actually did run on 12vdc and another RV’er -Adventurous Way- has a great post on their installation!  However, it no longer seems to be made and trying to get one is like trying to get unobtainium.  So, I kept searching and found that Honeywell makes a unit that is able to run on battery power alone!

I present to you, the cleverly named Honeywell TH6320ZW2003 Z-Wave T6 Pro Programmable Thermostat.

Honeywell TH6320ZW2003 Z-Wave T6 Pro

Note that you’ll need a couple more items besides the thermostat. At least, in our situation because:

  • Our roof mounted AC’s are actually Heat Pumps
  • We also have the AquaHot system which can supply heat via fan blown radiators
  • We have remote temperature sensor — separate from our thermostats
  • Coleman Mach systems use a 100 kΩ thermistor — not the standard 10 kΩ or 20 kΩ versions that most household units use.

Supplies:

  1. Honeywell TH6320ZW2003 Z-Wave T6 Pro Programmable Thermostat
  2. Remote temperature sensor: 10kΩ B3540 Thermistor
  3. 12v Automotive Relay
  4. Heat Shrink
  5. Soldering iron, supplies, skills

Why the relay?

When switching to heat mode, regular sticks & bricks heat pumps use 1 wire to run the compressor, 1 wire to run the blower, and 1 wire to toggle a ‘reversing valve’: this switches the flow of refrigerant inside the heat pump to make the heat pump ‘heat’ instead of ‘cool’.

RV Heat Pumps are slightly different.  They use 1 wire for the compressor in cool mode, 1 wire for the low blower (when in cool mode) and 1 wire for high speed on the blower (again, when in cool mode).  A ‘heat’ wire is used by itself to call for ‘heat’ from the heat pump.  This ‘heat’ wire handles compressor, reversing valve and blower all at the same time.  When calling for ‘heat’ via this ‘heat’ wire, the heat pump will ignore the blower wire signal (low or high, doesn’t matter).

The relay is used to convert the ‘compressor+reversing valve’ signal to just a ‘heat’ wire signal for the RV heat pump.  If your roof mounted AC is *not* a heat pump, you can likely omit this relay, but you’ll need to determine that for yourself since different units and different manufacturers may behave differently.

Operation of our setup:

  1. During cooling, the thermostat will automatically kick the fan speed to high if the temperature is too far above the set point.
  2. When set to ‘Heat’ mode, the thermostat will use the heat pump (with fall back to AquaHot heat if the room temperature is too far below the set point).
  3. If we want to use AquaHot as the primary source, we set the thermostat to ‘Emergency Heat’ mode.  This locks out the Heat Pump and uses the AquaHot heat only.

    Installation:

    1. Snap off the thermostat cover and remove the two screws securing it to the wall.

      Cover removed

    2. Disconnect the quick connector plugs.

      Quick connector plugs

    3. Cut the wires off the thermostat and strip 1/4″ of the insulation from the plug side wires.

      Old Thermostat Wires Cut

    4. Insert the stripped ends of the wires into the wiring base / new thermostat as shown.

      Wires stripped and inserted into the thermostat base.

      • Yellow – Y
      • Green – Y2
      • Gray – G
      • White/Black stripe – O/B
      • White – W2
      • Red – R
    5. Cut the Yellow and White w/ Black stripe wires at their mid point.
    6. Wire in the 12v Automotive Relay to the newly cut wires as shown:

      Relay connected

      • Relay 30 ( Common ) Blue Wire –> Thermostat side Yellow Wire
      • Relay 87a ( Normally Closed ) Red Wire –> Connector side Yellow Wire
      • Relay 87 ( Normally Open ) Yellow –> Connector side White w/ Black stripe
      • Relay 85 ( Coil ) Black –> Connector side Blue (Ground Wire)
      • Relay 86 ( Coil ) White –> Thermostat side White w/ Black stripe

      NOTE: The relay socket wire colors may be different — Use the relay pin numbers to confirm.

    7. Unscrew the old remote temperature sensor and cut the wire.

      Old temp sensor wires cut

    8. Separate the two wires ceiling side wires
    9. Strip about 1/4″ of the insulation and slide a good length of heat shrink over the ceiling side wires.

      Wires split & heat shrink in place.

    10. Bend the new 10kΩ B3540 thermistor leads as shown.

      New Thermistor – leads bent

    11. Solder the new 10kΩ B3450 thermistor to the ceiling side wires — which wire goes where doesn’t matter.
    12. Slide the heat shrink down over the thermistor, leaving about 1/8″ of the long side of the thermistor wires exposed and heat the shrink tubing to shrink it in place.

      Thermistor with heat shrink

    13. Push the thermistor wires back up into the ceiling and bend the remainder out to the side of the outline from the old room temperature sensor as shown.

      New Thermistor Pushed into place

    14. Slide the cut portion of the old room temperature sensor through the ceiling hole and screw it back into place — ensure the exposed thermistor wire is left out from under the old temperature sensor.
    15. Bend the thermistor down alongside the old thermistor.

      New Thermistor alongside old one.

    16. Test the resistance of the Green with White stripe wires at the thermostat connector and ensure they’re in the 7 kΩ to 12 kΩ range (depending on room temperature).

      Checking new thermistor reading — Yes, the next step is out of order from the photo.

    17. Plug in the newly wired thermostat base and screw it to the wall.
    18. Put the 3x AA batteries in the thermostat and begin the programming — do not plug it onto the wall plate until programmed.  Note that each setting has a numeric ID at the top of the thermostat. I’ve included them below for your convenience. Here are the settings we used:
      Expand for full settings...

      • #120 – Schedule Type: none (When using Z-wave, the schedule does nothing anyway)
      • #130 – Outdoor Temp Sensor: No
      • #200 – System Type: Heat Pump
      • #205 – Equipment Type: Air-to-Air
      • #218 – O/B On – Heat
      • #220 – Compressor Stages: 2
      • #221 – Aux/E Stages: 1
      • #253 – Both Aux/E Control
      • #255 – Aux Heat Type: Gas/Oil
      • #260 – Fossil Kit Control: Thermostat
      • #300 – Auto Changeover: Off — I’ll script this via Z-wave controls
      • #305 – High Cool Stage Finish: No
      • #306 – High Heat Stage Finish: No
      • #340 – Aux Heat Droop: 15
      • #350 – Up Stage Timer Aux Heat: Off
      • #355 – Balance Point: Off
      • #356 – Aux Heat Lock out: Off
      • #365 – Compressor 1 CPH: 3
      • #366 – Compressor 2 CPH: 3
      • #374 – Aux Heat CPH: 5
      • #387 – Compressor Protection: 5 minutes
      • #390 – Ext Fan Run Time in Cool: Off
      • #391 – Ext Fan Run Time in Heat: Off
      • #430 – Min Cool Temperature: 50
      • #431 – Max Heat Temperature: 90
      • #500 – Indoor Sensor: Yes
      • #515 – Sensor Type: 10k
      • #520 – Temperature Control: Wired
      • The rest are up to you…

      [collapse]
    19. Once programmed, click ‘Finish’
    20. Set the Date/Time
    21. You’ll likely get an immediate error about the wired temperature sensor, this is expected — it’s not connected yet.
    22. Press and hold the ‘Menu’ button to get back to the setup screens
    23. Use the left/right arrows to get to ‘System Test’
    24. Snap the thermostat onto the wall plate
    25. Press ‘Select’ and cycle through your systems heat/cool modes to confirm everything works as expected
      NOTE: Going from Heat to Cool on the Heat pump too quickly can damage the compressor — Allow at least 5 minutes between testing each mode.
    26. Once all seems right, exit the System Test menus and set your mode / temperature and ENJOY!

    Future plans…

    The main thing I’d like to change is to have the thermostat powered from a DC converter, not batteries. There are adjustable DC-DC converters on Amazon, but seems when the potentiometer on these ‘adjustable’ units fails, they fail in a way to cause the voltage to go high, thus likely damaging your sensitive electronics. I may test a standard ‘5v’ supply to see if the extra 0.5 volts is OK and just run them from that 5v converter.

    Documentation Links

    Airxcel Service Manual for Mach8 Heat Pumps
    On page 8, under ‘VII Low Voltage Wiring’, there is an explanation of what each color wire performs.

    Airxcel Thermostat Installation, Operation and Application Guide
    On page 6, Figure 1: A complete pin-out description for the original thermostat

    Honeywell T6 Pro Z-wave Thermostat Installation Guide


    Michael

    Resident Tesla nut and polymath. Raised in eastern Kentucky, joined the US Navy at 19 to operate a Nuclear Reactor on a Fast-Attack submarine. After finishing his enlistment, Michael has continued to follow his passions in technology, astronomy, and of course, traveling the country.

    Jim Beletti · 2019-09-29 at 18:17

    Love it Michael – can’t wait to add these to my coach in a couple weeks.

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