The apartment heatpump delivered cool air (16 degrees delta). The thermostat was changed out with a Radiothermostat CT50. CT50 switch configuration setting include HP (Heatpump) and Electric (see photos)

  1. Does the photo (2nd photo below) of the original thermostat confirm wiring color convention?

  2. Why does the new wifi thermostat produce heat (100+ degrees from ceiling registers) and not cool air?

enter image description here

The second question is important because, I believe that wiring color convention was used.

As I understand it, the convention is True = +26V-28V. Thermostat Wiring color convention:

White: Heat
Green: Fan
Yellow: Cooling
Blue: Common
Red: +26V AC

The voltmeter, measuring the wifi thermostat, indicated:

White = FALSE, 


The apartment heatpump was originally configured with an electronic (not wifi) thermostat. The old thermostat worked fine and is wired as such:

enter image description here

The wiring into the original thermostat leads me to believe thermostat wiring convention was followed.

The office heatpump (identical wifi thermostat) correctly pumps cool air when summoned:

enter image description here

I am happy to provide any other information, if asked.


JUMPER Experiment

Purpose: Confirm BLU=Common, YEL=cooling, RED=26-28V AC Expectation: Cooling is engaged

10AM: 86 indoor temperature: Thermostat Mode = OFF = [WHT,YEL,GRN,ORA]

Jumper YEL (cooling) & GRN (FAN) to RED:

enter image description here

Return =70 degrees: Jumpered: [GRN WHT] = >26V = ON = TRUE; Measurement: FALSE = [ORA,YEL] <2.0V AC

Conclusion: cooling is engaged as expected => WHT, YEL, GRN, ORA convention is utilized

Measurements: Thermostat = HEAT-Setting

Purpose: Does calling for heat actually invoke cooling?

Remove jumpers from JUMPER Experiement

Measure voltages at [WHT,YEL,GRN,ORA]; Measurement = [WHT,YEL,GRN,ORA]=FALSE

Set thermostat to Heat and set target above ambient temp

Measure voltages at [WHT,YEL,GRN,ORA]: Measurements: ORA=FALSE; [WHT,YEL,GRN]=TRUE

Measure return temperature:: 76 degrees=return at 9 minutes 83=ambient temp

Measurements: Thermostat=COOL-Settings

Purpose: What signals are sent? [WHT,YEL,GRN,ORA]

Set thermostat to off confirm fan /compressor off, then to cool.

Set target-temperature to below ambient:


measure output temp: 93+degrees at ~3Min

====> Disconect orange (Reverser valve?): measure ORA = 130Mv => **OUT_TEMP=76 @ 1 min**

Why would the controller (thermostat) send a TRUE to the ORA (Reverser Valve) in cooling mode?

  • 1
    Did you check the other end of the cable to ensure that Blue really is Common? It's unlikely that your old thermostat needed a Common wire, and I suspect that your Blue is actually calling for heat ...
    – brhans
    Jul 16, 2020 at 23:44
  • @brhans How should I check that BLU is actually common? When I measured voltages, I assumed that BLU was common and read 26 volts difference between BLU and RED, BLU and YEL. Does this confirm that BLU is actually common? Thank you
    – gatorback
    Jul 16, 2020 at 23:51
  • 1
    Does commanding the system to heat produce cold air instead? Jul 17, 2020 at 0:10
  • 1
    @gatorback -- my suspicion is that it's the reversing valve control (O wire) that's not following convention Jul 17, 2020 at 0:44
  • 1
    @ThreePhaseEel 3 Experiments were performed in the last hour: results are posted at the bottom of the OP under your name. Thank you for reviewing and for the good suggestions
    – gatorback
    Jul 17, 2020 at 14:52

1 Answer 1


Move the orange wire on the troubled thermostat from O to B

What you are seeing here are fairly classic signs of getting your system's reversing valve control backwards. In order to do their fancy tricks of being able to heat your house as well as cool it, heat pump systems need to be able to make the indoor and outdoor coils "swap places", and this is done using a special four-way solenoid valve called a reversing valve.

Sadly, there are two different conventions for the way the reversing valve works, and no clear mapping between them and thermostat wire color codes. Most systems operate where the reversing valve needs to be energized to put the system into cooling mode, which is conventionally marked by the use of an O terminal designation on the thermostat and in system wiring diagrams. Oftentimes, this corresponds to an orange wire in the thermostat cable, but different cables may have different color codes, and fat cables may be used differently by different installers.

However, Rheem and Ruud units are a bit different; they require their reversing valves to be energized in order to get the system to heat. This is denoted by the use of a "B" terminal designation on the thermostat and system wiring diagrams; however, the dark blue wire that is intended to be used for this is rarely available in thermostat cables, so its often wired using an orange or brown wire instead.

To add to the confusion, many electronic heat pump thermostats have a single reversing valve output, denoted as O/B and configured for correct operation through the setup menus, instead of having separate O and B outputs available, as in a mechanical heat pump thermostat. In your case, your experiments with the jumpers and mode settings show that your system is configured with a reversing valve that must be energized to heat. Since your new thermostat has separate O and B terminals, fixing this is a matter of moving the orange wire over; on your old thermostat, there was a setting that had been changed to do the same thing.

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