Daikin Heat Pump Not Stopping

[imagineertobe]

Hi there. I have replaced a Honeywell thermostat with a 3rd Gen Nest Learning Thermostat with my Daikin water supply heat pump. 

When testing the setup, it seems Daikin uses a B orientation heat pump, since when the Nest is set to O orientation the air blows the opposite temperature from the Nest setting (blowing cold when set to heat, and blowing hot when set to cool). 

I’ve run into a problem where the unit won’t turn off once the target temperature is reached in heating mode while the Nest is set to B heat pump orientation. However, when the target temperature is reached in cooling mode, the unit turns off as expected. (FWIW, if I put the Nest back in O heat pump orientation, then it’s the cooling mode that gets stuck and heating mode that shuts off as expected.) 

Does anyone know why this might be happening? 

[Patrick_Caezza]

@imagineertobe 

Can you post a picture of the original thermostat's wiring if you have it?

[imagineertobe]

@Patrick_Caezza

Yes, here you go.

Original Honeywell thermostat. You can see the C wire is disconnected. The thermostat had 2 AA batteries that seemed to be providing power.Another view of the old Honeywell thermostat to see some of the wire labels more clearly.A view of the HVAC system circuit board.The wiring diagram from the HVAC unit (Daikin brand). I've added a key to the wire colors that are connected to the circuit board. The white wire is not in use.My Nest wiring.

[Patrick_Caezza]

@imagineertobe 

Your first problem is that stranded wires do not work with the Nest base plate. Stranded wires have a higher resistance than solid wires which causes problems. This link should help you understand the problem Thermostat wire undetected, unconfigured, or unspecified .

If possible you should replace the stranded cable with a standard 5 or 6 wire solid thermostat cable. If you can't do that, then use some small wire nuts to add on some short pieces of solid 18-awg wire between the stranded wire and terminals.

Based on the wiring diagram that you provided, your heat pump default is the O mode.

With a heat pump, the O/B terminal is what switches it between heat and cooling and the Y terminal is what turns the heat or cooling on or off.

[imagineertobe]

Thanks. I was already planning on replacing the wire so I’ll try that and report back.