Old house heats only regardless of mode

[David-S]

Hello,

I installed the nest learning thermostat in an older house with a heat pump. Regardless of whether the nest is in heating or cooling mode, it would only heat. This was true regardless of the O/B configuration. I probed the original thermostat to find out which wires it was activating in each operational mode, because it was able to operate the heat pump correctly both before and after the attempted nest installation. Below is the behavior that I observed from the original working thermostat:

(W also O/B)

(W2 also Aux)

(R and Rc shorted)

-- Off (from cool state):

R->OB

-- Cool:

Rc->Y

Rc->G

R->OB

-- Heat:

Rc->Y

Rc->G

-- Heat(with aux heat):

Rc->Y

Rc->G

R->Aux

-- EmHeat:

Rc->G

Rc->Y2

R-Aux

-- Off (From heat state):

Nothing called

Is it possible to configure the Nest learning thermostat to behave like this original thermostat?

Thank you for your time!

[zoeuvre]

Hi David-S, 

Uh-oh! Yes, you can set up the Nest Learning Thermostat to act like your old thermostat. The Nest Thermostat can be set up in many ways to match different heating and air conditioning systems, including heat pumps. To get the right actions, you need to set up the wiring correctly in the Nest Thermostat's setup menu. Here are the steps you can follow:

1. Make sure the Nest thermostat is put in the right way and joined to your heating and cooling system.
2. On the Nest thermostat, go to the main menu and pick "Settings".
3. Go to "Equipment" and pick "Heat Pump".
4. In the Heat Pump settings, you can change things like O/B wire and extra heat (W2/Aux). Here's how you can configure them based on your observations:
   • If the O/B wire turns on the reversing valve for cooling, set it to "O".
   • If the O/B wire turns to "B", it will activate the valve for heating in reverse mode.
   • Turn the W2/Aux wire to "W2" if it turns on extra heat.
   • Keep the W2/Aux wire as "Aux" if it turns on both the extra heat and the second level of warming up.
5. Save the settings and then leave the menu.

By setting up the O/B and W2/Aux options properly, you can make the Nest Thermostat behave just like your old one based on its original wiring and operation.


Let us know if that helps.

Best,

Zoe