Defrost not kicking on heat strips

[GL4]

Hi All,

I’ve got a heat pump outside and an air handler in the garage with heat strips. For the past year, I’ve been running it as a dual system with the heat pump turning off at 40F and the heat strips taking over. It wasn’t efficient, but it worked!

Yesterday, my HVAC tech rewired my third generation Nest Learning thermostat so that the heat strips would work in conjunction with the heat pump. I believe that means he changed it to show as a single fuel system. The problem is that when the heat pump goes into defrost now, the heat strips don’t kick on, and cool air blasts into the house, causing the heat pump to continuously run as the inside  temp rises and falls along with the defrost intermittently coming on. Could this problem be caused by the fact that the tech wired the Aux to W1 instead of W2?

[CoolingWizard]

Depending on which model thermostat you have, it is actually correct that the auxiliary heat is typically wired to the W2/AUX terminal. The real question is, did the technician make changes at the air handler or the outdoor unit at the same time? I asked this question because in a heat pump system, it is the outdoor defrost control board that initiates the auxiliary heat.  Between the outdoor heat pump unit, and the indoor air handler, the defrost control board has a W2 terminal that is connected to the W2 terminal in the air handler. A dual fuel system would typically be a heat pump with a gas or oil fired furnace. You employed a workaround when you had it wired as a dual fuel system and the temperature cut over at 40°F.

The AC Cooling Wizard

[GL4]

The tech did do something in the air handler. But just to clarify, I have a Nest Learning thermostat (3rd gen), a heat pump, and heat strips in the air handler. The Nest wiring diagram currently shows a white wire into W1. Should it instead be in W2?

[CoolingWizard]

GL4, the way heat pumps work between the defrost control board and the indoor air handler is that the defrost control board uses a W2 signaling wire to activate the electric heat in the air handler. Typically a thermostat is wired to the very same auxiliary heat unit using an E wire to enable the homeowner to activate the emergency heat in the case of a failure of the outdoor unit. As for auxiliary heat, when activated by the outdoor unit, the signal wire sends a signal to the thermostat on W2 to allow the thermostat to know that auxiliary heat has been turned on. In some cases, smart thermostats can be programmed in such a way that if it detects that the heat pump is running, and the outdoor ambient temperature is so low that the heat pump cannot keep up with the demand for heat, the thermostat can also activate the auxiliary heat to boost the heating capacity of the system. The nest is one of these types of advanced thermostats.

The goal here is for you to, that is, if you are comfortable, to ensure that your thermostat has the white wire on W2/AUX and in the air handler, the same color wire is also on the W2 of the control board. Do you have access to the indoor Air Handler?

The AC Cooling Wizard

[GL4]

Getting into the air handler is a bit complicated. I spoke to the tech on the phone, and they said W1, W2, and W3 are all tied together in the air handler, so he implied they essentially act as one. If that’s the case, would switching the white wire from W1 to W2 on the Nest do the trick?

[GL4]

I moved the Nest wiring from W1 to W2. It was the white wire. But when I check wiring in the Nest app, it shows the wiring into W2 as brown instead of white…

[CoolingWizard]

If he indeed connected all three stages together then it should work. The problem is if the control board has indeed 3-stages of heat, and if the air handler does not have 3-stages of heat, it can act erratically. You should not tie W1, W2 and W3 together in the air handler.  For me to look up the control system, I need the model number of the air handler.  Some systems have multiple banks of heating strips and get activated is a sequence or as Stage 1 and Stage 2.

The AC Cooling Wizard

[GL4]

The air handler is Trane, 4TEE3F31A1000AB. Why would the Nest app show the white wire going into W2/Aux as brown?

[CoolingWizard]

Sometimes the installer uses differ colored wires instead following industry standards.  What ever color is in W2 should be in W2 at the other end.  Your Trane air handler can indeed have up to 3 stages of electric heating coils.  In order for that to be the case, separate electrical contactors are used for each stage. Additionally, you air handler has an Elecyriclly Commuted Motor (ECM) and can greatly vary the speed of the blower. Can you get me the model number of the outdoor unit?

The AC Cooling Wizard

[GL4]

The outdoor unit is a Trane XL14i.

[GL4]

When the white wire was plugged into W1 it showed up as white in the Nest app, but now that the same white wire is plugged into W2/Aux it’s shows up as brown in the Nest app. Not sure if that matters?

[CoolingWizard]

XL14i is the general product family. The model umber is listed on the manufacturer label on the unit. 

The AC Cooling Wizard

[GL4]

The model is 4TWX4030A1000AB.

[CoolingWizard]

GL4,

Your outdoor unit is a single stage Heat Pump. Not sure how many stages of electric heat you have in the indoor air handler unit.  The auxiliary heat kits are installed by the contractor and not by the factory.  Since the outdoor unit does contain an outdoor temperature sensor, the connection between W1 and W2 and W3 is not a valid configuration.  The proper configuration between the Nest thermostat, the air handler and the outdoor unit typically W1 from the thermostat to W1 in the air handler, W1 from air handler to W1 on the outdoor heat pump,  W2 from the outdoor heat pump temperature sensor W2 going to the air handler W2 to the W2 of the thermostat.  


The AC Cooling Wizard

[GL4]

If I understand correctly, it doesn’t matter if I plug into W1 or W2 on the Nest because it isn’t wired properly from the heat pump to the air handler and therefore neither option will allow my heat strips to kick in during defrost mode?

[CoolingWizard]

Defrost control board in the outdoor unit is what typically controls the indoor unit electric auxiliary heat, which is activated on the W2 on the indoor air, low-voltage control board. It doesn’t matter if his wired to W1, W2 and W3 together in the air handler itself, but for you to have the nest thermostat to be happy with the exhilarate heat, particularly with a nest learning thermostat, that should be on W2. 

The AC Cooling Wizard

[GL4]

It won’t be cold enough until tomorrow night for me to confirm with results, but now that I’ve moved the white wire to W2 inside the Nest, is there a chance the heat strips will kick in like they should when the heat pump goes into defrost?

[Jenelyn_O]

Hi there,

Thanks for reaching out. I'm sorry for the delay. How's your Nest Thermostat? I just want to confirm if you're still encountering the issue. Please let us know so we can further assist you. 

I appreciate your help, @CoolingWizard and @MikeClark.

Best,

Jenelyn

[Dan_A]

Hey there,

I wanted to follow up and see if you still need help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,

Dan

[Dan_A]

Hey there GL4,

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you need assistance or want to discuss topics, feel free to start a new thread.
 

Cheers,

Dan

[CoolingWizard]

The Nest App just uses different colors to simply differentiate the wires.  What matter is what color wire is in the terminal backplate and the other end. If you think about it, the thermostat is not smart enough to see the color of the wire.  So, the software uses the industry standard wire colors for each terminal. 

The AC Cooling Wizard