3rd Generation Nest Thermostat - heating not working below 20 degrees. Aux heat broken?

[j_isaiahyoung]

Hi All,

I’ve been having issues with my nest thermostat this winter and last winter but since we rarely get sub freezing temps I’ve been putting off bringing it to the community.

anytime the temps get below freezing and the thermostat has been heating for a while it starts blowing cold air. It seems there is no emergency heat and the heat pump cannot keep up after so many hours. It’s setup as single fuel (carrier model ch14nb030) in my new construction home that once used a Honeywell T60 Pro that I switched out for my nest that I’d had for a WHILE. On nights where it’s 40 AUX heat says it kicks on but now im not sure if it’s my aux or just the regular heat pump heat. I love my nest. This is its only flaw. And id love to understand what’s going on. Attached are pictures of the old Honeywell wiring and my current nest wiring. 

[j_isaiahyoung]

@CoolingWizard any thoughts?

[j_isaiahyoung]

 

[j_isaiahyoung]

old thermostat

 

[CoolingWizard]

@j_isaiahyoung , allow me to possibly clear things up for you. When you heat a house with a heat pump, typically there are 10kW to 15kW of electric heat strips installed in the air handler.  This auxiliary heat is controlled by the thermostat and/or the outdoor heat pump defrost controller.  As long as your indoor air handler is heat pump aware, when the indoor unit is placed into auxiliary heat mode, the blower speed should go down to medium speed, and the heat strips are energized. This happens whenever the defrost controller is activated to melt the ice on the outdoor unit. Secondly, the Nest Learning Thermostat will activate the auxiliary heat heat strips to help boos the heat if the heat pump cannot keep up. 

If you are getting cold air, your heat strips in all likelihood are not working. Try this, move the W2 wire in the Nest Backplate to the * position. Define this as Emergency heat. Set your Heat / Cool system to OFF. 
Now then, activate emergency heat using your Nest Thermostat.  Your blower should com on and warm heat should be felt after 5 minutes.  
If there is no heat at all , the heat strips need to be checked.  

AC Cooling Wizard

[j_isaiahyoung]

Thanks so much for your reply. There's a 10 kW marking on the fan in the attic. Would this be where the heat strips are placed? I saw a switch on the fan that looked like a breaker. Does this switch need to be flipped? The home is a brand new home and this happened last year as well.

[CoolingWizard]

@j_isaiahyoung , when they install 10K W, in your case, heat strips they have a separate circuit breaker assigned inside the unit for the air handler heat strips. if that circuit breaker is not turned on then that’s correct the heat strips will not energize although the system will think it’s turning them on. Check that circuit breaker that’s inside the cabinet for the air handler heat strips and make sure that it’s on

AC Cooling Wizard

[j_isaiahyoung]

I checked the breaker on the fan and it was flipped to ON. I also checked the 60kA switch on the breaker for HEAT and it was flipped to on as well. Any thoughts?

[CoolingWizard · Accepted solution]

@j_isaiahyoung ,

If possible, can you get me a picture of indoor unit air handler model number?  Also, you have an unused wire in the wall.  We can use that to set up an Emergency heat feature if you desire to 

AC Cooling Wizard

[j_isaiahyoung]

I believe it’s the Carrier FX4D!

[CoolingWizard]

The Carrier FX4D units can come from the factory with Heat Strips installed or they can be installed in the field by the HVAC contractor. The last three digits of the model number will indicate the options. 000=No Factory installed. 005 = 5kW factory installed.  008 =  8kW factory installed, 010 =  10kW factory installed and 015 = 15kW factory installed.

AC Cooling Wizard

The field installed heat strips can be up to 30kW.

AC Cooling Wizard