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Heat Pump Auxiliary Heat Question

jsneuman
Community Member

Hello--

I have had a Nest Thermostat for many years and my father in law (with a similar all electric heat pump system) just asked me this question:  

Does the Nest thermostat tell the outdoor heat pump to go into defrost mode or does the system itself (meaning void of any communication from the thermostat) tell the Nest to have the system go into defrost mode?  I guess we want to know "who" is in charge.  

When the Heat Mode is on, and the Heat Pump initiates a Defrost Cycle, it energizes the Aux. Heat wire back to the Furnace and Nest Thermostat.  Does the Nest Thermostat recognize that the Heat Pump has gone into a Defrost Cycle and not turn off the Heat Mode before the Heat Pump Defrost Cycle is over?   (Even though the Heat Mode setting has been reached).

We know how to set the lock out temperature and how to make sure the heat pump isn't working too hard, but generally it seems that our units run quite a while with frost on the coils before it is sent into defrost mode.  

Thanks in advance!  

Jeff

Seems like @CoolingWizard might know how to answer this based on his other answers that I have seen on similar thread.  

 

1 Recommended Answer

CoolingWizard
Silver Product Expert
Silver Product Expert

No thermostat can interrupt the defrost cycle unless it simply is the system off. The defrost cycle of a heat pump and the total control of that defrost cycle is 100% under the control of the defrost control board in the outdoor unit.

The thermostat itself takes no action when the call for the auxiliary heat is initiated by the outdoor unit. The key is the indoor furnace/air handler. The indoor air handler/furnace has to be smart enough to know that it is attached to a heat pump so it can take specific action with regard to the blower fan motor speed. 
likewise, if you command the nest thermostat to turn on the auxiliary heat, the outdoor unit takes no action based on that W-2 signal showing up on the defrost board  

Ken, the AC Cooling Wizard 

NestPro and HVAC company owner

View Recommended Answer in original post

5 REPLIES 5

CoolingWizard
Silver Product Expert
Silver Product Expert

Hello jsneuman,

The defrost cycle is controlled solely by the outdoor unit. There is a defrost control board that has a temperature sensor attached to the outdoor coil and one measuring ambient temperature.  The installer would have set the defrost time value on the defrost control board.  Basically, the control board monitors the ambient temperature and when the ambient temperature is 35°F or lower, and the outdoor compressor is running, at the value set in the defrost timer, the defrost cycle will start. Now, while in defrost cycle, the outdoor fan is turned off, and the indoor air handler is sent a call for Auxiliary Heat from the outdoor unit. This turns on the electric heat strips.  

The thermostat has the ability to manually activate the auxiliary heat, but the primary control of auxiliary during defrost cycle is left to the outdoor unit.  Some manufacturers with matched indoor and outdoor units, can slow the air handler blower down to low speed. Do understand that when the outdoor unit is in the defrost cycle, it’s basically acting like an air conditioner so we don’t want that cold air sent into the house so that’s why we control the auxiliary heat and sometimes the manufacturers will slow the blower down.

If at anytime the outdoor temperature sensor on the coil, reaches 75°F, that will signal the defrost board to terminate the defrost cycle and go back into heating cycle mode. As long as there is ice on the coil outside, the defrost cycle will be re-initiated if the ambient temperature is below 35°F and the outdoor unit has been running continuously for a set value typically set by the manufactures to be 30 minutes.

Now, as to the lockout feature of the nest thermostat. This is a calculation you need to make. At some point the outdoor unit will spend more time in defrost cycle then in heating cycle and it might be more economical to just force the thermostat to initiate auxiliary heat and turn off the outdoor unit. this feature is most useful and systems that are dual fuel. Specifically a heat pump made it to a gas furnace indoor.  

I am happy to answer any more questions you might have concerning the operation of your heat pump. If you can give me your manufactures model number on the outdoor unit and the indoor unit I can look up and tell you exactly what the timing values will be and how your defrost control board operates. 

The AC Cooling Wizard

NestPro and HVAC company owner

 

Our other question was regarding if the Nest Gen 3 thermostat can interrupt the Heat Pump initiated Defrost Cycle?   The Defrost Cycle won’t initiate unless the Nest has first started a Heat Mode Cycle, right?   When the Heat Mode is on, and the Heat Pump initiates a Defrost Cycle, it energizes the Aux. Heat wire back to the Furnace and Nest Thermostat.  Does the Nest Thermostat recognize that the Heat Pump has gone into a Defrost Cycle and not turn off the Heat Mode before the Heat Pump Defrost Cycle is over?   (Even though the Heat Mode setting has been reached, say 68 F)

Im the authors FIL.  I have a single stage Trane XL 15i Heat Pump System 2 ton

Furnace.  4TEE3F31B1000AA

Outdoor HP. 4TWX5024A1000AA

Thanks for your previous answer!

 

CoolingWizard
Silver Product Expert
Silver Product Expert

No thermostat can interrupt the defrost cycle unless it simply is the system off. The defrost cycle of a heat pump and the total control of that defrost cycle is 100% under the control of the defrost control board in the outdoor unit.

The thermostat itself takes no action when the call for the auxiliary heat is initiated by the outdoor unit. The key is the indoor furnace/air handler. The indoor air handler/furnace has to be smart enough to know that it is attached to a heat pump so it can take specific action with regard to the blower fan motor speed. 
likewise, if you command the nest thermostat to turn on the auxiliary heat, the outdoor unit takes no action based on that W-2 signal showing up on the defrost board  

Ken, the AC Cooling Wizard 

NestPro and HVAC company owner

Thanks again!

 

Markjosephp
Community Specialist
Community Specialist

Hello folks,

 

I wanted to ensure that everything was covered here, and I hope you've got the answers you're looking for. If you're still in need of any assistance, feel free to let us know.

 

You're a big help here in this thread, Ken!

 

Thanks,

Mark