Heatpump Defrost cycle

[kivi]

My heatpump has 3 wires, one for operation, second one for reverse valve, and another one for defrost cycle, my current thermostat has a connection for that wire while the Nest does not have that connection, I am wondering how the defrost cycle will work then. 

I guess the defrost is an automatic cycle and that wire only signals the thermostat that the heatpump is in defrost mode. but I am not sure

[CoolingWizard · Accepted solution]

Kivi,

Your assumption is basically correct.  What the “D”;defrost, wire does is tell the thermostat when the Heat Pump outdoor unit goes into defrost mode.  When the reversing valve is in the heating configuration and the outdoor ambient air temperature is very low, the Heat Pump outdoor coil will ice up. When it does, the heat pump must enter defrost cycle to melt the ice.  The “D” wire from the heat pump signals the heat pump indoor unit to turn off the blower, or turn on the auxiliary heat, The Nest Thermostat, for Heat Pumps,  leaves the control of the Blower Fan up to the indoor air handler control board and does not need to be notified of the Defrost cycle. 

now Kiki, let’s get real technical. In the old variations of the heat pump systems the indoor air controller was not very smart, it was not programmable  and it did not have software. All it was is contacts and relays. If the outdoor unit went into defrost, the control of  the fan could be affected by the thermostat having the blower fan switch in the ON position. This would override the defrost signal. So thermostats added the D signal in order to know that if defrost was turned on outside, the thermostat must override the fan switch and turn off the blower so defrost would work. Modern Heat Pumps systems can deal with this since indoor control board are much smarter now.  

Ken, The AC Cooling Wizard

[kivi · Accepted solution]

THank you Ken, I assume that my current terminal uses the D signal to turn on auxiliary heat to avoid blowing cold air in winter, it's old but it has a software, it has parameters to program. I am working to make a relay board to install Nest thermostat as  my thermostat has the fan relay embeded into it's board. one question I still have, if during the defrost, the ventilation goes off, the heatpump can still defrost? it's not required to heat up the condenser for defrost cycle?

[CoolingWizard · Accepted solution]

Kivi,

Your assumption is basically correct.  What the “D”;defrost, wire does is tell the thermostat when the Heat Pump outdoor unit goes into defrost mode.  When the reversing valve is in the heating configuration and the outdoor ambient air temperature is very low, the Heat Pump outdoor coil will ice up. When it does, the heat pump must enter defrost cycle to melt the ice.  The “D” wire from the heat pump signals the heat pump indoor unit to turn off the blower, or turn on the auxiliary heat, The Nest Thermostat, for Heat Pumps,  leaves the control of the Blower Fan up to the indoor air handler control board and does not need to be notified of the Defrost cycle. 

now Kiki, let’s get real technical. In the old variations of the heat pump systems the indoor air controller was not very smart, it was not programmable  and it did not have software. All it was is contacts and relays. If the outdoor unit went into defrost, the control of  the fan could be affected by the thermostat having the blower fan switch in the ON position. This would override the defrost signal. So thermostats added the D signal in order to know that if defrost was turned on outside, the thermostat must override the fan switch and turn off the blower so defrost would work. Modern Heat Pumps systems can deal with this since indoor control board are much smarter now.  

Ken, The AC Cooling Wizard

[kivi · Accepted solution]

THank you Ken, I assume that my current terminal uses the D signal to turn on auxiliary heat to avoid blowing cold air in winter, it's old but it has a software, it has parameters to program. I am working to make a relay board to install Nest thermostat as  my thermostat has the fan relay embeded into it's board. one question I still have, if during the defrost, the ventilation goes off, the heatpump can still defrost? it's not required to heat up the condenser for defrost cycle?

[CoolingWizard · Accepted solution]

Kiki, A heat pump basically changes the flow of the refrigerant to make the indoor coil hot for heating and cold when cooling. Now when the heat pump has low ambient temperature outside there’s a chance that the outdoor coil will freeze and cause ice buildup on the outdoor coil. So with that heat pump goes into defrost cycle it changes the reversing valve itself making warm air come from inside the house to be put in the coil outside, basically acting like an air conditioner. It does not matter if the indoor coil blower is running or not, because when the coil temperature sensor senses no more ice it will deactivate the defrost cycle.

The nest thermostat can control the fan without the addition of any additional hardware that you’re considering building. The “D” wire for the outdoor unit, has significance to the indoor air handler.  The indoor air handler will deal with both the auxiliary heat, it it exists, and the blower.  The problem is when there is no auxiliary heat the indoor unit must turn off the blower fan so we’re not blowing cold air into the house while in defrost cycle. On some older air handler‘s, if the thermostats blower fan switch was not in the AUTO position, it was in the ON position, it would override the defrost signal from the outdoor unit to the indoor unit and the fan would stay on. The industry simply corrected this by adding the D terminal in the thermostat that would sit on the thermostat to override the fan blower switch.

When the nest thermostat encounters a heat pump it Uses the preferred method of allowing the blower control to remain in the air handler. You do not need a D wires and the D wire does not initiate auxiliary heat. Auxiliary heat is typically on the W2 or the AUX terminal. And this nest thermostat can be forced to turn on the emergency heat indicates that the outdoor unit fails and you have no other heat.

Ken, The AC Cooling Wizard

[Jhonleanmel]

Hey folks, 

We appreciate your help here, @CoolingWizard.
@kivi, I wanted to follow up and see if you are still in need of any 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,
Mel

[EdwardT]

Hi folks, 

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

Thanks,

Edward

[EdwardT]

Hi kivi,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Thanks,

Edward