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Old 09-05-2013, 10:20 AM   #1
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HVAC Control


I am converting my HVAC control to a PLC (AB Micrologix).

I have a Carrier Heat Pump w/ 15kW electric backup. At my thermostat, I have the following connections: R, Y/Y2, G, C, W/W1, O/W2. I want to make sure I account for everything when writing the code for this and I had a few questions.

1) When the Aux. Heat (W/W1) kicks on, does the compressor turn off or continue running? I was always under the assumption that the heat pump still ran, but I came across some documentation online that says the heat pump will shut down when the electric strips come on.

2) Does the compressor lockout (timeout before restarting) take place in the thermostat or in the control board?

3) Does the defrost cycle take place in the thermostat or in the control board?

4) During the defrost cycle, does the control board turn the electric strips on automatically?

Thanks.


Last edited by phuz; 09-05-2013 at 10:34 AM.
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Old 09-05-2013, 10:48 AM   #2
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[QUOTE=phuz;1238395]

Quote:
1) When the Aux. Heat (W/W1) kicks on, does the compressor turn off or continue running? I was always under the assumption that the heat pump still ran, but I came across some documentation online that says the heat pump will shut down when the electric strips come on.
The compressor continues to run. When the compressor is in defrost the defrost control relays turn on the compressor, switch the reversing valve of the heat pump, turn on the interior electric heating element(s), and stops the condenser fan at the outdoor coil from spinning.

When the aux. heat kicks on it would depend on what the function is. Aux. heat will come on when the heat pump can not keep up with heating the house. It is also supposed to come on to temper the inside air when the condenser goes into a defrost cycle.

Quote:
2) Does the compressor lockout (timeout before restarting) take place in the thermostat or in the control board?
The defrost control board

Quote:
3) Does the defrost cycle take place in the thermostat or in the control board?
The defrost control board

Also depending on the set up, if you have a programmable thermostat you can lockout the heat pump from operating at a certain temperature or lockout the auxiliary heat strips based on your preferences.

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Old 09-05-2013, 11:11 AM   #3
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Thanks.
Does the control board stop the compressor momentarily when switching the reversing valve or does it do it on the fly?
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Old 09-05-2013, 12:28 PM   #4
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If the system is defrosting, the outdoor fan will stop running and the compressor will continue to run (making a buzzing or humming sound). The compressor is actually circulating hot refrigerant through the outdoor unit to melt the ice off the system. After a few minutes, the system will complete the defrost cycle, you will hear a "whooshing" sound, the fan will restart, and steam will blow out the top of the unit.
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Old 09-06-2013, 05:10 AM   #5
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Quote:
Originally Posted by phuz View Post
Thanks.
Does the control board stop the compressor momentarily when switching the reversing valve or does it do it on the fly?
Depends on which model you have. Some of carrier's upper end units do shut the compressor off for 1 minute before switching the RV for defrost.
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Old 09-06-2013, 06:05 AM   #6
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Originally Posted by beenthere View Post
Depends on which model you have. Some of carrier's upper end units do shut the compressor off for 1 minute before switching the RV for defrost.
But the control board on the main unit handles this, right? Not the thermostat?
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Old 09-06-2013, 07:52 AM   #7
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beenthere, thanks for the info about the Carrier heat pumps. I'll add that to my notes.

Yes, the defrost control board at the condenser controls the defrost mode actions.
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Old 09-06-2013, 08:00 AM   #8
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Great, thanks.
At this point, I am taking over control of the thermostat and don't want to remove any of the controls from the control board itself.
My 15kW electric backup consists of 3 strips and I am thinking about breaking them apart so that I can enable one, two, or three of them using solid state relays, instead of always turning on all 3 when it calls for backup. This will probably be a function of outside temperature, too.

And speaking of outside temperature, where does that signal come back to the inside, and is it a 0-5V signal or thermocouple?
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Old 09-06-2013, 08:58 AM   #9
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If your unit has an outdoor temp sensor. Its a 10,000 ohm resistance at 77F thermistor.

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