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  1. #1
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    Jul 2012
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    2-Stage HP, Temp difference Stage 1 vs. 2

    Recently had a Carrier 3-ton 2 stage heat pump installed. The temperature difference (cooling) between the supply air and the return for stage 2 is 19 degrees which I understand is OK. When it is running in stage 1 which is most of the time the difference is only 14-15 degrees. I know the compressor in stage 1 is only about 2/3 of stage 2 but since the the fan (FV4 air handler) runs at a lower CFM shouldn't the delta be close to if not the same. Wouldn't this make stage 1 less efficient.

  2. #2
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    Can you hear the fan speed up when the 2nd stage kicks in?

  3. #3
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    Jul 2012
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    Yes I sure can.

  4. #4
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    So no one is familiar with or has taken readings on a Carrier 2 stage unit? Just trying to make sure my unit is setup and running properly.

  5. #5
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    Did you measure this right after the coil or out of the registers? How long had it been running in 1st stage? Where are the ducts located? If the ductwork is in a hot attic, they will pick up the same amount of heat no matter what stage it's in.

    What are the outdoor temepratures? In higher outdoor ambient conditions I think 1st stage is closer to 70-75% capacity, but the airflow is normally set at 80% of 1st stage. So if the capacity is only 70 but airflow is 80% the temp drop will be a little lower.



    I personally have a carrier 2 stage but it's an Infinity system that changes airflow based on humidity levels and outdoor temepratures so I can't really compare. It runs very low airflows in 1st stage and mantain very low supply temperatures. THe actual supply temeprature will depend on the indoor return temperature and humidity, the outdoor temeprature and the system charge.

  6. #6
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    Jul 2012
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    Thanks for your reply. I took the reading at the registers. It was 90 degrees outside. The AC had been on stage 1 for some time and I took a reading. I then turned down the t-stat which bumped it to stage 2 and waited 15 minutes and took a reading. The ducts are in the attic but I would think I would have very close to the same loss in both stages.

    As far as the CFM's I think you are onto something. I looked through the installation manual and noticed it said that the compressor in low stage operates at 67% of full capacity. Since I have a 3 ton unit this means that on stage 2 it is running at almost exactly 2 tons.

    The air handler manual says that the CFM for stage 2 is 1050 which is 350 per ton. The CFM for stage 1 is 840 which is 420 per ton. If my logic is correct, it seems to me that I'm getting 20% more supply air per ton at stage 1 but this air is a a little more than 20% less cool then stage 2 so it looks like I am getting about the same efficiency and all is good. Does that sound right?

  7. #7
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    Makes sense to me. Also consider attic heat gain through ductwork is a fixed amount of BTU regardless of total CFM. How hot outside does it get typically get before stage 2 kicks on?

  8. #8
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    The compressor may operate at 67% of full capacity (the manual I read said it unloads 40%..so 60% capacity), but the system capacity is only reduced by 20-30% depending on operating conditions. Now that doesn;t mean effiecny increased by that much. The compressor is drawing more than 40% of the power it was in 2nd stage because with the coils being effectively oversized, it's transferring more ehat.

    Effciency also depends on the total power consumption which measuring air temperature won't tell you. The delivered tons may be 75%, but the power consumption might be 70% of 1st stage. Also since your duct size stays constant, the blowers efficency (CFM/watt) is greater in 1st stage.

  9. #9
    Join Date
    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by Triton1 View Post
    So no one is familiar with or has taken readings on a Carrier 2 stage unit? Just trying to make sure my unit is setup and running properly.
    The CFM to btu ratio is higher in first stage so your delta T will be lower meaning the temperature will be warmer in first as you have noted

  10. #10
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    I don't know how hot it has to get before stage 2 kicks on. It has not kicked on by itself yet but it has only been a high of 94 since the unit has been installed. It runs for a long time in stage 1 but will cut off for a while. I understand this is good for humidity control and comfort. We had a number of 100+ degree days several weeks ago so I imagine it will run in stage 2 then. I will learn more in the next few weeks.

  11. #11
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    Aug 2007
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    Quote Originally Posted by Triton1 View Post
    I don't know how hot it has to get before stage 2 kicks on. It has not kicked on by itself yet but it has only been a high of 94 since the unit has been installed. It runs for a long time in stage 1 but will cut off for a while. I understand this is good for humidity control and comfort. We had a number of 100+ degree days several weeks ago so I imagine it will run in stage 2 then. I will learn more in the next few weeks.
    A two degree difference from set point will activate second stage. If it has been 100+ where you are and it has been running in first stage mostly it is over-sized

  12. #12
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    Quote Originally Posted by motoguy128 View Post
    The compressor may operate at 67% of full capacity (the manual I read said it unloads 40%..so 60% capacity), but the system capacity is only reduced by 20-30% depending on operating conditions. Now that doesn;t mean effiecny increased by that much. The compressor is drawing more than 40% of the power it was in 2nd stage because with the coils being effectively oversized, it's transferring more ehat.

    Effciency also depends on the total power consumption which measuring air temperature won't tell you. The delivered tons may be 75%, but the power consumption might be 70% of 1st stage. Also since your duct size stays constant, the blowers efficency (CFM/watt) is greater in 1st stage.
    I think I understand what you are saying. So assuming my temperature readings are correct and it's running at the listed CFM's, it sounds like it is operating at or close to what it should be. Correct?

  13. #13
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    Jun 2001
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    Moore, Oklahoma, United States
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    Quote Originally Posted by second opinion View Post
    A two degree difference from set point will activate second stage. If it has been 100+ where you are and it has been running in first stage mostly it is over-sized
    I didn't know you could get a 2 stage in less than a 3 ton capacity. I suppose the OP could have saved a chunk of money and got a single stage 2 ton.

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