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  1. #1
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    Apr 2018
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    Determining optimal temperature to switch for Trane dual heat system

    Hi,

    I have recently had a Trane XR15 2.5 ton heat pump installed alongside a Trane 2-stage furnace #S9X2B080 with a TCONT850 thermostat. I live in Ontario, Canada, and was advised to only use the heat pump for heating during the "shoulder" months when temperatures are above freezing. I am currently trying to determine the optimal temperature to set the compressor lockout to to restrict heating to just auxillary heat from the propane furnace. I have read some past posts where some pros were able to determine the most cost effective temperature lockout... I have no clue how to do this and am hoping someone can help. I also don't know the COP, etc I would need for this.

    I have entered the details of my rates for propane and electricity below for calculation of the ideal Temp for me to set my thermostat at to shut off the heat pump and revert to propane furnace heating. If it matters, the thermostat implements a 4 degree differential to turn the heat pump on back again.

    Propane rate fluctuates, but is around 65 cents per litre


    Electricity:

    We have time-of-use rates which makes this more complicated:

    On-Peak 18.39 c/kWh
    Mid-Peak 14.69 c/kWh
    Off-Peak 11.69 c/kWh


    I suppose to make it simpler, the mid-peak rate could be used for the calculation.

    So, those are my rates. I'm not sure if and how you can factor in the time of use rates for electricity. If someone invented a truly smart thermostat that could factor in TOU and implement the heat pump accordingly to save money, I would buy it in a heartbeat! I imagine that some outdoor temperatures would be economical to run the heat pump in overnight when rates are low, but would be a money burner during the day during peak rates.

    Does anyone have the calculations to help? Thanks!

  2. #2
    Join Date
    Mar 2009
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    Mount Holly, NC
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    In order to do this the best for you, it is important to know your home load calc numbers, and the size of your heat pump...
    The TRUE highest cost system is the system not installed properly...

    Find a HVAC-Talk Contractor by clicking here

    Do you go to a boat repairman with a sinking boat, and tell him to put in a bigger motor when he tells you to fix the holes?

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  3. Likes JedTheHumanoid liked this post
  4. #3
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    Using the mid peak/average of the 2 rates. The heat pump is cheaper to use until its COP drops to 1.7 including defrost cycles. You need to know your heat pumps COP to determine outdoor temp to lock out for economical use. But probably around 17°F.

    However, you will probably reach your heat pumps thermal balance point before that.
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  5. Likes JedTheHumanoid liked this post
  6. #4
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    Apr 2018
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    Thread Starter
    Quote Originally Posted by vstech View Post
    In order to do this the best for you, it is important to know your home load calc numbers, and the size of your heat pump...
    Quote Originally Posted by beenthere View Post
    Using the mid peak/average of the 2 rates. The heat pump is cheaper to use until its COP drops to 1.7 including defrost cycles. You need to know your heat pumps COP to determine outdoor temp to lock out for economical use. But probably around 17°F.

    However, you will probably reach your heat pumps thermal balance point before that.
    Thank you both for your prompt replies. I did not receive home load calc numbers from my contractor (and I don't think he produced them either). If it helps, my house was built in 1989 and has just had attic insulation upgraded to R60 with blown in cellulose. Original windows. 2 story house, 2750 sq ft not including basement.

    The heat pump is the 2.5 ton version.

    I can't find the COP specs for the XR15, only the HSPF, which is up to 9.60. Google is revealing nothing except old threads from this forum, where the answer isn't clear (or relevant attachments that might contain the COP data are long deleted).

    The outdoor model number on the heat pump is: 4TWR5030H1000AA.

    Does any of this help refine the estimate of optimal temp for the compressor cutoff? Thanks!

  7. #5
    Join Date
    Apr 2018
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    Thread Starter
    I should also add that the furnace has an AFUE of 95%, thanks

  8. #6
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
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    Wheiw... a 2.5 ton in your area, heat pump likely can't keep up below 30 or 35... it will be efficiently producing heat below this, but not enough for your home load.
    The TRUE highest cost system is the system not installed properly...

    Find a HVAC-Talk Contractor by clicking here

    Do you go to a boat repairman with a sinking boat, and tell him to put in a bigger motor when he tells you to fix the holes?

    I am yourmrfixit

  9. #7
    Join Date
    May 2008
    Location
    Missouri
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    I'd simply set comp. lockout at a few degrees below freezing temp, run system and then once it fails to keep up the house temp. then raise it a degree or two. Kinda a trial and error method, but will work. Once system does not keep up, when running nearly all the time, you've gotten to the "balance point" or very near it.

  10. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    8,139
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    bal park estimate

    Quote Originally Posted by JedTheHumanoid View Post
    Hi,

    I have recently had a Trane XR15 2.5 ton heat pump installed alongside a Trane 2-stage furnace #S9X2B080 with a TCONT850 thermostat. I live in Ontario, Canada, and was advised to only use the heat pump for heating during the "shoulder" months when temperatures are above freezing. I am currently trying to determine the optimal temperature to set the compressor lockout to to restrict heating to just auxillary heat from the propane furnace. I have read some past posts where some pros were able to determine the most cost effective temperature lockout... I have no clue how to do this and am hoping someone can help. I also don't know the COP, etc I would need for this.

    I have entered the details of my rates for propane and electricity below for calculation of the ideal Temp for me to set my thermostat at to shut off the heat pump and revert to propane furnace heating. If it matters, the thermostat implements a 4 degree differential to turn the heat pump on back again.

    Propane rate fluctuates, but is around 65 cents per litre


    Electricity:

    We have time-of-use rates which makes this more complicated:

    On-Peak 18.39 c/kWh
    Mid-Peak 14.69 c/kWh
    Off-Peak 11.69 c/kWh



    I suppose to make it simpler, the mid-peak rate could be used for the calculation.

    So, those are my rates. I'm not sure if and how you can factor in the time of use rates for electricity.

    If someone invented a truly smart thermostat that could factor in TOU and implement the heat pump accordingly to save money, I would buy it in a heartbeat!
    I imagine that some outdoor temperatures would be economical to run the heat pump in overnight when rates are low,
    but would be a money burner during the day during peak rates.

    Does anyone have the calculations to help? Thanks!
    THERMAL BALANCE POINT = ~ 34'F

    UTILITY COST = $2,800 FOR HEATING SEASON ~ 7000 HEATING DEGREE DAYS

    With T.O.U., one needs a lot more Heat Pump capacity + Higher Efficiency
    to avoid use of the costly propane.

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  11. #9
    Join Date
    Apr 2018
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    Thread Starter
    Quote Originally Posted by dan sw fl View Post
    THERMAL BALANCE POINT = ~ 34'F

    UTILITY COST = $2,800 FOR HEATING SEASON ~ 7000 HEATING DEGREE DAYS

    With T.O.U., one needs a lot more Heat Pump capacity + Higher Efficiency
    to avoid use of the costly propane.

    ATTACHMENT
    Thanks everyone for all of your help!

  12. #10
    Join Date
    Mar 2009
    Location
    Mount Holly, NC
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    The drawback to off peak use, is its much colder during off peak, and the heat pump would usually be out of its stride...
    The TRUE highest cost system is the system not installed properly...

    Find a HVAC-Talk Contractor by clicking here

    Do you go to a boat repairman with a sinking boat, and tell him to put in a bigger motor when he tells you to fix the holes?

    I am yourmrfixit

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