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  1. #1
    Join Date
    May 2007
    Location
    Upstate SC
    Posts
    188

    Method for determining most efficient dual fuel changeover temp

    (My search skills must suck, because I know I've seen several threads to this effect over the last few months)
    ---
    I live in Upstate SC

    I'm trying to figure out what the most efficient changeover temperature should be from HP to the gas furnace. Because of a location issue with the outdoor remote temperature sensor for the tstat, last week we had some evenings that were less than the 40° F changeover setting, but the remote thermo only registered in the upper 40s... thus heating never switched to gas furnace. The house was still plenty warm and comfy on HP, but if it's more efficient ($/BTU added) to run the gas furnace at those temperatures, I'd like to. I know this value changes as the cost of electricity and natural gas change. I was just trying to find a way to determine, based on a set price for each and respective efficiencies of each heating unit, which the best changeover temp would be.

    Trane 3 ton XL16i HP
    Trane 80k BTU XV90 furnace
    I don't have model numbers in front of me but can provide them later if needed (crumbsnatchers gained unlawful access to the study and wrecked havoc with my files).

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323

    Thumbs up

    Quote Originally Posted by OOC View Post
    (My search skills must suck, because I know I've seen several threads to this effect over the last few months)
    ---
    I live in Upstate SC

    I'm trying to figure out what the most efficient changeover temperature should be from HP to the gas furnace. Because of a location issue with the outdoor remote temperature sensor for the tstat, last week we had some evenings that were less than the 40° F changeover setting, but the remote thermo only registered in the upper 40s... thus heating never switched to gas furnace. The house was still plenty warm and comfy on HP, but if it's more efficient ($/BTU added) to run the gas furnace at those temperatures, I'd like to.

    I know this value changes as the cost of electricity and natural gas change.

    I was just trying to find a way to determine, based on a set price for each and respective efficiencies of each heating unit, which the best changeover temp would be.

    Trane 3 ton XL16i HP
    Trane 80k BTU XV90 furnace
    I don't have model numbers in front of me but can provide them later if needed (crumbsnatchers gained unlawful access to the study and wrecked havoc with my files).
    With electric rate at $0.10/ kW and gas at $1.20,
    your economical switchover point could be ~ 28'F.

    C.O.P. 2.6 Effective electric rate = $0.10 / 2.6 or $0.0385 / kW-Hr

    x 29.3 kW /100,000 BTU ... $1.13 per 100,000 BTU
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    What Tstat do you have and where is the remote sensor located?

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    you almost have to remove the "effective electric rate" because you are paying for the line charges anyway whether you have a HP or not. As someone else mentioned in another post, you might want to wait on a time when its going to go from 40 during the day to maybe the low 20's at night and find the point where the HP will not keep up and set the blance point just above that by a degree or two. I would imagine in SC, the balance point is below your lowest daily temps anyway.

  5. #5
    Join Date
    May 2007
    Location
    Upstate SC
    Posts
    188
    Trane 803 tstat. The remote sensor was originally located inside the OU case but was pulled back into the wall penetration for the refrigerant lineset early last week. Last night I pulled it out of the wall pentration and suspended it a couple inches underneath the wall mounted electrical disconnect box. It showed more accurate outdoor temps as compared to a thermometer I have outside than it did at the previous locations the contractor tried. I'll have to check to see how much extra wire is with it and move it to permanent spot in a shaded alcove on the north side of the house.

  6. #6
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Does it have a separate cable from the Tstat to the remote sensor? It should be seperate from the control cable that serves the outdoor unit.

  7. #7
    Join Date
    Sep 2006
    Posts
    136
    Quote Originally Posted by second opinion View Post
    Does it have a separate cable from the Tstat to the remote sensor? It should be seperate from the control cable that serves the outdoor unit.
    Why is that?
    Thanks,

    Mark

  8. #8
    Join Date
    May 2007
    Location
    Upstate SC
    Posts
    188
    Yes. It has separate wiring from the control wiring into the OU. The remote sensor wiring (2-wire wrapped with black insulation) appears to terminate at the remote sensor as if it were built that way. It's a Honeywell part (which I know they make Trane tstats), or at least the model number on it is the same as the Honeywell part # for their outdoor sensor. I haven't traced where it goes once it runs back into the crawlspace, but is a separate line from the control wire set to the OU.

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