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  1. #1
    Join Date
    Dec 2005
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    No. VA.
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    Demand defrost on Carrier Heat Pumps

    Quick techie question:

    There have been a number of comments on various forums that Carrier Heat Pumps don't have demand defrost. After reading the docs. on my new Carrier Heat pump--25HNB6, 3 ton--there is a setting in the Infinty control for Auto defrost vs. a fixed time period.

    I also was doing some research and found this link:

    http://www.patentgenius.com/patent/6318095.html

    whiich is a patent some years ago awarded to Carrier for demand defrost.

    Question is: what is the difference in the logic between the Auto setting on my Carrier Heat Pump and the "demand defrost" logic in say a Trane or A/S heat pump?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,052
    Demand defrost uses thermistors that sense ambient temp and coil temp and use the logic in the board to determine of the coil is freezing up and how fast to decide when to defrost. Auto looks at past cycles to determine how often to see if the coil is below freezing. Better than a fixed timer but not true demand.

  3. #3
    Join Date
    Dec 2005
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    No. VA.
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    71
    Quote Originally Posted by BaldLoonie View Post
    Demand defrost uses thermistors that sense ambient temp and coil temp and use the logic in the board to determine of the coil is freezing up and how fast to decide when to defrost. Auto looks at past cycles to determine how often to see if the coil is below freezing. Better than a fixed timer but not true demand.
    OK--Carrier's patent app. appears to do what you've stated which appears to follow the documentation for the "Auto" setting in the Infinity Control for my HP.

  4. #4
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    May 2000
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    Indianapolis, IN, USA
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    The patent seems to be for a demand system where thermistors monitor ambient & coil temps and use preprogramed logic to determine if the coil has frosted.

    Auto on the Carrier is just as I said. Instead of a fixed 30, 60, 90 or 120 minute interval to look at a open or closed defrost stat, auto bases the time on the previous defrost cycle. If the cycle took less than 3 minutes, which would mean clear coil, mild out and no wind, the board won't look at the defrost stat for another 120 minutes of run time. A defrost time of 3-5 minutes means the board will look in 90 minutes, 5-7 minutes then it looks in 60 minutes. If a defrost lasts over 7 minutes, meaning it was fairly heavily frosted or cold & windy (even if the coil is clear) then the next look at the coil temp is 30 minutes. So even if the coil is clear, this system will defrost as frequently as 30 minutes. A demand system would know the coil was clear by comparing coil temp with ambient temp and not defrost.

  5. #5
    Join Date
    Dec 2005
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    Quote Originally Posted by BaldLoonie View Post
    The patent seems to be for a demand system where thermistors monitor ambient & coil temps and use preprogramed logic to determine if the coil has frosted.

    Auto on the Carrier is just as I said. Instead of a fixed 30, 60, 90 or 120 minute interval to look at a open or closed defrost stat, auto bases the time on the previous defrost cycle. If the cycle took less than 3 minutes, which would mean clear coil, mild out and no wind, the board won't look at the defrost stat for another 120 minutes of run time. A defrost time of 3-5 minutes means the board will look in 90 minutes, 5-7 minutes then it looks in 60 minutes. If a defrost lasts over 7 minutes, meaning it was fairly heavily frosted or cold & windy (even if the coil is clear) then the next look at the coil temp is 30 minutes. So even if the coil is clear, this system will defrost as frequently as 30 minutes. A demand system would know the coil was clear by comparing coil temp with ambient temp and not defrost.
    I assume the logic described in the Carrier patent must be similar to what other manufactuer's use in their "demand defrost" feature--if so, why do you think that Carrier sticks with time/temp. some 10 years later?--just curiousity on my part.

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