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  1. #27
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    Nov 2011
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    22
    So you may also be doing a lot of un-needed defrost then.
    If you look at the gragh, the frost limit set point coincides with air temperature drop.
    I can adjust the frost level set point to give optimum performance. I do this by tracking COP in real time and corrilate with a microprocessor.

    pilko

  2. #28
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    Jan 2004
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    Lancaster PA
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    Often, the colder it is outside, the less defrost cycles you need.
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  3. #29
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    Nov 2011
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    Quote Originally Posted by beenthere View Post
    Often, the colder it is outside, the less defrost cycles you need.
    Very true, that's why I control DF based on frost level. But, to ensure that I do not DF too early or too late I use trended COP to fine tune the process by including it in the input and equations.
    I still have to tweak the code once in a while, but that's part of the fun.

    pilko

  4. #30
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    Probably would have been easier just to use an on demand defrost board from another unit/brand. Since they already have the needed compressor protection programmed/built into them. And base defrost on actual performance by reading line, coil and outdoor temp.
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  5. #31
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    Nov 2011
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    22

    Confused

    When I bought the HP 11 years ago I had never heard of demand defrost. I just knew that the way it defrosted based on time was very inefficient so I started designing my own.I have been playing with the controls ever since.
    I guess it's time I learned more about the fluid/vapour side.

    pilko

  6. #32
    Join Date
    Jul 2004
    Location
    Charleston, SC
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    930
    My thoughts are the fan has to run, or you will get longer defrost times, as already stated. If longer defrost times are not a problem, then I am guessing that the heat pump is over-sized for the application.

    While in defrost you are not heating, but the house is still loosing heat.

    The heating capacity of a Carrier 3 1/2 ton heat pump at -2*c is 32430 btuh

    If the heat loss of the structure is more than the 32430 btu per hour at -2*c then a 10 minute defrost means that you only have 50 minutes of heat, 27025 btu. 5405 btu now needs to be made up with electric, not counting the heat removed for defrost.

    15 kW heat kit produces 51180 btuh, 853 btu per minute. You will need 6.3 minutes of the heat strips to offset heat loss during a 10 minute defrost. Again, this does not include heat removed for defrost.

    Longer defrost time equates to longer auxiliary heat times to make up the loss. The electric heat is going to run weather during defrost to offset the heat loss or during heating because the heat pump capacity is too small.
    Last edited by cartercrew; 12-04-2011 at 11:00 AM. Reason: Recalculated for -2* instead of -8*.
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  7. #33
    Join Date
    Nov 2011
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    22
    The answer lies in these images. The first image shows 2 defrosts, the first at 1000 CFM and the second at 500 CFM. When I zoom and put them together in image two, the area under the 20* return temp (-dT x Time) is only 29% larger with a 500 CFM defrost BUT considering the CFM is only half, the heat loss is only 64%.
    It is logical to conclude then that heat loss at 0 CFM will be a small fraction of heat loss at 1000 CFM
    The Calcs:-

    At 1000 CFMAverage dT of the air = -9.04
    Time = 138 seconds
    Heat Loss = CFM/60 x dT x Density of air x Specific heat of air
    = 1000/60 x 138 x 9.04 x 0.075 x 0.2375
    = 370 BTU
    At 500 CFMAverage dT of the air = -10.3
    Time = 156 seconds
    Heat Loss = CFM/60 x dT x Density of air x Specific heat of air
    = 500/60 x 156 x 10.3 x 0.075 x 0.2375
    = 238 BTUIt should follow that the heat loss at 0 CFM will be very low

    pilko
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  8. #34
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    Jan 2004
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    Let it run 10 normal defrost cycles at 500 CFM. See how well the coil defrost then. I gets lots of calls on heat pumps not defrosting right, that are caused by dirty air filters, not leaving enough air go through.
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  9. #35
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    Nov 2011
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    Quote Originally Posted by beenthere View Post
    Let it run 10 normal defrost cycles at 500 CFM. See how well the coil defrost then. I gets lots of calls on heat pumps not defrosting right, that are caused by dirty air filters, not leaving enough air go through.
    This is a reply from the VIP poster on the other forum

    "with a blocked filter, you get poor refrigeration performance which leads to irregular ice build up ( this comes down to distribution, vapour liquid fraction "x"), and can mislead the defrost sensor.
    When ice is heavily formed (due to faults) normal defrost (time limited) will not clear the coil."

    pilko

  10. #36
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    Jan 2004
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    Lancaster PA
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    When you get poor defrost on your heat pump. have him come out and defrost it. LOL...

    Blocked filter, and no air flow do the same thing. Your heat pump is NOT designed for the indoor fan to shut off during defrost.
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  11. #37
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    Nov 2011
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    It is time for me to make a decision.
    The VIP poster on the other forum has provided valuable technical information, this forum has not.The only usefull information this forum has provided is obvious, that the heat pump will run longer to defrost the coil. My graph already showed that ( and it is not much longer)
    I am still not sure that it is safe to do it as the consequences are great (it is my money)
    I will program my HP to reduce fan speed during DF, for a number of times at 50% CFM then reduce to 40% and so on.
    During this testing It would prudent to monitor important parameters, please advise which and I will set it up.
    Thank you everyone for your help.

    regards

    pilko

  12. #38
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    Jan 2004
    Location
    Lancaster PA
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    Ask the VIP at the other site what to monitor, since your taking his advise. The technical info you posted from him here, isn't technical.

    Too many cooks spoil the soup.

    PS: What band of heat pumps does he install, Japanese probably, the ones designed not to run the indoor fan.
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  13. #39
    Join Date
    Nov 2011
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    22
    @ beenthere,
    "Ask the VIP at the other site what to monitor, since your taking his advise."

    I didn't expect you to advise personally, you are more vested in winning an argument than being helpfull. Your statement has proven that.
    You are a moderator and as such you should be temporing critisism not creating it.
    I was asking for advise from the forum as a whole.

    pilko

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