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  1. #40
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    Sep 2005
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    Atlanta GA area
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    Note on defrost and heat strips; There is a difference between really COLD air coming out of the vents during defrost (no heat), and just cooler (chilly) air coming out. MeThinks 5KW of heat would be better than none. Usually in my area defrost lasts for 5-10 minutes... as long as one is not sitting in a chair in the draft of a vent... one does not really notice it much.

    We use HP's for basements most of the time... most basements are over 50% in the ground so heat and cooling needs are small (unless one has a party down there). A house I did the Spring PM on a month ago: 3000 ft footprint, with 2 tons of HP in the basement. Basement is about 65% in the ground, they say it does not run much year around. Only change the filter once a year, and it is not really dirty... could go 2 years if needed.
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  2. #41
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    Oct 2002
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    Minisplits, I don't understand where the heat comes from to deice the coil. Conventionally, its sensible heat at the inside coil which means it gets cold and is warmed back up with resistance heat.

    Only energy available would be compressor energy, and not the heat content of the air. I don't understand.

  3. #42
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    Sep 2005
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    Quote Originally Posted by timjimbob View Post
    Minisplits, I don't understand where the heat comes from to deice the coil. Conventionally, its sensible heat at the inside coil which means it gets cold and is warmed back up with resistance heat.

    Only energy available would be compressor energy, and not the heat content of the air. I don't understand.
    I am not sure I can answer the question any more than they DO work.

    I think since there are SOOOO many sensors all over the unit (inside and outside), perhaps they do not really get frosted up much before a mini-defrost may happen more often, smaller cycle time).

    Someone that knows will be along soon to comment.
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  4. #43
    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 timjimbob View Post
    Minisplits, I don't understand where the heat comes from to deice the coil. Conventionally, its sensible heat at the inside coil which means it gets cold and is warmed back up with resistance heat.

    Only energy available would be compressor energy, and not the heat content of the air. I don't understand.

    Both heat pumps use a reverse cycle strategy to defrost the outdoor coil. When operating in
    defrost mode, the compressor pumps hot refrigerant vapor through th
    e outdoor coil, melting ice
    that has accumulated on the coil. During this time, the indoor fan is disabled and the heat pump
    does not provide heating capacity. The defrost cycle reduces the integrated performance of the
    heat pump since there are periods of
    operation when the compressor is running but heat is not
    being delivered from the unit. AHRI Standard 210/240 dictates that the second defrost cycle be
    used to determine the defrost penalty to account for remaining moisture/frost not removed during
    the fi
    rst defrost cycle (AHRI 2008). Each unit implements slightly different defrost control
    strategies as a function of outdoor temperature, run time, and refrigerant pressures

  5. #44
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    Jan 2004
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    PA
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    Quote Originally Posted by WyrTwister View Post
    As I said , I do not do very much residential . So I apologize for being rusty . :-(

    Several times , I have been asked to bid a job where one or more A/C units were being retrofitted with heat pumps .

    Naturally , no one wants to hear they have to up-size their electrical panel or service . Sometimes that is close to impossible , and certainly cost prohibitive .

    I usually had to get with the mechanical contractor & find out exactly what he was proposing .

    I usually had to make sure the resistance heat strip did not exceed the ampacity of the heat pump . And extract a promise the controls would be wired so that both did not run at the same time , even in defrost .

    Even at that , I was careful to word my proposal with all those specifics .

    It can be a dicey situation .

    God bless
    Wyr
    Unless it would be fairly warm out. And the home owner decided to turn the stat up several degrees suddenly. The heat pump would never draw its rated amp while the aux heat was also on.
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  6. #45
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    Nov 2006
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    Southeastern Pa
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    The only legal way to install a unit (NEC legal) is to wire according to the MCA and the MOD. You use article 440 to wire the OUTDOOR unit, and article 424 for fixed electric space heating equipment: strip heat.
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  7. #46
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    Oct 2002
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    Plano, TX
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    HP needs to have heat added, at some point, to defrost outside coil.
    Typically, it is the heat picked up by the inside coil. Cannot be the case if inside fan is stopped. Like a typicval system with a clogged AirFilter or broke evap motor, It does not generate much heat at the condenser and compressor is in danger of liquid slugging.

    The way I see the minisplit in defrost mode is like an
    Quote Originally Posted by second opinion View Post
    Both heat pumps use a reverse cycle strategy to defrost the outdoor coil. When operating in
    defrost mode, the compressor pumps hot refrigerant vapor through th
    e outdoor coil, melting ice
    that has accumulated on the coil. During this time, the indoor fan is disabled and the heat pump
    does not provide heating capacity. The defrost cycle reduces the integrated performance of the
    heat pump since there are periods of
    operation when the compressor is running but heat is not
    being delivered from the unit. AHRI Standard 210/240 dictates that the second defrost cycle be
    used to determine the defrost penalty to account for remaining moisture/frost not removed during
    the fi
    rst defrost cycle (AHRI 2008). Each unit implements slightly different defrost control
    strategies as a function of outdoor temperature, run time, and refrigerant pressures

  8. #47
    Join Date
    Jun 2013
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    Quote Originally Posted by timebuilder View Post
    The only legal way to install a unit (NEC legal) is to wire according to the MCA and the MOD. You use article 440 to wire the OUTDOOR unit, and article 424 for fixed electric space heating equipment: strip heat.
    Yep .

    God bless
    Wyr

  9. #48
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    Jun 2013
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    Quote Originally Posted by timjimbob View Post
    HP needs to have heat added, at some point, to defrost outside coil.
    Typically, it is the heat picked up by the inside coil. Cannot be the case if inside fan is stopped. Like a typicval system with a clogged AirFilter or broke evap motor, It does not generate much heat at the condenser and compressor is in danger of liquid slugging.

    The way I see the minisplit in defrost mode is like an
    Do not know how the mini-split defrosts . As I said , have not switched it into heat , yet . That will be several months away .

    Will probably use my gas fired central air furnace as primary heat & the HP for secondary heat

    God bless
    Wyr

  10. #49
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    Sep 2005
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    Atlanta GA area
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    Quote Originally Posted by WyrTwister View Post
    Do not know how the mini-split defrosts . As I said , have not switched it into heat , yet . That will be several months away .

    Will probably use my gas fired central air furnace as primary heat & the HP for secondary heat

    God bless
    Wyr
    Might want to run that little mini and see how little elec it uses... you will be surprised.
    Also; mini's tend to produce lots of heat down into the teens outside. Not sure where you live, but in my area we rarely see the teens... mostly the mid 20's, and those only a few nites a year. Having said that... we may have a cold winter this year after a cool summer.
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  11. #50
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    Jun 2013
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    Quote Originally Posted by ga-hvac-tech View Post
    Might want to run that little mini and see how little elec it uses... you will be surprised.
    Also; mini's tend to produce lots of heat down into the teens outside. Not sure where you live, but in my area we rarely see the teens... mostly the mid 20's, and those only a few nites a year. Having said that... we may have a cold winter this year after a cool summer.
    It is a 12,000 btu unit . Currently pulling around 10 amps of 120 VAC .

    Right now , I have the tstat of the central system set at 80 degrees F . This winter , I might set it at 65 - 70 degrees F ( on the heat side of the tstat ?

    Wonder how much current the fan on the 125,000 btu ( gross ) gas fired condensing furnace pulls ? Maybe 1/3 to 1/5 of that the HP would pull ?

    You may be correct , the HP may be cheaper to operate , down to the low 40's or even the high 30's ? I have no personal experience with a HP .

    How does the equipment life of a straight A/C compare to the equipment life od a unit used for both heat and cool ?

    Thanks ,
    Wyr
    God bless

  12. #51
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    Sep 2005
    Location
    Atlanta GA area
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    Most split HP systems (conventional ducted equipment) tend to loose their heating capacity around freezing--give or take a few D. This is why they need elec strips backup heat.
    Mini's tend to produce usable heat down into the teens. Has to do with all the electronics and controls... long explanation.

    Might do a bit of testing (verify conditions are the same; low temp, humidity level, wind level--as these all affect heating needs). Probably better to do the test by reading the meters than waiting for the bill... weather is a component and is fickle...
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