Page 3 of 4 FirstFirst 1234 LastLast
Results 27 to 39 of 51
  1. #27
    Quote Originally Posted by timjimbob View Post
    If I remember My NEC AC and heat SERVICE load calculation called it a" non-coinsidental load" One or the other. It assumes your not running both. Things have changed since the 70s. Hardly any straight electric resistance heat now, except when defrost or second stage.

    Can not remember the term , but that about sums it up .

    I recently installed a 12,000 BTU mini split HP . It pulls ~ 10 amps at 120 VAC . Have not even switched it to HP mode , to see if it works ?

    Do not think it has any resistance heat . From what I read , when in HP mode , and it goes into defrost , the inside fan does not run ? So it does not blow cold air .

    Major source of heat will probably still be my gas fired furnace .


    God bless
    Wyr

  2. #28
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    20,896
    Quote Originally Posted by WyrTwister View Post
    Are you asking about service calculations or the branch circuits to the actual HVAC equipment ?

    Are you talking about a split system heat pump ?

    If so , the outside unit , size the wire and fuse / circuit breaker to the name plate Max fuse / circuit breaker ampacity .

    The indoor unit / air handler / fan coil unit will have a small fan moter and probably the control transformer . These do not pull much current . Then , will it have electric resistance heat strips ? If so , find out the kw of each heat strip . As has been said , they come in several kw rateings .

    I typically see 5 kw & 10 kw . I run a 30 amp circuit to 5 kw and a 60 amp circuit to a 10 kw . Best I remember , 60 amp is the max , unles the unit itself has fuses or circuit breakers to sub divide the power . For instance , a 90 amp circuit to feed a FCU with a 5 & 10 kw heat strip . With internal fuses or CB's to feed the individual heat strips .

    The fan and control transformer is typically wired with the first stage of resistance heat ( and are 240 VAC ) , since their load is small . The second stage of heat pretty much only has the heat strip connected to it .

    As far as how much this impacts the service load calculations , it depends , as has been said , how the heat pump compressor and the heat strips , are staged .

    Worse case senerio , the above example would be 90 amps plus what ever the name plate minimun circuit ampacity of the out dooe unit is . All these numbers have built in 125% rateing for continious duity .

    In a straight A/C unit ( no heat pump ) , for service load calculations , you use the largest of the resistance heat load or the A/C load , but not both , since you should not be running A/C & heat at the same time .

    Since you live in Atlanta , I am guessing you would not need as much resistance heat , as some one in , say , Maine ?

    All this does not even considerable the possiability of a dual fule heat pump . I suspose that would be another compination ?

    I , myself have a 2 stage 92% gas furnace with a 4 ton A/C . 99% of the time , the furnace never goes to the high flame heat mode .

    God bless
    Wyr
    THX, that was a nice review...

    Where I was going with the question was load on the service panel, not circuits.

    Sometimes I will go to a house and they want to convert to Elec heat, or they want Dual-Fuel. Atlanta has NG most everywhere, so HP's are not the norm.

    As we know, the difference between gas heat and AC vs HP load is the elec strips.

    Where I was going with the question is this: If I get to retire (well semi-retire, I really enjoy working with my hands as long as my back does not hurt... ), I would like to live in the country. As we know... country homes sometimes have really OLD elec systems... and some have no propane at all. Would really like to learn a little more about how to size an elec panel for a HP.
    Here is an example:

    House is around 1500 ft, average insulation and windows. Kitchen is elec and so is the DWH (water heater). then of course the usual lighting and utility circuits like dedicated to dishwasher, to washer, to clothes dryer, etc. House has a 150A service.
    If one added a 3 ton HP with, lets say, 10 KW (this would be around 30A for for the outdoor unit and 50A for the indoor unit)...

    What rules of thumb can I follow to determine if the circuit box (and service entrance) will handle that added amperage? Specifically, when I am on a sales call and I look at the circuit panel and the gauge of wire coming in (I can figure out really quickly if it all matches), what do I need to handle what size of HP and strip load...

    THX!
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  3. #29
    Quote Originally Posted by ga-hvac-tech View Post
    THX, that was a nice review...

    Where I was going with the question was load on the service panel, not circuits.

    Sometimes I will go to a house and they want to convert to Elec heat, or they want Dual-Fuel. Atlanta has NG most everywhere, so HP's are not the norm.

    As we know, the difference between gas heat and AC vs HP load is the elec strips.

    Where I was going with the question is this: If I get to retire (well semi-retire, I really enjoy working with my hands as long as my back does not hurt... ), I would like to live in the country. As we know... country homes sometimes have really OLD elec systems... and some have no propane at all. Would really like to learn a little more about how to size an elec panel for a HP.
    Here is an example:

    House is around 1500 ft, average insulation and windows. Kitchen is elec and so is the DWH (water heater). then of course the usual lighting and utility circuits like dedicated to dishwasher, to washer, to clothes dryer, etc. House has a 150A service.
    If one added a 3 ton HP with, lets say, 10 KW (this would be around 30A for for the outdoor unit and 50A for the indoor unit)...

    What rules of thumb can I follow to determine if the circuit box (and service entrance) will handle that added amperage? Specifically, when I am on a sales call and I look at the circuit panel and the gauge of wire coming in (I can figure out really quickly if it all matches), what do I need to handle what size of HP and strip load...

    THX!

    I am going to be honest with you . It has been ages since I did load calculations for a house . I deal in mostly commercial & if given a choice , I would run from anything residential ( except my own ) .

    But , it comes down to this ;

    Seems like I remember , for a house , about 3 watts per square foot .

    Then you add all the required " special " circuits , laundry circuits , kitchen circuits , HVAC circuits and water heater .

    But , if I had to give an answer , off the top of my head . You said a 150 amp service . Adding 50 kw of electric strip heat ? You might get by , but I would feel safer with a 200 amp service . I feel better if I have a little extra designed in .

    I have the material gathered up . The plan is , when it cools off , to replace the service on our house with a 200 amp service . And if I can talk myself into getting off my lazy behind .

    Will probably do it in 2 stages . Build the meter base / service riser ( overhead service ) , then hang an outdoor loadcenter to feed all the old 120/240 VAC loads and any new loads .


    If you wish to calculate it all out

    http://ecmweb.com/code-basics/dwelli...r-calculations

    God bless
    Wyr

  4. #30
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    20,896
    Hey Wyr,

    THX for the link... I can figure out what I need from there.

    My current rule of thumb is: An average size house (1500 ft ranch), needs a 200A service to be ALL electric (including 10KW of strip heat).
    It appears I need to stick to this...

    And god bless you also Bro!
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  5. #31
    Quote Originally Posted by ga-hvac-tech View Post
    Hey Wyr,

    THX for the link... I can figure out what I need from there.

    My current rule of thumb is: An average size house (1500 ft ranch), needs a 200A service to be ALL electric (including 10KW of strip heat).
    It appears I need to stick to this...

    And god bless you also Bro!
    I pretty much agree . Same size house with gas works OK on a 100 amp service .

    God bless
    Wyr

  6. #32
    Join Date
    Oct 2002
    Posts
    1,248
    wyrtwister-
    Your link is great. The link clearly shows HP load is much more than non-heat pump options.

    As we are expected to use a Manual J vs rule of thumb for ac equipment, an electrician (or AC contractor) should not rely on a rule of thumb to determine service method either.

    A couple times a winter, I come accross a customer where their main breaker has tripped.

  7. #33
    Quote Originally Posted by timjimbob View Post
    wyrtwister-
    Your link is great. The link clearly shows HP load is much more than non-heat pump options.

    As we are expected to use a Manual J vs rule of thumb for ac equipment, an electrician (or AC contractor) should not rely on a rule of thumb to determine service method either.

    A couple times a winter, I come accross a customer where their main breaker has tripped.
    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

  8. #34
    Join Date
    Oct 2002
    Posts
    1,248
    In defrost, Heat strips are needed to keep from blowing cold air. How do you get around this?

  9. #35
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    20,896
    One possibility that could be done is to stage the heat strips.

    If only 5KW comes on with the HP or in defrost, then the rest only when the HP (or defrost cycle) is locked out by the T-stat.

    Problem with this is: Another co may service it and change the wiring... then there is a hot-potato game to see who gets blamed for a main breaker tripping.

    Better IMO to take the high road. Happy customers are a good thing...
    Unhappy customers mess with referrals.
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  10. #36
    Join Date
    Oct 2002
    Posts
    1,248
    5 KW negates 1 to 2 tons of cooling. (15,300BTU) Anything larger then that will be cold air, as I see it.

  11. #37
    Quote Originally Posted by timjimbob View Post
    In defrost, Heat strips are needed to keep from blowing cold air. How do you get around this?
    Not really sure ? From my ( selfish ? ) point of view , it was a simple matter that the avalable electrical supply would not accomidate both . Or , I could not in good faith bid it that way .

    If it would not fly , then some one needed to come up with another HVAC solution . But I would not take on the liability .

    One of these occasions , it was an appartment complex . Had a boiler & a chiller . They said the metal underground pipeing was rusting out . ( I think the chiller was shot and would not be supprised if the boiler was not too far behind ? )

    They would have had to dig up / tear up a LOT of piping & a LOT of that was under concrete .

    But they would have to do a huge amount of demolition to up grade the electrical to each apartment .

    I do not know how it turned out , we did not get the job , so I went down the road .

    As to your origional question . The mini split I just installed at our house is a HP . Pretty sure it has no resistance heat ? I obviously have not tried it on heat . So I have no real idea what it does in defrost ? I seem to remember reading it does not run the indoor fan on defrost ? ( Does that make any sense ? )

    A question for all of you all . What are the conditions that initate a defrost cycle . I seem to remember hearing of defrost timers for older equipment ? I am guessing the electronica do the " deciding " now ?

    God bless
    Wyr

  12. #38
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    20,896
    Most mini's do not have heat strips... there may be a few that do. Daiken makes a model that attaches to a Goodman AH which can use a heat strip... kinda pricey.

    Defrost cycles initiation depends on the type of defrost board. The older ones were timed... some of the newer ones are time/temp.
    The older ones defrost every 30-60-90 minutes of HP operation (installer choice).
    The 'on demand' models (time/temp) usually defrost every 5 hours (time) and have one or two temp probes which defrost when a frozen coil is detected.
    The reason for 'on demand' types is different climates. A rainy and almost freezing climate would need more frequent defrosts than a dry and cold climate.
    GA-HVAC-Tech

    Quality work at a fair price with excellent customer service!

    Romans Ch's 5-6-7-8

    2 Chronicles 7:14

  13. #39
    Thanks ,
    Wyr
    God bless

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event