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  1. #1
    Join Date
    Oct 2006
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    15

    4 ton AC coil with 5 ton Fan air flow ?

    Question on 4 ton coil with 5 ton Fan.


    I was looking over the Manual J Load for my house this weekend that the Hvac crew left for me.

    Is the fan to high speed for coil ?

    The fan speed’s on the 5 ton furnace to be installed are variable. 1270 low, 1840 Med -Heat, 2270 Cool- High.

    Sensible Latent Split : 75% / 25% . This because I told them Im adding a dehumidifier and to add this to the load calculation.

    Does this fan speed of 2270 seem correct going over a 4 ton coil? I have not done any Hvac for while but this does seem a little high to me. I also thought maybe because of the southwest climate, New Mexico. I was told the speed of 2270 would be ok. Now Keep in mind all the Ducts are to being redone with round and square metal, brand new at 0.5 sp.

    The house is close to 2200 sqft. Half this house has a lot of glass facing west . You would not believe how hot it get on the west side of this house with no AC. 90 degrees peak from 2-5 pm indoors in the summer. That’s why I told them new duct work all way and kicked in some extra cash for the job to be done right.

    The J calculation seems very correct as to all the upgrades insulation. Etc. But the furnace seems to be a little oversized. Im thinking for the fan speeds and the climate conditions

    Also I was thinking of maybe looking for furnace with the fan running on 240V Any suggestions ?
    The company I hired deals mainly in residential and had no clue as to what furnaces have 240V fans and neither do I for residential. I only hook up the electrical side of the commercial unit those I know.

  2. #2
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    The furnace has multiple speeds that deliver various cfms at different static pressures(resistance to air flow of the ducts ,coil,filter,etc.),they should have selected the speed based on this info..Hopefully they did .

  3. #3
    Join Date
    May 2002
    Location
    Houston Texas
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    6,323
    Gas furnaces are 110V not 220V electric air handlers use 220V motors.

    2270 CFM on 4-ton system is 567 CFM per ton that will give you the most sensible BTU's and very minimal latent capacity. I doubt that you will have 75% SHR with that kind of CFM per ton.

    That would seem to be what you need in a dry climate like New Mexico. You are adding cooling load to the house with a dehumidifier so I suspect it should only be run at night or you could have trouble keeping up during peak cooling periods.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,120
    You might want to make sure that both you and they know the difference between .5"SP, and .5" FR.

    Big duct on a furnace with a 4 ton coil to have a .5" SP
    A 4 ton coil can have a .25 to .3"pd at 1600 CFM, and .35" or more at 1800 CFM.

    So make sure they're not thinking FR.
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  5. #5
    Join Date
    Nov 2000
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    Eastern PA
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    Where are you getting those cfm numbers from for the fan speeds? It doesn't sound like you are taking static pressure into consideration at all. As a matter of fact, with the 240v questions, it sounds like you are installing this system yourself.
    Government is a disease...
    ...masquerading as its own cure…
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  6. #6
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    Jan 2004
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    Lancaster PA
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    LOL... Why wouold anyone want a 240 volt furnace for their house anyway.
    Its not like it uses less electric.
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  7. #7
    Join Date
    Nov 2000
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    Eastern PA
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    Quote Originally Posted by beenthere View Post
    LOL... Why wouold anyone want a 240 volt furnace for their house anyway.
    Its not like it uses less electric.
    Yea, but if they are replacing an existing all electric system........
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  8. #8
    Join Date
    Oct 2006
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    15
    No, Im not istalling it myself. I could do it myself it would take me a while though and I don't have the time for one. I got other Electrical job to do. I wish I had that kind of time. not to mention Im not to uptp date on the hvac equipment out there.
    the numbers are from the furnace speeds and the load calculations, (Standard J ) , yes there is static pressure pressure number on the load calculation the duct work comes out very big. And my other question was if they made blowers with 240V for residential. I I know they make 120V blowers usually that is the norm. Just thought There might be something out there like that these days with all these Efficiency units being made. It would be nice though. Like I said I dont do very much Hvac anymore but keep a licence.

  9. #9
    Join Date
    Oct 2006
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    15
    Quote Originally Posted by RoBoTeq View Post
    Yea, but if they are replacing an existing all electric system........
    I did replace the entire electrical system. 240V pulls less current. than 120V. the meter spins slower. Im the electrician.

  10. #10
    Join Date
    Oct 2006
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    15
    Quote Originally Posted by dash View Post
    The furnace has multiple speeds that deliver various cfms at different static pressures(resistance to air flow of the ducts ,coil,filter,etc.),they should have selected the speed based on this info..Hopefully they did .
    Im going to ask that same question because I think they based this on the old duct system with some new duct parts but mainly keeping the old duct system. but I told them the other day just to take out the old duct system all togeather.

    I know you cant just slap duct work in and hope for the best. They based this off a Ducane model. Im thinking I could still have them change this unit out even though they started the job. there not into the job that far yet of removing the old duct work.
    Im going to get some clarification on this Standard J some number seem a little pushed here. don't make any sense. it more looks like the old duct system to me with some changes. then I said removeit all. I sure that why this dont make any sense. I leaning towards the Carrier two stage system 90 In any case it will new a new load calculation.

  11. #11
    Join Date
    Oct 2006
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    15
    Quote Originally Posted by RoBoTeq View Post
    Yea, but if they are replacing an existing all electric system........
    Ok I see what you mean. to an electrician like I though you ment replace the entire electrical system. well did that.

    I see your point electrical blowers 240V that must be where I thought maybe a gas unit might have a 240V fan. I do know they have 120V , But I swear I seen a 240V gas unit someplace, maybe what I say was the electrical blower.

  12. #12
    Join Date
    Oct 2006
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    15
    Quote Originally Posted by classical View Post
    Gas furnaces are 110V not 220V electric air handlers use 220V motors.

    2270 CFM on 4-ton system is 567 CFM per ton that will give you the most sensible BTU's and very minimal latent capacity. I doubt that you will have 75% SHR with that kind of CFM per ton.

    That would seem to be what you need in a dry climate like New Mexico. You are adding cooling load to the house with a dehumidifier so I suspect it should only be run at night or you could have trouble keeping up during peak cooling periods.
    Yep, the load calcualtion is wrong. it' based on the old duct system Im sure of it. Im going to need a new load calcualtion. because I told them the other day to just get rip of the old duct system. after the load cal was done. they were going to add the some replament duct in the old system..

    New load calculation will be needed. Im sure that what im looking at.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    A 1 horse power 120 volt motor pulling 6 amps, uses as much electric as a 240 volt 1 horse power motor pulling 3 amps.

    Makes no difference. 240 volt is NOT more efficient current wise then 120 volt.
    Just smaller wire.
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