4-ton AC with a 5-ton indoor coil?
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  1. #1
    Join Date
    May 2005
    Posts
    8
    Hello,
    My existing furnace has 5-ton blower in it. I'm trying to have a central AC installed. Most of the contractors who looked at my house indicated that I should be installing a 5-ton AC unit. One contractor, however, performed a manualJ? analysis and concluded that a 3-ton unit would be the right size. From what I've learned, it would not be a good idea to install a 3-ton coil on top of my furnace as this restricts the airflow too much. What the contractor suggests is that I should opt for a 4-ton AC unit and have a 5-ton coil mounted on the furnace. Supposedly this is a compromise between a 5-ton and 3-ton AC units. Anyway the question that I have is that does this make sense? Would there be any problems with a mismatched AC components e.g. 4-ton AC with a 5-ton indoor coil?
    Thanks in advance,
    Matt

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Mismatched is a no-no,but a 3 ton may have a "factory" approved match with a 4 ton coil.

    The 3 ton coil,should not be a problem for air flow in cooling,If the heating cfm(airflow) required is great than the cooling cfm then it can be a problem.

    I've heard if the coil is physically smaller then the size of the plenum on the furnace,they can install a "plate" to force the air thru the coil in cooling and remove it for heating allowing proper air flow.You might want to ask about this.

  3. #3
    Join Date
    May 2005
    Posts
    8
    Thanks for the reply.
    My heating CFM is higher hence the problem with the installation of a 3-ton indoor coil The 4-ton outdoor unit coupled with a 5-ton indoor coil unit is supposedly an OK thnig to do (according to the manufacturer). Alternatively I can opt for a 5-ton outdoor unit as well. The reason that the 4-ton unit has been suggested is that this would help with lowering the humidity level as the AC unit would run longer. However going with the 4-ton AC requires that I switch to a 5-ton indoor coil.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,811
    We frequently put 3 ton A/Cs on furnaces with blowers capable of 5 tons. Never a problem. We measure static pressure, use the manufacturers chart to determine what speeds to set. OFTEN HEATING IS SET HIGHER THAN COOL!!! There is nothing wrong with that.

    Many mfrs make several widths of each capacity coil. Get the widest one you can for lowest restriction.

  5. #5
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    4 tons,is oversized if the load is 3 tons,and will not do a good job dehumidifing,beter then a 5 ton,but not what it should be.

    Did the measure the External Static Pressure of the duct system to determine it's too small?

  6. #6
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    A 5-ton blower will probably be too large for a 3-ton condensing unit at sea level, particularly if there is a humidity problem.

    Some blower data I have handy shows over 1600 CFM on low speed at 0.5" esp.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,579
    Ask that contractor what your house needs for heat, and if you current furnace is over sized, you should replace it with the right size, and get the proper blower for your 3 ton a/c.

    JMO.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  8. #8
    Join Date
    May 2005
    Posts
    8
    No pressure measurements were taken. The house heating needs are for a 5-ton unit as I live in the pacific northwest area and heating requirements far surpass the cooling requirements. As I said in my original post, the cooling requirements were based on Manual-J analysis. This pointed to a 3-ton AC unit. Given my existing blower, however, I'm unable to go with a 3-ton system as the smaller indoor coil would restrict the furnace too much during the heating season. Also the AC manufacture has a maximum limit of one ton in over sizing the indoor coil. So in all I have the following two options:

    1) A 5-ton AC with a 5-ton indoor coil or
    2) a 4-ton AC with a 5-ton indoor coil.

    I'm leaning towards the latter option and in my original question, I was inquiring about people's experience in this area. Thanks for all the replies.

  9. #9
    Join Date
    Apr 2005
    Location
    San Luis Obispo County, CA
    Posts
    215
    Originally posted by ahdofu

    1) A 5-ton AC with a 5-ton indoor coil or
    2) a 4-ton AC with a 5-ton indoor coil.

    I'm leaning towards the latter option and in my original question, I was inquiring about people's experience in this area. Thanks for all the replies.
    Sounds like you have a fairly decent contractor, at least he DID the Manual J.

    Now, if it were MY system, and those were MY options, I would install the 4 ton A/C with the 5 ton indoor coil, and use a TXV (thermal expansion valve) on the indoor coil as the metering device.

  10. #10
    Join Date
    May 2005
    Location
    midwest
    Posts
    9
    I'd have to agree with dave_slo. I recently installed a 3 ton outdoor section with a 3.5 ton A-coil w/TXV in my daughter's house. Works great, subcooling figured out to a 19 degree superheat, manufacturer's specs called for 15-20.

    Also, I've heard that using a larger evap improves dehumidifaction due to increased surface area.

  11. #11
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by ahdofu
    No pressure measurements were taken. The house heating needs are for a 5-ton unit as I live in the pacific northwest area and heating requirements far surpass the cooling requirements. As I said in my original post, the cooling requirements were based on Manual-J analysis. This pointed to a 3-ton AC unit. Given my existing blower, however, I'm unable to go with a 3-ton system as the smaller indoor coil would restrict the furnace too much during the heating season. Also the AC manufacture has a maximum limit of one ton in over sizing the indoor coil. So in all I have the following two options:

    1) A 5-ton AC with a 5-ton indoor coil or
    2) a 4-ton AC with a 5-ton indoor coil.

    I'm leaning towards the latter option and in my original question, I was inquiring about people's experience in this area. Thanks for all the replies.
    Since no Static was measured ,I think they have no way of knowing if a 3 ton coil,with or without the ypass ,I mwentioned will work.

    Can you post the model number of your furnace.

    If you don't measure it's just a guess.

  12. #12
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,811
    Given my existing blower, however, I'm unable to go with a 3-ton system as the smaller indoor coil would restrict the furnace too much during the heating season.
    Still trying to figure out where you got that horsehockey from. Manufacturers put multi-speed motors in their equipment for just such a purpose. Most furnaces run on medium low or medium for heat to get in the center of the heat rise range. If you need 1200 CFM for your 3 ton A/C, I'd bet low would be very close.

    Also, I've heard that using a larger evap improves dehumidifaction due to increased surface area.
    Larger coil improves SENSIBLE, not dehumidification. It runs warmer with more surface area. Smaller coil runs colder, sucks more juice.

  13. #13
    Join Date
    May 2005
    Posts
    8
    BaldLoonie,
    The so called "horsehockey" quote comes from the contractor. Basically he will not put in anything smaller than a 4-ton AC on my system. I've talked to other contractors and even though they all had initially suggested a 5-ton AC, none of them is willing to go below 4-tons again (I have tried dealers representing Carrier, Trane, Bryant & Lennox). As for my furnace, it's made by Payne & the model number is 383KAV.

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