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Thread: Wrong Size AC Unit?

  1. #1
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    Wrong Size AC Unit?

    Hi All,

    I moved in to a new home here in the Kansas City, Missouri, area late last year. Our home has a traditional midwest HVAC system, forced air gas heat and outside AC condenser unit. In my case, it is a Carrier CA16NA048-A outside unit and CNPVPALAAAAA inside coil, mated with a Carrier 59TP5A100E211220 furnace.

    This summer, we've dealt with high humidity in our home. We consistently have humidity levels at 69-72% in our home with the thermostat set to 72 at night and 74 during the day. We know this isn't acceptable for a whole host of reasons, not the least of which is mold, dust mites and comfort. We've been fighting our home building to correct the issue, but thus far their only response is to slow the fan speed down to the minimum 350CFM and ask me to buy a dehumidifier. We did that, picking up a 70pint unit from Home Depot and placing it in our finished basement. The humidity levels have still remained high, dropping down to 63-67% on average.

    Yesterday, I finally convinced the builder to send me the design and load calculations. Two things immediately jumped out to me as a consumer:

    1. The load sheet shows the system was designed for a home that faces the South. My home does not. It faces the W-SW.
    2. The load sheet shows the system says the Latent Cooling Equipment Load Sizing cooling capacity is 31291 Btuh / 3.4 tons, yet my Carrier system is a 4 ton system.

    Based on the above it would seem to indicate the system is oversized for my home, since the Carrier unit is rated for 44,500 Btuh Don't oversized units cool the home quickly and cycle frequently, so they do not have the run time needed to pull the humidity?

    I'd love to get your thoughts. Thank you in advance.

    Mike

  2. #2
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    That for reading our recommendations. How did you determine the cfm of air flow? What is the cooling cycle during the peak heat of the day, mins. on/mins. off? What is the return and supply temp/%RH during the longest run? Typical winds mph in area?
    Fresh air ventilation, how much and when? Humidity in the basement? How much water is removed by the dehumidifier?
    If you down sized your a/c, would the %RH change in your home, slightly??
    Looking forward to your answers to better answer the questions.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  3. #3
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    31,000 btuh is 2.5 tons... and they installed 4!
    The TRUE highest cost system is the system not installed properly...

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  4. #4
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    Quote Originally Posted by teddy bear View Post
    That for reading our recommendations. How did you determine the cfm of air flow? What is the cooling cycle during the peak heat of the day, mins. on/mins. off? What is the return and supply temp/%RH during the longest run? Typical winds mph in area?
    Fresh air ventilation, how much and when? Humidity in the basement? How much water is removed by the dehumidifier?
    If you down sized your a/c, would the %RH change in your home, slightly??
    Looking forward to your answers to better answer the questions.
    Regards Teddy Bear
    I don't have all the answers to your questions, but attached is a copy of the calculations they gave me. Name:  Capture.JPG
Views: 1473
Size:  147.1 KB

    What is the cooling cycle during the peak heat of the day, mins. on/mins. off? Unknown. I've not had a chance to measure/time it.
    What is the return and supply temp/%RH during the longest run? Unknown. They did test both the return and supply air with a meter prior to dropping the fan speed down to 450CFM and found it "acceptable."
    Typical winds mph in area? Somewhat windy, but I'm not sure how to quantify.
    Fresh air ventilation, how much and when? We don't open windows. I know the unit itself pulls in fresh air, as we have two pipes (one exhaust and one fresh air) on the furnace.
    Humidity in the basement? Humidity in the basement is now 45% since the dehumidifier has been running (about two weeks)
    How much water is removed by the dehumidifier? Unknown. it pumps out directly to a floor drain. There was a lot of water removed initially.

    I've redacted it only to remove contractor and my information.

  5. #5
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    By their own report Manual S You Are Oversized you 32% 45.5KBTU output on 31.29 KBTU needed.......You are allowed 15 % Oversize on Single Stage Equipment

    Also Kansas City 1% Design is 93 Degrees, they gamed it to 96 making your actual oversize worse

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  7. #6
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    96K output on the furnace for a 43K loss, wow.

    Sounds like 60K and 2.5 or 3 ton would be MAX.

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  9. #7
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    Quote Originally Posted by BaldLoonie View Post
    96K output on the furnace for a 43K loss, wow.

    Sounds like 60K and 2.5 or 3 ton would be MAX.
    Just to clarify - are you saying my furnace is twice the size necessary?

  10. #8
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    Quote Originally Posted by msglsmo View Post
    Just to clarify - are you saying my furnace is twice the size necessary?
    Your heating load is only 43.124 KBTU the furnace selecting is 96.3 KBTU Output, though it is 2 stage so It can be locked in Low Stage

  11. #9
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    They probably could have installed a 3 ton, at going with .8shr. And you would be tickled pink with it.

    They probably went with such a large furnace to get the air flow the over sized 4 ton A/C needs.

    PS: You can go as low as 320 CFM per ton with your high indoor humidity.
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  13. #10
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    Why no duct loads????? What is the location of your ducts?
    Doug

  14. #11
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    The sensible/latent loads are wrong. The latent load is +2X higher than the estimate. Not sure of sensible.
    Measure the cycles in mins. on/off during peak cooling load and ^F/%RH return-supply before going any further!
    This is info needed before further comment from me.
    Changing equipment with the current info is worse than the original selection of equipment.
    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  15. #12
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    So here is the response from my HVAC contractor:

    "Concern #1 is you feel that your system is too large, so it will not dehumidify correctly. When we, or anyone for that matter sizes the cooling for a home, we always round up to the nearest 1/2 ton size. This is standard practice, and meets certifed energy star guidlines. The 3.4 ton size is the bare minimum size, and whenever the outside temperature exceeds 93 degrees, your home would lose ground and struggle to keep up. A 3.5 ton system is actually rated at 3.3 tons, and the 4 ton is rated at 3.8 tons. The energy star guidlines on your home would size your hvac between 43,000 and 49,000 BTUs for cooling. Your ac unit now specs out at 46 to 47,000 btus of cooling, and that is when it is running in a brand new, clean status."

    Thoughts?

    Mike

  16. #13
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    Fudged the Manual J to read what they wanted it to, and no idea how to do a Manual S.
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  17. #14
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    Quote Originally Posted by msglsmo View Post
    So here is the response from my HVAC contractor:

    "Concern #1 is you feel that your system is too large, so it will not dehumidify correctly. When we, or anyone for that matter sizes the cooling for a home, we always round up to the nearest 1/2 ton size. This is standard practice, and meets certifed energy star guidlines. The 3.4 ton size is the bare minimum size, and whenever the outside temperature exceeds 93 degrees, your home would lose ground and struggle to keep up. A 3.5 ton system is actually rated at 3.3 tons, and the 4 ton is rated at 3.8 tons. The energy star guidlines on your home would size your hvac between 43,000 and 49,000 BTUs for cooling. Your ac unit now specs out at 46 to 47,000 btus of cooling, and that is when it is running in a brand new, clean status."

    Thoughts?

    Mike
    Enery Star Guidelines.....Select the smallest unit possible taking 50% excess latent and applying towards sensible...Energy Star allows 5% under and 15% over capacity on single stage equipment .....You DO NOT round up 1/2 ton unless ( for example) you were using 2 stage equipment and it does not come in 1/2 ton increments ...and the previous increment would not meet the load....This IS NOT. The case in your situation.....Since they are Citing Energy Star...Google ACCA 5 that is Energy Star and it is very well documented....roumg up is not acceptable...

  18. #15
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    Measure the on/off cycle in mins, and record the temp/%RH return and supply. The rest is just commentary and constructive at this point unless you are just looking for a fight with your contractor.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  19. #16
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    Quote Originally Posted by lkapigian View Post
    Enery Star Guidelines.....Select the smallest unit possible taking 50% excess latent and applying towards sensible...Energy Star allows 5% under and 15% over capacity on single stage equipment .....You DO NOT round up 1/2 ton unless ( for example) you were using 2 stage equipment and it does not come in 1/2 ton increments ...and the previous increment would not meet the load....This IS NOT. The case in your situation.....Since they are Citing Energy Star...Google ACCA 5 that is Energy Star and it is very well documented....roumg up is not acceptable...
    As I read ACCA5 section 3.3.1, it appears rounding up is acceptable (which is unfortunate):

    "Total equipment capacity is between:
    - 95% and 115% of total cooling requirements (for air conditioners
    and heat pumps)
    or
    - 95% and 125% of total cooling requirements (for heat pumps with heating dominated requirements)
    or
    - the next largest nominal piece of equipment, per OEM increment, that is available for either to satisfy the latent and sensible requirements."

    I agree that it doesn't make sense to round up. I've always been told that slightly undersized it better than oversized.

  20. #17
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    On additional thought. Also on the load report, it lists my house as facing the South. However, my home faces West-Southwest, 231degrees on a compass. What impact does that on the report?

  21. #18
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    Quote Originally Posted by msglsmo View Post
    As I read ACCA5 section 3.3.1, it appears rounding up is acceptable (which is unfortunate):

    "Total equipment capacity is between:
    - 95% and 115% of total cooling requirements (for air conditioners
    and heat pumps)
    or
    - 95% and 125% of total cooling requirements (for heat pumps with heating dominated requirements)
    or
    - the next largest nominal piece of equipment, per OEM increment, that is available for either to satisfy the latent and sensible requirements."

    I agree that it doesn't make sense to round up. I've always been told that slightly undersized it better than oversized.
    Which would mean the largest allowed for your house is 3.5 tons, not four tons unless its a heat pump.

    However, if Manual S was used, you would find that 3 tons would probably been more then enough.
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  22. #19
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    Quote Originally Posted by Gunslinger View Post
    Why no duct loads????? What is the location of your ducts?
    I'm not sure about this, but hopefully this helps:

    My basement is finished, so all duct work is in finished/condition spaces of the home. We have one large air return vent on each level of the home - 2nd floor, main floor and basement. Vents are in the floor for all rooms, except one bedroom which has a wall mounted vent.

    Hope that helps.

    Mike

  23. #20
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    Quote Originally Posted by teddy bear View Post
    Measure the on/off cycle in mins, and record the temp/%RH return and supply. The rest is just commentary and constructive at this point unless you are just looking for a fight with your contractor.
    Regards Teddy Bear
    I'll measure the on/off cycles. I don't have the ability to measure the return and supply temp and humidity.

    I'm not looking for a fight; rather I'm looking for my builder and HVAC contractor to ensure my system was designed/engineered correctly. Your Bear Rules of <50% RH in the summer makes complete sense to me, but I'm sitting at upper 60's/low 70's RH. Something isn't right.

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