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  1. #1
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    Default Can someone help me size a mini-split heat pump

    I have a 450 square foot studio with 8 foot ceilings over top of my garage. Since it's a studio I have two layers of 1/2 drywall over resilient channel. It is well insulated and only has two small windows and a door. I am looking at the Mitsubishi Mr Slim 21 Seer heat pump system. I hope someone can tell me if I should by a 9000 btu or the 12000 btu system. I have heard that it's better to undersize a little but I don't know if 9000 btu's is too undersized. I live in eastern Ky if that helps. If someone out there has installed one of these can you also tell me what size wire I will need to run this system. I have a 10/3 wire with nothing on it near where I am installing it if that would work. I like the design of the LG Art Cool system but I have read some complaints about the LG systems. I appreciate any advice I can get.

  2. #2
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    JD 4310

    Default

    Good rule of thumb is 1 ton of cooling per 400sf; 12,000btu = 1 ton. I'd go for the 12,000 myself.
    GL5240
    KX057

  3. #3
    Platinum Member ustmd's Avatar
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    May 2009
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    Manor, TX (outside of Austin)
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    Kioti CK27 HST

    Default Re: Can someone help me size a mini-split heat pump

    Quote Originally Posted by Jenkins003 View Post
    I have a 450 square foot studio with 8 foot ceilings over top of my garage. Since it's a studio I have two layers of 1/2 drywall over resilient channel. It is well insulated and only has two small windows and a door. I am looking at the Mitsubishi Mr Slim 21 Seer heat pump system. I hope someone can tell me if I should by a 9000 BTU or the 12000 BTU system. I have heard that it's better to under size a little but I don't know if 9000 btu's is too undersized. I live in eastern Ky if that helps. If someone out there has installed one of these can you also tell me what size wire I will need to run this system. I have a 10/3 wire with nothing on it near where I am installing it if that would work. I like the design of the LG Art Cool system but I have read some complaints about the LG systems. I appreciate any advice I can get.
    I have almost the exact same situation as you. I am in Central Texas with a walk in attic space (~300sq ft) with two 2 x 4 skylights that I converted into a home office with a 1/2 bath.

    I went with the Mr. Slim unit and it has been great. More money than the other units, but worth it (so far-18+ months). I have the 9000 BTU unit. This summer with >90 days at 100+, the unit had no trouble keep up. The office space is surrounded on 3 walls by hot attic.

    During the winter, the unit does a good good of keeping the space comfortable, though I am not in Ky.

    I had mine professionally installed and he ran a dedicated circuit for the unit.

  4. #4
    Epic Contributor murphy1244's Avatar
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    Kioti DK-40, MF-135, Ventrac 4500Y

    Default Re: Can someone help me size a mini-split heat pump

    Quote Originally Posted by 45V View Post
    Good rule of thumb is 1 ton of cooling per 400sf; 12,000btu = 1 ton. I'd go for the 12,000 myself.
    Thats kinda overkill. In ohio we use 1 ton -600 square feet.
    Murph

  5. #5
    Veteran Member
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    N.E. PA
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    BX22

    Default Re: Can someone help me size a mini-split heat pump

    The concept of "not going too big" is related to too much cooling capacity, which can not reduce humidity effectively. With modern variable or multistage units, this is not much of a factor as it was in the past. On the other side, larger capacity will provide more heat capacity at colder temperatures. So if you get winter weather that can linger below 30 deg, I would go with the larger unit to provide more heating. If you are in a temperate area, go smaller. In the hot south I would again lean towards the bigger unit.

    I have an addition that is about 550 sq-ft, with 6 windows but well insulated. I put in a 1.5 ton (18000 BTU) heat pump. Absolutely no regrets. No problem with cooling, and enough heating capacity that I have seen the backup heat on two times in 6 years, when it was near zero deg outside.

    paul

  6. #6
    Veteran Member gsganzer's Avatar
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    L3800 w/FEL and BH77, BX 2200 w/FEL and MMM

    Default Re: Can someone help me size a mini-split heat pump

    I have a 600 sq/ft in-law apartment with 10' ceilings. I put in a 2-ton unit, to facilitate rapid cooling when it's unoccupied and I want to go out and piddle (it will be a workshop and home office, after the in-laws are gone.)

    It definitely heats and cools quickly, but a 1.5 ton would have also worked fine.
    I plan to screech to a sideways stop at heavens gate, jump out with a cigar in one hand, a scotch in the other, yelling "WOOHOO!! WHAT A RIDE!!!"

  7. #7
    Silver Member
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    Default Re: Can someone help me size a mini-split heat pump

    I have a 480 square foot insulated office and laundry/bathroom in my shop that I heat and cool. There are two windows and two walk in doors. I had a 2 ton high efficiency Lennox heat pump installed which does an excelent job. I could have used a 1.5 ton unit, but a 2 ton was almost the same price. The unit has two 4kw heat strips for the emergency heat, but I disconnected one 4kw element so it now has only a 4kw heat strip which does fine if the outside temperature is below freezing and the heat pump goes into its defrost cycle.

    Bill
    '14 Kubota L5460HSTC
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  8. #8
    Veteran Member Mickey_Fx's Avatar
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    Vancouver Wa.
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    Yanmar Fx24D, Cub 3204

    Default Re: Can someone help me size a mini-split heat pump

    If heating is important to you and your decision what to purchase. the rating system for heating is HSPF. SEER is the rating system for cooling. These two rating systems are not tightly connected.

    One can find HP's who's SEER rating can vary by 3 points and have the same HSPF rating.
    Yanmar Fx24D,
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  9. #9
    Veteran Member gsganzer's Avatar
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    L3800 w/FEL and BH77, BX 2200 w/FEL and MMM

    Default Re: Can someone help me size a mini-split heat pump

    Quote Originally Posted by Jenkins003 View Post
    I like the design of the LG Art Cool system but I have read some complaints about the LG systems. I appreciate any advice I can get.
    I've had a LG Art Cool 2-ton unit for about 9 months. No issues yet.
    I plan to screech to a sideways stop at heavens gate, jump out with a cigar in one hand, a scotch in the other, yelling "WOOHOO!! WHAT A RIDE!!!"

  10. #10
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    Default Re: Can someone help me size a mini-split heat pump

    Thanks to everyone who has so far replied. The only reason I'm even considering a 9000 btu unit is this place is so well insulated that the regular rules may not apply here. I was working in my studio today doing some painting and it was in the mid 20's here today. I heated the room up with a small torpedo but the rest of the day all I had for heat was my 1000 watt work lights and it never really got cold in there all day. The two layers of drywall and the 1/2 dead air space behind that really makes a difference. I wonder which unit would be the cheapest to operate. The 12000 btu unit obviously would draw more power but it seems like it would get to a set temperature quicker. I plan on adding a small ventless gas fireplace as backup heat in case the power goes out or it's gets so cold the heat pump can't keep up.

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