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Community and Q&A

mini split sizing for my cottage

cottage94 | Posted in General Questions on

I am new to the site but have read many responses by Dana and it certainly sounds like you could help me.
I have a cottage outside Toronto which I live at in the summer and also go up 3-4 times in the winter.
my cottage is 760 sq ft, rectangle in shape with a cathedral ceiling.  Open concept with 3 bedrooms off the main area.
I have a 14600 btu portable heat pump/AC  that does not do the trick in cooling the place so I am going to put in a mini-split heat pump.
I have attached the second page of a manual J type calculation and it indicates I need an 18000 btu heat pump for cooling( which is what I am mainly concerned with although I will use in the winter as supplemental heat to my wood stove.
One contractor recommends the Fujitsu 18RLXFWH, while another recommends 15RLS3H.
Both seem to meet the requirements but I am concerned the 18K unit since it cycles down to 7000btu while the 15K unit goes down to 3000btu.  I am thinking the 15K will be better to reduce humidity in the summer.
Your help or recommendations would be appreciated.

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  1. Jon_R | | #3

    > the 15K unit goes down to 3000btu. I am thinking the 15K will be better to reduce humidity in the summer.

    Without looking up the specs, it could well be the opposite. CFM/ton is the primary determinate of humidity reduction. Contrary to popular view, match to load has a minor effect.

  2. HoverDA | | #1

    Do you want to use mini split as main heat source in winter?

    18RLXFWH has bigger outdoor condenser compared to 15RLS3H. So 18RLXFWH has less defrost frequency and potentially better real world efficiency when heating in cold winter.

    15RLS3H has lower minimum cooling capacity. So it removes the humidity better in the summer.

    If you mainly consider use mini split for cooling, I'll choose 15RLS3H.

  3. cottage94 | | #2

    Thanks for the reply HoverDA. I am planning to use the mini split mainly for cooling, heating is a bonus and will supplement my main source of heat which is a wood stove.

    Would there be an issue with the 15K unit running at max since the calculation calls for about 18K btu. Is there a buffer built into these type of calculations?

  4. cottage94 | | #4

    Thanks for the replies. So based on what I have read on this site it is better to spec out the mini-split so that the unit is running at 90-100% capacity on the hottest day I would see in the summer (34C).
    Also, it is best to pick a unit that has a low minimum cycling capacity.
    Based on the specifications on the 2 units (15,ooo unit with 3100-18400 btu vs 18000 unit with 7000-23000 btu) and my requirements of having a heat load of about 18,000 BTU on the hottest day for a 7600 cu ft space the 15K Fujitsu would be a no brainer choice for my cottage.

    Does that make sense or am I missing something?
    Not sure why one contractor would recommend the 18,000 RLX unit....

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