Mini Split AC

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1. ObservedJul 12, 2026Conversation3 turnsPlacementAfter turn 13 turns · After turn 1Replay chat
   Observed conversationJul 12, 2026 · interface recreation

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   Mitsubishi ductless mini split, open plan room of 21ft by 35ft and 8ft tall what size BTU should be used for this zone.

   For an open-plan room that is 21 ft × 35 ft with an 8 ft ceiling, the floor area is:

   • 21 × 35 = 735 square feet
   • Ceiling height: 8 ft (standard)

   A quick sizing estimate for a well-insulated home is about 20–30 BTU per square foot, depending on climate, insulation, windows, and sun exposure.

   • 20 BTU/sq ft: ~14,700 BTU
   • 25 BTU/sq ft: ~18,400 BTU
   • 30 BTU/sq ft: ~22,100 BTU

   For a Mitsubishi ductless mini-split, the likely choices are:

   • 18,000 BTU (1.5 ton): Suitable if the room is well insulated, has average windows, and is in a moderate climate.
   • 24,000 BTU (2 ton): Better if the room has large windows, significant afternoon sun, high ceilings (not in this case), poor insulation, or is in a hot climate.

   My recommendation

   For a 735 sq ft open-plan living area, a 24,000 BTU Mitsubishi indoor unit is generally the safer choice. Many installers would select a 24k unit because open living/dining/kitchen areas often have:

   • Heat from cooking and appliances.
   • Multiple occupants.
   • Large window areas.
   • Air movement into adjacent spaces.

   However, if this is a newer, well-insulated home with modest window area and you're in a mild climate, an 18,000 BTU variable-speed Mitsubishi may still be sufficient because inverter systems can ramp their output up and down efficiently.

   To size it more accurately, I would need:

   • Your location (city/state or climate zone).
   • Whether this is primarily cooling, heating, or both.
   • Approximate window size and orientation (north/south/east/west).
   • Insulation level (new construction vs. older home).
   • Whether the kitchen is included in the 21 × 35 ft space.

   With that information, I can estimate the load much more closely and recommend the appropriate Mitsubishi model size.

   

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   If the Insulation level was sub standard and have many parties requiring extra people which increases BTU, not to mention having 110f temp days would that change the BTU of the room?

   Yes. Those factors can significantly increase the cooling load, and they push the recommendation toward a larger system.

   For your 735 sq ft room:

   • Substandard insulation increases heat gain through the walls and ceiling.
   • 110°F outdoor temperatures create a much larger temperature difference than a typical design day, increasing the cooling demand.
   • Frequent parties add heat from people. As a rough guide, each additional person beyond the first two contributes around 500–700 BTU/hr (depending on activity level).
   • If the room includes a kitchen, cooking appliances can add several thousand BTU/hr while in use.
   • Large west- or south-facing windows can add a substantial amount of solar heat, especially during the afternoon.

   Under those conditions, it's not unusual for the cooling load to be in the 24,000–30,000+ BTU/hr range.

   A few considerations:

   • A 24,000 BTU Mitsubishi inverter mini-split can often increase its output above its nominal rating during peak conditions, so it may provide more than 24,000 BTU/hr when needed.
   • If a proper load calculation comes out above about 28,000–30,000 BTU/hr, then a 30,000 BTU or 36,000 BTU system—or two smaller indoor units—may be the better solution.
   • Oversizing isn't ideal either. Even inverter systems perform best when they're not significantly oversized, because excessive capacity can reduce humidity control and cause more cycling at light loads.

   Given what you've described—735 sq ft, poor insulation, 110°F design temperatures, and frequent large gatherings—I would expect a 24,000 BTU unit to be the minimum worth considering, and I would not be surprised if a professional load calculation recommended 30,000 BTU instead.

   If you can tell me:

   • your city and state,
   • whether this is cooling only or heating and cooling,
   • how many windows are in the room (and which direction they face),
   • and whether there's a kitchen in that 21 × 35 ft area,

   I can estimate the cooling load much more closely.

   

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   What is the suggested Zone BTU size for this then?

   Based on the conditions you've described, I'd recommend sizing the zone approximately as follows:

   • Room size: 735 sq ft (21 × 35 ft)
   • Ceiling: 8 ft
   • Insulation: Below average
   • Summer temperatures: Up to 110°F
   • Occupancy: Frequent parties with many people
   • Likely open-plan living space

   Suggested zone size: 30,000 BTU/hr

   Here's why:

   • 18,000 BTU: Too small.
   • 24,000 BTU: Could work in a well-insulated home with average occupancy, but it will likely struggle on 110°F days with lots of people.
   • 30,000 BTU: A better fit for the extra heat load from poor insulation, extreme outdoor temperatures, and higher occupancy.
   • 36,000 BTU: Consider only if the space has very large west-facing windows, extensive glass, or other unusually high heat gains.

   For a Mitsubishi system, a 30,000 BTU zone would generally provide a good balance of capacity and efficiency under the conditions you've described.

   One additional point: if this is a multi-zone Mitsubishi system, check the outdoor unit's capacity. The total capacity of all indoor units and how they're expected to operate simultaneously matters. It's common for the combined indoor unit ratings to exceed the outdoor unit's nominal capacity, but the amount of oversubscription should follow Mitsubishi's design guidelines.

   If you're planning the entire house, I can also help size each zone and recommend an appropriate outdoor unit based on the full floor plan.

   

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