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Community and Q&A

Cooling with a Ductless Minisplit

kieran973 | Posted in General Questions on

Hi, I had two ductless Fujitsu wall units installed last December, both on the first floor of our two-story house – one 15K btu unit in our dining room, one 9k btu unit in our family room. What is the best way to use these units for air-conditioning, both in terms of efficiency and long-term maintenance? On hot days, the 15K unit does the air-conditioning for most of the first floor, but I’ve been turning it completely off at night when going upstairs to bed because it just seems like a waste of energy to air-condition the empty first floor overnight. The unit is set to 70F (in “cool” mode, fan speed on high) from about 7 AM – 11 PM, then turned off for the night. When we come down the next morning, the temp on the wall thermostat is between 72-74F, though it’s noticeably more humid (but not severely humid). Should I be turning the unit off at night? Should I just “set it and forget it” at 70F 24/7? Should I set the temperature to 75-77F or so at night but not turn the unit completely off? Does turning a mini-split on and off everyday accelerate long-term wear and tear? Thanks.

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  1. brp_nh | | #1

    I'm not a HVAC expert and I don't have data, but we do use a mini split for cooling in the summer. We're in Zone 6, so our AC use is mostly during a few heat waves for a few days or a week at a time, then we open the house back up when we can.

    I would recommend "set it and forget it" and just put the fan on auto. We just put our Mitsubishi unit on 70F, auto fan, and leave it.

    If you want to save some energy during times of peak demand, you could bump it up to 72F (for example), but don't constantly shut it off overnight or bump the set temp way up. You're just making the mini split work harder when you turn it back on or when you go back down to 70F. When you shut it off or turn the set temp above the room temp, you're also losing the dehumidifying part of the AC, which is why your humidity spikes.

    If you do turn the cool mode off when you don't need AC, I believe it's good to run it in just the fan mode for a bit (an hour or two?) to dry any condensation off the coils. You might see something in the manual about this.

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