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Minisplit short cycles only in heating mode

_matt_p | Posted in General Questions on

Hi, I had a Fujitsu 15RLS3 mini-split heatpump installed for the first floor of our house (~1000 sqf and fairly open floor) in March of this year. During the heating season, even with fairly low outdoor temperature, such as high 30s or low 40s, the unit would short cycle tremendously, sometimes at intervals of only a few minutes. It did not seem to reduce inverter speed much when approaching the temp set point, but just shut off at high speed, rest for a few minutes and start running at high speed again, etc It would also use much more electricity than I would expect, I guess due to ramping up and down all the time… I had a rep from the installer come out, he did not seem to understand the concept of short cycling and said there is nothing wrong with the heatpump. During the cooling season that just started, the unit modulates nicely and pretty much runs continuously at low speeds as soon as the temperature differential (between inside and outside) is at least approx. 10-15 degrees. The latter observation makes me think the problem is not over-sizing per se. My guess is that the problem lies in heat distribution that affects the sensor differently than distribution of the cold air. The unit is installed about 1.5 feet below the ceiling and maybe 10 inches away from one corner of the room (there was not an obvious better location). I was told the sensor is on the side of the unit facing the corner of the room. Is it possible that the unit quickly puts out so much heat when it is cold that the hot air accumulates fast in the “pocket” where the sensor is located that it shuts off way too early, then after a few minutes when the hot air has dissipated, it ramps up again quickly and this repeats itself? If so, my guess is that only an external temperature sensor would help? If anyone has had similar experiences or has any ideas of what might be causing the short cycling in the winter, I would appreciate a reply. Thank you – Matthias.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matthias,
    Lots of people have reported the problem you describe. The usual solution is to install a remote (external) thermostat, which isn't cheap.

    Here are links to three previous Q&A threads on the issue:

    Fujitsu wired thermostat dilemma

    Remote thermostat for Fujitsu mini-split?

    Mitsubishi mini split behaving very differently with external thermostat vs. without

  2. STEPHEN SHEEHY | | #2

    Matthias- have you tried orienting the vanes away from the corner? That might help.

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