How to Control Ductless Mini-Split Temperature Issues

To maintain temperature, first we need to understand how a Ductless-Mini Split is controlled. Instead of a thermostat on the wall with a temperature sensor and a setpoint, a Mini-Split has unit mounted controls. The Return Air sensor (RA) is the control sensor. The RA sensor is in the return air opening of the indoor fan coil. The remote-control, wireless or wired, are not thermostats. The “Follow Me” mode on the remote control will incorporate the remote-control mounted sensor but this does not eliminate the return sensor.

On a High Wall unit, the RA sensor could be 6 to 7 feet off the floor. On a ceiling cassette the RA senor could be 8 to 12 feet off the floor. On a floor console the RA sensor could be near the floor. On a ducted unit the RA sensor is in the return and may not even be in the same space that we are controlling. Outside heat sources can also affect RA temperature such as sun light, computers, or coffee pots. Windows, or unsealed wall penetrations may introduce cold air that can affect the RA sensor readings and comfort.

Is the equipment oversized for the space? Mini-Splits are modulating by nature however they do have a minimum output. They also have a minimum run time of 5 minutes. If the equipment is too large for the space, operating for 5 minutes at the minimum heating output could overheat the space. This will be most evident on milder days.

How do we address these control issues?

First look at the clearances listed in the installation instructions for the indoor unit. If these clearances weren’t followed, recirculating air could be causing our problem. Make sure there are no obstructions to good air flow. Is this just a very large space with poor air circulation? Next place a good quality accurate probe thermometer (Not Infrared) near or on the return air sensor and monitor. If this temperature is much higher or lower than the set point temperature, we will need to figure out why. If it’s an external heat source, leaky duct work or open wall penetration we will need to correct it. On high wall units or ceiling cassettes, you may find that we are just reading much warmer air from being so high above the floor. The “Follow Me” mode may help or simply increase the set point.

Many factors can create temperature control issues. Sometimes it’s not possible to correct them reasonably. In these cases, the solution may be the 24 Volt Interface. The 24 Volt interface will allow you to use any 5-wire thermostat to control the Mini-Split system. This will control the Mini-Split more like a traditional On/Off system while maintaining some system modulation.

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