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Community and Q&A

Thermostat Set Point and Room Temperature

davidsmartin | Posted in Energy Efficiency and Durability on

I have two rooms, each less than 300 square feet with a 15K Fujitsu unit connected to a single outside unit.  I know that a single unit with a short duct could have handled both rooms but we wanted separate thermostats for each even if it means that the units will cycle more than they should.  Anyway, both rooms are usually 4 to 6 degrees cooler than the setting on their thermostats regardless of the outside temperature.

The outside temperature is only in the 30’s and obviously they have plenty of capacity to heat.  If it gets colder outside they will produce more heat but still not enough to get up to the thermostat setting.  They are not in “economy” mode.  I have used an infrared thermometer and determined that the temperature above the unit is much below the thermostat setting.  Somehow the units seem to decide that the room is warm enough and they don’t need to put out enough heat to warm the room.

Any ideas about why this might be happening?

Thanks for any suggestions,

Mount Holly, VT

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  1. user-5946022 | | #1

    Their internal tstat may be calibrated wrong?
    Can you just set the t-stat for 4-6 degrees higher and get the comfort and temp you want?

  2. Expert Member
    NICK KEENAN | | #2

    Is the unit running and not providing enough heat? Or is it not running even though the thermostat should be telling it to run?

  3. davidsmartin | | #3

    The green power light is on and it does produce heat, but it stops giving the heat when the temperature gets to within 4-6 degrees of the room temperature. If I set the thermostat for 76 degrees then it will usually be around 70. But I sort of doubt the problem is with the internal thermostat since it is true of both units.

    1. tommay | | #5

      If you still have the instruction manual for the t-stat, check and see if it has a set or adjustable differential setting. Use several thermometers around the house to see if they are comparable to the the temperature reading on the t-stat itself. T-stat location may have some influence, eg. is it in the sun or sunny room, on an outside wall, to near/far from the heating unit...
      Try using a fan, or ceiling fan if you got one, to mix up the air in the room to even things out and see if that has any effect. A heat supply in a small area works a lot different than say baseboards that create circulation along a greater portion of the room.

  4. walta100 | | #4

    To my ear the way most mini splits sense the rooms temp from the air moving thru the unit seems unlikely to get a good sample that seems to line up with your complaint.

    Would this remote sensor be compatible with your unit?


  5. this_page_left_blank | | #6

    Based on your description this can be completely solved by adjusting the temperature offset. Look in your manual. There's separate adjustments for heating and cooling. If it turns out that the temperature is correct sometimes but not other times, that would likely be due to the temperature being measured inside the head rather than a more central thermostat. You can get aftermarket wireless solutions for this. I have a Sensibo branded unit which works fine.

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