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I just moved into a house with a Nest thermostat and this is the first time I've ever had a thermostat of any kind (just had window units and radiators for the past 20 years).

I don't understand why I have to choose between heating and cooling modes. Isn't the point of a thermostat that I tell it what temperature I want the house to be, and it makes it happen (e.g. if I want it to be 70 and it's currently 75 then it cools, but if it's 65, then it heats)?

So, say it's a hot (85) and a cool (50) night - at 1pm I come home, and the house is 75 degrees so I set it to cool with a temperature of 70. I go to sleep, the temperature drops, and once it drops below 70 it can never go back up because it's set to "cool." So when I wake up, the house temperature is 60. Note - this hasn't happened (yet, because the temperature swings aren't that extreme), but could it?

  • my guess is the default is meant to re-enforce traditional thermostats to keep users more familiar with the modus operandi. – dandavis Nov 20 '17 at 19:44
  • Traditional thermostats have switches you must throw to select mod. I have no earthly idea why, but it's important the heating and cooling systems not work against each other, especially if they are a heat pump. – Harper Nov 21 '17 at 0:56
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You don't. There is a feature called Heat*Cool:

https://nest.com/support/article/What-is-Heat-Cool-mode

  • Ah, interesting - thank you! But just curious; given that feature, why would anyone ever want to use regular "cool" or "heat" mode (which would presumably would have the problem that I described above)? – Jer Nov 20 '17 at 19:23
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    It would be a bad thing for the AC to turn on in the middle of winter when it's covered under a couple of feet of snow ... – Stavr00 Nov 20 '17 at 19:28
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    Personally, I don't want that level of automation. If it's cool in the morning (and heat was on that night) and it gets warmer in the afternoon and I decide to open the doors, I don't want the A/C coming on. Also, for systems like a heat pump, it's not good for them to switch from A/C to heat too close together. – JPhi1618 Nov 20 '17 at 19:29
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You have to have a thermostat that has an "Auto" mode (not the same as a fan auto mode), and you'll have to set the heat/cool temperatures 3-4 degrees apart.

If there's no gap between these temps, you could end up with dueling heating/cooling systems. The heat will kick on, and warm it to 71. Then the A/C will kick on, and cool it to 69. Etcetera.

I'm not sure if Nest has this mode.

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