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I've noticed it over the years in various living quarters: an inside A/C temperature setting that's appropriate and comfortable for a given outside temperature is quite unbearable when the outside temperature changes (to result in a different number of A/C runs).

One may think this due to the positioning of the thermostat, but I think the problem is elsewhere — humidity.

As such, I'm curious why is it so common that air conditioning control units only have a thermostat, which so often requires such frequent adjusting when the outside temperature changes? Wouldn't it make more sense to have some sort of a humidistat instead, which would be set at a given point, and would, supposedly, require no further adjustment?

Is there any solution to this effect?

Maybe a control unit that lets you set a cooling value based on humidex, instead of °C / °F? I'd much rather set my HVAC to 27 humidex for the whole cooling season, than keep adjusting it between 21°C and 27°C, up and down, all the time.

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I can think of several reasons.

  1. A thermostat is a reliable, proven, simple, and inexpensive device which works well enough in most cases.

  2. A humidistat and its derivatives rely on a comparatively expensive relative humidity sensor plus some finicky electronics. A thermostat would still be necessary to prevent potentially dangerous results, like shutting off when the humidity is below 75% but it is still 40°C/104°F (admittedly unlikely), but also the humidity threshold may not be satisfied until the temperature is 5°C/41°F. Not so good for many living things.

  3. Humidistat technology of old has a limited range, limited responsiveness, and a short lifetime of it being accurate. Stable and reliable sensors have been around only since the 1990s.

In short, it is reasonable to expect new devices to commonly incorporate this functionality. As manufacturing volume increases, it reduces the price and consumers expect more.

  • I just wanted to point out that point 2 is not really correct -- if you set the setting to 27 humidex, it's guaranteed that the AC will always run when the temperature is above 27degC, or when below 27degC, only if the humidity is high, and below 20degC, humidex is supposedly undefined, thus it'll automatically stop being engaged, so, I don't see a problem with such scale. – cnst Jun 1 '15 at 23:37
  • Relative humidity cannot be calculated without a temperature reading, so #2 is doubly invalid (no extra cost - rh sensors always include temp, unless they are estimating humidex directly using an actual wet bulb). It should also be pointed out that #3 is not unique from #1, but an "as opposed to" for #1. IMHO, if the answer were "#1 as opposed to #3" it would be perfect. – Nathan Wiebe Aug 24 '16 at 2:15
  • @NathanWiebe: I have a sensor which does not provide temperature but semi-directly indicates relative humidity. I obtained it in 1994; here is a modern version of it. If you read that specification, you'll see it does not indicate temperature. DigiKey's price is $6.50; in 1994 its ancestor was $22! – wallyk Aug 24 '16 at 3:26
  • @cnst: See my comment above. It was how I added humidity sensing to my home automation system in 1994. I was not aware of humidex sensors or temperature/humidity. In my application, I already had outdoor temperature and wanted only to add RH. – wallyk Aug 24 '16 at 3:29
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Some newer thermostats now offer a humidity setting. So you can either choose to run your a/c to achieve a specified temperature setting or a specified humidity setting. (I have an LG window A/C unit that has a humidity setting.)

I live in an area where our humidity level is pretty constant (6%-10%) except for maybe August, so humidity is not a big factor here. I can see that in some places humidity may have a greater factor.

If a thermostat had a "feels like" setting, which took into account temperature and humidity, that would be ideal. When the humidity in our area does jump up, my wife often complains she is freezing even though the thermostat is set to the same 72°F it always was.

If you do replace your thermostat look for one that also includes a "fan circulation mode" that will run every once in a while to move the air around without turning the compressor on. This can create a dramatic cooling effect without changing the humidity level or wasting electricity.

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