When I turn on the AC to heat my living room, it usually takes a long time, but when I also boil some water in the kitchen(The rooms are connected), the living room heats very quickly.

I ran a small experiment at home to test this up. Here is what happened.

At 6:00 AM, it was -2C outside and 14.2C in my living room with 29% humidity. I turned the AC and the gas in the kitchen but didn't put the water to boil. At 7:00 AM, it was -3C outside and 19.1C in my living room with 30% humidity.

The next day at 6:00 AM, It was -5C outside and 13.9C in my living room with 29% humidity. I turned the AC and the gas in the kitchen and put some water to boil. At 7:00 AM, it was -5C outside and 21.3C in my living room with 44% humidity.

How does the humidity help increasing the temperature?


  • I didn't include the outside humidity because it's extremely inaccurate.
  • I got three temperature sensors, one in the middle of the living room and two next to the kitchen. They always show the same temperature, but just one with 0.1 accuracy.
  • I also got two humidity sensors. They stand one next to each other and are not so accurate. They can have up to 6% difference.

Related question

Why does humidity cause a feeling of hotness?, this question helps to understand why we feel warmer in a humidified environment.

  • $\begingroup$ Who turns on an AC to heat a room? $\endgroup$ – Bob D Dec 27 '20 at 16:08
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    $\begingroup$ @BobD AC referring to cooling only is a US idea. And you can buy a "window unit" or "air con" at Home Depot which does both heating and cooling. $\endgroup$ – Bill N Dec 27 '20 at 16:28
  • $\begingroup$ Yeah, my AC can do both heating and cooling. $\endgroup$ – Ilya Gazman Dec 27 '20 at 16:39
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    $\begingroup$ @BillN Then the OP is talking about a heat pump. $\endgroup$ – Bob D Dec 27 '20 at 17:07
  • $\begingroup$ How much gas do you think you used? $\endgroup$ – Chet Miller Dec 27 '20 at 18:32

There are several things happening in your situation. If you are boiling water, the steam which comes off the stove is going to have many more joules per gram due to the specific heat of vaporization of water. That energetic water vapor will mix with the air and raise its temperature faster than air passing across the air-con coils.

Also, you have two heating sources in the 2nd case: air-con and boiling water. The higher humidity is most likely not the cause of the increased temperature but is a consequence of boiling the water.

On the other hand, the human feeling of warmth or cold does depend on the local humidity. Our sensation is partly dependent on how rapidly our skin warms or cools. If the humidity is high, we tend to lose heat more slowly because of slower evaporation of skin moisture; evaporation is a cooling process for our skin because it requires heat from our bodies. High humidity slows the evaporation; low humidity allows faster evaporation.

Also, more humidity allows the atmosphere to retain thermal energy due to the relatively high specific heat of water vapor compared to N$_2$ or O$_2$. It also means that if the humidity is close to saturation, the air will take longer to warm up because the water vapor requires more energy per gram than the N$_2$ or O$_2$.

  • $\begingroup$ Note that I turned the gas on in both cases. In the first case, it heated air, and in the second, it heated water. $\endgroup$ – Ilya Gazman Dec 27 '20 at 16:50
  • $\begingroup$ Again, the large energy content of the steam combined with its large specific heat once it's distributed in the room will keep the temperature higher (you're losing energy to the outside all the time) than simply heating air. Also, the final internal room temp is 2C higher in the 2nd case compared to the first. The outisde temp is ALSO 2C higher. As I implied, the humidity is increasing alongside the temperature. You don't have a good control on the initial humidity ( high in one case, low in another), so you can't say your experiment shows that starting humidity allows more rapid heating. $\endgroup$ – Bill N Dec 27 '20 at 16:58
  • $\begingroup$ FWIW we always put a pan of water on top of the gas or wood burning heaters to increase humidity. $\endgroup$ – Adrian Howard Dec 27 '20 at 17:41
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    $\begingroup$ @IlyaGazman Okay. My mistake about the outside temp. Do you understand the concept of specific heat? Which has a higher value, nitrogen gas or water vapor? Spend some time thinking about that. $\endgroup$ – Bill N Dec 28 '20 at 3:06
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    $\begingroup$ @IlyaGazman To the outside air, from the inside air which had less water vapor than in case 2. More water vapor means the temperature will be more stable because you must lose more energy to drop temperature. Also, higher temperature air will have a larger maximum possible water content. For example, 30% relative humidity at 22C has more water vapor than 30% relative at 19C. Boiling the water puts energy and increases temp stability against heat loss. $\endgroup$ – Bill N Dec 28 '20 at 14:55

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