Why do heaters and air conditioners "dry" the air?

And why in winter it is so important to have a full-fledged humidifier with a tank of at least 5 liters in the house

Graph 1 shows how quickly the relative humidity of air drops when heated from -10°C (blue curve) and from 0°C (orange curve) to room temperature. Note that in the first case, the discomfort zone for humidity is reached already at +10°C, while in the second case, it is reached at +20°C
Graph 1 shows how quickly the relative humidity of air drops when heated from -10°C (blue curve) and from 0°C (orange curve) to room temperature. Note that in the first case, the discomfort zone for humidity is reached already at +10°C, while in the second case, it is reached at +20°C

I will reveal to you the main secret right away, with the rise in air temperature, its relative humidity drops very quickly

And with low relative humidity, the ability of the air to "absorb" water vapor sharply increases. As a result, your skin and mucous membranes lose more moisture through evaporation, dry out, weaken their protective properties.
The opposite is also true, with the cooling of the air, the relative humidity quickly increases, evaporation of moisture from the skin is hindered, the cardiovascular system bears an additional load, mold grows vigorously in the corners of the house and fills the air with such a wonderful allergen as spores.

More about health and humidity - in the article "How air humidity affects your health". Here we will try to assess the scale of the disaster with the microclimate in the house, which can be caused by a common, conscientiously working heating (or air conditioning) system

Graph 2 shows how rapidly the air's capacity to absorb moisture increases with temperature in absolute terms
Graph 2 shows how rapidly the air's capacity to absorb moisture increases with temperature in absolute terms

More on topic

First a little theory

It is important to consider the relative humidity of the air to assess the quality of the indoor microclimate. It is measured in % and indicates the air's ability to absorb additional moisture. For example, if the air humidity is 20%, it means the air is very dry and can absorb a lot more water vapor (80% in addition to what is already present). At 20% humidity, wet clothes will dry very quickly, and the majority of people will experience respiratory problems. At 100% humidity, wet laundry will never dry, and in a warm enough climate, mold will also start to grow.

What is considered optimal humidity?

For most people, paper books, paintings, wooden furniture, musical instruments, the optimal humidity is considered to be between 40% and 60%.
BUT! In practice, air humidity is a very individual concept. Some people are comfortable at 30%, while for others, 60% would be too dry. Unfortunately, understanding comes only through trial and error in each specific case.

EXTREMELY IMPORTANT and fundamental to understanding the mechanism of microclimate formation in the home, I want to emphasize this once again: as the temperature rises, the relative humidity of the air drops very quickly.

Graph 3 illustrates the required mass of water vapor (in grams per cubic meter of air) that must be added during heating to maintain relative humidity at the comfort level of 50%
Graph 3 illustrates the required mass of water vapor (in grams per cubic meter of air) that must be added during heating to maintain relative humidity at the comfort level of 50%

I won’t tire you with calculations, it’s easier to show on the picture:

  • In graph 1, you can see how quickly the relative humidity of the air drops when heated from −10°C (blue curve) and from 0°C (orange curve) to room temperature. Note that in the first case, the discomfort zone due to humidity occurs already at +10°C, and in the second case — at +20°C
  • Graph 2 shows how quickly the air’s ability to “absorb” moisture increases with temperature in absolute terms
  • Graph 3 — how many grams of water vapor need to be added per m3 of air when heating to maintain humidity in the comfort zone (50%)
  • Finally, graph 4 — the water consumption by the air humidifier that is minimally necessary to maintain a comfortable microclimate in the bedroom (50% humidity) at outside temperatures of 0°C and −10°C.

Note that even at a bedroom temperature of 20°C, the minimum comfort requires a humidifier water consumption of at least 0.75 liters per hour!

Graph 4 illustrates the minimum water usage rate of an air humidifier needed to sustain a comfortable indoor climate (50% relative humidity) in a bedroom, with external temperatures of 0°C and -10°C
Graph 4 illustrates the minimum water usage rate of an air humidifier needed to sustain a comfortable indoor climate (50% relative humidity) in a bedroom, with external temperatures of 0°C and -10°C

As you can see, aquariums, decorative fountains, indoor plants, wet towels, and other popular remedies will not help you maintain comfortable humidity indoors during winter. Only a full-fledged and sufficiently powerful air humidifier with a 5-6 liter tank can do that. (8 hours of sleep X 0.75 liters per hour = 6 liters per night).

The calculation provided, however, does not accurately reflect the real situation in most bedrooms.
It all comes down to ventilation. In our calculation, we assumed that the bedroom meets the minimum ventilation standard - 30 m3 per person per hour (60 m3 for two people). In practice, in most modern apartments, this standard is not even close to being met. Even with windows open for ventilation, the ventilation rates are almost always significantly worse than the standards, and when windows are closed - by tens of times.
With such a setup, humidity is closer to the optimum, with tightly closed windows, two adults can easily "breathe in" 50% or even more, but the CO2 concentration skyrockets.
Do you feel weak and averse to any kind of activity in the mornings? It is quite possible that the issue lies in poor ventilation.

But that's a whole different story. Read about how to properly organize ventilation in a smart home in a separate article.

Find out what to look for when choosing an air humidifier in the article "What to Look for When Choosing an Air Humidifier for a Smart Home".

PS.

In a warm and humid climate, everything happens in reverse. When the air conditioner is running, the hot and humid air from outside is cooled, which increases the relative humidity up to 100%. Therefore, to maintain minimally comfortable conditions indoors, excess moisture needs to be removed. The count here is also in buckets, but fortunately, modern split systems handle this task perfectly. You may have noticed that on a hot day, condensate drips from the air conditioner through a special pipe. Under certain circumstances, you can even overdo it and drive the humidity in the air-conditioned room into an uncomfortably dry zone. In my opinion, the best solution is, once again, a Smart Home, where air humidity is automatically controlled in real-time, and all the home's climate systems are managed from a single center with feedback including humidity.