Does turning up the thermostat humidify or dehumidify a room?

I was told that when an A/C cools down a room, it draws moisture out of the room’s atmosphere. Does turning up the heat do the same likewise (dehumidify), or the opposite?

It dehumidifies, generally. I think, and I am not any kind of expert, that AC draws moisture out of the room to use in cooling it, and heat just dries it out.

Generally, the warmer the air is, the more moisture it can hold without any water condensing out.

In an A/C unit, the air is cooled down, can’t hold as much water and water may condensate - therefore the cooler air returned to the room contains a lower absolute amount of moisture, although the % water saturation may be higher than in the original warm air.

On the other hand, heating the room reduces the water saturation without changing the absolute amount of water in the air, because the amount of water needed to reach 100% saturation is higher.

Is it an electric or gas heater?

You could plot the dry and wet bulb temperatures of the air on a psychrometric chart to find out what the humidity is before and after.

Just to add, the water *definitely *condenses out when it passes over the cooling coils.

The absolute amount of moisture in the air is reduced. As the air is heated, it expands, increasing the pressure and causing air, along with its moisture, to leak out of the room/building. Unless you are heating an absolutely air-tight chamber.

When central heating started to become popular and affordable in the 60s, people found that their wooden furniture suffered from drying out. Panels split and glue became unstuck, so many people would put a tray of water under a radiator. Of course, those people who used the radiators to dry their washing, suffered the opposite problem.

The short answer is that it depends on whether you’re talking about absolute or relative humidity. Absolute humidity is the actual mass of water vapor in a given volume of air and that will remain more or less constant as you raise the temperature in an indoor environment. But turning up the temperature will lower the relative humidity (and vice versa). Relative humidity, not absolute, is the crucial factor in how dry or moist the air feels. The lower vapor pressure in dry air is responsible for things drying out more quickly, while higher relative humidity on a hot day is why you feel more uncomfortable than in dry air.

It’s quite different outdoors, though. There, the tendency is for relative humidity to stay somewhat constant on average, so that higher temperatures over the long term increase the absolute humidity, since most of the world is ocean from which evaporation increases. This is an important factor in climate change because water vapor is a strong greenhouse gas so it amplifies the warming.

This is true for the continental climates, here on The West Coast we see the opposite … as temperatures increase, relative humidity decreases … thus the not-as-famous-as-that-other-quote “It’s not the triple digit temperatures that get ya, but the double digit humidities” … typically when the temps pass 100ºF, RH are below 10% …

Does “turn up the thermostat” mean raising the temperature on a cold day, or lowering the temperature on a hot day? My thermostat controls both heating and cooling.

Wolfpup is right in making his distinction. Where I live, in summer, the relative humidity is always 100% at sunrise, but falls to the 40s in the afternoon, which means absolutely nothing, because the amount of moisture in the air remains constant, measured by the dewpoint.

TV weathermen rarely mention the dewpoint, but it is the only way to judge the likely comfort level on a summer day, even more relevant than the temperature. Dewpoint of 70 or less is comfortable, and over 75 is uncomfortable, no matter what the temperature is. I know that has little to do with the humidity in a room, but I mention it to put air moisture into perspective.

Raising the temperature on a cold day.

When you turn on the air conditioning, as the air cools, the water tends to condense out of it. This tends to happen where the air is cooling, i.e. inside the AC unit, and water will collect inside the unit. Different types of AC units have different ways of dealing with this water. Some drain it, and others blow it out the exhaust air, for example.

When you raise the temperature, you increase its ability to hold moisture. But, if you don’t have a source of moisture nearby, you won’t add any moisture to the air. If you have an old fashioned steam system and it leaks a bit of steam, then there’s a source of moisture. If you take a nice hot shower, there’s a source of moisture. If you tracked a bunch of snow inside, there’s a potential source of moisture as it melts and evaporates.

Baseboard water heating systems and forced air heating systems don’t inject moisture into the air when they heat.

The air in most homes in the winter is fairly low in humidity, so much so that some folks use humidifiers so that the air isn’t quit so dried out.

One noticeable effect of dry winter air is that your body can store a greater static electricity charge in dry air than it can in humid air (the charge bleeds off faster in humid air). This, combined with clothes that naturally produce more static (like wool) means that you are much more likely to get a higher zap from static in the winter than you are in the summer.

… that’s when petting cats can be dangerous …

I take exception to your claim that a dew point of 70 is comfortable. I’d put the comfort point at 55 or even 50. And I live in an area where the dew point is routinely 70 in July and August. Believe me, it ain’t comfortable.

You can think of the humidity in the air of your home like water in a glass.
To start with you have a glass that is partially full.
If you raise the temperature, the amount of water stays the same, but the glass gets bigger. So instead of being maybe 50%% full, it’s now only 25% full. There is the same amount there, but it feels drier.
Using the AC is like sticking a sponge in the glass, and wringing the sponge out in the sink. The glass is the same size, but there is less water in it. Using a dehumidifier is a version of turning on the AC.
Using a humidifier is pouring more water into the same glass you have had.

If you have a humidifier on your furnace, as you increase the temperature the relative humidity will eventually stay the same as the humidifier dumps more moisture into the air to counteract the immediate reduction in relative humidity. Therefore, the absolute amount of water in the air will increase until the relative humidity returns to the humidifier’s setpoint.

My reading after finding out that a window unit didn’t drain is that it’s now considered more energy efficient to not let it drain, but instead actually let it evaporate, creating additional cooling

Depending on the outdoor temperature, it may not be possible to raise indoor humidity much without causing problematic condensation on windows. This is especially true of windows that have blinds closed over them, which reduces air circulation next to the window and allows the interior surface of the window to become colder. The water can accumulate in pools at the bottom of the window, enabling mold growth and causing damage to wooden window frames and paint.

Some heating systems, especially forced hot air, have humidifiers to help add moisture.

Not that it’s relevant to the OP, but since discussion of both heating and cooling has been going on, mention should be made of the favorite way to condition air in desert climates: the swamp cooler. A swamp cooler uses the evaporation of water to cool the air being conditioned. Dry warm air is blown over/through a water-soaked medium; the water evaporates, resulting in a reduction in the temperature of the air. If the system employs direct evaporative cooling, the air also gets more humid. Thus, in a dry climate like the desert, a win/win result occurs.

Of course, when the relative humidity of the air in the area goes up, the efficiency of the cooler goes down. I grew up with a swamp cooler; the worst days were those right after a day of summer rain (from the annual monsoon). You’d get triple-digit temps and relatively high RH, and the swamp coolers just couldn’t get the job done. When we moved into our own house in 1972, one of the best parts of the new house was that it had “real” air conditioning.