# Cooling down by switching the heating on?

1. Jun 30, 2009

### Count Iblis

Suppose the temperature in your room is 25 °C with 100% humidity. You are feeling very hot and sweating a lot. Most of the sweat does not evaporate on your body, it just drips onto the ground.

If you switch on the heating in your room, would you feel more comnfortable? It seems to me that by increasing the temperature by a few degrees the relative humidity would go down and the sweat would evaorate more easily. Of course, the evaporated water must be allowed to escape. So, you could open a window and sit close to the heating. If the outside temperature is 25 °C and the humidity is 100%, then close to your heating the temperature will be a bit higher but the humidity will be far lower.

2. Jun 30, 2009

### mgb_phys

If you open a window the humidity will quickly reach whatever the outside humidity is.
If you don't open a window and it's already 100% then increasing the heat isn't going to change the humidity, dropping it will at the point that water begins to condense out but then you wouldn't be too hot.

3. Jun 30, 2009

### Count Iblis

The temperature in the room is not uniform. As the outside air saturated with mosture passes the heating it heats up and its relative hunidity will be a bit lower.

4. Jun 30, 2009

### Staff: Mentor

Convection dissipates two completely separate forms of heat:

Sensible heat is heat transferred via dry convection. When air passes over your skin, it warms up, transferring heat.

Latent heat is transferred via evaporation. When air with less than 100% relative humidity passes over your skin, some moisture will evaporate into it, transferring heat.

Now, since air that starts at 25C and 100% humidity gains sensible heat (it warms up) as it passes over your skin, it will also gain latent heat, as the relative humidity drops as your skin warms it, which then allows moisture to evaporate. The total heat transferred is thus a function of the combination of latent and sensible heat of the air. If you pre-warm the air (say, to body temperature), you will increase the latent heat loss, but you will decrease the sensible heat loss to zero. The net effect will be a reduction in heat transfer.

The relationship between latent and sensible heat can be seen on the psychrometric chart: http://irc.nrc-cnrc.gc.ca/images/bsi/83-psy_E.gif [Broken]

Start with an air temperature at the bottom edge, then trace up to a relative/absolute humidity. From there, trace up and to the left along the diagonal to find the total heat content (enthalpy) of the air, along the left edge. Note that you can hold the absolute humidity fixed and increase the temperature (as you suggested) and that increases the heat content of the air.

The problem you are dealing with here is something engineers use to predict the performance of an evaporative cooling tower - a person behaves in a similar fashion. The air next to your skin will be at 37C and 100% humidity, while the air in the room with be at 25C and 100% humidity. Connect those two dots to find the sensible and latent heat difference and energy difference. Performance of a cooling tower is typically 75% of the difference between the two and depends on airflow. For a person, performance depends on airflow as well and a strong breeze will get your skin very close to the room's enthalpy.

That said, personal comfort is not just a matter of maintaining proper body temperature. If the humidity is too high, even if the overall heat transfer is sufficient to keep you cool, you'll sweat more and the sweat will linger on your skin longer, making your skin feel "clammy".

Last edited by a moderator: May 4, 2017