Hello,

 

This is probably very basic to most here, but I was wondering if someone could explain to me why/how running a furnace or heater causes humidity to drop in a room.

 

Running an air conditioner with the cold evaporator coil on the inside of a room causes water to condense on the coil and run out the back to the outside, lowering the humidity in a room. I would have guessed that a furnace would have the opposite effect on humidity. But instead, it too seems to lower humidity.

How does relative humidity vary with temperature?

Air at a higher temp holds more moisture. But what i'm referring to is, say, a room that is held at constant temperature with a thermostat - say 75F. The temperature in the room stays roughly constant with a furnace running on a cold day, but the humidity will decrease due to the furnace. Why, and where does the water go?

the absolute humidity doesn't change, it is only the relative humidity that does.

 

with the example of the refridgeration coil, the relative humidity increases to 100% then when condensation happens and water is removed from the air you get a decrease in absolute humidity. but there isn't such a decrease with a heater.

If absolute humidity doesn't change & relative humidity does, that would require the temperature to change. But suppose a room started out at 40% relative humidity and 75F. It's an extremely cold day and a furnace runs almost constantly to keep the temp at 75F. Is it fair to say that so long as the temp stays at about 75F all day, the relative humidity shouldn't decrease any by the end of the day?

 

To put this in a little more perspective, a friend and myself were having a discussion about this and he said that furnaces dry the air out (lower relative humidity) if they are run for a long time in a house. This doesn't really make sense to me and i'm trying to understand what the mechanism of action would be if that's correct. Where would the water go if the furnace was condensing it somehow? And why would the heat from a furnace cause it to condense rather than further evaporate?

In a closed system, nothing would change. But you generally don't have a closed system. Cool air enters and gets heated up, while some warm air escapes. The cool air contains little moisture, so it has a low RH by the time it reaches the target temperature. It's also possible you can get condensation in cold parts of the house, taking water out of the system.

and the fact that the air outside has a much lower water content means that the water vapour is going to tend to diffuse out.

Thanks that makes more sense to me. It sounds like the heat from the furnace really has nothing to do with relative humidity dropping during the winter when the furnace is turned on - it's the blower on the furnace that is mostly to blame, causing dry air from outside to diffuse into the house more quickly through cracks.

On a very cold day, the air outside may feel moist (that's the relative one), but it's actually quite dry (on an absolute scale).

 

If you have a fire inside, assuming your chimney works, the exhaust gases leave the house. That means that fresh air enters the house somewhere. This fresh air might have a relative humidity at 0 deg C, but at 20 deg C, it's quite dry.

 

In case you thought that the fire produces water (it does!), that water will leave through the chimney. If any significant amount of gases from the fire would enter your house, you'd be in trouble.