Can anyone explain the science behind hot fan heaters drying up the air

katy123

Hi

My friends mentioned that a hot fan heater dries up the air much faster than an oil filled radiator. Why is this? This question came about as we were talking about the dyson fans.

Also, they say that cold air doesn't retain much moisture and when heated it loses more moisture. But, where does the moisture go? Eg: in winter, a house is filled with cold air from the outside which naturally has low moisture, you turn on the radiators and it heats up the air in your house, surely, the moisture is the same as when the air was cold. Can anyone please explain, thanks?

HappyMJ

The air's relative humidity figure goes down as temperature rises. The air hasn't lost any moisture. It can hold more moisture as it gets warmer so feels drier.

marleyboy

Hot air dryers reduce moisture quicker because the heating element temperature is sapping up the moisture much quicker as it fans out the air, an oil radiator radiates out heat at a much slower rate yet can be just as effective.

The key to controlling the flow of how moist or dry any room is, is ventilation.

thescouselander

It's true that cold air holds less moisture - in terms of where it goes it will condence on cold objects, like a window or a cold wall, and that is how you get condensation in your house.

In a closed room, assuming minimal ventilation, you won't lose any moisture because there's nowhere for it to go. As HappyMJ says its more to do with the relitive humidity due to hot air being able to hold more water. To be honest I wouldn't have thought the method of heating would make much difference.

katy123

HappyMJ wrote: »

The air's relative humidity figure goes down as temperature rises. The air hasn't lost any moisture. It can hold more moisture as it gets warmer so feels drier.

Thank you for your reply, as you've said, the relative humidity goes down as temp rises and the air hasn't lost any moisture, then how does it make it feel drier? Has it got anything to do with your skin losing water vapor as the warm air is sucking up water?

katy123

marleyboy wrote: »

Hot air dryers reduce moisture quicker because the heating element temperature is sapping up the moisture much quicker as it fans out the air, an oil radiator radiates out heat at a much slower rate yet can be just as effective.

The key to controlling the flow of how moist or dry any room is, is ventilation.

So, simply because a fan heater can heat up a room quicker than an oil filled radiator, it causes the relative humidity to fall a lot quicker?
But, surely, an oil radiator once it reaches the same temperature will make the room feel just as dry as the fan heater which suggest there is no difference to the dryness of the room, or is there something to do with ventilation, where because it takes longer for an oil filled heater to heat up the air, ventilation gives it enough time to replenish the moisture in the water from fresh cold air? But then again, I could say that you could wait for a fan heated room to be well ventilated to achieve the same results? All very confusing,..

[Deleted User]

katy123 wrote: »

Thank you for your reply, as you've said, the relative humidity goes down as temp rises and the air hasn't lost any moisture, then how does it make it feel drier? Has it got anything to do with your skin losing water vapor as the warm air is sucking up water?

The absolute humidity is the amount of water in a given volume of air. Relative humidity is this quantity of water divided by the maximum the air can hold at that temperature and pressure. Warmer air can hold more vapour, but heating the air won't get rid of the moisture that it already holds, so the absolute humidity stays the same and the relative humidity goes down.

The rate of evaporation, which is how you perceive dryness, is the net effect of two factors: the rate of evaporation, and the rate of re-condensation. The rate of evaporation is only dependent on the temperature, but the re-condensation is dependent on temperature, relative humidity and air pressure.

When you heat the air the evaporation rate increases, and the lower relative humidity also reduces the rate of re-condensation, so the room feels much dryer even though the absolute humidity hasn't gone down. At 100% RH by definition there is no evaporation because the air can't hold any more vapour, and if you cool the air until it reaches 100% RH this is known as the dew point, and the moisture condenses. That's how clouds form: warm air absorbs water, and then cools as it rises, until it's temperature drops below the dew point, and a cloud forms.

katy123

jack_pott wrote: »

The absolute humidity is the amount of water in a given volume of air. Relative humidity is this quantity of water divided by the maximum the air can hold at that temperature and pressure. Warmer air can hold more vapour, but heating the air won't get rid of the moisture that it already holds, so the absolute humidity stays the same and the relative humidity goes down.

The rate of evaporation, which is how you perceive dryness, is the net effect of two factors: the rate of evaporation, and the rate of re-condensation. The rate of evaporation is only dependent on the temperature, but the re-condensation is dependent on temperature, relative humidity and air pressure.

When you heat the air the evaporation rate increases, and the lower relative humidity also reduces the rate of re-condensation, so the room feels much dryer even though the absolute humidity hasn't gone down. At 100% RH by definition there is no evaporation because the air can't hold any more vapour, and if you cool the air until it reaches 100% RH this is known as the dew point, and the moisture condenses. That's how clouds form: warm air absorbs water, and then cools as it rises, until it's temperature drops below the dew point, and a cloud forms.

Having read this 7 times, I think I finally get this, many thanks.......:):):):):)

katy123

jack_pott wrote: »

The absolute humidity is the amount of water in a given volume of air. Relative humidity is this quantity of water divided by the maximum the air can hold at that temperature and pressure. Warmer air can hold more vapour, but heating the air won't get rid of the moisture that it already holds, so the absolute humidity stays the same and the relative humidity goes down.

The rate of evaporation, which is how you perceive dryness, is the net effect of two factors: the rate of evaporation, and the rate of re-condensation. The rate of evaporation is only dependent on the temperature, but the re-condensation is dependent on temperature, relative humidity and air pressure.

When you heat the air the evaporation rate increases, and the lower relative humidity also reduces the rate of re-condensation, so the room feels much dryer even though the absolute humidity hasn't gone down. At 100% RH by definition there is no evaporation because the air can't hold any more vapour, and if you cool the air until it reaches 100% RH this is known as the dew point, and the moisture condenses. That's how clouds form: warm air absorbs water, and then cools as it rises, until it's temperature drops below the dew point, and a cloud forms.

Oh, Jack, I forgot to ask, so does it make any difference in terms o dryness when using an oil filled radiator vs fan heated one? Is it a myth?

[Deleted User]

I can't think of any reason why the method of heating would make any difference per se, but if you stand in the moving air from the fan water will evaporate faster, in the same way that your clothes dry faster on a windy day.

changed_username

Just to throw a spanner in the works I'll state that not all oil filled radiators release heat at the same rate.

The cheap model radiators take longer to heat but if you look at the top quality radiators they have many features which can heat the air faster.

These include a chimney effect which pushes hot air out of the radiator faster as well as advanced fin designs for more efficient release of hot air.

That said fan heaters release hot air near instantanously so will heat an area quicker than an oil filled radiator.

With radiators, like many things, you get what you pay for.