Can heaters really keep a loosely enclosed space dry? humidity theory

[kaon]

Hi all,

There seems to be a common practice of using simple heat sources for dehumidification. (for tropical climates at least, I guess excess moisture is less of a problem in temperate climates)

Eg. fixing an always-on lamp inside wardrobes, and various cabinets which are non-airtight. For protecting clothing or electrical equipment from moisture damage.

http://img206.imageshack.us/img206/303/heaterhumidityxd5.png

I can imagine that when the lamp is first turned on, the interior temperature increases until the heat loss rate equals the lamp power. Relative humidity drops accordingly until the temperature stabilizes. At this stage, this should not be controversial.
But I do not expect the low-RH to be sustainable, given that the cabinet is not air-tight or moisture tight. My intuition is that moisture would migrate into the heated cabinet until the RH is equalized inside and out. Is this correct?

Another possibility is that water vapor tends to spread so that absolute humidity is equalized...

So what is the relevant theory of humidity?

TIA

(I am not talking about electrical "dry-boxes" marketed for storing cameras and lenses, those I'm sure have a way of pumping out moisture, and those are practically air-tight. I am talking about a low-tech heater-in-cabinet arrangement.)

[Carnak]

leave the closet door open or undercut it and draw some return from it

[Carnak]

heat lowers RH but does not change the amount of moisture in the air.

heat makes mositure evaporate out of materials

[teddy bear]

The answer to the question is that the relative humidity in the box is 45%RH. Provided there is no additional liquid moisture in the box. With a liquid water source in the box, the %RH of the air in the box will rise. If the box has a perfect vapor barrier, the %RH will rise to 100%RH or until the liquid is evaporated. Air leakage or marginal vapor retarder allows moisture to move to the outside which will reduce the %rh of the air in the box. Regards TB

[Hrdworkingacguy]

warmer air can hold more moisture...So if you have 95% rh at 25c if you increase the temp to 40c the airs ability to hold moisture goes up...The %rh will be less with the same amount of moisture in the air...:eek:

[Mikeylikesit]

kinda like the light bulb in the dead refrigerator we use to store welding rods?



however, the refer is mostly airtight, so that may not be apples to apples.

[kaon]

Hrdworkingacguy and teddy bear have roughly the same idea.
The RH inside drops initially that's for sure.
But suppose the cabinet is only loosely covered, as suggested in the diagram, and we leave it undisturbed with the heater on, for a few days/months until we reach steady state. We assume there is no moisture source inside.

In case the diagram doesn't display for you, the example figures I used are:
OUTSIDE:
temp: 25 degrees C
RH: 95%

INSIDE:
temp: 40 degrees C
RH: ???

I think that IF water vapor tends to diffuse across temperature differentials so as to equalize RH, then inside RH eventually increases to 95% as well.

Any thoughts?

[Carnak]

vpaour pressure is what makes water vapour diffuse. The higher the temperature, the higher the vapour pressure

moisture migrates away from heat and towards the cold

Rain gets a brick wall wet. Then the sun comes out, beats on the wall and drives the mositure through the brickl

[teddy bear]

Hrdworkingacguy and teddy bear have roughly the same idea.
The RH inside drops initially that's for sure.
But suppose the cabinet is only loosely covered, as suggested in the diagram, and we leave it undisturbed with the heater on, for a few days/months until we reach steady state. We assume there is no moisture source inside.

In case the diagram doesn't display for you, the example figures I used are:
OUTSIDE:
temp: 25 degrees C
RH: 95%

INSIDE:
temp: 40 degrees C
RH: ???

I think that IF water vapor tends to diffuse across temperature differentials so as to equalize RH, then inside RH eventually increases to 95% as well.

Any thoughts?
The grains of moisture per pound of air are the same on inside and outside of the enclosure.
TB

[kaon]

The grains of moisture per pound of air are the same on inside and outside of the enclosure.
TB
Thanks, that answers my question.

If it is true that water vapor tends to spread so as to equalize absolute humidity (by mass) across connected air masses at different temperatures, then the loosely-closed and heated cabinet would maintain a stable state of lower RH inside.

[jogas]

Different situation, but when I rebuilt my harley, frame up, I had the frame sandblasted. I have a heated/insulated standalone garage and the frame stayed flash-rust free for the 2 weeks it took for my powdercoater guy to be ready to coat it.
I also rebuilt the Evo engine the same way, with no rust.
I kept the garage at 70 degrees the whole winter.
I did tighten up the garage as much as possible.
But my Modine heater was the real dehumidifier when it came to rust prevention.
jogas

[Carnak]

dry winter air infiltrating was your dehumidifier

[teddy bear]

Different situation, but when I rebuilt my harley, frame up, I had the frame sandblasted. I have a heated/insulated standalone garage and the frame stayed flash-rust free for the 2 weeks it took for my powdercoater guy to be ready to coat it.
I also rebuilt the Evo engine the same way, with no rust.
I kept the garage at 70 degrees the whole winter.
I did tighten up the garage as much as possible.
But my Modine heater was the real dehumidifier when it came to rust prevention.
jogas

Add a good dehumidifier for the same rust-free storage for the summer. Maintain <50%RH to eliminate potiential summer rusting or musty odors of quality items. To test the humidity, get a good %RH meter. Accurite at wal-mart or Therma-Stor <$15. RegardsTB

[BigJon3475]

I like reading these articles if your interested.

http://www.buildingscience.com/documents/digests/bsd-138-moisture-and-materials/?topic=/doctypes/digest

This is the base page for that document but if you have time search through there lot's of neat info.

http://www.buildingscience.com/doctypes/digest/
http://www.buildingscience.com/buildingphysics/moisturecontrol

[jogas]

dry winter air infiltrating was your dehumidifier

Winter air (10* OAT) is not "dry" until it is heated to room temperature (70*)..... psychometrically speaking.
jogas

[Carnak]

Winter air (10* OAT) is not "dry" until it is heated to room temperature (70*)..... psychometrically speaking.
jogas


Considering the fact that it was a heated garage, pyschrometrically speaking it is the infiltration that is keeping the humidity down.

If you were in their breathing like a horse with the heater off, infiltration would still lower the RH

[jogas]

Considering the fact that it was a heated garage, pyschrometrically speaking it is the infiltration that is keeping the humidity down.

If you were in their breathing like a horse with the heater off, infiltration would still lower the RH

My point was: winter (cold) air is not dry until heated.
jogas

[jeffk91467]

Here in Hawaii, almost all the chilled water control valve actuators I see have tropical heaters in them. Some VFD's have them here. The RH here is usually about 60% year around during the day.
I think the heaters are mainly to increase the temp of the enclosure so you are above the dew point.
I can see the chilled water actuators would probably fill up with condensate since the lower part of the valve near the pipe is somewhat cold. So the heater probably does a good job protecting the electronics.
For the VFD's, it probably is only necessary for an outside installation. The dewpoint for 30 btu/lb enthalpy (60% RH @ 78 deg F daytime) would be 65 deg F at night. So you can get some condensation if the VFD is mounted outside, where it gets cool at night.
We did a job last summer, and the electrician ran the 3-phase wiring inside the VFD a couple inches right above the tropical heater. (Next to the big yellow sticker saying DO NOT RUN WIRES NEXT TO HEATER). After a couple months, the insulation on the wires finally melted off and shorted out the 460. You can guess what happened to the drive.

[Carnak]

My point was: winter (cold) air is not dry until heated.
jogas


the amount of moisture 'held' in cold air is low, does not matter what its RH is, is the point

[Shophound]

Hrdworkingacguy and teddy bear have roughly the same idea.
The RH inside drops initially that's for sure.
But suppose the cabinet is only loosely covered, as suggested in the diagram, and we leave it undisturbed with the heater on, for a few days/months until we reach steady state. We assume there is no moisture source inside.

In case the diagram doesn't display for you, the example figures I used are:
OUTSIDE:
temp: 25 degrees C
RH: 95%

INSIDE:
temp: 40 degrees C
RH: ???

I think that IF water vapor tends to diffuse across temperature differentials so as to equalize RH, then inside RH eventually increases to 95% as well.

Any thoughts?

Box at 40 degrees C will be around 41% RH.

I agree with Carnak...vapor pressure will be elevated inside warmer box than environs surrounding box. In reality, if box is not airtight, warmer air will tend to exit box near top and make up displaced air from the bottom. Effect over time would be reduction of relative humidity in box not only due to increase in temperature but due to lower moisture content of infiltrating air (provided box had higher grains of moisture content than surrounding environs initially...otherwise if grains of moisture of box and surrounding air was the same, and air exchange is continuous, relative humidity drop is only due to temperature increase. Grains of moisture remain unchanged).

[jogas]

the amount of moisture 'held' in cold air is low, does not matter what its RH is, is the point

Here's my thinking:
As air temp drops, it shrinks in volume, it's RH goes up (Discharge air off a DX evaporator is 100% RH, otherwise we wouldn't need a condensate drain).
Right, now say it's 20* and 79% RH outside and 72* 20% RH in my house. That's because when the air is heated it's volume increases. There is less pounds of air in my house at 72* than 20*. Grains can be equal, but if volume /pound of air is increased, then there is less pounds of air in my house, and net moisture in my house is also reduced.
I could be wrong. If so, please enlighten me.
jogas

[Shophound]

Here's my thinking:
As air temp drops, it shrinks in volume, it's RH goes up (Discharge air off a DX evaporator is 100&#37; RH, otherwise we wouldn't need a condensate drain).

I could be wrong. If so, please enlighten me.
jogas

You're on the right track. Just a few things:

Supply air off a cooling coil isn't exactly at 100% relative humidity. Bear in mind coil bypass factor. Not all air that passes through the coil is cooled or dehumidified. It is more common for me to see supply %RH running in the high eighties/low nineties.

The coil surface temperature, if at or below the dew point of the air passing over the coil, will cause condensation of moisture from this air. The air that contacts the coil surface will experience relative humidities of 100% and be in a saturated state. The air that bypasses the coil unaffected will mix with the saturated air exiting the coil, lowering the overall relative humidity in the supply plenum.


Right, now say it's 20* and 79% RH outside and 72* 20% RH in my house. That's because when the air is heated it's volume increases. There is less pounds of air in my house at 72* than 20*. Grains can be equal, but if volume /pound of air is increased, then there is less pounds of air in my house, and net moisture in my house is also reduced.As air warms, it expands, increasing in volume as you state. If the amount of moisture in a given sample of air does not change throughout the warm-up process, the relative humidity will decrease while the grains of moisture for the given sample of air will remain the same.

A pound of air is a pound of air. At a given temperature it exerts a given pressure on its environs, for the volume it occupies. Increasing the temperature of this pound of air increases its volume. If a vessel containing this pound of heated air is airtight, there will be no gain or loss of moisture vapor within the air, only an increase of pressure.

If the vessel is the typical average construction of an American house, the pressure exerted by heated air within the structure seeks escape paths to the outdoors, where the air is colder and more dense. At the same time, air that is escaping the residence will be replaced by air from outdoors due to not only wind blowing against the structure, but a pressure difference created by the escaping air requiring make-up air to achieve a relative air pressure balance.

This is where the "stack effect" comes in...air as it warms becomes more buoyant and rises, increasing in volume as it does so. The lower regions of the house are slightly depressurized from the rising air, which creates a pressure imbalance between the indoor lower region air and air outside the structure. Infiltration occurs to make up this pressure difference.

How a house "loses" moisture in winter occurs primarily through infiltration. The moisture content of the cold outdoor air is low, the air being pushed out of the house due to heating/stack effect carries with it any interior generated moisture with it. If supplemental humidity is not added to the indoor air, it will eventually become equal to outdoor air in terms of grains of moisture per pound of dry air, albeit the relative humidity levels will differ.

So...I'd say in the end the relative humidity drops due to expansion of the air volume. The make-up air having an overall lower moisture content will reduce the grains of moisture inside the house until it is equal with the outdoor grain level, unless supplemental interior humidity is generated.

[kcwong]

The lamp just decrease the RH with the moisture content remains the same. The RH is indicating distance to the dew point (100% RH). If you lower the RH, the condensate will not easily appear. The lamp is used for condensate prevention.

[Carnak]

Here's my thinking:
As air temp drops, it shrinks in volume, it's RH goes up (Discharge air off a DX evaporator is 100&#37; RH, otherwise we wouldn't need a condensate drain).
Right, now say it's 20* and 79% RH outside and 72* 20% RH in my house. That's because when the air is heated it's volume increases. There is less pounds of air in my house at 72* than 20*. Grains can be equal, but if volume /pound of air is increased, then there is less pounds of air in my house, and net moisture in my house is also reduced.
I could be wrong. If so, please enlighten me.
jogas

Winter and summer there will not be a big difference in the pounds of dry air in your house

Maybe look at your house volume and see how many pounds of dry air and how many pounds of moisture are in there in the summer and winter. Think about it winter you may be 68 to 72F, summer 75 to 78, not a huge change.

Sensibly heating air lowers RH but does not change the moisture content.

Adding heat and lowering RH does not remove moisture from the air, but it does encourage solids to dry out

[jogas]

Adding heat and lowering RH does not remove moisture from the air, but it does encourage solids to dry out

And that is the answer/confirmation to the starting post of this thread.
jogas