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pstu
01-13-2005, 07:20 PM
Please help me fill in a gap in my understanding -- does a gas forced-air furnace actually work to dramatically reduce humidity? Or is there something else that makes it seem so?

What I observe is that any few days when the heat is on, the house humidity goes down dramatically. Finally I understand that cold outside air contains far less grains of moisture, even when saturated (didn't used to know that). Before I always figured the heater drove out the humidity somehow, but then I asked myself "well where does it go?" -- and couldn't think of an answer.

You could really help me if you could explain the situation. Like most things, I expect it to be absurdly simple when someone clues me in on what's actually happening.

Carnak
01-13-2005, 07:36 PM
As you suspect, when it gets cold enough to require heat the amount of moisture held in the outside air gets low. Winter air holds little moisture.

In my hometown, customers used to refer to gas heat as the 'dry heat'. Gas heated homes were always 'dryer' than ones with electric heat for example.

This was because they had older gas furnaces that used a pilot light and had a drafthood. Significant air would go up that chimney 24/7, and these chimneys ended up 'naturally ventilating' a home. Air going up the chimney, would cause cold air to infiltrate into the homes all the time. This would dry out the home.

Xavier
01-14-2005, 12:35 PM
STUDENT, below is an explanation and chart that will help you understand the RH in your home. It applies to both heating and cooling, but this explanation and chart is for heating.

Here is another analogy that will help to explain Relative Humidity. Lets take a balloon and blow it up. Now lets assume the temperature of the air in the balloon is 70 degrees F with 40% Relative Humidity. Now if we place that balloon in the refrigerator and cool the temperature to 45 degrees TWO thing happen: First the volume of the balloon will be smaller and second the RH will increase to 100%! However, the amount of water in the balloon is still the same!

Now place the balloon in the oven and heat it to 80 degrees F. The size of the balloon will increase and the RH will decrease to 29%! What this explains is there is an inverse relation to temperature and humidity. Temperature goes up RH goes down, Temperature goes down RH goes up!

Here is a chart from Skuttle showing the affect of heating outside air! For example if the outside air is 10 degrees F and has an RH of 70% heating that air to 72 degrees F will increase the volume and reduce the RH to 6%. This 6% RH air mixes with the air in the home to reduce the overall RH and "dry" out the home!

http://www.skuttle.com/humid.html

UNCONTROLLED OR TOO MANY INDOOR AIR CHANGES IS THE REASON HOMES ARE DRY, NOT HEATING THE INSIDE AIR, AS THE AMOUNT OF WATER REMAINS THE SAME! This is why I recommend providing outside combustion air to the home. In addition, this is why I recommend opening a window near the dryer when you operate it. This will reduce the amount of "dry" outside air that is sucked/pushed in the home!

Hope this more detailed explanation with data helps!

right way
01-15-2005, 06:23 AM
Wow! Student If you weren't confused before, you are now.

Simply heat evaporates moisture. The circulation of air thruogh a forced air furnace cycles the same air to be evaporated again and again, until the moisture content in the inside air is nil.

As for where does it go? Your house isn't a balloon, there are things in your home that can absorb moisture and slows its dilivery back into the atomosphere.

Then to go to the balloon analogy. Actually thats wrong too.
You understand that moisture in the air is nothing but water in the process of evaporating and evoporating is nothing but a liquid in the process of changing states.
From liquid to gas. At which waters releases into the gases hydrogen and oxygen. (Hence H2O)
Now if the Balloon is hermetically sealed and the air content in the balloon at the initial reading is 70 deg. with relative humidity of 40%. Then when the Temp. inside the balloon drops to 0 Deg. the trapped moisture content then would freeze. Dropping the airborn water molecules to the exterior surface of the balloon. Thus reducing the airborn molecules represented in the humidity content.
(again changing states into a solid)

Once we reheat the air inside back to 70 Deg. the evaporation process begins. Yet lacking the air movement to speed the process along your humidity might increase slightly compared to the frozen reading. Without the moving air to carry the the water molecules the moisture would again sit on the outer interior of the balloon surface in the form of water droplets again not being in the airborn state required for humidity readings.

I hope this helps you out.

Carnak
01-15-2005, 11:14 AM
Actually for a change X's analogy was not bad except in the balloon the pressure is changing all the time and the pressures in the balloon are above atmospheric.

With the same amount of water in a fixed amount of air, the relative humidity will increase as the air cools and decrease when heated, but the pressures are all out of whack and his percentages would be off. The precentages would still increase and decrease.

The atmosphere is a mixture of gases including water vapour. Each individual gas, exerts its own partial pressure. The atmospheric pressure is the sum of these individual partial pressures.

Increasing temperature increases the vapour pressure of water. At 100% RH, the partial pressure exerted in the atmosphere by water is equal to its vapour pressure for that temperature. It is a finite limit of how much water can be 'held' in the air set by the atmospheric pressure. A formula for RH is the ratio of the partial pressure of water vapour in the atmosphere to its saturated vapour pressure for that temperature.

If you were out camping and the camp fire was out, a pot of water would slowly evaporate away, and the rate of evaporation is driven by its vapour pressure 'forcing' its way into the total atmospheric pressure.

If you light that campfire its a different story. When water is heated to its boiling point, its vapour pressure equals atmospheric pressure and it can rapidly boil away. It is not 'held back' by the atmospheric pressure any longer.

Now when you compare a city at sea level to Denver (Mile high) and the Dead Sea (below sea level), the atmospheric pressure changes but that is another story.

[Edited by Carnak on 01-15-2005 at 11:20 AM]

pstu
01-15-2005, 01:57 PM
Thanks to everybody who has offered help so far, and to any people who might add some messages in the future.

Xavier's advice fit my questions reasonably well, I understood the balloon analogy would work OK so long as I didn't try to map every little detail onto it. So I overlooked the pressure aspect which I am sure Xavier meant me to do. Ditto the "Relative Humidity Conversion Chart" since its max temperature was 35 degrees. Remember I am in S.Texas and for us 40-50 degrees is more our typical winter range.

Since I got a psychrometrics mini-app (converter software), have plugged in some outdoor temp and humidity values most appropriate to our weather and it is eye-opening. Your messages that the winter dry house is *all* about psychrometrics tells me a lot.

Best wishes -- P.Student

Carnak
01-15-2005, 06:06 PM
Originally posted by right way
Wow! Student If you weren't confused before, you are now.

Simply heat evaporates moisture. The circulation of air thruogh a forced air furnace cycles the same air to be evaporated again and again, until the moisture content in the inside air is nil.

As for where does it go? Your house isn't a balloon, there are things in your home that can absorb moisture and slows its dilivery back into the atomosphere.

Then to go to the balloon analogy. Actually thats wrong too.
You understand that moisture in the air is nothing but water in the process of evaporating and evoporating is nothing but a liquid in the process of changing states.
From liquid to gas. At which waters releases into the gases hydrogen and oxygen. (Hence H2O)
Now if the Balloon is hermetically sealed and the air content in the balloon at the initial reading is 70 deg. with relative humidity of 40%. Then when the Temp. inside the balloon drops to 0 Deg. the trapped moisture content then would freeze. Dropping the airborn water molecules to the exterior surface of the balloon. Thus reducing the airborn molecules represented in the humidity content.
(again changing states into a solid)

Once we reheat the air inside back to 70 Deg. the evaporation process begins. Yet lacking the air movement to speed the process along your humidity might increase slightly compared to the frozen reading. Without the moving air to carry the the water molecules the moisture would again sit on the outer interior of the balloon surface in the form of water droplets again not being in the airborn state required for humidity readings.

I hope this helps you out.

you must be informedhomeowner's evil twin

Xavier
01-17-2005, 09:00 AM
Student thanks again for your comments and I am glad you understand my explanation. Too often posts do not answer the question, they only respond to the words. You are correct I did not address the affect of pressure on the balloon so not to confuse the main reason homes are dry in the winter.

As you know in every class, there is always a student who must raise their hand and attempt to show what they know or do not know. I could have used a very large trash bag and only fill it a little with inside air for my analogy, but it would not have been as dramatic. Then when it was heated or cooled there would be no significant affect of pressure, so it can be ignored, like one of the posters who always attempt to show us, what he does not understand (the question)!

The Skuttle chart clearly shows the affect of air changes on RH in homes, not the heating and re-heating of the same inside air. If this was true, would cooling and re-cooling the inside air raise the RH in the summer?

Student, continue your research, use your equipment and some of the links provided to understand and control the RH in your home summer and winter and remember to be KISS. I still disagree with one of your comments that it is more difficult to control IAQ in the South then North, but a different topic.

Finally, I still do not think the majority of HVAC and homeowners understand the root cause on why the RH changes in homes!

P.S. I do not understand why someone would become a professional member and hide his or her information/credentials?

pstu
01-17-2005, 10:09 AM
Guess I should retract the comment that it is easier to control IAQ in Northern homes. What I should have said is, there are a lot more engineers (and others) who live in those conditions and are working on that problem. Hopefully you won't find anything to object to in *that* statement <g>. For example Aprilaire seems to have made a humidifier for many many years but just now decided that a DEhumidifier made sense in their business plan. And it seems to me that the machinery for adding humidity must be simpler than that for removing humidity.

I still am amazed that psychrometrics explains it all so neatly. A few weeks ago I downloaded a psychrometric calculator and laid out a few relevant data points about what outdoor RH would warm to a target indoor 72 degrees, 48% RH. That told me a lot, and some experts here have informed me that gas furnaces don't add anything to what that chart tells me (I do understand how infiltration and leaky ducts affect things).

Thanks again to everyone who has replied to this thread!

Best wishes -- P.Student

P.S. Did you know "psychrometric" and "psychometric" are BOTH perfectly good words meaning different things? No fair! <g>

Carnak
01-17-2005, 01:50 PM
I was actually sticking up for you for a change X. Seems a friend of 'informedhomeowner' was being quite critical of your balloon analogy. How many names are you registered under here?

Xavier
01-17-2005, 04:50 PM
Thank you Carnak, I missed the irony!

I too did not understand his points, but as he was new to the site, I elected not to ask him to explain, thinking someone else may. Because no one did, I still wonder how many homeowners understand his or her home’s HVAC & IAQ?

Why would you ask if I have more then one name as all my information is in my profile or credentials for anyone to contact me. Many contact me directly and I answer their question with data and my "analogies,” good ones and “not so good ones!”

7X
01-20-2005, 07:24 PM
Well stated responses Carnak and Xavier.