Does Heating Air reduce moisture content? (For HVAC Folks...and other smart people..

[SeaKaye12]

Hi,

I've never been clear as to whether or not heating air (like in a house in winter) will reduce the humidity.

When air is heated....where does the moisture go; if in fact it is reduced?

I can see that with a gas furnace that is vented to the outside....that is where moisture could go. But; what about electric heat? Electric heat does not have outside vents...and it does not have a moisture collecting system (like a de-humidifier) so if electric heat reduces moisture....where does the moisture go?

I have always been puzzled about this. Can any of you explain it in terms that I can understand?

Thanks, Chuck

 

[bigdee]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

It goes outside....the heat is just the driving force. Warm air flows towards low pressure (cold) and carries the moisture (latent heat) with it

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I'll venture some semi-educated guesses.

Forced air electric, the moisture may end up as condensation in the ductwork.

Electric baseboard heaters probably don't really eliminate moisture, but again, it would most likely be in the form of condensation, either on the metal exterior of the baseboard heater, or it may get absorbed by the walls/ceiling in the room (most typically in the corners where there is reduced air flow).

Those little in-wall fan heater (Cadet) probably allows for the condensation to accumulate on the inside of the units, and the moisture ends up in the walls.

I think a lot of the moisture is removed through the building envelope, which is never truly air tight. The warm, wet air rises into the attic, and out through the ridge/soffit ventilation of the roof structure. Unless a heating system has a moisture collection system, I think this is where most of the moisture ends up. The rest of it probably gets absorbed by household items.

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

It goes outside....the heat is just the driving force. Warm air flows towards low pressure (cold) and carries the moisture (latent heat) with it

Yes and no. The exterior insulation has a moisture barrier that is designed to keep moisture (condensation from the exterior side) out, but at the same time keeps interior moisture in. Attic floor insulation typically (unless the wrong type was used) does not have the moisture barrier (the paper backing), which allows it (moisture) to get into the attic, and subsequently vent to the exterior.

Heat always rises, it never goes sideways (which is why gable vents are no longer used).

 

[GA_Boater]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

In combustion heating the moisture is a by product of burning the fuel and most goes up the chimney . Not the case with electric heat. Attic insulation does have a vapor barrier against the living space. Otherwise the vapor would soak the insulation rending it useless and soaking wet.

 

[foodfisher]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I am working on a mobile home with paneling that has a picture of wood on the not so pretty paneling. The picture is peeling off everywhere do to none vented heating. My guess is moisture is being absorbed by the underlying wood. Air is sucked in at floor level, heated, then pushed back in at ceiling level with nowhere for the moisture to exit.

 

[bigdee]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Yes and no. The exterior insulation has a moisture barrier that is designed to keep moisture (condensation from the exterior side) out, but at the same time keeps interior moisture in. Attic floor insulation typically (unless the wrong type was used) does not have the moisture barrier (the paper backing), which allows it (moisture) to get into the attic, and subsequently vent to the exterior.

Heat always rises, it never goes sideways (which is why gable vents are no longer used).

The vapor barrier does a good job of slowing it down and if your house was 100% airtight the humidity would increase over time and we are beginning to see that especially with modular houses because they are built so tight....the only way the humidity decreases is for it to leave the building,period

 

[dwco5051]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Infiltration of cold outside air with a low amount of moisture and then heating it causes the relative humidity of the heated air to be less even though the total amount of water vapor in the heated air is a constant. Most dwellings constantly exchange outside air with the heated inside air. As this process goes on the relative humidity of the inside air decreases since warm air can hold a higher percentage of water vapor than cold air. Thus outside air that may have a RH of 30 could be less than 10 once heated to room temperature. This is the reason you need humidifiers in the winter to be comfortable. If you want all the boring formulas go to your local library and get a copy of the ASHRAE handbook and get yourself really confused about it works. Condensation on cold surface is caused when conditioned air is cooled to a point where the film of air on the surface falls below the dew point.

 

[r.j.dawg]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I noticed most of the replys have come from the southern U.S. so maybe I'll give an opinion from the frozen north. It's next to impossible to have a completely 100% airtight house. Because of building codes, there have to be exhaust fans in bathrooms. People want exhaust hoods in their kitchens. All are pneumonia holes when it comes to an airtight building envelope. Up here, when the temp is -4F, the relative humidity outside is generally around 15 - 20%. We have to add humidity to the home for not only comfort but to maintain wooden furniture and flooring from cracking and splitting. (try taking off a woolen sweater when the humidity is very low......OUCH) Recommended levels at that temp is 30 - 40%. So to answer your question, heating most definitely does reduce your humidity levels, up here. In So Cal, not sure. It depends where you live.

 

[rbh]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

YUP on most things, BUT if you have lived in a house with electric or wood or a boiler heating system your house is extremly dry!!
Most oil or gas furnaces have a humidifier built in so the home is always feeling moist and clammy if it gets cold.
If there is a break in the oil or gas furnace plenum?? (the part were the heat and exhaust are separete) you will be getting alot of exhaust gasses, co2 moisture etc.
Have your furnace checked for cracks.

 

[Tim Frank]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Hi,

I've never been clear as to whether or not heating air (like in a house in winter) will reduce the humidity.

When air is heated....where does the moisture go; if in fact it is reduced?

I can see that with a gas furnace that is vented to the outside....that is where moisture could go. But; what about electric heat? Electric heat does not have outside vents...and it does not have a moisture collecting system (like a de-humidifier) so if electric heat reduces moisture....where does the moisture go?

I have always been puzzled about this. Can any of you explain it in terms that I can understand?

Thanks, Chuck

Some interesting responses....

But :
1) Heating or cooling..(unless there is actual condensation, see "dew point") air does NOT reduce....or increase the humidity, unless you do something else in tandem....like add a humidifier...or dehumidifier, etc.
2) You need to understand the relationship between absolute humidity and relative humidity. The moisture is not "going anywhere" it just feels like it has ....

 

[Tim Frank]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Infiltration of cold outside air with a low amount of moisture and then heating it causes the relative humidity of the heated air to be less even though the total amount of water vapor in the heated air is a constant. Most dwellings constantly exchange outside air with the heated inside air. As this process goes on the relative humidity of the inside air decreases since warm air can hold a higher percentage of water vapor than cold air. Thus outside air that may have a RH of 30 could be less than 10 once heated to room temperature. This is the reason you need humidifiers in the winter to be comfortable. If you want all the boring formulas go to your local library and get a copy of the ASHRAE handbook and get yourself really confused about it works. Condensation on cold surface is caused when conditioned air is cooled to a point where the film of air on the surface falls below the dew point.

^^^^^^

The answer to the OP's post. BZ!

 

[JB]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Yes. Heat air and it expands, thus the same amount of water make less humidity. Ever see anyone running a humidifier in the summer??

 

[bigdee]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

I have two houses with electric heat, one is modular and the other is site built. The site built has a problem with low humidity in the winter and the modular is just the opposite. The difference is the modular is extremely tight...in the modular we have to run the bath fans and range hood fans alot to reduce humidity. In the site built I purposely leave the bath fans OFF when showering to add humidity. I can't stand static shocks so I prefer the modular.

 

[joed]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Heating air does not reduce or increase the amount of moisture in it. It does however change the relative humidity since that is a calculation based on air temp. Warm air can hold more moisture relative to cold air.

 

[drrpm]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

The short answer is no. Heating does not effect absolute moisture content. Heating does effect the relative humidity. Warm air can hold more water vapor than cold air. If you heat cold air the relative humidity will go down because it can hold more water vapor at the higher temperature but the absolute quantity of water vapor is the same.

 

[WIMUSKY]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Yes. That's why we use humidifiers in winter. Somebody mentioned floors and furniture benefit, I use a humidifier mainly because it makes you breath better. You can tell when it just feels "dry" in the house...... That reminds me, I need to fill mine again.....

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

In combustion heating the moisture is a by product of burning the fuel and most goes up the chimney . Not the case with electric heat. Attic insulation does have a vapor barrier against the living space. Otherwise the vapor would soak the insulation rending it useless and soaking wet.

You are correct, but the moisture that goes up the flue is not airborne moisture being removed from the house. That is moisture condensing inside the flue, because natural gas is a (relatively) cooler fuel. Oil and wood heat burn hotter, which creates a swifter moving updraft, and the exhaust doesn't cool down enough inside the flue. That's why gas systems have metal flues/liners, and oil & wood have clay/brick liners.

The air that comes through the return, and is heated by the furnace never comes in 'contact' with the air in the flue. If it did, that would mean the reverse was happening as well, and carbon monoxide would then be exhausting into the living space, which would be a very bad thing.

Also, attic insulation should not have the vapor barrier against the living space. People will use paper-backed batt insulation in this application, but that doesn't make it right. Insulation against the living space (attic structure, not exterior walls), should always be loose fiber, or cellulose (old stuff), asbestos (older) or wood chips, (even older). I've even seen shredded newspaper treated with a flame retardant (c. 1930's).

Just so you know, I didn't stay at a Holiday Inn Express, last night, but I spent 12 years as a home inspector.

 

[Beefer]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

YUP on most things, BUT if you have lived in a house with electric or wood or a boiler heating system your house is extremly dry!!
Most oil or gas furnaces have a humidifier built in so the home is always feeling moist and clammy if it gets cold.
If there is a break in the oil or gas furnace plenum?? (the part were the heat and exhaust are separete) you will be getting alot of exhaust gasses, co2 moisture etc.
Have your furnace checked for cracks.

Incredibly important!!! If you have a gas furnace, you can do it yourself. This is something that may save your family. Since most of us here are DIY'ers, I'll tell you how to do this, it's really easy, and should be done once a year (before first use of the season is wisest).

Tools needed - flashlight and a small mirror (possibly a screwdriver or 1/4" hex head, depending upon your panel)

1) Remove outer panel
2) You should now be able to see the burners, and directly above the burners is the heat exchanger. This is what you are going to check.
3) Take flashlight and mirror, and look all over the heat exchanger for cracks, rust, seam separation, etc. The smallest problem can kill your family.
4a) If everything looks good, close it back up, and repeat next year.
4b) Saw something that didn't look right? ie, crack, etc., DO NOT USE!!!! Call a furnace guy right away, and get it taken care of immediately.

In a perfect world, we would all get our furnaces checked/tuned up annually, which would include a complete inspection, eliminating the need for you to do this yourself. Since we don't get them serviced like we should, at least do this annually.

Oh, and I grew up in NY, and was a home inspector in the cold northwest. FL is my semi-retirement home.

 

[Fishing Dude too]

Re: Does Heating Air reduce moisture content? (For HVAC Folks...and other smart peop

Absolute dry air is gained at -40. This can be done by supper heating, or cooling. I deal with this issue in my job so heatting or cooling can dry air. The trouble why air dries in winter is air exchange from outside to inside. A house that is to tight is unhealthy as a house that is to loose cost to much to heat. Once the ouside temp drop below freezing outside air starts to dry out, and when you bring it inside it is drier than inside air. When the air is to dry the moisture that your body releases evaporates quickly lowering comfort levels. On the same note if moisture is t high you can grow mold in your house.