Forced air heating systems (as opposed to radiant hydronic floor) tends to draw moisture out of everything

I have heard that before, but I am trying to understand why that might be and is it an effect you are trying for? Does forced air create or destroy moisture that is already within the envelope of the house? Where does that moisture go?

Forced air may increase infiltration and infiltration reduces moisture levels (winter). Other things being equal, I expect furniture to have the same moisture level with ducted or radiant. More if you use a humidifier (which is easier with ducts).

Well I know it is a concern for people who own valuable vintage guitars in a forced air furnace system. The forced air (they claim) draws the humidity out of the wood and glues, and can damage the guitars. They have their own forums where it is discussed extensively.

The same concern parallels furniture, specifically valuable antiques.

Of course, forced air also gives people fits with dry skin , but that is probably evaporation effect on the skin, that dries out skin and nasal passages. I have an add on humidifier to my present home in Kansas and it makes a difference on my dry skin and my guitars, in the winter.

My future home in No Cal will have the opposite problem. High humidity in the winter, winters not quite as cold, marine layer fog off and on year round, 30" of rain over the 3 winter months, and a radiant hydronic system. I presume will have to pay special attention to my ventilation system to avoid mold issues.

Of course the moisture doesn't leave the envelope unless it's exhausted out, right?

Ventilation, airtightness, dehumidification - all will be important.

Posted By JohnRLee on 06 May 2013 10:16 AM
Well I know it is a concern for people who own valuable vintage guitars in a forced air furnace system. The forced air (they claim) draws the humidity out of the wood and glues, and can damage the guitars. They have their own forums where it is discussed extensively.

The same concern parallels furniture, specifically valuable antiques.

Of course, forced air also gives people fits with dry skin , but that is probably evaporation effect on the skin, that dries out skin and nasal passages. I have an add on humidifier to my present home in Kansas and it makes a difference on my dry skin and my guitars, in the winter.

My future home in No Cal will have the opposite problem. High humidity in the winter, winters not quite as cold, marine layer fog off and on year round, 30" of rain over the 3 winter months, and a radiant hydronic system. I presume will have to pay special attention to my ventilation system to avoid mold issues.

Of course the moisture doesn't leave the envelope unless it's exhausted out, right?

N. Cal doesn't have high humidity ANY time of the year.  It has a rainy season, but when it's raining out it's at relatively modest outdoor dew points.  The mid-winter average dew point in Arcata (see link- pull up a dew point graph) is about 41-42F, and the mid-summer mean is about 52F (yes, the air is more humid in summer than in winter, despite lower rainfall.)  This is a never-to-dry, never-to-humid type of climate- it's "just right", MOST of the time! (Unlike Kansas, with a 23F mid-winter mean dew point and getting-on-sticky 64F summertime mean dew point.) 

In Arcata's winter a 42F dew point is 35%RH @ 70F, a perfectly comfortable & healthy humidity for both humans & guitars.  In summer a 53F dew point correlates to a similarly comfortable and healthy 46% RH @ 75F. Your mold hazard is VERY low year round, as long as you at least some ventilation to purge moisture from indoor sources. This is in fairly stark contrast to KS humidity averages.

Forced hot air when blowing directly on an stringed instrument raises it's temperature, which can have a drying effect.  In KS air-handler driven infiltration would also lower the indoor dew points to less than ideal humidity levels.  The humidity cycling is much more severe where you've been living than what you'll see in N. CA.  The absolute highest dew point recorded in Arcata last summer was 64F, which is just the average in Wichita.

N. Cal doesn't have high humidity ANY time of the year.

Ha Ha. You should have seen the drywallers up here when I told them that the best source of dry air was right outside the door, despite the fact it was 35F and raining. Their eyes literally bugged out and from that point on I was officially an idiot.

They thought the sling psychrometer was nunchaku or "nunchuks" and that I came over to the new build because my wife wouldn't let me swing them in the house.

The forced air (they claim) draws the humidity out of the wood and glues

As jonr and Dana1 have noted, that is probably all from inadequate construction; infiltration and duct losses. In a heating dominated climate, the colder it gets, the harder and hotter the furnace runs, the faster the ducts leak air and the more the superdry air outside is able to infiltrate and suck the moisture out of everything.

If your house is properly sealed, moisture should actually build up within the envelope, but proper ventilation should control it.

And in that part of CA, ai-handler driven infiltration doesn't matter, at least from a guitar or human health point o view. The outdoor air is never so dry or changing humidity so quickly as to become a problem for musical instruments or humans. Uncontrolled air leakage DOES have energy use consequences in temperate coastal NoCal, but controlling indoor humidity there is easier there than in most of the US.

It takes awhile for some people to fully get that just because it's raining outside doesn't mean that the same air at indoor temperatures is going to be humid. The temperature at which it's raining actually matters (as I discovered during a raging 105F downpour in Tucson the summer I lived there. :-) ).

Posted By Dana1 on 06 May 2013 04:32 PM
And in that part of CA, ai-handler driven infiltration doesn't matter, at least from a guitar or human health point o view. The outdoor air is never so dry or changing humidity so quickly as to become a problem for musical instruments or humans. Uncontrolled air leakage DOES have energy use consequences in temperate coastal NoCal, but controlling indoor humidity there is easier there than in most of the US.

It takes awhile for some people to fully get that just because it's raining outside doesn't mean that the same air at indoor temperatures is going to be humid. The temperature at which it's raining actually matters (as I discovered during a raging 105F downpour in Tucson the summer I lived there. :-) ).

Dana,

Great info, as usual.

Despite your claims, of which I'm not disputing...my concerns in Trinidad (not Arcata, I'm 15 miles to the north of Aracata) would not be of too dry air, but mold issues.

So as long as the house is wrapped and sealed properly, you don't feel I'll have high humidity issues in this home?  Marine layer fog is more prevalent here than in Arcata (which is a mile or more in land form the coast).  My home will be 150' from the edge of coastal bluff (125' above sea level though).  Maybe I'm overly worried about nothing.

Still, don't you feel it's important to pay close attention to a properly designed ventilation design as part of my hydronic design?  If so, what should constitute the ventilation system?  Fresh air inlet and exhaust to some degree?  Or just air circulation?  You can't exhaust air without replenishing fresh air from outside right?

Posted By JohnRLee on 06 May 2013 05:02 PM
Still, don't you feel it's important to pay close attention to a properly designed ventilation design as part of my hydronic design?  If so, what should constitute the ventilation system?  Fresh air inlet and exhaust to some degree?  Or just air circulation?  You can't exhaust air without replenishing fresh air from outside right?

Ventilation isn't a part of hydronic design, but rather part of a well sealed house design regardless of the heating system used. I'm guessing you haven't had a chance yet to research what's available for ventilators. Google both heat recovery ventilator and energy recovery ventilator. They both exhaust stale indoor air and bring in fresh air through a heat exchange medium. In the winter the indoor heat is transferred to the incoming air, and in the summer the incoming heat is transferred to the exhaust air. The primary difference between the two is the energy recovery is kind of a block to moisture leaving or entering thus saving the energy cost to cool or heat moisture as well as air.

My ventilator runs about 20 minutes every two hours at low speed. It's also connected to timers in both bathrooms and a humidistat in the master bath so it serves the function of bathroom exhaust fan. I have an UltimateAir 2000 unit. Some, if not all, ventilators have settings to slightly pressurize the house, i.e., the incoming blower tries to bring in more air than the exhaust blower discharges.

The mantra in energy wise building is, "Build it tight and ventilate it right." Houses in the past didn't have a moisture problem because they were so leaky the air leaving the house carried the excess moisture out because it was warmer than the air coming in. Warmer air holds more vapor. That's why in the past hot air heating systems dried out the air, because as the temperature went up, the relative humidity went down.

You mention mold. Where in the house structure are you concerned mold may form?

Posted By dmaceld on 06 May 2013 11:08 PM
The mantra in energy wise building is, "Build it tight and ventilate it right." Houses in the past didn't have a moisture problem because they were so leaky the air leaving the house carried the excess moisture out because it was warmer than the air coming in. Warmer air holds more vapor. That's why in the past hot air heating systems dried out the air, because as the temperature went up, the relative humidity went down.

You mention mold. Where in the house structure are you concerned mold may form?

Thanks for the post and info.

Your question is a good one, but also an abstract one.  But to be specific, I simply don't know.  And of course, I would be concerned about mold anywhere, but I don't have any targeted areas of concerned.  Perhaps I'm overly concerned about mold, but I have no idea where I should be concerned about mold forming (on the inside surface of the wall sheathing?)

Maybe a good ventilation system should be in place, but not because of mold fears... but for comfort.  And I assume that a good ventilation system will stir the air and move the heat from the radiant hydronic floor to be distributed better?  If true, wouldn't this make the radiant system a bit more efficient too?

I will research heat recovery ventilation systems, even though in my climate HRVS may not be critical (4400HDD).

Again, thanks for your post.

All homes should have a ventilation plan. It may not be so important to the "efficiency" of your radiant system as it is to keeping your place comfortable, particularly when the radiant is not even running. An HRV can be part of that.

Posted By JohnRLee on 06 May 2013 05:02 PM

Posted By Dana1 on 06 May 2013 04:32 PM
And in that part of CA, ai-handler driven infiltration doesn't matter, at least from a guitar or human health point o view. The outdoor air is never so dry or changing humidity so quickly as to become a problem for musical instruments or humans. Uncontrolled air leakage DOES have energy use consequences in temperate coastal NoCal, but controlling indoor humidity there is easier there than in most of the US.

It takes awhile for some people to fully get that just because it's raining outside doesn't mean that the same air at indoor temperatures is going to be humid. The temperature at which it's raining actually matters (as I discovered during a raging 105F downpour in Tucson the summer I lived there. :-) ).

Dana,

Great info, as usual.

Despite your claims, of which I'm not disputing...my concerns in Trinidad (not Arcata, I'm 15 miles to the north of Aracata) would not be of too dry air, but mold issues.

So as long as the house is wrapped and sealed properly, you don't feel I'll have high humidity issues in this home?  Marine layer fog is more prevalent here than in Arcata (which is a mile or more in land form the coast).  My home will be 150' from the edge of coastal bluff (125' above sea level though).  Maybe I'm overly worried about nothing.

Still, don't you feel it's important to pay close attention to a properly designed ventilation design as part of my hydronic design?  If so, what should constitute the ventilation system?  Fresh air inlet and exhaust to some degree?  Or just air circulation?  You can't exhaust air without replenishing fresh air from outside right?

Rapid drying or excessive can crack antique guitars, mold issues are from moisture accumulation in susceptible materials. Neither are a problem in your climate. ( Trinidad dew points and temperatures are pretty much identical with Arcata's.)

A good capillary break on the exterior (such as back-ventilated siding plus a crinkle-type housewrap) keeps the bulk moisture from accumulating in the walls, and are important in rainy marine climates. (It's required by code in Vancouver B.C.) The mean wintertime outdoor temps are above the dew points of healthy indoor air levels, so even air leakage from the interior (which should still be avoided for energy reasons) won't cause mold-inducing moisture levels on the sheathing the way it would in colder climates.  Simply ventilating at a rate sufficient to keep moisture from building up indoors, letting indoor dew points track the outdoor dew points would keep the building assemblies mold free. It doesn't take a super high ventilation rate to achieve this either (unless you have an indoor sauna/hot tub) and you can use ventilation as your primary dehumidification year round, unlike the steamy southeastern US.

Heat Recovery Ventilation (HRV) is recommended, but a continous exhaust-only approach would also be fine in your climate. Of course you can't exhuast air without replenishing it, your house would collapse fairly quickly from the depressurization and you'd get altitude sickness.  

HRVs are balanced systems, delivering & retrieving a designed-in amount of air to every room, whereas with exhaust-only the incoming air locations will vary.  There are inlet ports devices designed to allow the fresh air in at designated points, but it takes a VERY tight house for those to truly be the fresh air sources. Air follows the lowest impedance paths, and everything from weatherstripping to dryer vents and random other leaks become your air input points in most houses.  A properly installed HRV really DOES work at even-pressure, with known incoming & outgoing air locations.

HRV ducting is quite small compared to AC or heating ducting- 6" round trunks with 3" & 4" branches are typical for residential apps, and  with the low air volumes they're whisper-quiet by comparison too. Typical installed costs on new construction would normally come in between $2K-4K, depending on the complexity and size, local labor rates, etc.

Skimming the last year's numbers, Arcata gets weather that will cause indoor humidity to go to 80% and it gets weather that will, with no internal moisture gain, cause an interior humidity of 18%. Luckily this isn't common and doesn't last long. I don't know anything about guitars, but if this is a problem, then you may need a dehumidifier and a humidifier respectively.

While I do own a few guitars, I'm not hat concerned with them drying out in Trinidad, when I move there. I brought up the guitar scenario, because ICF asked why forced air heating systems tend to dry things out, suggesting moisture evaporation, which isn't the case, but yet something happens to that moisture.

My real concern, as I've stated many times is high humidifies and potential mold growth in this area, not guitars or furniture drying out.

I'm also working with a Title 24, H.E.R.S. analyst (required for a building permit in Calif) and she works and lives in Arcata. She indicated yesterday that mold growth is a noted concern in this area and a ventilation system is code... and a GOOD ventilation system is suggested, along the lines of a heat recovery system, even though the climate isn't cold enough to demand a heat recovery system, it is suggested to balance the humidity in the envelope.

So, per Dana1 and my HERS analyst, I need to start researching HRVs

but yet something happens to that moisture

It goes from the guitar wood into the air in the home. In the room air, it mingles with all the other water vapor from cooking, showering, plants, respiration, etc. Why doesn't the water vapor build up, trapped inside the house like that?

Posted By jonr on 07 May 2013 06:38 PM
Skimming the last year's numbers, Arcata gets weather that will cause indoor humidity to go to 80% and it gets weather that will, with no internal moisture gain, cause an interior humidity of 18%. Luckily this isn't common and doesn't last long. I don't know anything about guitars, but if this is a problem, then you may need a dehumidifier and a humidifier respectively.

Show me the temp & humidity bin that makes it 80% RH @ 75F. (And how many standard devations out is that?) That's a dew point of ~69F, which is UNGODLY rare there.

Last year the highest recorded dew point in Trindad was 64F, on 15-16 October between 8PM and midnight, and it fell back to 55F by noon on the 16th.

Most homes have at least SOME humidity buffering capacity, and would ride out the extremes without causing mold even without air conditioning or dehumidifers. In that climate mold issues would be predominantly from bulk-moisture wetting from the exterior, or underventilated very tight houses.

Show me the temp & humidity bin that makes it 80% RH @ 75F.

I used 70F and it's at the link you provided, somewhere in the last year. As I said, it is rare. I wouldn't worry about it but guitars might.

But when the outdoor dew points are that high the outdoor temps are above 65F, and thus would be normally be above 70F indoors without active cooling or massive ventilation rates, and active cooling would also be dehumdifying. The daily high on 15 October was a "bright clouds" 65F, but as the humidity rose over the course of the evening it became a foggy 64F at midnight.

It is truly rare event to have fog at temps above 60F in that location, though they're quite common at temps in the 50s. Turn on the temperature option in the dew-point graph to see when they collide:

http://weatherspark.com/#!dashboard;a=USA/CA/Trinidad

It takes sustained exposure to higher humidity to unglue or crack a guitar. The transients are just too fleeting to matter. Mean dew points are a more important measure than 3 & 4-sigma outliers that last but a few hours.

But when the outdoor dew points are that high the outdoor temps are above 65F, and thus would be normally be above 70F indoors without active cooling or massive ventilation rates,

Normally maybe, but right now, my house it at 68F inside while it is 66F outside and in a few minutes, the outside will be higher than inside (due to thermal mass). So I'm very close to being at the same %RH as outside and will soon be experiencing higher %RH than outside.

I agree that as long as you aren't talking about condensation, short term %RH is of little importance.