Fresh air in humid climate

[pstu]

This is probably an old question, but it is important to me:

Is it unwise to have a fresh air inlet on the AC return plenum, because of being in a hot-humid climate?

Would that really increase the humidity in the house, or would the fresh-air intake tend to displace infiltration elsewhere in the home? Is there any good info on this?

Thanks in advance -- P. Student

[uktra]

It all depends on how tight the home is. Fresh air intakes to the return work well, in very tight homes (less than .30 NACH), with an electronic damper control unit (ex. Aircycler) that lets one control the amount of air coming into the home.

[Edited by uktra on 10-15-2004 at 10:19 AM]

[teddy bear]

Originally posted by perpetual_student
This is probably an old question, but it is important to me:

Is it unwise to have a fresh air inlet on the AC return plenum, because of being in a hot-humid climate?

Would that really increase the humidity in the house, or would the fresh-air intake tend to displace infiltration elsewhere in the home? Is there any good info on this?

Thanks in advance -- P. Student

A frseh air inlet on the return is better than no fresh air. Operating air handlers create possitve/negative pressures throughout the ducts and rooms. The pressure difference cause fresh air ventilation. Adding a fresh air return decreases the negative pressure on the home. This stops humid air from being sucked in to wall cavities etc. That's good. But it is not an effective ventilation device because it only works when the air handler operates on high speed. We need fresh air whenever the home is occupied, regard of heating/cooling needs. Opeating an air handler during the occupied hours is expensive compared to a small dedicated fan. A small dedicated fan could be setup to blend, filter, and circulate the fresh air throughout the home. Fresh air is critical to health regardless of humidity, so humidity must be controlled(<50%RH). Humid climates should have reheat or dehumidification to avoid mold potiential and have fresh air ventilation.

[Carnak]

Use the fresh air intake with fan on Auto

[pstu]

The house I am particularly wondering about, is 3430 sqft, in Houston area. A blower door test was done and it tells me "Estimated Manual J Air Change Rate" was

Winter = 0.40 per hour or 249 CFM
Summer = 0.24 per hour or 149 CFM

This would be in the "medium" category for tightness, would it not? The summer figure would seem to meet the criteria Uktra mentioned, for being tight enough (in summer at least) that a fresh air intake would perform as recent literature says.

I suppose there would indeed be some extra latent load, or is that not necessarily so? I am wondering if the natural infiltration would tend to be via the dozen old fashioned (i.e. leaky by design) ceiling canister lamps. My hypothesis is air from that source may carry even more humidity from the attic air, and a fresh air intake may conceivably not add latent load.

Does this make sense to you? Does this modify any of your advice? I really appreciate everyone who has responded.

[Carnak]

With natural infiltration the humidity from the outside air has to be removed from the room air by the air conditioner. You try to remove extra humidity out of air that is already dry.

With a fresh air intake and the air handler fan in auto mode, the house gets pressurized intermittently meaning dry air is trying to escape the house therefore humidity is not entering the house. At the same time the evap coil is facing warmer and more humid air. There is an additional load on the coil but it is easier for the coil to remove sensible and latent heat from the miced air, than it is for it to remove extra humidity from the dry air.

Using a fresh air intake is like wearing a condom. Not using the intake is like getting an abortion. Keep the humidity out of the house in the first place.

[Lifer]

I have done some work for a mech. engineer that did a blower test on my duct work and a blower door test on the whole house at start up and we set a balancing damper on the 6" run that was about 15' from the blower itself. I cant remember what the setting was but when I questioned him he said that he included it in his heat load calc.

[building solutions]

P student
Remember law 1. One cfm in equals 1 cfm out. When ventilation air eclipses ach natural, the ventilation rate dominates the moisture load. They are not cumlative.

[airman1]

hey carnac What the hell are you smoking out there?

[Carnak]

I smoke whatever I can get my hands on supplies little scarce here these days. The fresh air intake is the best info provided on your site. You looking to rumble?

[energy_rater_La]

.35 ACH per hour is the standard in Louisiana,
Florida standards are .25 ACH
Even with the Fla standard your house is not 'too tight'.
Was a recommendation to add fresh air suggested by the
person who tested you home?
Air from your recessed lights should be stopped.
Airtight trim kits do the job well. Hot humid attic
air enters through these areas and reduces comfort
and IAQ.
Once these cans are sealed, adding a fresh air may
become necessary.
A cost effective way to do this is to add a fresh
air intake, with a filter back grill, to filter air
before bringing it inside. Here hvac companies are
using a 6" duct with a manual damper ducted to the
return. Air passes though the filter usually located
under a soffit, to the return where it is dehumidified
before entering the conditioned areas.
Makeup air is 10 cfm per person, and 15 cfm per gas
appliance in conditioned areas.
I have seen barometric dampers also. They seem to work
well, but there is some discussion as to the long term
solution.

[teddy bear]

Originally posted by energy_rater_La
.35 ACH per hour is the standard in Louisiana,
Florida standards are .25 ACH
Even with the Fla standard your house is not 'too tight'.
Was a recommendation to add fresh air suggested by the
person who tested you home?
Air from your recessed lights should be stopped.
Airtight trim kits do the job well. Hot humid attic
air enters through these areas and reduces comfort
and IAQ.
Once these cans are sealed, adding a fresh air may
become necessary.
A cost effective way to do this is to add a fresh
air intake, with a filter back grill, to filter air
before bringing it inside. Here hvac companies are
using a 6" duct with a manual damper ducted to the
return. Air passes though the filter usually located
under a soffit, to the return where it is dehumidified
before entering the conditioned areas.
Makeup air is 10 cfm per person, and 15 cfm per gas
appliance in conditioned areas.
I have seen barometric dampers also. They seem to work
well, but there is some discussion as to the long term
solution.


How does this strategy get fresh air into the home when routinely occupied? This type of ventilation only works when heating/cooling at peak load which is a small percent of the time that you need ventilation. In addition, you are over-ventilating when unoccupied during peak load heating/cooling. In a modern house, I suggest a small 100 watt fan to blend a specific amount of fresh air with house air, filter the blend, and circulate the blend throughout the house using the heating/a/c ducts. The fan may be operated on a routine time schedule to match at at home schedules or constant operation. Natural infiltration or controlled ventilation in green grass climates requires supplemental dehumidification to avoid high indoor humidity during cool damp weather. If your climate only has occasional cool wet weather, a simple residential dehumidifier may do the job. In humid climates, it is much more efficient to use a dedicated high efficiency ventilating dehumidifier to optimize indoor air quality.

[pstu]

ENERGY_RATER_LA wrote:

>>Was a recommendation to add fresh air suggested by the person who tested you home?

No, in fact nobody I have talked to locally has been very encouraging. Mostly I perceive they are unfamiliar with the concept, so I don't consider them expert opinion compared to a number of experienced people from this forum.

>>Air from your recessed lights should be stopped.

I totally agree, just have not done it yet. It seems wrong to me these lights are even offered for sale, but we seem to live in an unenlightened world.



TEDDY_BEAR wrote:

>>How does this strategy get fresh air into the home when routinely occupied?

One solution would be a timer device which makes the air handler run at a minimum duty cycle. I believe both Aprilaire and Lipidex (Aircycler) make such things.
http://www.aircycler.com/
http://www.aprilaire.com/pdfs/10005994.pdf

As I understand it, both will refrain from adding any runtime if the airhandler is already doing enough, and some models can operate a motorized damper to put a maximum on air intake in any given hour.

Your preferred method of a dedicated intake fan sounds good also. My main observation is -- the passive intake to the air handler return, will tend to give positive house pressurization at those precise times the air handler might tend to produce negative pressurization (e.g. via supply leaks). That sounds like a benefit to me.

Another observation about the 1st method is -- will a variable speed air handler make it exceedingly difficult to understand the correct size for a passive intake?

Thanks -- P.Student

[Edited by perpetual_student on 10-25-2004 at 11:53 AM]

[teddy bear]

Originally posted by Carnak
With natural infiltration the humidity from the outside air has to be removed from the room air by the air conditioner. You try to remove extra humidity out of air that is already dry.

With a fresh air intake and the air handler fan in auto mode, the house gets pressurized intermittently meaning dry air is trying to escape the house therefore humidity is not entering the house. At the same time the evap coil is facing warmer and more humid air. There is an additional load on the coil but it is easier for the coil to remove sensible and latent heat from the miced air, than it is for it to remove extra humidity from the dry air.

Using a fresh air intake is like wearing a condom. Not using the intake is like getting an abortion. Keep the humidity out of the house in the first place.

A friendly comment. Fresh air is not like sex, we need fresh air whenever we are in the house with the windows closed, not just when we have a high heating/cooling load. We have months of only occasional heating or cooling therefore occasional ventilation.

[energy_rater_La]

teddy bear,
As the op wrote no one seems to want to deal with these type of issues. That is also my experience. Slowly some
hvac companies in my area are learning ventilation stratigies, usually after notice that they are being sued.
As there is no longer mold insurance availible here, the
lawyers are now going after the hvac companies insurance.
I get called after meeting several times on other jobs where the hvac installer calls me some choice names for
questioning the install, sizing and duct leakage. Once the hvac company has notice of a pending lawsuit, then I am
called in to diagnose the problem.
Long way around to get the point.
I personally have no problem with barometric dampers and
have hsd them installed with good results on several homes.
I would like to learn more about the system you wrote about
and see if it is workable here also.
I am learning about hvac but have only a few years of experience. Lots of questions, few answers.
The more I learn, the more questions I have.
There are different stratigies to keep homes under positive pressures, but the education process is not keeping up with the problems that are occuring.
I have seen erv's installed on the underside of the roof decking and questioned why make up air would be from the
hottest area of the house. hrv's which shouldn't even be used in our climate. One house had a 6' duct to the soffit
with no filter. I don't get it.
When installing an erv, why not learn how to properly install it? This not a cheap piece of equipment.
Some homeowners are installing whole house dehumidification
systems which seem to do the job when (again) they are
properly installed, often they are not.
The price of these systems are out of the ballpark for most
homeowners.
Sorry if my post did not provide an answer to the original post. Clouding the issue was not my intent.
I will check back and do some research on the solution
teddybear posted. For now I have to get back on the road!

[energy_rater_La]

I found this on the Building Science Corp. website.
Here is the article, without diagram. Cut & paste the link to see diagram.
Below are the links..

Central Fan Intergrated Supply Ventilation - The Basics

The simplest, most effective, and most economical way to introduce fresh air in homes
with central forced air systems is to use the central fan to pull in and distribute a
controlled amount of outside air. This central fan intergrated supply ventilation appraoch
depends on two patented processes:

FAN CYCLING: Fan cycling assures that the central air handler fan will run enough to
distrubute ventilation air and evenly mix air throughout the house, even when there is no demand
for heating and cooling. But rather than operate the fan continously or by a simple timer, the
FanCycler (tm) method factors in prior operation - it does not run the central fan for ventilation
when operating for heating or cooling has already accomplished the necessary ventilation and
mixing. In this way, the FanCycler method saves energy as well as wear and tear on equipment.

VENTILATION DAMPER CYCLING- Intergrated a motorized ventilation damper
with fan cycling limits the potential for over-ventilation and saves the energy of unnecesairly
conditioning this 'extra' outside air. The damper opens when the fan comes on, but if the fan
stays on longer than needed for the introduction of fresh air, the damper automatically closes.
Air is simply re-cycled for as long as the fan continues to operate.

While BSC strongly recommends fan cycling with motorized damper control, the two
climate regions where it is most important are hot/humid (because of the problems associated with unnecessairly introducing moisture-laden outside air) and severe cold (because of the energy
penalty and potential comfort problems associated with unnecessairly introducing extremely
cold outside air).


Article
http://buildingscience.com/resources/mechanical/CFIS_Basics.pdf

fancycler
http://www.fancycler.com/

aprilaire
copy & paste link should work...
http://www.aprilaire.com/product.asp?ID=4BC76056DD7246EAB4AD7A01B4B4B5FD&categoryID=9DDC176A7AD948DCB0D14F348EB3FF1e

tstat
http://content.honeywell.com/yourhome/ac-automated_control/hc.htm

climate specific info
http://www.buildingscience.com/housesthatwork/

[teddy bear]

Originally posted by perpetual_student
The house I am particularly wondering about, is 3430 sqft, in Houston area. A blower door test was done and it tells me "Estimated Manual J Air Change Rate" was

Winter = 0.40 per hour or 249 CFM
Summer = 0.24 per hour or 149 CFM

This would be in the "medium" category for tightness, would it not? The summer figure would seem to meet the criteria Uktra mentioned, for being tight enough (in summer at least) that a fresh air intake would perform as recent literature says.

I suppose there would indeed be some extra latent load, or is that not necessarily so? I am wondering if the natural infiltration would tend to be via the dozen old fashioned (i.e. leaky by design) ceiling canister lamps. My hypothesis is air from that source may carry even more humidity from the attic air, and a fresh air intake may conceivably not add latent load.

Does this make sense to you? Does this modify any of your advice? I really appreciate everyone who has responded.
A couple interesting points. Blower door testing assumes 7 mile per hour wind etc. I have been expirementing with a Telaire CO2 meter attached to Hobo data logger. An interesting feature of the meter is calculating the cfm of fresh per occupant at a given CO2 level. At the ideal 1000 ppm CO2, 15 cfm of fresh air per person is entering a space. Therefore 4 adults in home with 1000 ppm CO2 are recieving 60 cfm of fresh air. A moderately tight master bedroom with 2 adults during calm winds and no heating/cooling exceeds 2,000 ppm CO2. This is 4cfm fresh air per adult. This is not life threating but far from ideal. During this time, oxygen is low, pollutants are high. Next in importance to maintaining <50%RH, oxygen/CO2/pollutant levels are the critical issues for indoor air quality. Least important is air cleaning which occurs mostly naturally by particulate settling out rather than air filters on the air handler.

The data logged CO2 meter demonstrates the real ach rate on an occupied space. The home that I tested is .2 ACH home with a blower door test. Clearly the delivered cfm very with weather and a/c/heating requirements. On a cold windy day, the fresh air exceeds the standard of .3 ACH. But the home is unhealthy much of the time and occasionaly wet enough to grow mold/dust mites.

[fishin]

from all i'm reading. are you guys saying that the tighter a home is the higher the humidity is going to be?

[Carnak]

Originally posted by teddy bear

A friendly comment. Fresh air is not like sex, we need fresh air whenever we are in the house with the windows closed, not just when we have a high heating/cooling load. We have months of only occasional heating or cooling therefore occasional ventilation. [/B]

Teddy

I have gone from one extreme to the other, from Canada to the Caribbean.

An old story of mine, an HRV in a new construction home in Canada. First winter it is constantly on high speed, due to all the moisture being released from new construction.

Typically these units are balanced on high speed to give code required fresh air.

During the second winter, even if they are run on low speed,(lower than the code requirements) they will over dry the house in a cold climate. People add humidifiers. You ventilate to give fresh air AND to control excessive humidity and therefore prevent window condensation, yet they run a humidifier to keep their lips from splitting open. It defeats the purpose.

What creates this excessive humidity in the winter? Mainly people breathing, they exhale among other things water vapour and carbon dioxide.

In my experience in Canada, you set the dehumidistat on the HRV to control window condensation, and this setting depends on window quality, but it is possible to have 35% RH and no condensation problems during sub-zero weather.

Even the worst hypochondriacs I ever worked for had no complaints whatsover. Mositure and CO2 are being created at the same time, so by controlling moisture you control CO2, and have decent IAQ.

Now I am down in the Caribbean, been here 6 years now.

In a hot humid environment, if the equipment is properly sized you are going to get long run times. Even if you are below design conditions a run time of 15 minutes on the hour will bring in significant amounts of fresh air.

If a system over ventialtes when no one is home is no big deal, the people are returning to a home full of fresh air. ASHRAE 62 recognizes this in commercial buildings and states how long the systems have to run before a space is re-occupied.

If you are in between seasons and ambient air has dewpoints say greater than 60F, then for those short times they may need a dehumidifier. A prolong rainy summer period in the low 70s may be a problem. I am fortunate here that it is always cooling season.

A dehumidifier is a last resort. Every place has a unique climate, but my opinion is intermittent ventilation gives good IAQ, and properly sized AC gives you virtually non-stop run time during design conditions, and will still give decent run times in off design conditions. You design for 95 and it is only 81 out, the system is still going to run.

It would be interesting to see some reports of schools in the carolinas, (possibly SC). They already cut the ASHRAE ventilation rate in half.

[energy_rater_La]

So intermittent fresh air or constant?

[Carnak]

intermittent ventilation gives good IAQ

[teddy bear]

Originally posted by energy_rater_La
So intermittent fresh air or constant?

Take your pick on how. We all agree, adequate clean air when occupied to provide oxygen, purge pollutants, and CO2. Keep the relative humidity <50%RH in the space exposed to the people. The discussion is "How?". Consistence of performance, comfort, investment, cost of operation, ease of maintenance, and durability are the issues. All of us personally live with varying levels of performance. Like the shoemaker's children seldom have the best shoes.

The expectations of our customer maybe different. Especially if we lead them to expect the best. I suggest for a concept to be the best, we should be able to demonstrate performance with simple monitoring, like a <50%RH meter for humidity control and fresh air change rate with <1,000 ppm CO2. Our business is changing but this allows us to differentiate the professionals from the rest of the industry.

[Carnak]

If they expect indoor RH consistantly less than 50% in cooling season, then they will need reheat.

I typically can give 50-55% without the reheat.

[cem-bsee]

of interest is the statement by Aprilaire that their controller does not allow vent air in when indoor RH% is >60. One does not need ventilation then?

My RH% = 60- 70 for past 6 wks, w/o need for a/c, indoor = 67- 78F, outdoor = 50- 82F with 9pm -9am RH%=100, daytime 70- 90%.

[energy_rater_La]

Carnak & Teddybear,
Can I CE credits from you both?
I have learned more useful information from this post than
all the - maybe this will work and maybe that will
work that I get everywhere else.

Thanks, and keep it going.

[teddy bear]

Originally posted by fishin
from all i'm reading. are you guys saying that the tighter a home is the higher the humidity is going to be?
It's a balancing act. Outside air varies from dry or to wet. Inside, we are always adding moisture. When outside air is dry and the home is leaky, outside air dries the inside. When outside air is wet, inside is made wet by outside plus we add "people moisture". The air change rate determines how fast the home is dried or "wetted". Unventilated air tight homes are dried or "wetted" more slowly but tend to be wetter. There is an advantage with the tight home. Ventilating tight homes with the right amount of dry air avoids over-drying. Also,a moderate amount of dehumidification will remove the moisture from fresh air and the occupants during the wet outside weather. The leaky home will require much more moisture removal during wet outside conditions because of over-ventilation. A home with four needs 75 cfm of fresh air and generates 25 pints of internal moisture. During wet outside, removing a 100 pints of moisture per day maintains less than 50%RH. During cool wet weather, illegal reheat or dehumidification are the only reliable methods to assure humidity control. A long answer to a short question.

[Carnak]

By illegal reheat, you are most likely implying electric heaters.

A dehumidifier is a reheat system as well, only it is using the heat of rejection for reheat. Commercially there are heatpipe, hot gas units, liquid line systems out there as well providing reheat.

Still curious as to why it has to be less than 50%. What dewpoint are you trying to maintain in the space, less than 55F?

[teddy bear]

Originally posted by Carnak
By illegal reheat, you are most likely implying electric heaters.

A dehumidifier is a reheat system as well, only it is using the heat of rejection for reheat. Commercially there are heatpipe, hot gas units, liquid line systems out there as well providing reheat.

Still curious as to why it has to be less than 50%. What dewpoint are you trying to maintain in the space, less than 55F?


The level of %RH is debatable. Dust mites need +58%RH to survive. Carpeting on concrete requires drier air because it's cooler between under the pad and relative humidity is higher. So 55%RH maybe OK if not no carpeting on concrete. Every degree cooler elevates the %RH +2% at the same water content. 50%RH or 50^F dew point @ 70^F provides a small saftey factor.

[Carnak]

50 dewpoint is tough in a humid climate, even with reheat. Outside air here has a dewpoint of 80F for six months straight.

You are referring to 50F dp at 70F as a 'winter' indoor design for a southern home? Works out to almost 50% RH.

The %RH is generally a confusing term, I think the industry needs to focus more on dewpoints.

[teddy bear]

Originally posted by Carnak
50 dewpoint is tough in a humid climate, even with reheat. Outside air here has a dewpoint of 80F for six months straight.

You are referring to 50F dp at 70F as a 'winter' indoor design for a southern home? Works out to almost 50% RH.

The %RH is generally a confusing term, I think the industry needs to focus more on dewpoints.

Mold and dust mites are dependent on %RH. An interesting point is that higher temperature raises the dew point for <50%RH operating at 78^F, 50%RH is a 57^F dew point. An unoccupied home +80^F, 50%RH, +60^F dew point is easy with a good dehumidifier. The ultimate for controlling a vacation home is no a/c but maintaining <50%RH with adequately sized dehumidifier. I tested a FL vacation home with a dehumidistat/ac/mold problem the previous year. Documented <50%RH, reduced the electric $400 for the summer. No mold. Maintaining <50% with supplemental 100 pint of dehumidification/2,500 sq.ft. of home with a solid structure/duct work is a reasonable expectation.

[pstu]

>>I have been expirementing with a Telaire CO2 meter attached to Hobo data logger.
>>An interesting feature of the meter is calculating the cfm of fresh per occupant
>>at a given CO2 level. At the ideal 1000 ppm CO2, 15 cfm of fresh air per person is entering a space...

>>...The data logged CO2 meter demonstrates the real ach rate on an occupied space.

The more I reflect on this, the more I think this is exactly the best approach (if you can afford the equipment). Thank you VERY MUCH for sharing this idea with us!

If you know of any inexpensive sources for instrumentation to measure CO2, please share it. Absent that, I will be keeping my eyes open for instruments to measure CO2.

And to think most homeowners don't even have a hygrometer... sigh!

Best wishes -- P.Student

[TConnors]

I have a Thermaster (Sante Fe) dehumidifier that brings fresh air into the house. I question whether it is a good thing during humid days. I do not set it's fan to run all the time, only when there is a call for dehumidification. If I run the fan 24/7, the dehumidifier cycles about 800% more. This is because it's bringing in a lot of humidity. This also heats the house up a lot - the air temp increase is about 20 degrees, which in the summer is VERY hot air. So this in turn causes the A/C to run a lot more. Running it 24/7 also cloggs the filter MUCH faster (you're bringing in 8 times as many mold spores by running it full time).

I find letting it run only on humidity call works well, it runs maybe 10% of the day, and keeps the house under 50% RH very easily.