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Originally Posted by unc99

TB and others, a few additional questions if you don't mind:

1. If I put a venting dehumidifier on just one floor (ie, either in the crawlspace or attic for a two story, two unit home), would it be the first or second floor?

2. Could I put an ERV on on floor and a venting dehumidifier on the to other floor? Maybe this is overkill, but just thinking through all options.

3. Should I get a variable speed HVAC or regular?

Thanks!

1. If you really need a dehumidifier, put it in the lowest floor since that usually is colder and the dehumidifier adds heat (all the electricity it uses ultimately becomes heat plus the latent heat of water vapor)

2. I don't see a reason for an ERV going from conditioned to just slightly differently conditioned space

3. Variable speed will dehumidify better (if same total size), be more efficient under partial load and will wear out less due to less cycling. so yes to that. In residential you mostly get a 2 or 3-speed compressor, for example the "Carrier Infinity 21". Some other forum member have more knowledge on specific models than I do.

TB and others, a few additional questions if you don't mind:

1. If I put a venting dehumidifier on just one floor (ie, either in the crawlspace or attic for a two story, two unit home), would it be the first or second floor?

2. Could I put an ERV on on floor and a venting dehumidifier on the to other floor? Maybe this is overkill, but just thinking through all options.

3. Should I get a variable speed HVAC or regular?

Thanks!

Originally Posted by teddy bear

Interesting, who is going to tell all those people in the more arrid climates?
I suggest that mold free is healthier than a high indoor humidity in the winter time. Most of the discussion is about 70^F, 50%RH verses 30^F 50%RH.

"Recent theoretical work has shown that slight seasonal
changes in transmission efficiency can produce the strong seasonal
cycle in influenza incidence through dynamical resonance
(21). The findings presented here indicate that the seasonal cycle
of AH may be the factor producing this resonance. Specifically,
low wintertime AH levels, both indoor and outdoor, increase
influenza IVS and thus increase IVT efficiency.
These findings also suggest that humidification of indoor air,
particularly in places where transmission to those at high risk for
complications, such as nursing homes and emergency rooms,
may help decrease the spread and the toll of influenza during
influenza season. Previous studies using RH (8–10, 13–19, 22)
also supported similar air humidification practices; however,
these assertions were perhaps undermined by the marginal
association of RH with IVS and IVT. Our findings, which
attribute a much stronger effect of AH on both IVT and IVS
(Figs. 1 and 3), suggest that humidification measures may be
warranted."
Most suggest that after getting adequate fresh air to purge indoor pollutants, humidify to just below the point of condensation on the cool surfaces. No problem for me. Humidification to the point of growing biologicals like mold and dust mites is not suggested. Normal temperatures indoors and 50%RH is comfortable. The coldest climates will occasionally require 35%RH to avoid condensation. What has research on the effect of inadequate fresh air change on occupied space suggests high pollutants and virus concentrations? I assume not good.
Regards TB

humidifying to "just under condensation" is difficult with many thermal bridges as you have in buildings. And this also means you will be right under 100% RH at the perimeter wall. Some air also goes inside the wall where it is even colder. (that's why ASHRAE recommends negative pressure in winter, positive in summer... most designers just make it positive all year).

And focusing on just influence virus is a problem when that ignores all the other issues (look at ASHRAE humidity/bio growth chart). I'm not a doctor, but influence happily spreads from human to human without much need to be in the (artificially humidified) building. and higher humidity may grow even worse other bugs. In winter everyone is just exposed to influenza, you can't sterilize the world.

Humans lived in outside air for thousands of years and can take it. Now when we live in buildings, we create artificially more humid environments (since we produce water). Keeping it naturally more dry seems more healthy and natural to me. the people in arid winter areas like Colorado never seem to have a problem with that.

Originally Posted by pmeunier

TB, Isn't it possible to have, with the right equipment and building, healthy humidity, fresh air and no mold? I hear you about the customer that was "out of money", it may not be possible to have all 3 and a choice needs to be made. However, is it really extravagant to want all 3?

It depends on "healthy humidity, fresh air and no mold" is. Healthy humidity is the part needs clarification. I fell that 40%RH winter and 50%RH for the summer is healthy. This is doable with some form of mechanical ventilation, a humidifier, and a dehumidifier. High quality windows and a moderately air tight structure are part of the equation.
Regards TB

TB, Isn't it possible to have, with the right equipment and building, healthy humidity, fresh air and no mold? I hear you about the customer that was "out of money", it may not be possible to have all 3 and a choice needs to be made. However, is it really extravagant to want all 3?

Originally Posted by pmeunier

Thanks TB, but I believe that this approach is incompatible with the well-being of the occupants. It's been found that low absolute humidity (dew point) increases virus transmission (not relative humidity). Below about 15 mbar of water vapor pressure, the influenza virus greatly increases its survivability and infection rates (see "Absolute humidity modulates influenza survival, transmission, and seasonality", Shaman and Kohn 2009, Proceedings of the National Academy of Sciences, http://www.pnas.org/content/early/20....full.pdf+html). That's a dew point of about 55^F. Air with less than a 50^F dew point is a health issue, besides causing discomfort. Recommending increased ventilation to reduce the dew point down below 32^F dew point or less in winter is risky for human health.

Interesting, who is going to tell all those people in the more arrid climates?
I suggest that mold free is healthier than a high indoor humidity in the winter time. Most of the discussion is about 70^F, 50%RH verses 30^F 50%RH.

"Recent theoretical work has shown that slight seasonal
changes in transmission efficiency can produce the strong seasonal
cycle in influenza incidence through dynamical resonance
(21). The findings presented here indicate that the seasonal cycle
of AH may be the factor producing this resonance. Specifically,
low wintertime AH levels, both indoor and outdoor, increase
influenza IVS and thus increase IVT efficiency.
These findings also suggest that humidification of indoor air,
particularly in places where transmission to those at high risk for
complications, such as nursing homes and emergency rooms,
may help decrease the spread and the toll of influenza during
influenza season. Previous studies using RH (8–10, 13–19, 22)
also supported similar air humidification practices; however,
these assertions were perhaps undermined by the marginal
association of RH with IVS and IVT. Our findings, which
attribute a much stronger effect of AH on both IVT and IVS
(Figs. 1 and 3), suggest that humidification measures may be
warranted."
Most suggest that after getting adequate fresh air to purge indoor pollutants, humidify to just below the point of condensation on the cool surfaces. No problem for me. Humidification to the point of growing biologicals like mold and dust mites is not suggested. Normal temperatures indoors and 50%RH is comfortable. The coldest climates will occasionally require 35%RH to avoid condensation. What has research on the effect of inadequate fresh air change on occupied space suggests high pollutants and virus concentrations? I assume not good.
Regards TB

Originally Posted by teddy bear

...
I have fixed many air tight homes with condensation problems in attics/walls by supplementing natural fresh air infiltration/exfiltration by introducing enough additional dry cold fresh mak-up air to get the lower the inside dew point to more normal inside conditions. Generally, <32^F dew point exfiltrating air typically will not cause condensation problems northern US.
...

Thanks TB, but I believe that this approach is incompatible with the well-being of the occupants. It's been found that low absolute humidity (dew point) increases virus transmission (not relative humidity). Below about 15 mbar of water vapor pressure, the influenza virus greatly increases its survivability and infection rates (see "Absolute humidity modulates influenza survival, transmission, and seasonality", Shaman and Kohn 2009, Proceedings of the National Academy of Sciences, http://www.pnas.org/content/early/20....full.pdf+html). That's a dew point of about 55^F. Air with less than a 50^F dew point is a health issue, besides causing discomfort. Recommending increased ventilation to reduce the dew point down below 32^F dew point or less in winter is risky for human health.

Originally Posted by kaleun

the first thing (energy efficiency wise) to dehumidify would be to use an energy recovery ventilator to ventilate. this dries some of the fresh air. Seal the house to avoid infiltration.

a 2-stage or variable speed AC helps too since that cycles less and dehumdifies more than an oversized single-stage AC.

this should take care of all issues, if not you have a building and not an HVAC problem.

% RH.

Yes ERVs do transfer 40-50% of the moisture from the wet air to the dry air provide the you are exhausting dry air. During wet, cool weather and evenings, 50%-60% of the moisture goes into the home. At an air change in 4-5 hours after a couple days, the air inside the home is as wet as the outside air. Mold only needs a couple days of high humidity to grow. Under a carpet or behind the insulation of basement old floor or cold wall, the humidty may be near 80-90%RH.
Two stage a/c without any cooling load in a home with proper ventilation and active occupants will have months of 65-80% RH at the cool surfaces. Yes dehumidifiers should be set at 50%RH not any lower. Good high efficiency dehus will maintain 50%RH throughout the home when the a/c is operating enough. Get a good %RH meter and place it in the cool corners. If the %RH is <60% occasionally, do nothing. If the %RH is +70%RH, get the %RH down to 50%RH. Not "so be it" or in a couple years your basement will smell like a basement.
Air tight home without fresh air ventilation and low occupancy may not have big trouble during dry hot weather like some years. The wet cool weather with an air change of fresh air in 4-5 hour and 3-5 occupants require 30-60 lbs. of moisture removed every day to maintain <50%RH.

Originally Posted by kaleun

theAll the dehumidifier does is dry the air and reheat it with its own condenser. So in summer you actually heat up your building with electricity. the only difference to an AC is, that the AC has the condenser outside and rejects the heat to outside. I only would use that as last resort if humidity really is an issue. If humidity goes up for some days in the shoulder season, so be it. On a rainy day humidity is high. unless it rains for months, this won't grow mold.

The only critical space is the basement if yo have cold basement walls in summer. thsi might justify its own standalone de-humdifier to stay below 60% RH. ASHRAE recommends 60% is the cutoff for mold grow. No need to de-humdify to 40%. when you look at the psychrometric chart you see that in a 60°F basement it with virtually impossible to de-humidy to 45% with regular refrigeration. all the de-humidifiers are rated to operate at 75-80°F. why people think they can set them to the same setting in a cold basement is beyond me. Every dehumdifier i see is set to t30% or whatever the lowest setting is. then they run all day. More moist air from the houise goes into the basement and people see the buckets of water and see that as evidence how moist their basement is. humidity is one of the things 99.999999% of the population don't understand. Many engineers don't understand it either.

i used to have a de-humdifier in my tri-level house basement. I now finished the lwoer level and condition it. teh need for dehumidification ithe basement went away becasue the neighboring space is dry enough to keep the basement below 60% RH.

A well designed dehu like a Santa Fe/Ultra-Aire/Honeywell etc will not operate as long as the a/c runs enough to maintain your desired %RH. All of monitoring I have shows 5-6 hours of running in the wettest cool weather.

The inside moisture levels are the dew point of outside infiltration/ventilation plus the moisture from the occupants and their activities. Ultimately the occupants are responsible for the indoor air quality in their homes. Indoor air quality is fresh air change to purge indoor pollutants and <50%RH to avoid mold and dust mites. Comfort is also an issue. Lower %RH allows a high indoor temperature. With moving indoor air 77^F-78^F, 50%RH is very comfortable. During rainy cool weather, not much cooling load.
The debate goes on. As my a/c salesman answered my question, will this system keep my house <50%RH, Yep pretty much.
After a rainy week, I called and ask the question. Well, its been raining for a week, what do you expect.
So be it!
Regards TB

the first thing (energy efficiency wise) to dehumidify would be to use an energy recovery ventilator to ventilate. this dries some of the fresh air. Seal the house to avoid infiltration.

a 2-stage or variable speed AC helps too since that cycles less and dehumdifies more than an oversized single-stage AC.

this should take care of all issues, if not you have a building and not an HVAC problem.

All the dehumidifier does is dry the air and reheat it with its own condenser. So in summer you actually heat up your building with electricity. the only difference to an AC is, that the AC has the condenser outside and rejects the heat to outside. I only would use that as last resort if humidity really is an issue. If humidity goes up for some days in the shoulder season, so be it. On a rainy day humidity is high. unless it rains for months, this won't grow mold.

The only critical space is the basement if yo have cold basement walls in summer. thsi might justify its own standalone de-humdifier to stay below 60% RH. ASHRAE recommends 60% is the cutoff for mold grow. No need to de-humdify to 40%. when you look at the psychrometric chart you see that in a 60°F basement it with virtually impossible to de-humidy to 45% with regular refrigeration. all the de-humidifiers are rated to operate at 75-80°F. why people think they can set them to the same setting in a cold basement is beyond me. Every dehumdifier i see is set to t30% or whatever the lowest setting is. then they run all day. More moist air from the houise goes into the basement and people see the buckets of water and see that as evidence how moist their basement is. humidity is one of the things 99.999999% of the population don't understand. Many engineers don't understand it either.

Many people have the furnace in the basement. It always creates some negative pressure and draws (moister) air from the rest of the house. Same with dryers in basements. It is a losing battle to attempt to de-humdify the basement to 50% or less. All it does is use electricity and heat up the house, creating even more AC load. the AC is much better suited to dry the entire house.

i used to have a de-humdifier in my tri-level house basement. I now finished the lwoer level and condition it. teh need for dehumidification ithe basement went away becasue the neighboring space is dry enough to keep the basement below 60% RH.

Originally Posted by pmeunier

Having air "forced into the house" is what you want when it's more hot and humid outside than inside. It causes a very small positive pressure that reduces the infiltration of humidity and dust. There are plenty of leaks for the air to go out, it doesn't cause problems. However, when it's much colder outside, positive pressure can cause condensation in the walls and then it's not a great idea.

Some more recent thoughts about positive pressure ventilation in cold climates.
We build a state of the art air tight 2,500 sqft. home in cold climate. A blower door test shows that the home leaks and air change in 10 hours. This is 40 cfm of air leakage out the top of the home and down wind. Put the home in WI with the average outside temp at 17^F and 7.5 mph wind. If we have 3 occupants putting 1.5 lbs. per hour of moisture into the home from breathing and activities. As the 40 cfm of fresh air passes through the home, the air is warmed to 68^F and humidified to 67%RH which is a dew point of 57^F. As the damp exfiltrates the home, the dreaded condensation occurs in the attic and exterior cold surfaces. We know from past history that homes with more air leakage do not have any condensate problems exfiltrating the moisture from the occupants as long as the dew point of the exfiltrating air is low.
I have fixed many air tight homes with condensation problems in attics/walls by supplementing natural fresh air infiltration/exfiltration by introducing enough additional dry cold fresh mak-up air to get the lower the inside dew point to more normal inside conditions. Generally, <32^F dew point exfiltrating air typically will not cause condensation problems northern US.
This is a good strategy for homes that need make-up air to function.
Also consider that this same house on the coldest windy day, gets twice as much fresh air while the occupants are in and out of the home. The indoor moisture drops rapidily to point of needing humidification at the extreme.
Homes with bathfans, clothes drier and kitchen hood also add to the complexity.
Most homes leak more air and only need occasional supplemental fresh air during cold weather, which makes supplemental make-up fresh air a suitable choice when we are able to control the when and how much is added to the home.
I was amazed when called to a home with 6 occupants with condensation from the attic roof deck was leaking out of the siding during a warm thawing spell in the middle of winter in WI. To make things worse, the clothes drier was back drafting the ng furnace during some wind/temp conditions. The home owner was out of money and desperate. A duct fan mixing 100 cfm of fresh air into the home fixed everything. The attic dried and the furnace functioned. The heating bill went up <$100 for the year. That was 20 years ago and I have heard nothing. Since, all WI homes are required to a fresh air inlet as a minimum.
Enough for a holiday eve.
Lets start preparing for the feast tomorrow. I must beark away and plug the address for Mel's Diner into my GPS so I don't miss diner tomorrow.
Regards TB

Originally Posted by unc99

Does the venting dehumidifier expel indoor air, or just bring in out door air? In other words, does it work like an air exchanger? If not, is it problematic to having air forced in the house and not having indoor air mechanically expelled?

Having air "forced into the house" is what you want when it's more hot and humid outside than inside. It causes a very small positive pressure that reduces the infiltration of humidity and dust. There are plenty of leaks for the air to go out, it doesn't cause problems. However, when it's much colder outside, positive pressure can cause condensation in the walls and then it's not a great idea.

Originally Posted by unc99

Thank you very much Teddy Bear. If you don't mind, I have a couple more questions:

1. From the Ultra-Aire website, it appears that the ventilated/fresh air is added to the supply side of the HVAC. Doesn't the mean the air will enter the home uncooled/unheated? Maybe it doesn't make enough difference to affect the whole house temp, but seemed odd to me that the ventilated air would not be added to the return side so that the HVAC would then cool/heat it.

2. If I also add a Perfect 16 (or other air cleaner/filter) to the return side, I suppose the issue in 1 above would mean that the air supplied by the ventilator would not be cleaned until it circulated through the home and back into the return. Again, maybe this isn't too worrisome since you mentioned the Ultra Aire offers Merv 14 filtration.

3. If I use a ventilating dehumidifier for ventilation instead of an air exchanger, should I be concerned that I probably won't get any fresh air in the winter months as there will be no need for dehumidification? If you live in a climiate with hot/cold months is a air exchange with a separate dehumidifier a better option?


Thanks!

1. 80 cfm of fresh air blended into a 600-800 cfm of cold air steam means little to the temp of air entering the rooms. The a/c runs a small percentage of time compared to the fresh air ventilation. All of our units have a merv 11 minimum air filter. Most pollutants enter the home on clothes and through structural cracks.
2. Merv 11 is very good filtering. The Merv 14 is an optional filter that can be added to any dehumidifier.
3. Fresh air ventilation is independent of the dehumidifying of the air in the home. They can a occur at the same time indenpendently. Fresh is dependent on occupancy while dehumdification is dependent on the %RH setting which usually <50%RH.
During the windy coldest weather, most homes get enough fresh air naturally. But you can have fresh air whenever you want it. Diffenitly, less fresh air is need during cold windy weather. Also your clothes dry and exhaust fans need make-up air when operating.

Hope this answers your questions, if not, repost.
Regards TB

Thank you very much Teddy Bear. If you don't mind, I have a couple more questions:

1. From the Ultra-Aire website, it appears that the ventilated/fresh air is added to the supply side of the HVAC. Doesn't the mean the air will enter the home uncooled/unheated? Maybe it doesn't make enough difference to affect the whole house temp, but seemed odd to me that the ventilated air would not be added to the return side so that the HVAC would then cool/heat it.

2. If I also add a Perfect 16 (or other air cleaner/filter) to the return side, I suppose the issue in 1 above would mean that the air supplied by the ventilator would not be cleaned until it circulated through the home and back into the return. Again, maybe this isn't too worrisome since you mentioned the Ultra Aire offers Merv 14 filtration.

3. If I use a ventilating dehumidifier for ventilation instead of an air exchanger, should I be concerned that I probably won't get any fresh air in the winter months as there will be no need for dehumidification? If you live in a climiate with hot/cold months is a air exchange with a separate dehumidifier a better option?


Thanks!

Originally Posted by unc99

Thanks to all for the responses. I'm leaning towards going with a venting dehumidifier on the first floor, and two air cleaners (such as either the Perfect 16 or the Lennox Hepa model) on each floor. Although perhaps this is too over the top.

A couple of questions:

1. Is there any issue with putting an air cleaner along with a venting dehumidifier? If I understand them correctly the air cleaners are just boxes with filters, so I'm guessing not but am not sure.

2. Does the venting dehumidifier expel indoor air, or just bring in out door air? In other words, does it work like an air exchanger? If not, is it problematic to having air forced in the house and not having indoor air mechanically expelled?

3. Is anyone aware of an EVR which also dehumifidies? If so, it seems this would erase if any issues I noted in point # 2 above.

Thanks very much in advance....this site has been a wealth of information...much appreciated.

You also have some options on the filtering of the fresh air/house air blend flowing through the dehumidifier. There is a Merv 14 option option available with the Ultra-Aire. Suggest a minimum of a Merv 11 for the air handler.
Regarding the make-up air issue, the ventilating whole house dehumidifier blends fresh air with the house air, filters, and circulates the filtered blend throughout the home. The amount of air should be enough to change the air in the home in 4-5 hours. There is no exhaust of stale air. You have several appliances that need make-up air to function. The clothes drier, kitchen hood, and bath fans exhaust air and benefit from the make-up air. Also, make-up air reduces untreated air from naturally infiltrating the home which is benefical. See no need to ventilate when the home is unoccupied for extened periods of time, but maintaining <50%RH is important to avoid mold, mildew, and dust mites. The DEH 3000 controller has a good occupancy schedule provide fresh air when you are in the home on a routine basis.
Being able to control fresh air on a routine basis that matches occupancy schedules also reduce cost of conditioning the make-up air.
Keep us posted.
Regards TB

Thanks to all for the responses. I'm leaning towards going with a venting dehumidifier on the first floor, and two air cleaners (such as either the Perfect 16 or the Lennox Hepa model) on each floor. Although perhaps this is too over the top.

A couple of questions:

1. Is there any issue with putting an air cleaner along with a venting dehumidifier? If I understand them correctly the air cleaners are just boxes with filters, so I'm guessing not but am not sure.

2. Does the venting dehumidifier expel indoor air, or just bring in out door air? In other words, does it work like an air exchanger? If not, is it problematic to having air forced in the house and not having indoor air mechanically expelled?

3. Is anyone aware of an EVR which also dehumifidies? If so, it seems this would erase if any issues I noted in point # 2 above.

Thanks very much in advance....this site has been a wealth of information...much appreciated.

Originally Posted by teddy bear

...
I am open to other suggestions.
Regards TB

After years of trial and error, I found that what worked best for my house was to have a tight-band dehumidistat on the main floor, with the dehu's output directly in the open basement with a long stretch of straight flex duct to absorb noise, and drawing air from the return ducts. A relay cuts off the dehu when the A/C runs. The dehumidistat is a "Columbus Electric", which has internal Honeywell parts essentially identical to the Honeywell dehumidistat, but set at the factory to operate with a much narrower deadband.

Fresh air is fed with a "T" junction to the dehumidifier's intake, after being filtered with a HEPA filter running at a low speed. An autotransformer (variac) controls the HEPA filter speed, and a pressure switch at the HEPA filter's intake turns on a light if the HEPA is unable to maintain enough suction on the fresh air intake. When this happens and the variac has no more power to give to the HEPA filter, or it's too noisy, it's time to change the HEPA filters. Outside air is surprisingly dirty...

I think that the dehu configuration that works best is a function of the house's design and dehumidification needs. There isn't a single configuration that's the optimal one for all houses. Discharging the dehu in the basement has evened the temperatures between floors in my house, besides the desired effects on mold and insects.

Originally Posted by TalkingHead

TB: Where do you connect the discharge duct from the ventilating dehumidifier: before or after the evaporator coil? Doesn't Honeywell say before and Thermastor say after? Second question: Where should the return to the dehumidifier come from: a separate return opening or simply attach to the existing return duct to the A/C unit?

Ideally, connect the dehumidifier to the supply duct down stream of the fan/a/c coil. This connection maximizes the amount of moisture removed by the a/c. If the supply duct has +.4" of positive pressure, avoid the supply side or interlock the avoid dehu operation when the a/c is operating. When connecting a dehu to two a/cs, split the supply of the dehu to both of the a/c supplies.
There various methods of getting distribution of the dry air throughout the home and not require the a/c blow to operate when the dehu is operating.
A properly setup a/c will maintain <50%RH during lengthly operation. The dehu should only need to operate during low/no cooling loads. I am open to other suggestions.
Regards TB

and another link

http://www.buildingscience.com/docum...humid-climates

another good one.

http://www.buildingscience.com/docum...w-case-studies

http://www.buildingscience.com/docum...ventilate/view

PDF file from Building Science web site about ventilation in all types of climates.