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  1. #1

    ERV vs whole house Dehum with fresh air?

    We are building a new house in NC and we are a bit at odds with the ventilation requirements of the structure. Its a completely foamed house ( Floors walls, and roof deck)with an Infinity Green Speed HP unit. I'm wondering if installing an ERV ( Balanced) is the best route or installing a separate whole house de-hum with a fresh air vent would make more sense? I had thought about the Honeywell logical fan switches for the exhaust portion hooked up to a few bath fans with the whole house dehum. but I'm sure that is not a balanced way of doing it? What are the risks? I have been told the Infinity will do a fine job of dehumidifying the house and to just go with the ERV, but I just dont see it dealing with the hot humid days as well as a dedicated unit. Maybe I can run my air less with just a dehum?

    Thanks for the help.

  2. #2
    Join Date
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    With a tight house, you shouldn't have any problem with a high humidity on the hot humid days.
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  3. #3
    Quote Originally Posted by beenthere View Post
    With a tight house, you shouldn't have any problem with a high humidity on the hot humid days.
    i guess my thought was that an ERV would run several times per day and the tighter the house the less the AC would run... so wouldnt the ERV bring in the humid air from outside?

  4. #4
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    Feb 2009
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    Tallahassee, FL
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    Yes but it does remove some of the heat and humidity first.

  5. #5
    Join Date
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    An ERV doesn't bring in as much humidity as an HRV does. It also brings in less heat then a whole house dehumidifier will.
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  6. #6
    Join Date
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    Quote Originally Posted by URTOAST View Post
    We are building a new house in NC and we are a bit at odds with the ventilation requirements of the structure. Its a completely foamed house ( Floors walls, and roof deck)with an Infinity Green Speed HP unit. I'm wondering if installing an ERV ( Balanced) is the best route or installing a separate whole house de-hum with a fresh air vent would make more sense? I had thought about the Honeywell logical fan switches for the exhaust portion hooked up to a few bath fans with the whole house dehum. but I'm sure that is not a balanced way of doing it? What are the risks? I have been told the Infinity will do a fine job of dehumidifying the house and to just go with the ERV, but I just dont see it dealing with the hot humid days as well as a dedicated unit. Maybe I can run my air less with just a dehum?

    Thanks for the help.
    Clothes drier, kitchen exhaust, and bath fans need make up air to function. Negative pressure sucks in moisture and dirty air through the few remaining air leaks in these air tight homes. Also positive pressure in these air tight homes stops infiltration through the unwanted air leaks in these home. Thus using filtered make-up air as the main method of purging indoor pollutants and renew oxygen may have some benefits. All a/c needs a significant cooling load to remove moisture. when there is no cooling load, the a/c will not be able to remove the 2-6 lbs. of moisture per hour from the occupants and the needed fresh air ventilation. ERVs do transfer some moisture and heat from the incoming fresh air to the exhaust air, when the exhaust is drier than the incoming fresh air. The typical transfer is 39-49% efficiency. with this low efficiency, after several air changes, the home will be as wet as the incoming outside air. Well ventilated ERV homes require a whole house dehumidifier to maintain <50%RH during low/no cooling load times of the year.
    True, the mention varible speed a/c removes more moisture during partial cooling load conditions but is unable to remove without significant cooling loads. I am currently monitoring a ERV/mentioned a/c. Last fall and this spring, dehumidification has been required when there was no cooling load. The monitoring is continueing with a couple hours of cooling and many hours of dehumidification.
    During typical winds and stack effect this winter, there was enough natural ventilation in this foam insulated home to not need to operate the ERV. The ERV was used to ventilate the during calm moderate temp weather. The target was to maintain an air change in 4-5 hours.
    Clearly, whole house dehumidification is required in these home if you want to maintain <50%RH when the outdoor dew points are +55^F and there is low or no cooling loads.
    100 cfm of fresh air at 65^F dew points is couple lbs. per hour and the moisture from the occupants may be couple lbs. of moisture per hour. During wet cool weather, you need to remove 3-4 lbs. of moisture per hour. The variable a/cs do not run enough to maintain <50%RH.
    The a/c guys try to avoid fresh air ventilation to avoid the moisture loads. Get these statements about humidity control in writing.
    I will post the monitoring results as these conditions occur this summer.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  7. #7
    Quote Originally Posted by teddy bear View Post
    Clothes drier, kitchen exhaust, and bath fans need make up air to function. Negative pressure sucks in moisture and dirty air through the few remaining air leaks in these air tight homes. Also positive pressure in these air tight homes stops infiltration through the unwanted air leaks in these home. Thus using filtered make-up air as the main method of purging indoor pollutants and renew oxygen may have some benefits. All a/c needs a significant cooling load to remove moisture. when there is no cooling load, the a/c will not be able to remove the 2-6 lbs. of moisture per hour from the occupants and the needed fresh air ventilation. ERVs do transfer some moisture and heat from the incoming fresh air to the exhaust air, when the exhaust is drier than the incoming fresh air. The typical transfer is 39-49% efficiency. with this low efficiency, after several air changes, the home will be as wet as the incoming outside air. Well ventilated ERV homes require a whole house dehumidifier to maintain <50%RH during low/no cooling load times of the year.
    True, the mention varible speed a/c removes more moisture during partial cooling load conditions but is unable to remove without significant cooling loads. I am currently monitoring a ERV/mentioned a/c. Last fall and this spring, dehumidification has been required when there was no cooling load. The monitoring is continueing with a couple hours of cooling and many hours of dehumidification.
    During typical winds and stack effect this winter, there was enough natural ventilation in this foam insulated home to not need to operate the ERV. The ERV was used to ventilate the during calm moderate temp weather. The target was to maintain an air change in 4-5 hours.
    Clearly, whole house dehumidification is required in these home if you want to maintain <50%RH when the outdoor dew points are +55^F and there is low or no cooling loads.
    100 cfm of fresh air at 65^F dew points is couple lbs. per hour and the moisture from the occupants may be couple lbs. of moisture per hour. During wet cool weather, you need to remove 3-4 lbs. of moisture per hour. The variable a/cs do not run enough to maintain <50%RH.
    The a/c guys try to avoid fresh air ventilation to avoid the moisture loads. Get these statements about humidity control in writing.
    I will post the monitoring results as these conditions occur this summer.
    Regards TB


    So the Greenspeed's dehum ability is less then a dedicated whole house unit...

    If we run an ERV we will in most cases need dehumidification in the spring and fall.....

    If I keep the house pressurized it will benefit the structure and people living in it.

    Would it not be sensible to tell them to install a whole house dehum with a fresh air intake port and maybe a fresh air damper that opens and closes per the demands of the T-Stat and fresh air requirements of the home? That way we get the Dehum ability's we need in the spring and fall, we get the fresh air, and I'm not sure if that keeps the house positive but maybe that damper thing will help with that?

    All in all I think they need to come up with an ERV-Dehum in one box that can sense when the house is going negative and bring in more air and run the dehum when needed but not all of the time...

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by URTOAST View Post
    So the Greenspeed's dehum ability is less then a dedicated whole house unit...

    If we run an ERV we will in most cases need dehumidification in the spring and fall.....

    If I keep the house pressurized it will benefit the structure and people living in it.

    Would it not be sensible to tell them to install a whole house dehum with a fresh air intake port and maybe a fresh air damper that opens and closes per the demands of the T-Stat and fresh air requirements of the home? That way we get the Dehum ability's we need in the spring and fall, we get the fresh air, and I'm not sure if that keeps the house positive but maybe that damper thing will help with that?

    All in all I think they need to come up with an ERV-Dehum in one box that can sense when the house is going negative and bring in more air and run the dehum when needed but not all of the time...
    Keeping the home positive is not the biggest issue. Fresh air when occupied and there is not enough natural fresh air infiltration is important. Maintaining <50%RH is important.
    Getting these issues dealt with for a reasonable investment is important to most. The simplest method of getting fresh air into a home is via a fresh air inlet, an occupancy timer or CO2 controller on a high efficiency whole house ventilating dehumidifier is the most practical. A merv 11 air filter for fresh air the air circulated through the home has merit. The up charge for the high SEER VS heat pump and ERV are personal decissions. These are expensive options that may have merit but will probably not pay for themselves.
    Also suggest the Ultra-Aire whole house ventilating dehumidifier like the UA 105H which the most efficient dehu made, +8 pints per Kw.
    Keep us post on how this adventure works out.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  9. #9
    Quote Originally Posted by teddy bear View Post
    Keeping the home positive is not the biggest issue. Fresh air when occupied and there is not enough natural fresh air infiltration is important. Maintaining <50%RH is important.
    Getting these issues dealt with for a reasonable investment is important to most. The simplest method of getting fresh air into a home is via a fresh air inlet, an occupancy timer or CO2 controller on a high efficiency whole house ventilating dehumidifier is the most practical. A merv 11 air filter for fresh air the air circulated through the home has merit. The up charge for the high SEER VS heat pump and ERV are personal decissions. These are expensive options that may have merit but will probably not pay for themselves.
    Also suggest the Ultra-Aire whole house ventilating dehumidifier like the UA 105H which the most efficient dehu made, +8 pints per Kw.
    Keep us post on how this adventure works out.
    Regards TB
    Apparently the Green speed unit uses an optional pre-heat function to dehumidify without over shooting the temperature so I'm wondering if this will let it dehum without the AC demand. I'm sure the elect. strips come on, and I'm not sure how efficient this method is but it sounds as if it will dehum lower then a normal variable speed unit.

  10. #10
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    Quote Originally Posted by URTOAST View Post
    Apparently the Green speed unit uses an optional pre-heat function to dehumidify without over shooting the temperature so I'm wondering if this will let it dehum without the AC demand. I'm sure the elect. strips come on, and I'm not sure how efficient this method is but it sounds as if it will dehum lower then a normal variable speed unit.
    The benefit of the VS a/c unit is that they reduce their cooling speed to avoid the dreaded off cycle that allows moisture on the cooling coil to re-evaporate back into the home in a short time. This works well until the load declines to <40% of the capacity units capacity. As the cooling load declines below that, the VS units over-cool upto 3^F of the t-stat setpoint. During cool wet weather with low/no cooling, these units are unable to remove moisture. During +60^F outdoor dew points with adequate fresh air ventilation/infiltration and occupants adding moisture, you need to be able to remove 1-3 lbs. of moisture removal per hour. The only way to maintain <50%RH is with reheat or dehumidification. Reheat with the a/c removes <1 lb. of moisture per kw plus re-evaporation at the end of the cycle. High efficiency dehumidifiers like the Ultra-Aire dehu remove 4-7 pints per kw. In addition to the being a dehumidifier, this unit is capable of fresh filtered air ventilation and maintaining <50%RH regardless of cooling load or indoor temperature.
    Additionally this concept provides make-up air for exhaust devices like the clothes drier, kitchen hood, and bath fans.
    Weither you have a VS a/c or a simple high SEER a/c, you need a whole house dehumidifier to maintain <50%RH during low/no cooling loads and dew points +55^F.
    The VS a/c cost 50-100% more than simple a/c. The long term maintaince and repair is an unknown. They are expensive and complex. This will challenge a/c techs and the home owners finances as they grow old.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  11. #11
    Join Date
    Jul 2008
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    Rochester NY
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    Quote Originally Posted by URTOAST View Post
    Apparently the Green speed unit uses an optional pre-heat function to dehumidify without over shooting the temperature so I'm wondering if this will let it dehum without the AC demand. I'm sure the elect. strips come on, and I'm not sure how efficient this method is but it sounds as if it will dehum lower then a normal variable speed unit.
    So this unit is straight heat pump, not a hybrid?

    If it has a reheat feature, that IS a whole house dehumidifier. Can you post a link to the details?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  12. #12
    Join Date
    Jun 2003
    Location
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    There are several pre and post heating strategies to extend the cooling cycles. Preheating does not work. Post or reheating does work but is very inefficient. You could turn on the kitchen stove or anyother heat generating device in the home to extend the a/c cycle. These ideas remove <1 lb. of moisture per KW electricity. The Ultra-Aire/Santa Fe dehus remove 4-8 lbs. per KW.
    The biggest problem is that all of these devices load moisture on their cooling coils/drain pan before the moisture goes down the drain. It is a minimum of 1lb. of moisture per ton of capacity. The loading moisture is left on the coil at the end of the cooling cycle. 3lbs. of moisture re-evaporating back into the home at the end of the cooling cycle raises the indoor %RH 7-9%. A grossly over-sized dehumidifier has the same problem. Raising the indoor 7-9%RH with moisture from the coil, restarts the dehumidification cycle all over again.
    Ideally, get a simple high seer a/c sized to handle the highest cooling load you anticipate. Including enough for visitors and the hottest days is not the end of the world. Get the a/c setup with a cold cooling coil to achieve the temp/%RH, <50%RH. If the home is unoccupied 6-8 hours a day, use t-stat setup or set back. Many of the smart stats remember the recovery time needed to reach the normal temp and start early. During this extended cooling cycle, the home will be throughly dried.
    include a small whole house dehumidifier as part of the pkg. This will maintain <50%RH when the a/c is off or the outdoor dew points are high. If you want the best indoor air quality, include a small amount of fresh air ventilation when the home is occupied during warm calm weather. You will have a comfortable, healthy home with minimal operating cost.
    After having this, you will understand the benefits and the simplicity. What is so hard about this?
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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