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  1. #1
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,476

    Pros and cons of HRV vs Fresh Air Ventilation in a dry heating/cooling climate.

    I'm considering a HRV or Fresh Air Ventilation to improve my indoor air quality. The house is recently-built and fairly tight. Our summer design temp is 95 and winter is 16 (as I recall). Outdoor humidity is very low most of the time, with only short periods of maybe 60% on a few summer days. I keep the house at about 30% in the winter with a Aprilaire bypass humidifier. Summer usually runs about 30-50% with the AC running. I have dual-fuel (gas aux) with balance point set to 40F due to the current low price of natural gas here.

    I'm looking for opinions on these two options. The Fresh Air Ventilation would use something like the Aprilaire 8126 or equivalent (6" duct with damper). Of course the HRV is going to have a higher up-front cost, and the Fresh Air Ventilation would cost a little more over time due to the outside air being brought in. The air here is normally very dry, so dew point really isn't an issue.

    I would have this controlled by my VisionPro IAQ thermostat, either using ASHRAE or timed mode. My HVAC guy is very flexible and is willing to put in whatever I decide.

    Thoughts and opinions?

  2. #2
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    6,915
    How about another option

    Have you considered an ERV? This would recover not only most of the heat but also some of the moisture in the winter that it is exhausting out of your house. An ERV would put less of a load on your humidifier than the HRV or fresh air options. ERVs typically have fewer moving parts to potentially fail than HRVs.

    In Albuquerque your winters are mild enough (I think) that the extra installed cost of HRV/ERV option may not pay off. You would have to calculate the amount of heat you would recover in a years time taking into account the total cfm of air exchanged and the efficiency of the HRV/ERV.

  3. #3
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,476
    Yes, HRV or ERV would be options. If I was in some other area of the country I probably wouldn't even consider simple Fresh Air Ventilation, but I think it could work well here. I have a friend who does energy surveys (LEED stuff) and he said I should consider the HRV.

  4. #4
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    I was just wondering about the fresh air ventilation. Without a HRV or ERV, I would have mixed air from outside air that may be a lot cooler than inside in the winter. I run my blower (VS ECM) on low all the time. Would this cool the air enough to notice, coming out of the supply vents when the furnace isn't running?

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,064
    A home needs an air change in 4-5 hours when occupied to purge indoor pollutants and renew oxygen. When the wind blows and its cold outside, expect an increase in natural infiltration. When the wind is calm and temps outside are warmer, supplemental fres air ventilation is usually required.
    Therefore very little fresh air is needed during cold weather. Investigating the rise of the inside air dew point verses the outside dew point will indicate the amount of natural fresh entering the home. Currently your outdoor dew point is 24 ^F. What is you inside temp/%RH/dew point? If your current indoor dew point is 43^F and you have 2 occupants, you are getting 70 cfm of fresh air moving through your home. This may be enough fresh air infiltration to indicate no need for more fresh air during this type of weather.
    Two occpants add 1 lb. of moisture per hour.
    Give us your inside air conditions to help determine the amount of fresh air naturally entering your home.
    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"

  6. #6
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Indoor coditions:
    Indoor: 72F, 30%, 2 adults and 2 children
    We heat to 72F and cool to 76F.
    When cooling we generally have 30-50% RH indoors.

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,064
    t
    Quote Originally Posted by garya505 View Post
    Indoor coditions:
    Indoor: 72F, 30%, 2 adults and 2 children
    We heat to 72F and cool to 76F.
    When cooling we generally have 30-50% RH indoors.
    Thanks for the interest.
    72^F, 30%RH, = 39^F dew point
    Outdoor for your area is 23^F currently. The air passing through your home enters at 23^F and leaves at 39^F. Your family typically adds an average of .5 lbs of moisture per hour respiration, presperiation, and activities. If your doors were wide open with high air flow, the air entering and leaving would be the same. If your home was a plastic bag with no air moving through it, expect 100%RH or saturation.
    Assuming you were typical and adding 1.5 lbs. of moisture per hour, how much air moving through your home? Using the I phone app "Ultra-Aire" estimates that you have 110 cfm of fresh air passing through your home. The infiltration varies with wind and stack effect. Also clothes drier, bath fans, fireplace, and kitchen hood will change the air flow through your home. During calm weather with moderate temps, expect much less infiltration.
    110 cfm is adequate fresh air for 3,666 sqft of home at .2 ach. I would size a make-up air duct fan for the fresh air you want, connect it to your return duct. Operating the make-up air when you are in the home and the winds are calm/moderate temps. An ERV/HRV will not have pay back because you do not need additional fresh air during cold weather. In fact you may benefit eliminating from additional house tightening during very cold, windy conditions.
    Also you may want to add a humidifier if you indoor %RH is <30%RH for extended periods.
    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"

  8. #8
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Teddy Bear, thanks for all the info.

    We have a humidifier that manages to hold 30% RH indoors.

    I understand what you are saying about infiltration and air moving through the house. A simple ASHRAE calculator says I need about 48 CFM (continuous), so if I'm getting 110 already during all but moderate conditions, it seems like ventilating during cold, hot, or windy condtions is a waste of energy.

    Why couldn't the need for ventilation be based on CO2 in the home? Then, under conditions were you have enough "natural" ventilation, it would be OFF, and it would open the vent under moderate condtions when the CO2 goes up. Somebody must have thought of this before.

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,064
    Quote Originally Posted by garya505 View Post
    Teddy Bear, thanks for all the info.

    We have a humidifier that manages to hold 30% RH indoors.

    I understand what you are saying about infiltration and air moving through the house. A simple ASHRAE calculator says I need about 48 CFM (continuous), so if I'm getting 110 already during all but moderate conditions, it seems like ventilating during cold, hot, or windy condtions is a waste of energy.

    Why couldn't the need for ventilation be based on CO2 in the home? Then, under conditions were you have enough "natural" ventilation, it would be OFF, and it would open the vent under moderate condtions when the CO2 goes up. Somebody must have thought of this before.
    I am impressed! You are absolutely right. A CO2 control is ideal to activate ventilation when the home is occupied and natural ventilation is short. I have been making that argument for a couple years. The biggest problem is that there is not much of a martket for that kind of a device for retail.
    Eventually, enough will understand the need for an ideal control to provide fresh air when needed.
    Keep us posted on how you provide fresh air during the calm weather.
    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"

  10. #10
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Quote Originally Posted by teddy bear View Post
    I am impressed! You are absolutely right. A CO2 control is ideal to activate ventilation when the home is occupied and natural ventilation is short. I have been making that argument for a couple years. The biggest problem is that there is not much of a martket for that kind of a device for retail.
    Eventually, enough will understand the need for an ideal control to provide fresh air when needed.
    Keep us posted on how you provide fresh air during the calm weather.
    Regards TB
    I think maybe one of the problems is that the controllers are not cheap. I see the Honeywell C7232A1008 that would do such a thing, but requires a 4-hour unoccupied period for "Automatic Background Calibration". I would imagine that this works well in commercial applications where the building is vacant every night, but for a residence maybe not.

  11. #11
    Join Date
    Feb 2010
    Location
    In a boiler room
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    6,915
    Quote Originally Posted by garya505 View Post
    I think maybe one of the problems is that the controllers are not cheap. I see the Honeywell C7232A1008 that would do such a thing, but requires a 4-hour unoccupied period for "Automatic Background Calibration". I would imagine that this works well in commercial applications where the building is vacant every night, but for a residence maybe not.
    Most commercial co2 sensors require that 4 hour period, it is needed to correct for sensor drift. It could be during the day too. Is there occasionally a time when your house is empty with everyone at work? It doesn't have to be every day.

  12. #12
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,476
    Quote Originally Posted by chuckcrj View Post
    Most commercial co2 sensors require that 4 hour period, it is needed to correct for sensor drift. It could be during the day too. Is there occasionally a time when your house is empty with everyone at work? It doesn't have to be every day.
    I was thinking about that. There are times when the house is empty for 4 hours, but maybe only once or twice a week.
    According to the manual for the honeywell C7232A sensor, it only needs one unoccupied 4-hour period per week.
    "Sensor must be mounted in a location which sees at lease one 4-hour unoccupied period per week so that
    the CO2 level drops to outdoor levels. Automatic Background Calibration will not work properly in locations
    without four hours of unoccupied time per week, or where there are sources of CO2 other than people
    (breweries, mushroom farms, etc)"


    So, I think it would work in our house, but I guess it would rule out having a home brewery.

    Using CO2-based ventilation seems like such an obvious improvement over a timed or ASHRAE control algoritm, I wonder why I can't find more information about people actually using a system like this. The controllers aren't cheap, but they're not hideously expensive either.

  13. #13
    Join Date
    Jun 2007
    Location
    Minneapolis, MN
    Posts
    705
    We specify a lot of foam and ERV is the answer. I personally use one in my renovated 1921 farm house and it is needed (wind blowing or no). We also specify and install ERVs as bathroom exhaust fans with master timer and individual 20 minutes push timers...RenewAire most of the time.

    I lived in Albuquerque for three years and now reside in Minneapolis. However you are "conditioning" the air, you certainly don't want to waste what you paid to condition.

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