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  1. #1

    Help with HRV System

    We have a ranch style home, 1600 sq ft on the main floor and 1400 sq ft in the basement of which 1000 sq ft is finished. Our home was built in 2013 and it's an Energy Star home so it is very air tight. Our indoor air quality is suffering due to this. I haven't been able to locate a local HVAC company who installs HRV systems, so I will likely be installing it myself unless I can find an installer. I'm pretty handy so I'm not worried about the actual installation process, but I do have a few questions/concerns:

    1. We live in Northern Colorado on the front range where winter night time temperatures can get down to about -20F and most winter nights are in the single digits and teens. From what I've gathered from my research, we would require an HRV as ERVs don't do well in colder climates. Is this correct?

    2. What size unit would we need? According to the ASHRAE 62.2 formula, we would need 7.5 CFM per occupant (4 occupants = 30 CFM) + 3 CFM per 100 sq ft ((3000/100) * 3 = 90 CFM) for a total of 120 CFM. Is this an accepted formula for determining HRV airflow and would a 120 CFM unit be sufficient or would I want some extra capacity?

    3. The unit will be installed to an existing HVAC system. From the various HRV installation instructions I've seen, the fresh air supply duct from the HRV would connect to the air return supply duct, close to the furnace, and the exhaust ducts from the HRV system would connect to the same air return supply duct, but further from the furnace. Is this correct? Are there any other installation options to consider?

    4. Does the HRV run all of the time? If so, does the furnace/AC blower fan also run all of the time?

    5. Are there any brands/models of HRV units I should avoid? Any that you would recommend? Given my allergies, HEPA filtering is a must for any unit I get.

    Any advice and recommendations are greatly appreciated.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,643
    Most homes air tight home get enough fresh air during the windy cold winter weather. Signs of inadequate air flow is excessive condensate on the windows of the home and stuffy odors. If so start by operating a small exhaust fan until the excess moisture declines. This will raise your heating bill a very small amount. As the temperatures rise and wind decline in milder seasons, the amount of natural infiltration declines to the point that mechanical ventilation is a must. But the benefit of an HRV declines because of the warming of the fresh air. In the end of the story, mechanical fresh air ventilation is a must during the milder season when the home is occupied and windows closed. You also have exhaust devices like your clothes drier and exhaust fan, bath and kitchen that need make-up air. Many are using make-up air ventilation systems to provide fresh when occupied. If a wood burner is involved, the need for makeup air is even more important. HRVs do not provide make-up air.
    The last issue is the indoor moisture control during the damper time of year. Any extended time of +60%RH inside the space like basement will grow mold. Adequately ventilated home during +60^F outdoor dew points plus the moisture from the occupants easily exceed the above recommendations require supplemental dehumidification. Even short periods like a week or two will start the mold grow process.
    For ideal comfort and indoor air quality, you may need more than a simple HRV. You are probably about right on the amount of air flow from the make-up point. Avoid installing anything that requires operation of the central air handler during occupancy hours unless it is VS fan. For supplemental dehumidification and make-up filtered fresh air, the site sponsor, the small Ultra-Aire.
    Keep us posted on the level of winter moisture problems you have experienced and the use of wood burner.
    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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