Results 1 to 10 of 10
  1. #1
    Join Date
    Feb 2012
    Location
    colorado
    Posts
    12

    Long runs for ERV

    Asked our local contractor to give us an estimate on an HRV install. The house is new construction and (when we close) it will have two 8" insulated flexi ducts to serve as the fresh air intake and stale air exhaust. There are two 90 degree bends, one fairly sharp, the other much less so. Unfortunately, the total run of each is about 50-60 feet. Going straight through the utility room wall to the outside is not an option.

    House is a ranch with a finished basement. 3900 sq ft total, 4 br, 3.5 ba. In dry Colorado.

    Is this going to work, or is there too much resistance to flow with the long lengths?

    Any recommendations on a unit that would do better with these long runs. A Broan HEPA ERV is their default unit.

    Thank you very much!

  2. #2
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,943
    Quote Originally Posted by teahouse1 View Post
    Asked our local contractor to give us an estimate on an HRV install. The house is new construction and (when we close) it will have two 8" insulated flexi ducts to serve as the fresh air intake and stale air exhaust. There are two 90 degree bends, one fairly sharp, the other much less so. Unfortunately, the total run of each is about 50-60 feet. Going straight through the utility room wall to the outside is not an option.

    House is a ranch with a finished basement. 3900 sq ft total, 4 br, 3.5 ba. In dry Colorado.

    Is this going to work, or is there too much resistance to flow with the long lengths?

    Any recommendations on a unit that would do better with these long runs. A Broan HEPA ERV is their default unit.

    Thank you very much!
    Since they are 8" ducts, there should be no problems. Have vents installed in the floor or low wall behind all of your toilets for best results.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  3. #3
    Join Date
    Mar 2006
    Location
    Denver/Boulder
    Posts
    2,231
    Quote Originally Posted by teahouse1 View Post
    Asked our local contractor to give us an estimate on an HRV install. The house is new construction and (when we close) it will have two 8" insulated flexi ducts to serve as the fresh air intake and stale air exhaust. There are two 90 degree bends, one fairly sharp, the other much less so. Unfortunately, the total run of each is about 50-60 feet. Going straight through the utility room wall to the outside is not an option.

    House is a ranch with a finished basement. 3900 sq ft total, 4 br, 3.5 ba. In dry Colorado.

    Is this going to work, or is there too much resistance to flow with the long lengths?

    Any recommendations on a unit that would do better with these long runs. A Broan HEPA ERV is their default unit.

    Thank you very much!
    Where in colorado?
    What lies behind us and what lies before us are small matters compared to what lies within us.


    Two pressures, four temperatures = SUCCESS!


    Boulder Heating Contractor


    For Consumers:

    For HVACR Professionals:


  4. #4
    Join Date
    Feb 2012
    Location
    colorado
    Posts
    12
    In Denver, thank you.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,372
    Quote Originally Posted by teahouse1 View Post
    Asked our local contractor to give us an estimate on an HRV install. The house is new construction and (when we close) it will have two 8" insulated flexi ducts to serve as the fresh air intake and stale air exhaust. There are two 90 degree bends, one fairly sharp, the other much less so. Unfortunately, the total run of each is about 50-60 feet. Going straight through the utility room wall to the outside is not an option.

    House is a ranch with a finished basement. 3900 sq ft total, 4 br, 3.5 ba. In dry Colorado.

    Is this going to work, or is there too much resistance to flow with the long lengths?

    Any recommendations on a unit that would do better with these long runs. A Broan HEPA ERV is their default unit.

    Thank you very much!
    Expect a 50% reduction in ventilation capacity with these long runs. My experience says that if your home is typical of current construction that you will have +.2 Air changes an hour on a cold windy day from natural ventilation without any operation of the HRV. An air change in 4-5 hours is adequate to purge indoor pollutants and renew oxygen according to ASHRAE/EPA. You need some mechanical ventilation during calm winds and moderate temperature. 8" ducted HRVs are rated for 200-300 cfm of fresh air. 50% is 100-150 cfm of real air flow. This is probably enough to handle your needs during calm winds.
    I would suggest good bath fans for your key baths that you use everyday. Also a good kitchen hood.
    Control is the key to effectively managing your fresh air. No ventilation is required when the home is unoccupied. When occupied and the winds are up to +7 miles per hour, no ventilation is needed. But when the building is occupied and the winds are calm, mechanical fresh air is a must. Most current controls do not deal with this well. Most homes are grossely overventilated during winter weather and underventilated the rest of the year.
    Monitoring the CO2 levels in the home for determining your real need for fresh air. You only need fresh air ventilation when the CO2 levels are +650 PPM which indicates occupancy without enough fresh air infiltration. I am monitoring a foam insulated home in WI that only needed 6 hours of ERV operation during the month of FEB because of +7 mph winds. Yet during moderate temps and calm winds, the home needs 100 cfm fresh air when occupied.
    CO2meter com has monitors for <$200.
    Most are using moisture levels in your home to operate the HRV/ERV. When the home is damp fresh air ventilation is triggered. Unfortunately, a couple occupants in homes like yours can be dry and yet not being get enough fresh air. On the other hand, when outdoor dew points rise, adequate fresh air raises the indoor %RH indicating the need for fresh air. Yet the %RH control can never be satisfied with ventilation. Confusing?? Remeber minimal fresh air ventilation during calm weather and occupancy year around, is a must.

    Keep us posted on your methods and results.
    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
    Feb 2012
    Location
    colorado
    Posts
    12
    Thanks TB. Always appreciate your insightful posts.

  7. #7
    Join Date
    Mar 2006
    Location
    Denver/Boulder
    Posts
    2,231
    Quote Originally Posted by teahouse1 View Post
    In Denver, thank you.
    Just a couple of things...


    1) In my experience most HRV's ducts are run in flex duct to the outside. Flex duct impedes airflow, especially for longer runs. The last time I ran HRV ducting like that(longer run) I installed hard pipe, completely sealed the ducts, then insulated them. each step is important, and takes time to do correctly. The needs of your system depend on your design characteristics: desired peak airflow, length of duct, type of duct, number of turns, etc.

    2) The particular HRV airflow requirements are determined by thorough analysis as defined in ASHRAE standard 62.2. not all contractors know how to do this.
    What lies behind us and what lies before us are small matters compared to what lies within us.


    Two pressures, four temperatures = SUCCESS!


    Boulder Heating Contractor


    For Consumers:

    For HVACR Professionals:


  8. #8
    Join Date
    Feb 2012
    Location
    colorado
    Posts
    12
    Quote Originally Posted by darctangent View Post
    Just a couple of things...


    1) In my experience most HRV's ducts are run in flex duct to the outside. Flex duct impedes airflow, especially for longer runs. The last time I ran HRV ducting like that(longer run) I installed hard pipe, completely sealed the ducts, then insulated them. each step is important, and takes time to do correctly. The needs of your system depend on your design characteristics: desired peak airflow, length of duct, type of duct, number of turns, etc.

    2) The particular HRV airflow requirements are determined by thorough analysis as defined in ASHRAE standard 62.2. not all contractors know how to do this.
    Thank you. I wish it was constructed as you speak, agree pressure drop would be much less, and much better design for longer life and cleaning. Problem is that the flexi ducts are now placed between the floor joists in the basement ceiling, which is now finished. One end, the 2 ducts drop into the basement utility room with our HVAC equip, the other duct ends run through (about 15 feet) some accessible crawl and to the outside.

    The number's I'm looking at show ~0.2-0.25"water pressure drop at 140 CFM over 100 feet of length with 15% compression. Seems to go up quickly with more compression/sag. I'm sure bends add to that.

    Could you test the resistance in the current system to see if it would be ok? There would just be so much added expense to doing it any other way. Could change the portion in the crawl-space (which actually has more sag) to sheet metal duct as you speak somewhat more realistically ($).

    What would be an acceptable amount of resistance to flow? Would be happy with 140 CPM max flow. Considering a Broan HEPA ERV right now.

    May send you an email off line.
    Thanks!

  9. #9
    Join Date
    Mar 2006
    Location
    Denver/Boulder
    Posts
    2,231
    Quote Originally Posted by teahouse1 View Post
    Thank you. I wish it was constructed as you speak, agree pressure drop would be much less, and much better design for longer life and cleaning. Problem is that the flexi ducts are now placed between the floor joists in the basement ceiling, which is now finished. One end, the 2 ducts drop into the basement utility room with our HVAC equip, the other duct ends run through (about 15 feet) some accessible crawl and to the outside.

    The number's I'm looking at show ~0.2-0.25"water pressure drop at 140 CFM over 100 feet of length with 15% compression. Seems to go up quickly with more compression/sag. I'm sure bends add to that.

    Could you test the resistance in the current system to see if it would be ok? There would just be so much added expense to doing it any other way. Could change the portion in the crawl-space (which actually has more sag) to sheet metal duct as you speak somewhat more realistically ($).

    What would be an acceptable amount of resistance to flow? Would be happy with 140 CPM max flow. Considering a Broan HEPA ERV right now.

    May send you an email off line.
    Thanks!
    can't really say, too many variables in play. yes, it can, and should be measured as part of commissioning the system. If you use the system with constant fan you typically only need a fraction of the hrv's capacity, so you could very well be ok. It's only when higher CFM's are required by the system design or by user preferences (home owner wants to "freshen up" the house in a big hurry ) that a HRV's full capacity is used in my experience.

    Personally I'm partial to the Lifebreath HRV's but Broan has their fans too.
    What lies behind us and what lies before us are small matters compared to what lies within us.


    Two pressures, four temperatures = SUCCESS!


    Boulder Heating Contractor


    For Consumers:

    For HVACR Professionals:


  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,372
    Quote Originally Posted by darctangent View Post
    can't really say, too many variables in play. yes, it can, and should be measured as part of commissioning the system. If you use the system with constant fan you typically only need a fraction of the hrv's capacity, so you could very well be ok. It's only when higher CFM's are required by the system design or by user preferences (home owner wants to "freshen up" the house in a big hurry ) that a HRV's full capacity is used in my experience.

    Personally I'm partial to the Lifebreath HRV's but Broan has their fans too.
    More things to be concerned about. Clothes drier, bath fans, and kitchen hood need makeup air. Fireplaces, water heaters, and heating systems all need make-up air to function.
    I recently started a monitoring project of a foam insulated home. An impressive list and installation of all the equipment you could think of, including a large CO2 controlled ERV. I monitored the CO2 levels, indoor/outdoor ^F, dew points,%RH, and run times of ventilation for 4 mos. While the house ventilation maintained CO2 levels with minimal operation during +10 mph winds and extensive runs during calm winds and moderate temps. I could not determine sensible/latent recovery of the ERV.
    I took a closer look at the install of the ERV and found a dampered exhaust hoods on both sides of the ERV. In other words no fresh air inlet, only exhaust. This is a lesson I learned before. My mistake. Than I measured the fresh air in and stale out. 200 cfm of exhaust 8 ft of duct and 100 cfm of fresh air on 30 ft. of inlet dust, mostly because of the difference in duct length on the fresh air inlet. Does this make the case for comissioning??
    In the words of the owner, I would have used this set-up the rest of my life and beed satisfied. Fortunately, all sealed combust devices in the home. If we had an open combustion, with clothes drier, kitchen hood, and bathfans, back drafting for sure, even with a properly installed ERV.
    If there are any open combustion devices in these air tight homes, use make-up fresh air ventilation for the basic fresh air system. Homes in cold climates with normal winds need little fresh air to purge pollutants during winter. Yet they need fresh air ventilation during calm moderate weather.
    A lot concerns about our stategies we are using in these complex homes.
    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"

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event