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  1. #1
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    Adjust CFM levels for Bypass Humidifier

    I have a 3 zone system (basement, main, 2nd floor) and I'm trying to get the heating CFMs dialed in for each zone. How does a bypass humidifier impact the CFM for each zone? Does it reduce the CFMs by a percentage, a set number or what?

    For example, if I want to get 600 CFMs through my 12" round trunk feeding my basement, what would that zone need to be set to to overcome the CFM loss to the bypass?

    Thanks.

  2. #2
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    How are you measuring the CFM,s?

  3. #3
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    I don't. The zoning requires setting the CFM levels per zone. It is a fully communicating furnace so it takes care of itself as far as maintaining CFMs and adjusting blower speeds.

    I assume that if the zone needs 600 CFM, I would have to set the output for that zone to be somewhere around 700 CFM but I don't have anything to base that guess on. Maybe it is only a 50 CFM difference. I don't know how to guess at this.

  4. #4
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    It tries to maintain. If the ducts are too small they generally fail after a few years.

    Have a contractor come out and actually measure the air flow then you can decide what to do.

  5. #5
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    What manufacturer and which control are you using?
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  6. #6
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    How did the installer set it up?

    Why are you changing their set up?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  7. #7
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    I'd like to keep this from being manufacturer specific or installation specific. I am just trying to figure out how would you typically calculate the furnace heating CFM settings to account for a zone need and the lost CFM to a bypass. Does that make sense? I'm not sure I'm wording this the best way.

    If everyone must know the specifics, I believe my installer set the CFM settings too low. The humidifier has a hard time keeping up (clean pad, full hot water flow) and I think my upstairs CFM levels are not quite high enough which makes my rooms furthest from the supply trunk upstairs a bit cooler than other rooms.

    Lennox SLP98V 70k
    Healthy Climate 20x25 5" MERV 16 filter
    Aprilaire 600 bypass humidifier
    XP16 2-ton w/ matching 2-ton coil (this is not used for heating, only the furnace is)
    iComfort T-stat
    iComfort zoning
    3 zones (basement, main, 2nd floor)
    9x20 supply main floor
    10x16 supply 2nd floor
    12" round supply basement
    ~3300 finished sq ft (2400 above ground)


    I did find some decent resources on how to measure the static pressure across the furnace and based on that estimating the air flow through the bypass. I think I understand the proper process for doing it but I know my installer did not take such measurements and they don't generally get into details of design like I would prefer. I was hoping to get an idea of what some typical adjustments would be if possible.

  8. #8
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    What you are trying to do is system and house specific. Now that there are some details someone familiar with the I-Comfort control may be able to help you. If this were a Carrier/Bryant system I could point you in the right direction using their controls. I will say the VS blower should compensate for the bypass humidifier. Set the zones to the proper airflow based on the heating and cooling needs.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  9. #9
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    Thread Starter

    Thumbs down

    Quote Originally Posted by comfortdoc View Post
    What you are trying to do is system and house specific. Now that there are some details someone familiar with the I-Comfort control may be able to help you. If this were a Carrier/Bryant system I could point you in the right direction using their controls. I will say the VS blower should compensate for the bypass humidifier. Set the zones to the proper airflow based on the heating and cooling needs.
    The iComfort system does set CFMs in the furnace but it has no way to know how much CFM is going through the bypass. I don't see how any system would be able to automatically do that compensation.

    I'll try asking this another way. If you have a single stage furnace supplying a whole home with no zoning and the heating design is for 800 CFMs....then add a bypass humidifier. Would you up the furnace CFM setting to 850, 900, or 950 CFM so that with the bypass humidifier operating you are still sending the full 800 CFMs to the rest of the house?

  10. #10
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    The answer to your question is no.

    The VS blower compensates if there is a static pressure change through the furnace. Carrier can increase airflow by 10% for bypass humidifiers if necessary, but this is setup and tested for safe operation by the installing contractor.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  11. #11
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    Quote Originally Posted by comfortdoc View Post
    The answer to your question is no.
    Thank you for that. So if you were supplying 800 CFM before the addition of the bypass you would not increase the CFM of the blower and still produce 800 CFM. This results in less air flow to the house than what was originally designed for. May I ask why that is? Wouldn't you want to maintain 800 CFM to the house as per original design? Why would the addition of a bypass humidifier allow a lower CFM delivery to the house?

  12. #12
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    You are not losing any, or at least not significant, CFMs with a bypass humidifier. The air is staying in the ductwork.

    Depending on the set up, I would be running the humidifier even if there is no call for heat. Many companies set up the humidifier to run only with a call for heat and that amount of run time is usually not enough to satisfy the need.

    70K for 3300 sq' and 3 zones may not be the correct set up needed for your situation. You said they did not do details so how was the system chosen?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

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