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Thread: Booster fan question - sorry ;)

  1. #1
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    Booster fan question - sorry ;)

    Hi everyone, I need help on finding the right booster fan for my application. I know for a fact that you must already have a bad opinion at that point but hopefully I can explain why I think that this time, it just might help with my situation.

    Initially the ductwork was made for a basic central system, that I finally replaced with a carrier greenspeed due to the insulation and airtightness of the house (long story short, everything is awesome and cost of operation is great)
    only thing I now had as a problem is that it wasnt efficient to climate the whole house down to 17C during summer when I only needed the 3 rooms of the 2nd floor at that temperature.
    I managed to make modifications to the ducts and added a zoning system.

    Now my problem : the second story bedroom vents (2 per room, 1x5+1x6, 6totals) run from the the mech room with lots of elbows. my new zoning duct is 14x8 or something like that and managed to connect the bedrooms to it.

    anyhow, even with a good zoning damper as close as possible and everything full open and optimized as it can be with that setup, I require a high cfm from the carrier system to be able
    to push air at night.

    Now, the real problem I have is that there are 3 returns on the 2nd floor, one per bedroom, and there is a 18x18 register on the bottom of the the staircases (open/modern stairs) and there are two 12x12 opening on the intake side of the furnace. I had one cutout initially, but had to add a second one as it required a lot of return air when running full speed and there is just too much elbows, turns and duct lenghts on the returns of the 2nd story that the machine was cavitating without it.

    Now you see where I am getting at! sorry if this was a long journey So my problem now is that I can have sufficient AC in the bedrooms, its comfortable and such, but if I leave the doors open it will never be able to cool and the whole house will be cooled down for nothing. If I close all the bedroom doors before night cooling, there is a lot, i mean a lot of air exiting underneath the door, which is sucked all the way down through the house by the basement mech room 18x18 register. So again, the whole house goes down to 17C at night even if the damper/zoning shoots all
    the air to the 3 bedrooms with the doors closed. Even on 22-25C nights we sometimes go down for breakfast and its 16C so we start the gas fireplace. (Sometimes I wake up for work earlier like 4:30am and I still want to keep the bedroom at 17 until girlfriend wakes up, and makes no sense to cycle from heating/cooling when its 25 outside).

    So what I am looking for would be to add a booster fan on the return air from the bedroom. The three return ducts are joined togeter in a 8inch duct in the mech room. (just a reminder, these are the only returns ducts I have, the main returns is two 12x12 cutouts on the side of the vertical duct prior to the filter that feed from the 18x18 opening in the mech room wall at the bottom of the staircase)

    Ideally, I could have a 8 inch booster fan running on that return duct at low speed when the system is working, but it would kick in fast speed when the system has the damper set to shoot air only for the bedrooms.

    Note that right now, a kleenex does hold in place in front of the return vents in the bedrooms, but if I blow on it sideways it falls.

    Or just a booster that doesnt turn unless zone 2 becomes priority and then starts. I can wire any of those scenarios easy (i think), its a matter of knowing if such fans exist, are they energy efficient, and what do you guys think?

    Thank you very much for your time and your help,


    Sent from my iPhone using Tapatalk

  2. #2
    Join Date
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    I consider a booster fan to be a band-aid rather than a solution.

    The equipment is sized for the entire house. It requires a specific amount of air flow depending on demand and the duct system. Zoning MUST be thoroughly designed and matched to the equipment in order to work as expected. Few contractors are good at it.
    If you really want drastically different temperatures in areas of the house, a multi-zone ductless system would be a better choice. Perhaps a single zone ductless would give you the cooling you need for one room.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  3. #3
    Join Date
    Dec 2005
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    I've never seen a booster fan on a return duct. I don't think it's a good idea nor will it help.

    Are your returns high on the wall in the upstairs rooms? I have seen many newer homes where there's a grill and they use the wall cavity as a return duct but at the floor they don't want to cut the opening in and add fire stops so they just drill a few holes to let some air through. The point is you may not have a full return opening. You also need to make sure all the returns are connected at the furnace, I've found plenty that were never hooked up.

    If you're sucking that much air back then it's starving for return air. You need to get someone to look at the duct system, verify it's all connected, and see where and how you can improve it.
    Good? Bad? I'm the guy with the gun.

    Statements made by me are strictly my opinions and do not reflect the opinions of my employer. I am not authorized to make any official statements on behalf of my employer.
    Any technical advice offered by me is for educational purposes only, all HVAC related repairs should only be attempted by qualified personnel.

  4. #4
    Join Date
    Jul 2019
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    Thread Starter
    Hello again.

    thanks for the feedback.

    returns from 2nd floor are ducted all down to the intake of the unit.

    let me throw you some numbers : system pushes 468 cfm in 3 bedrooms. the return from those
    three bedrooms from a reading is 130 cfm.
    system prefers to pull its air from the bottom of the staircase as the second story ducts have lengthy runs and elbows etc.

    I do know that a booster is a band aid and we rarely see them especially on return ducts, but my problem is highly specific. zoned, with too much air bleeding underneath the doors ending up cooling the rest of the house.

    Look its a 3 floors x 1000 sqft house, im near Montreal with cold winters and hot summers, and I pay 500$ CAD per year for heating and cooling. I think its pretty good. but I want added comfort for summer season where I dont want to run the system in AUTO mode allowing it to heat the basement and 1st floor during summer due to too much cold air not being sucked properly.

    As for adding a second system, I think that a booster fan is a way smaller bandaid compared to adding a whole second unit which, in the end, will not increase comfort, it would add a 3rd zone in the master bedroom, (unless you blow air on the whole 2nd floor and we have to keep the doors open (no way) and in the end the air will just drop down just as right now, and this will consume more energy than a booster.

    If I can size a booster properly and have the air circulate within the second floor bedrooms without dropping downstairs too much Im thinking I might spend a couple a dollars more on the booster fan, but end up having more comfort in the morning when the system is in COOL mode and the 1st floor is also at 17C when im eating breakfast. I might even save a bit of money during spring and fall at which I run the system in AUTO.

    In other words, the problem is not a matter of zoning or capacity or comfort in the zone that are trying to achieve a certain temperature, its that the loop of the air is just wrong since its not being pulled at the right places and goes through the whole house instead of going in their dedicated return ducts.

    BTW : most of my problems come from the fact that I had to add a larger I Beam under the second floor that messed the ducting up a lot.

    I know what im asking is uncommun and thats why I want your input, im a mechanical engineer myself with a good amount of experience in construction and I have a HVAC specialty (from back when I was in university) and I myself never seen or did this before so this is why im asking.

    once again thank you! really appreciate it.

  5. #5
    Join Date
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    It appears you are hoping to triple the air flow in the returns with a booster fan. That will not happen.

    How did you measure the air flow?
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  6. #6
    Join Date
    Jul 2019
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    Thread Starter
    Hello, sorry I was on vacation

    I am hoping to increase the 2nd floor return flow from roughly 150-200 cfm to 400-500 in order to match more closely the 470 cfm being pushed in the bedrooms at night.

    to prevent the air being pushed on the 2nd floor to be sucked entirely by the return across the house in the basement


    Sent from my iPhone using Tapatalk

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