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  1. #1
    Join Date
    Jul 2008
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    86

    How to best supply make up air for a bathroom vent

    I'm building a duplex with two interior bathrooms in each unit (no windows). To do what I can to control moisture when it is tenant occupied, and totally dependent on mechanical ventilation, I have installed higher end bathroom fans that will automatically come on when sensing humidity and will supposedly move 110 CFM.

    I'm wondering what is the best way to supply make up air in a bathroom where the door will be closed when the fan is going (shower time) and no return air supplied (because it is a bathroom) to make it possible for the fans to do their job without starving for air. What would you do? The units should be fairly tight, infiltration wise. I have not yet had a fresh air supply to the return of the air handler installed but am seriously considering having that done.

    Anyone know how many CFMs a decent bathroom vent fan can pull under a 2' 8" door with a 1" gap at the bottom if there is make up air available outside of the bathroom?

    BTW, not sure how it is elsewhere in the country, but around here the bathroom vents are usually installed and supplied by the electrician.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,339
    The undercut on the bathroom door works fine. Only 500 ft per min. velocity under the door. The duct is 1" long, vertilily no pressure drop.
    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"

  3. #3
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Quote Originally Posted by DavidJ View Post
    I'm building a duplex with two interior bathrooms in each unit (no windows). To do what I can to control moisture when it is tenant occupied, and totally dependent on mechanical ventilation, I have installed higher end bathroom fans that will automatically come on when sensing humidity and will supposedly move 110 CFM.

    I'm wondering what is the best way to supply make up air in a bathroom where the door will be closed when the fan is going (shower time) and no return air supplied (because it is a bathroom) to make it possible for the fans to do their job without starving for air. What would you do? The units should be fairly tight, infiltration wise. I have not yet had a fresh air supply to the return of the air handler installed but am seriously considering having that done.

    Anyone know how many CFMs a decent bathroom vent fan can pull under a 2' 8" door with a 1" gap at the bottom if there is make up air available outside of the bathroom?

    BTW, not sure how it is elsewhere in the country, but around here the bathroom vents are usually installed and supplied by the electrician.
    You could just rely on random infiltration for this make up air. What ill effects do you imagine if the ventilation is exhaust only?

    I am in the hot-humid South and the method you propose would not be effective for humidity control. This is why: even indoor "bad, humid" air will have a lower dew point than the outside air most of the year. Presumably you are in a climate where this is not the case?

    According to a 2002 publication by Tamarack Technologies, a 1-inch crack under a 30-inch door will flow 50 CFM at a pressure difference of 0.01 inch water column. Combining that with a 6-inch jumper duct would allow 100 CFM at that same pressure. This is about the pressure difference allowed by the Florida building code. My own house creates pressure differences about twice that much -- not ideal but I think it will not create a problem.

    The following link has valuable info about airflow:
    http://www.buildingscience.com/docum...-grille-sizing

    However BSC seems to recommend you not worry about make up air for intermittent loads, as I recall.

    Hope this helps -- Pstu

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,317
    Turn on the bathroom fart fan and close the door. Stand on the outside of the door with a digital manometer and stick the open end of the hose, that is attached to one of the two ports on the manometer, under the door into the bathroom.

    If you measure a notable pressure difference, the door undercut isn't allowing the full CFM the fan can pull into the bathroom for makeup air. If the pressure difference is zero, you're golden. Set the manometer to measure in pascals.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Jul 2008
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    86
    Quote Originally Posted by pstu View Post
    You could just rely on random infiltration for this make up air. What ill effects do you imagine if the ventilation is exhaust only?
    I was concerned that in a tight structure, it's going to have a hard time finding 110 CFM of random infiltration and it can only create so much of a vacuum in the duplex unit. My plumber was telling me about his range hood exhaust fan. He related that for the first minute or so it works great and you can see the smoke/steam being sucked up through the hood. After about a minute it's visibly clear that the smoke/steam is just kind of hovering over the range. If he cracks open a window the smoke/steam immediately starts back up the hood.

    Now, I don't know how many CFM's his fan is pulling so it might not be a fair comparison but I just want to make sure the vent will have the air it needs to operate. In my bathroom vent case, the fan will continue to run until it has dropped the humidity to whatever level the adjustment is set at. I was hoping that the unit would work efficiently enough to stay ahead of the moisture to keep it from dripping off the walls.

    I just did some simple math to think through this further. 1,200 square feet X 8' ceilings is 9,600 CF in the unit. 9600 CF/110 CFM is 87 minutes for the bathroom fan to do one complete air exchange in the unit. 60 minutes in an hour / 87 minutes puts the fan at .69 ACH. Now I'm sure that in the real world even with the windows wide open the fan probably doesn't move that much air as numbers are always inflated but it provides food for thought as to what is going on when you turn on the fan. Of course, you would hope that the fan is only running for a few minutes and not bringing in that much air. Incidentally, the fan can be manually turned on but I'm using a timer instead of a switch so that it doesn't run all day.

    Yes, here in central Florida, I don't like the idea of going against my attempts at building a tight structure by providing a ready source of make up air that will be hot and humid. One could argue that any outside make up air would contain less moisture than a bathroom with a shower running full tilt.

    Thank you all very much for your thoughts and the links that were shared. I'll check them out. Anyone else want to weigh in?

  6. #6
    Join Date
    Dec 2003
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    2,390

    Talking

    110 cfm is a lot of air. How did you decide this was how much you need? Personally, I would just put the fan on a wall switch. Im not to keen on the humidistat, and what about a stinkerstat?

  7. #7
    Join Date
    Oct 2006
    Location
    So. NH
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    746
    Quote Originally Posted by DavidJ View Post
    I was concerned that in a tight structure, it's going to have a hard time finding 110 CFM of random infiltration and it can only create so much of a vacuum in the duplex unit. My plumber was telling me about his range hood exhaust fan. He related that for the first minute or so it works great and you can see the smoke/steam being sucked up through the hood. After about a minute it's visibly clear that the smoke/steam is just kind of hovering over the range. If he cracks open a window the smoke/steam immediately starts back up the hood.

    Now, I don't know how many CFM's his fan is pulling so it might not be a fair comparison but I just want to make sure the vent will have the air it needs to operate. In my bathroom vent case, the fan will continue to run until it has dropped the humidity to whatever level the adjustment is set at. I was hoping that the unit would work efficiently enough to stay ahead of the moisture to keep it from dripping off the walls.

    I just did some simple math to think through this further. 1,200 square feet X 8' ceilings is 9,600 CF in the unit. 9600 CF/110 CFM is 87 minutes for the bathroom fan to do one complete air exchange in the unit. 60 minutes in an hour / 87 minutes puts the fan at .69 ACH. Now I'm sure that in the real world even with the windows wide open the fan probably doesn't move that much air as numbers are always inflated but it provides food for thought as to what is going on when you turn on the fan. Of course, you would hope that the fan is only running for a few minutes and not bringing in that much air. Incidentally, the fan can be manually turned on but I'm using a timer instead of a switch so that it doesn't run all day.

    Yes, here in central Florida, I don't like the idea of going against my attempts at building a tight structure by providing a ready source of make up air that will be hot and humid. One could argue that any outside make up air would contain less moisture than a bathroom with a shower running full tilt.

    Thank you all very much for your thoughts and the links that were shared. I'll check them out. Anyone else want to weigh in?
    The range hood your plumber is talking about may be capable [2000 cfm] of depressurizing the whole house, not so likely with a 110 cfm bath fan. Humidistats and/or timers are an excellent idea, if not fan on with the light works too, or a time delay off after the light is turned off. Point is to make sure it is at least used most of the time it should.

    Also consider if your heating/cooling is a ducted system, when not operating provides a bypass to other rooms via the ducts. When operating [with no return in that room] it supplies make up air.

  8. #8
    Join Date
    Sep 2010
    Location
    NY
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    325
    It would probably pull enough air from the supply register and the door undercut. You can purchase a cheap instrument , ie u-tube manometer to check "s of water coulomb to see if differential between rooms is excessive.

  9. #9
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    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    You make a good observation that in a really tight house, makeup air will make a difference. Lstiburek and team observe that with their houses, ASHRAE levels of ventilation will create a 2-3 Pa pressure difference. They also endorse supply-only ventilation as being inexpensive and not harmful in such houses, even in climates with low outdoor dew point. So a question is why is there not already a fresh air intake running to the return plenum?

    I would like to inquire about the ability of that bath fan to actually reduce humidity. You probably have not taken measurements, but I wonder about the %RH of the air being exhausted, vs. that of the makeup air (wherever it comes from). Actually the ABSOLUTE humidity is the important factor, so answer in terms of dew point or grains.

    The scenario that concerns me is this: Indoor humidity is likely 50% RH and 75F, or about 55-60 dew point. Many summer climates chronically have higher dew point than that all day. So the makeup air gets cooled to 75F, then what dew point is it? Without some form of dehumidification that humidity sensor may never have a reason to shut off that bath fan. Please tell me why this will not happen in your house.

    Best of luck -- Pstu

  10. #10
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    Fort Worth, TX
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    Quote Originally Posted by pstu View Post
    You make a good observation that in a really tight house, makeup air will make a difference. Lstiburek and team observe that with their houses, ASHRAE levels of ventilation will create a 2-3 Pa pressure difference.
    This pressure difference being between indoors and outdoors with all of the exhaust fans in the house operating? Or the house is positively pressurized with a fresh air intake at ASHRAE ventilation rates?




    I would like to inquire about the ability of that bath fan to actually reduce humidity. You probably have not taken measurements, but I wonder about the %RH of the air being exhausted, vs. that of the makeup air (wherever it comes from). Actually the ABSOLUTE humidity is the important factor, so answer in terms of dew point or grains.
    Let's take a mock bathroom, say with a 10 x 10 footprint and nine foot ceilings. That's 900 cubic feet. A 120 CFM exhaust fan actually moving that much air will turn over the bathroom air around 8 times per hour (120 CFM x 60 minutes = 7200 cubic feet per hour / 900 cubic feet = 8).

    The humidity load on the bathroom is intermittent. Once the occupant stops showering or bathing, flushing the commode, using the sink, etc. and then leaves the bathroom, there is no more extra humidity generation occurring other than if there's any infiltration from outdoors taking place inside the bathroom with the fan running. The incoming air from the house is at a lower dewpoint than the air being exhausted from the bathroom, so at some point the higher dew point air will be completely replaced by the lower dew point air from the house a/c system. If the a/c is running and the supply vent to the bathroom is open, this will occur faster than with just the fan by itself running.

    The scenario that concerns me is this: Indoor humidity is likely 50% RH and 75F, or about 55-60 dew point. Many summer climates chronically have higher dew point than that all day. So the makeup air gets cooled to 75F, then what dew point is it?
    If the makeup air must first go through an operating evaporator, it will first mix with the return air, altering its dew point, and both together pass over the cooling coil and subsequently emerge with a dew point of around 55 or lower, if the system is operating correctly.


    Without some form of dehumidification that humidity sensor may never have a reason to shut off that bath fan. Please tell me why this will not happen in your house.

    Best of luck -- Pstu
    It will shut off if the sensor is located and set correctly. Keep in mind the momentary nature of bathroom moisture generation.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
    Join Date
    Jul 2008
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    86
    Quote Originally Posted by pstu View Post
    You make a good observation that in a really tight house, makeup air will make a difference. Lstiburek and team observe that with their houses, ASHRAE levels of ventilation will create a 2-3 Pa pressure difference. They also endorse supply-only ventilation as being inexpensive and not harmful in such houses, even in climates with low outdoor dew point. So a question is why is there not already a fresh air intake running to the return plenum?
    Yes, it was some of Lstiburek's writing that had/has me considering the make up air source fed into the return plenum.

    It seems to me that when you are talking about any ventilation such as a bathroom vent that is exhausting air out of a structure, there is no free lunch. Any air that is removed will have to be replaced as the building pressure reaches equilibrium with its outside environment. The air has to come back into the structure whether it be through a dedicated makeup source, or other holes in the envelope, however small they may be.

    If you had a perfectly sealed house and no make up air, you would create a depressurized structure where the level of depressurization would be determined by the power of the fan. In this scenario, if there was absolutely no leakage, intentional or otherwise, it would just get sucked back in through the same vent that was exhausting it once the fan turned off. It's not like you can create a permanent vacuum by removing air and not replacing it.

    It seems like the bottom line is that the air will come from outside the envelope, one way or another. Is there something that I'm not understanding?

  12. #12
    Join Date
    Oct 2010
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    SW Michigan, near Battle Creek
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    The bathroom exhaust air is going to be more humid than the hallway air that comes under the door (et is hallway ai plis humidity from the shower). If enough hot humid outside air enters the building the ac kicks in. A cool humid day might be a problem. Thats why they make dehumidifies.

  13. #13
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    Oct 2010
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    SW Michigan, near Battle Creek
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    921
    (it is hallway air plus humidity from the shower)
    doesn't some one proofread this stuff

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