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I have a new tight home and am advised to run my bathroom fan 4 to 6 hours a day

GBA Editor | Posted in General Questions on

…to promote healthy air change and eliminate moisture. However, my interior humidity is 32% and I feel we need more to be comfortable. Is running the bathroom fan counterproductive by eliminating moisture? How do we determine “bad air”? If we don’t run the fan the prescribed time we don’t notice any difficultly breathing , congestion etc. Is it sufficient to run the forced hot air fan continuously to circulate humidity from showers etc. rather then exhausting the air?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dick,
    You need to provide more information.

    1. Your interior humidity is 32%. Do you think that is too high or too low? Why?

    2. You write that you need to be "more comfortable." In what ways are you now uncomfortable?

    3. I'm glad you don't have any difficulty breathing and you don't suffer from congestion. Maybe everything is fine the way it is.

    4. Don't run the furnace fan continuously for circulation. The fan provides no useful benefits but uses a tremendous amount of energy. Some furnace fans draw 700 or 800 watts.

  2. Dick | | #2

    The recommended indoor humidity is 39% which reduces nighttime dryness and resulting colds etc. What detrimental effects would ensue from not mechanically ventilating a tight home? My furnace fan is a variable speed which my HVAC provider tells me would only cost a couple of dollars a year to run continuously. My thought is to circulate humid air from the bathroom throughout the house.

  3. GBA Editor
    Martin Holladay | | #3

    Dick,
    There is a wide variability when it comes to preferences concerning indoor humidity. If you want to try to hit 39% on the button, go for it — although it's hard to hit any number dead on, because weather changes all the time.

    I think the jury is still out concerning whether "nighttime dryness" results in colds. I'll leave that discussion to you and your doctor.

    Do you suffer from frequent illness? If not, relax.

    Most furnace fans are off more than they are on. I'm guessing that the typical furnace fan runs for only 4 to 6 hours per day on a year-round average, but this number will vary from house to house. If you run your furnace fan for an additional 18 hours per day, just to circulate the air in your house, and if your electricity costs $0.12 per kWh, then running the fan will cost you:
    A. For a 200-watt blower: $157.68 per year.
    B. For a 400-watt blower: $315.36 per year.
    C. For an 800-watt blower: $630.72 per year.

  4. homedesign | | #4

    Dick,
    I am surprised that your tight home is so dry.
    Where do you live and how tight is your house?
    Did they do a blower door test?

  5. Dick | | #5

    I'm in Ma. and blower test measured leakage to outside 30CFM Duct leakage 30 CFM25, Infiltration Htg: 599 Cig: 599 CFM50. HERS index. 59. Still looking for an answere to my main questain.

  6. GBA Editor
    Martin Holladay | | #6

    Dick,
    I think we've been trying to help. So tell me — what's your main question that you feel has not yet been answered?

  7. homedesign | | #7

    Dick,
    Your main question is interesting.
    did they calibrate/measure your exhaust system?to arrive at the run time?

  8. homedesign | | #8

    I think he is asking if he should ramp down his ventilation

  9. GBA Editor
    Martin Holladay | | #9

    Dick,
    Although I advise builders to provide mechanical ventilation systems that meet ASHRAE 62.2 requirements, it's up to the homeowners to decide how to operate the equipment. You can ventilate a lot or ventilate a little — it's up to you.

    As long as your windows aren't dripping with condensation, you probably aren't damaging your house.

  10. Dick | | #10

    My main concern was if any detremental health effects would ensue from not ventalting. Or is it only possible damage to the house? I think the condensation on the windows is a good measure.

  11. GBA Editor
    Martin Holladay | | #11

    Dick,
    As I suggested earlier, it's best to consult your doctor or a medical forum with medical questions. Here at GBA, we're builders, architects, and energy experts. We're not medical professionals.

    I'm sure plenty of the regulars here are eager to chime in with advice on whether high humidity or low humidity is best for your lungs. Take it all with a big grain of salt. Let me put it this way: humans have evolved to live healthy lives in deserts as well as very humid jungles. Human lungs work fine in both environments.

    Your house; your lungs. Run it how you want.

  12. homedesign | | #12

    Dick,
    I still think that you should question your contractor about the actual measured flow.
    I had to insist that they return and measure my fresh air flow...otherwise they are just guessing and you may be ventilating beyond code (or below code)

  13. Tom | | #13

    One way to use your bathroom to exhaust air is to use a programmable time switch.
    There are several on the market. The Tamarack Advantage 1A is the only one that allows you to variable speed your fan. You can find them at Tamtech.com

  14. Dick | | #14

    Martin, This is not a medical question. I merely wanted to know if the ventilation requirement was to protect the individuals or the house, after doing some additional research elsewhere I have concluded that it may affect either. Thanks for trying. Thanks to John for the info and Tom I do have a programmable timer on my fan.

  15. Riversong | | #15

    Dick,

    The code ventilation requirements are to protect both the house and the occupants. Old leaky homes were often too dry in the winter, but new tight homes of modest size are typically too humid.

    Excess humidity can cause:
    1) window condensation and consequent staining of wood trim
    2) condensation within the thermal envelope and consequent mold and rot
    3) damage to electronic equipment
    4) increase in dust mites and potential for insect infestation
    5) increased outgassing of formaldehyde from binders in manufactured lumber products

    A new house starts out with hundreds of gallons of excess moisture in the building materials, particularly lumber and concrete. The typical family introduces 4-5 gallons per day to the interior environment from normal activity. Without sufficient ventilation, including spot ventilation in bathrooms and over the kitchen range, that daily humidity adds to the moisture contributed by the drying of the house and can build to dangerous levels.

    Excess ventilation not only wastes energy, both the electricity to run the fans and the fuels required to heat the replacement air, it can also over-dry the conditioned space, particularly in a large house whose volume tends to dilute the daily anthropogenic humidity.

    Insufficient humidity can cause:
    1) shrinkage and damage to wood floors and furniture
    2) respiratory problems
    3) skin and throat irritation

    A house prefers relative humidity between 20% and 40% in cold climates. Humans prefer humidity between 30% and 70%. The best compromise, then, is to maintain indoor relative humidity between 30% and 40%. Some inexpensive hygrometers are notoriously inaccurate. Make sure you have a good quality digital hygrometer that keeps track of lows and highs.

    But whole house ventilation is not simply for controlling humidity (though that's often a good indicator of overall indoor air quality). Most new homes also have formaldehyde (from carpeting, furniture, cabinets and building materials) which is a trigger for generalized chemical sensitivity, and carcinogenic radon gas is common in all parts of the country (particularly the northeast).

    There are 17,000 petrochemicals available for home use, only 30% of which have been tested for safety. The typical American household has 63 synthetic chemicals, and the bloodstream of the average American contains 145 chemicals.

    Chemicals commonly found in the home include:
    - Persistent Bioaccumulative Toxins (PVC, CPVC, CSPE)
    - Persistent Organic Pollutants (PBDE, HBCD, PFOA)
    - Volatile Organic Compounds (formaldehydes, phthalates)
    - Heavy Metals (mercury, lead, cadmium)
    - Inhalable Particulates (fiberglass, wood smoke)
    - Combustion Byproducts (C02, CO, aldehydes, phenols, benzoprene, cresols)

    So there are many good reasons to maintain the ASHRAE approved level of ventilation, with or without heat recovery, of 1 cfm per 100 sf of conditioned area plus 7.5 cfm per bedroom (plus one). If you feel this overly dries your house for comfort and health, you can use a humidifier or more potted plants.

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