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Community and Q&A

Use of bathroom exhaust fan for ventilation in a small building

8aHA2u3EL9 | Posted in Mechanicals on

I questioned in a previous thread if a bathroom exhaust fan could suffice for exchanging air in a tiny, but well-insulated and tight 250 square foot dwelling.

Someone confirmed, given the small space, that it should be fine and to put it on a timer as it wouldn’t need to be constantly on.

Another said: “assuming that the fan on a timer needs to be cycled on and off frequently (for example, every other 5min interval) during occupancy, to compensate for lack of low-enough cfm settings, how significant of an effect on power draw do those frequent start-ups have?”

So, how much would such a fan need to be on to encourage adequate air exchange and would the power draw on a bathroom fan be more efficient than another method or not?

Thank you for any insights!

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  1. GBA Editor
    Martin Holladay | | #1

    You want to follow the ASHRAE 62.2 recommendations, explained here: Designing a Good Ventilation System.

    Here's the formula: 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. So a 300 square foot house with one occupant needs 10.5 cfm of ventilation.

    So, if you bought an exhaust fan that moved 42 cfm, you would operate the fan for 25% of the time -- say, 15 minutes every hour.

  2. karlkorp | | #2

    In Canada, Section 9.32 of the Building Code is typically used rather than ASHRAE as it is far easier to calculate. Simply use 20cfm for Master Bedroom and Unfinished Basement and 10cfm for every other room.
    Bathroom fans are the most common ventilation method in my part of the country only because it is better than the other commonly used simplified HRV connection to the furnace duct work which requires both the HRV and furnace fan to run continuously.

  3. HGWNwRfBXY | | #3

    I'm not sure you'll find a fan that is designed to run that often that will operate at such a low speed. I've done many of these set-ups and in Canada it is required to run for 2 - 4 hour sessions per day. I highly recommend a Panasonic fan, something like this:

    Tied to a switch like this:

    I have had it set up either intermittently throughout the day for shorter periods and using the Canadian standard for this type of ventilation and the longer duration cycles provided much higher occupant comfort than the short bursts more often. Timings are set usually for a morning period and for an evening period, during the highest activity times and coincidentally during the central furnace system operation (which likely contributes to better air circulation and ventilation.)

    Sounds like a fun project, good luck!


  4. 8aHA2u3EL9 | | #4

    Thanks Michael,

    Another person recommended the panasonic fans with switch on the other post as well, so two separate recommendations sound like a good confirmation to me! Thank you for providing feedback. I will not have a furnace, but will be using electric radiant floor heat instead due to the tiny space (eventually with a little vented propane marine fireplace or tiny pellet marine fireplace for back-up, just in case the electricity is out or I want to heat it up more quickly.) For this reason, I wanted to make sure I have an independent ventilation system.

    It's been fun planning, and I hope the building part goes smoothly. It seems like the reclaimed materials are coming together nicely and I'm getting great feedback here to help me with building details. The dissonance on whether foam insulation is free enough of off-gassing discussion has already steered me toward using Roxul stone wool instead. And these comments have helped me understand and become more clear on my heating and ventilation needs.

    Thank you all!

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