Building Science-How to Test a Bath Fan.

Recently I was at a house conducting an energy audit when the homeowner indicated they were experiencing high humidity in the master bathroom.  The windows were fogging up and they had some mold growing in one of the ceiling corners.  They operate a bath fan regularly but it didn’t seem to remove any moisture.  There are a few tests I can conduct to see if the fan is moving air.  Test one, the toilet paper test.  I take a sheet or two of toilet paper and see if the fan sucks it up to the grill.  Test two, use a smoke generator to see if the smoke is drawn into the fan.  

Both are good tests to show if the fan is moving air, but neither tell me how much air is being moved.  The proper way to test a bath fan is with a flow pan or flow hood.

This is my exhaust fan flow hood, the hood along with my DG-700 pressure and flow gauge will measure the CFM rate of most fans.  This tool will measure fans with up to 125 CFM of exhaust, more than enough to measure the typical bath fan and even some kitchen exhaust fans.  Once the DG-700 gauge is set up, simply fit the hood over the fan and read the gauge.  Very simple.

As you can see, this fan was moving 44 CFM (the number on the right side of the gauge) at a pressure of around 4.5 Pascals (the number on the left side of the gauge.   This fan would not meet the current code requirement of 50 CFM intermittent but would meet the 20 CFM continuous requirement, but the fan would need to be wired to operate continuously.   This bathroom does have a window so a bath fan was not required.

The flow meter has three settings for different CFM rates.  The opening settings (the sliding door on the right side of the flow meter) are listed E1, E2 and E3 with E1 being the largest opening for fans that move more CFM.  Most fans I test are at the E2 setting (where the sliding door is set in the picture).  The Pascal reading needs to be between 1 and 8 Pascals to achieve the most accurate measurement.

I also recently tested the Humidex Garage Exhaust Fan that was installed in the Code Minimum House.  This fan is designed to operate continuously until the moisture sensor on the unit tells the fan to shut off.  This nearly continuous operation allows for a much lower CFM rate but still meet the need for humidity reduction.

You might have noticed the pressure side of the pressure gauge had a reading of .1 Pascals.  I didn’t have a hose connected to that port so no reading was given.

We’ve seen a couple different tests using The Energy Conservatory’s Exhaust Fan Flow Meter.  An easy test to commission exhausting equipment.  What about the home in the opening paragraph, the home with high moisture and mold?  Well, that bath fan wasn’t moving any air.  There wasn’t an exhaust damper anywhere on that side of the home.  I suspect the duct was missed and the end of the fan is plugged with something.  Unfortunately there was no access to the attic in that portion of the home, repairs will be difficult.  The fan was simply a noise maker.  My suggestion to the homeowner, contact a HVAC company and have the bath fan ducted correctly.

As a side note, this is the first blog posting with video.  As much practice for me as giving information to you.  What do you think?  Please leave me a comment.

2 Replies to “Building Science-How to Test a Bath Fan.”

  1. Hi Randy, the videos worked ok.

    Perhaps it would be useful to reference them (eg “As you can see **in the video above**, this fan was moving 44 CFM …”). I had been looking for the photo showing the gauge!

    Summing up on the bath fan was helpful. What did you think about the garage fan? I’m not familiar with those – what is the required cfm?

    1. Hi Sue, thanks for the feedback!

      The humidex Garage Fan is good for garages up to 750 square feet and operate at 75 CFM. They are available on the Home Depot website, there are two options, one that just monitors humidity and a second model that monitors both humidity and carbon monoxide. I have installed two units for customers and they work well. Cost is around $500 for CO monitoring version. You can get a little more info here:

      I’m planning on updating this specific post on how to test a bath fan, I have recently purchased a new tool for testing air flow for both fans and forced air systems. Hopefully will have time to produce the videos and write that blog post in the next couple months.

      Thanks again for the comments!


Leave a Reply

Your email address will not be published. Required fields are marked *