Construction Materials-Humidex Garage Ventilation

Last winter, during the International Builder’s Show in Las Vegas, I found a garage ventilation system that I thought would work well for humidity control in my very cold climate.  I finally have a project where I could install and test the system.  The Humidex GVS-SD2 was installed at the #codeminimumhouse.  I wrote a blog post about attached heated garages in a cold climate, read that posting here.

The attached and heated garage at the code minimum house was constructed so that the garage floor is 7 inches (one normal step) below the house floor.  This step down eliminates any chance for water that may accumulate on the garage floor from entering the home.  I also feel that any fumes from chemicals stored in the garage, such as gasoline, may settle to the floor and have less of a chance to enter the home.

The garage also has a large floor drain so that most liquid water will move out of the garage before it has a chance to become vapor.  The space is heated by a hot water radiant system placed throughout the concrete floor.  A very nice system for quickly warming a cold vehicle and also speed floor drying.  Drawback is this drying is changing liquid water to water vapor and increasing the humidity within the garage.  The garage is air sealed and insulated similar to the home.  With the potential of a new moisture load entering the garage daily from a wet or snow covered vehicle, it’s important to address this increased humidity level, and the best way is to provide some sort of mechanical ventilation.

The garage floor drain rough-in at the Code Minimum House.

The best type of garage ventilation is with balanced mechanical ventilation such as a heat recovery ventilator (HRV) which would need to be dedicated for the garage and sized accordingly.  (Do not tie any forced air heating or ventilation equipment from the house with the systems used for a garage.  Always keep these systems separate.)  The drawback is that an HRV unit is expensive.  A second option would be to install a dehumidifier which will only remove moisture, not supply fresh air to the garage.  I see these often used for humidity control during energy audits.  The issue is they are a high electricity use appliance typically increasing electric bills $40 or more per month.  I think that in the case of a garage, exhaust only ventilation is a good option.  Typically there will be plenty of air leakage around the garage doors to more than offset the negative pressures from an exhaust only system.  The Humidex is a good middle of the road solution.

The Humidex GVS-SD2 comes as a kit containing the cord and plug surface mounted fan, a short piece of flexible duct, exterior damper and mounting screws.  One of the nice things about the kit, it doesn’t require any kind of rough in or pre-planning when used in new construction.  The product works well in both new and existing garages.  I installed the unit at the code minimum house after the garage walls were painted and the siding complete on the exterior.

The instructions suggest installing the unit 12 inches from the floor close to an electrical outlet.  The GVS-SD2 only allows for cord and plug as a power source, there is not an option for hard wiring the fan.  I deviated slightly from the instructions in that I choose to use metal piping instead of the plastic flexible duct.  I installed the duct at a slight angle to hopefully drain any moisture that should accumulate in the duct to the outside.  I taped the inside of the duct to the wall damper, again so that any moisture would drain out the exterior damper and not into the wall.

Cutting the LP Smartside and installing a mounting block and the exterior damper completed the exterior installation.  This is one area where pre-planning the location before siding would be beneficial.  Integrating the WRB to the mounting block to maintain a water-tight connection would be a best practice.  Installing a metal head flashing and tying to the WRB when remodeling is challenging.

The interior mounting of the fan unit is straight forward, attach the fan to the duct and screw the unit to the wall.  The instruction tell you to seal the plastic flex duct to the fan.  In my case, I could only friction fit the fan to the metal duct, the only way to add a seal would be to remove the damper fins on the exterior and try to seal the inside through the 4 inch metal vent pipe.  Next to impossible with a sticky tape, I chose just to provide the friction connection.  This will also make removing the unit for service or replacement easier if needed.

My one complaint in installing the GVS-SD2 was mounting to the wall.  There are several different screws sent with the unit, 4 small head screws, two large head screws and several more very short screws.  The short screws are used to attach the flex duct to the fan and exterior damper.  I assumed the 4 small head screws were to attach the exterior damper to the wall and the 2 large head screws were to attach the fan to the wall.  Well, there are two different sized holes in the fan unit to mount the fan to the wall.  One large head screw worked in the larger mounting hole, but did not fit in the smaller mounting hole.  I ended up using one screw from mounting the damper on the smaller hole fan mount.  Not clear in the directions.  In my mind, the fan mounting holes should be the same size, just me venting.

Now that the installation is complete, I could test the fan.  Plugging in the fan lit up several indicators on the control panel.  There is a single button for changing the desired humidity level setting, which are 25%, 35%, 45%, 55% and 65%.  There was also a flashing exclamation point which, according to the instructions, will flash for up to 48 hours on start-up to warm up the CO sensing electronics.  The fan could be heard but it was not overly noisy.  Nice that at least you can hear if it’s operating or off.

The nameplate rating sticker is located on top of the fan and can easily be seen.  The fan has a continuous duty rating meaning it is designed to operate all the time.  It also has the electrical draw but does not indicate the cubic feet per minute of air movement through the fan.

I used my Kill-O-Watt meter to verify the electrical consumption of .22 amps, which was less than the nameplate rating of .3 amps.  Converting the .22 amps to wattage is done by multiplying the amperage and voltage.  .22 amps x 120 volts = 26.4 watts.  The fan will cost roughly $2.50 per month to operate in my area.

Now for testing the CFM rate, I used my flow meter to test the ventilation rate which ended up just under 25 CFM.  So, how many air exchanges does that equal?  Lets do some more math, exciting, right?  The garage is 667 square feet and has a sidewall height of 9’6″ giving us a total volume of 6336.5 cubic feet.  The 25 CFM exhaust rate needs to be changed to cubic feet per hour.  25 CFM x 60 minutes = 1500 cubic feet per hour.  1500/6336.5=.24 air changes per hour or roughly one complete air exchange every 4 hours.  Is that enough air exchange for this garage?  The answer to all building science questions, it depends.  How much moisture loading inside the garage will depend on how much snow, ice and water is dragged in by a vehicle.  How quickly the ice or snow changes from a solid to a liquid, indoor air temperature and desired humidity levels will also be a factor.  My guess is it won’t be able to maintain 25% humidity.  35% to 45%, probably.  Time will tell.  If this were a larger garage, a dedicated HRV unit would work best.  HRV’s are capable of much higher CFM exhaust rates than the 25 CFM rate of the Humidex.

Let’s explore costs.  An HRV unit installed is going to run $1500-$2500 depending on unit size and complexity of installation.  HRV’s energy consumption will be 60-100 watts, three to five times that of the Humidex.  A dehumidifier will cost from $100-$1000 to purchase and more than ten times the operating cost of the Humidex, nearing $500 per year if operating continuously.  The Humidex GVS-SD2 will be around $600 to purchase and install, again depending on complexity of the install and only cost $30-$40 per year to operate.

My opinion, as long as the garage is less than 700 square feet, roughly 6500 cubic feet, the Humidex is a good option.  It will definitely be included as one of the choices to control garage moisture levels in my energy assessment/audits and building diagnostic reports.

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