Tuesday, July 12, 2011

A Solution For Hot Garages

After months of consideration, I finally decided to do something about my hot south facing garage.  I'd previously insulated the doors and made sure my entry door was sealed well, but in the Arizona desert, it seems to take much more than that.

I'm not so concerned about how hot the garage gets, but rather the living spaces above the garage (my daughters bedroom in particular).

The solution - a through the wall ventilation fan operated by a programmable timer switch.

Basics of Garage Ventilation Designs

There are a ton of ventilation systems available, but all consist of two main components:

  1. Exhaust Fan
  2. Intake Vent
Most ventilation systems are based around the concept of replacing a specific volume of air. For example, a garage may have a specific volume.  For example, your garage may have ceilings of 9', be 18' wide and 20' long...that is a volume of 9x18x20 = 3240 cubic feet.

Different applications requiring exhaust have a requirement to replace air at a specified rate.  They call this Air Changes per Hour (ACH).  For example, some toxic chemicals may have a higher ACH requirement than a woodworkers bench.

Exhaust fans capability to move air is measured in CFM - Cubic Feet per Minute.  Using the above 3240 cubic foot garage as an example, a 160 CFM fan would exhaust 3240 cubic feet in about 20.5 minutes, or resulting in just under 3 ACHs.  That is pretty low, but perhaps adequate for simply cooling (or keeping a garage cool).

Additionally, intake is a necessary requirement of a system. If your garage door is sealed tight, then you will need to add an intake vent. More on this later.

Exhaust Fan Selection

Considerations for selecting a fan are:
  1. CFM rating (to achieve the desired air changes per hour)
  2. Sones rating - this is a measure of the sound level of the fan.  A higher value is a louder fan.
  3. Power usage (efficiency)
  4. Design of the unit - through the wall or through the ceiling/roof?
  5. Is it rated for outdoor use?  Yes, there are many exhaust fans designed for interior use only.

For me, I went with the <a href="http://www.amazon.com/gp/product/B000WT8FOM/ref=as_li_ss_tl?ie=UTF8&tag=natusarch-20&linkCode=as2&camp=217145&creative=399369&creativeASIN=B000WT8FOM">Broan 509 180 CFM 6.5 Sones Through Wall Ventilation Fan, White Square Plastic Grille</a>.  I chose through-the-wall because I had a nice open area up high on my wall, and a through the wall installation seemed easier and faster.

Garage Exhaust Design Considerations

As mentioned above, I am assuming the only use is to ventilate the garage in order to cool it.  With that in mind, these are the primary considerations:

  1. Where will you place the fan?  Place it up high (duh...heat rises).  And place it on the opposite side of your air intake vents.
  2. Related to #1, place your air intake vents on the opposite side of the garage from the fan.  Place the intake low so cooler air can be drawn in.  And if possible, chose a cooler side of the house (north side or east side if possible).
  3. How will you operate the fan?  You can connect it to a standard wall switch, to a programmable timed wall switch, or any other sort of switch with an appropriate amperage rating for your fan.

And make sure your garage entry door is well sealed! You don't want your new exhaust fan to be pulling cool air from your home's interior into the garage. And this is especially true for those installing high powered fans.

I chose a programmable timer wall switch (<a href="http://www.amazon.com/gp/product/B000F3ZR8C/ref=as_li_ss_tl?ie=UTF8&tag=natusarch-20&linkCode=as2&camp=217145&creative=399369&creativeASIN=B000F3ZR8C">Aube by Honeywell TI033/U 7-Day Programmable Timer Switch, White</a>).  I have it run in the evening for a few hours (about 8 PM to 11 PM) to exhaust the "heat of the day", and again in the morning before it starts to heat up (4 AM to 6 AM).

Does This Project Save Electricity?

I think it is safe to say that this project does not save any money on my monthly electric bills. It does, however, make for a more comfortable room for my daughter, and helps exhaust noxious fumes from the cars and chemicals in my garage.

And who knows? Maybe it will become a net saver in the future when it keeps my daughter from turning the thermostat down lower (when she is tall enough to reach it!)

1 comment:

  1. why would you not run it during the day, to expel the heat as it builds up? it seems to me that you would want to run it from at the latest noon and until the sun goes down.