Any time you exhaust air out of your studio (be it in your house, garage or out-building), you need to provide replacement air from outside in equal amount to what is being exhausted.
The process of exhausting air forms a “partial vacuum” (also referred to as an area of lower pressure) inside the structure where the exhaust fan is located. Because of this “partial vacuum”, air will find any way it can to flow towards the fan.
Replacement fresh air is often overlooked by the do-it-yourselfer. The rule of thumb for replacement air is to provide fresh air at the same rate that combustion air is being exhausted. This means that if you are exhausting 400 CFM (cubic feet per minute) of combustion contaminated air, you also have to provide incoming fresh air (from outside the structure) at the rate of 400 CFM.
During the first energy crisis in the 1970’s, home builders and building code writers started to improve the design of houses to make them more energy efficient. One of the biggest changes has been to ‘tighten” up the houses or make them less prone to air leakage. Over the next 20 or so years, there seemed to be an increase in the amount of mold and mildew in houses and the next solution was to add in-house ventilation systems which provide fresh outside air, and exhaust used inside air. The net result of all these changes over the years is that there is much less air leakage and a much greater need to provide fresh air. Today, when mechanical systems are put into houses, the HVAC (Heating, Ventilation, and Air Conditioning) companies ALWAYS install a fresh air line into the house, usually somewhere near the furnace and/or hot water heater to provide make up air, or combustion air to these devices. This is current mechanical code across the United States.
Current code also calls for the fresh air supply to be a minimum of 10 linear feet away from any exhaust duct.
What happens if you don’t supply any fresh air? If there is no fresh air supply available for the studio exhaust system, the exhaust system will pull the air from the surrounding room and building. The air pressure in the room and building will begin to drop. As we all know, nature abhors a vacuum. Air will begin to flow back into the room and building from anywhere it can. Usually, this will be an opening that already exists in the building – such as the vents for plumbing (which results in sewage smells permeating the building) or the exhaust vents for furnaces and hot water heaters (which results in nitrous oxides and carbon monoxides being drawn back into the house). Anywhere air can flow back into the semi-partial vacuum created by the exhaust fan, it will. Eventually, as the vacuum pressure falls, the fan will begin to stall and overheat, but you will notice the smells and odors from back flow long before that happens.
The biggest complaint I have seen (and it is very frequent especially during the winter months and the concurrent heating season) is “I just cannot open the [window/door] right now because it is so cold outside!!”.
This is a valid issue and I certainly don’t want to lessen the importance of the complaint. First of all, the homeowner is spending a large amount of money to keep his/her house warm when it is cold outside. Opening a window or door is going to allow that heat to escape outside, or, as my mother would say when I was a child “I’m not paying to heat the outdoors you know”. And if the heat from inside doesn’t escape outside, you are drawing cold air inside and cooling off the whole house and the vicious cycle begins.
So, what to do?
The most energy efficient (note that I did not say least expensive!) way to solve the fresh air/makeup air dilemma is to duct your fresh air from its source directly to the workstation.
There are a variety of ways that this can be done:
Option 1, professionally ducted through the roof
The picture above shows both the incoming ducted fresh air (at upper left, wrapped in insulation) and the ducted exhaust systems. It is important to insulate the incoming fresh air ducts especially if your studio is in an area where the outside temperature falls below freezing during the winter months. Bringing cold air inside through a duct will create condensation on the outside of the duct, which will accumulate and drip onto your floor.
Option 2, ducted through an existing door/window
Although it is a little difficult to see, the ducting running below the bench is the fresh air intake system, and runs out through the slab door to the right in the picture, below the exhaust fan.
Here’s an outside view, showing the fresh air ducting coming out the slab door and running alongside the garage to achieve the minimu 10 foot separation between the exhaust and fresh air intake. (Also shown are the temporary supports to hold the duct in place while it was being assembled.) I raised the ducting up 12″ further to prevent entry by animals.
Because my studio is located in my garage, I don’t worry about condensation on the fresh air duct, but if your studio is located inside your house, I strongly encourage you to insulate the duct to help protect your floors and walls.