Makeup Air, or Your Fresh Air Supply

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.

9 thoughts on “Makeup Air, or Your Fresh Air Supply

  1. Hello!
    Very Interesting post! Thank you for such interesting resource!
    PS: Sorry for my bad english, I’v just started to learn this language 😉
    See you!
    Your, Raiul Baztepo

  2. Mike, are there any calculations to determine the surface area for the makeup air vents? Always before I just opened my garage door to supply the makeup air for my vent system. I am setting up a new studio though and plan to duct the air under the bench so I need to figure out how large of duct work to install. I keep hearing one size larger than the outgoing, but I would like something a bit more “engineered” than that since I will be running two or three 600-800 cfm fans for multiple stations. I am still trying to decide how big my enclosures will be to determine my fan sizes.

    • Paul — the surface area should be the same as or larger than the duct size you are using for exhaust.

      For example, if you are using 8 inch ducting, the area of the supply opening should be at least 50 square inches.

  3. Mike, thanks for posting information about ventilation and exhaust systems. I’m converting a garage into a studio in Minnesota and I wonder how the under-the-bench replacement air duct affects room temperature [if it does at all] in winter. If I understand how the system works [fresh air pulled in from outside and routed to the bench hood where it is vented through the bench top; contaminated air pulled up through hood and vented to outside], does extremely cold air get mostly pulled out with exhaust or does the air replacement act like a air conditioner and freeze me at my torch? Is there a way to dehumidify replacement air in summer? I’m not concerned with summer temperatures–I’m sitting next to a burner, I’m going to be hot–but I have an O2 concentrator and it doesn’t function well in really high humidity [75%+]. Any advice at all is appreciated.

    • You get a little cooling in the immediate torch area, but it’s not that bad. My studio is in my garage just north of St. Cloud Minnesota and it really wasn’t that bad. I’ve got an insulated and heated garage with a bedroom on top of it, so it is important to me (and my son!!) that the garage stay warm!!

      As far as humidity is concerned, yes, that is a factor, but more from the general humidity in the garage than what’s coming in through the replacement air ducting. Remember, about 95% or better of the replacement air is being drawing right back out again, so there is very little in the way of “infringement” on the regular room air.

      • Thanks! It all sounded pretty obvious, but after reading a variety of contradicting opinions from multiple sources, I wanted to be sure I really understood what you were talking about. I appreciate you taking the time. Thanks again.

  4. Mike,
    My HVAC guy isn’t sure that make up air fed to the bench will pass code–but he has never seen anything like this before. Can you give me any localities in Minnesota that have inspected this system? He currently wants to make the replacement air duct provide for the torch hood and the heating but that won’t do what I want [exhaust torch fumes] without requiring a monster furnace blowing out a massive amount of BTUs to keep the place warm. Again any help is much appreciated.

    • Chuck – I used this system in Sauk Rapids and the inspector had no problem with it. The only thing he “requested” was that additional fresh air ducting be routed to the existing furnace so that the furnace wouldn’t “starve” for air.

      There’s no reason at all to have a furnace in-line with the fresh air, as you say, it will do nothing but pump all that heat outside. Foolish to say the least.

      If he’s concerned about starving the furnace, have him run a smaller duct direct to the furnace. That’s code anyway on all houses built since sometime in the 1980’s.

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