More on makeup air issues

As I’ve mentioned in the past, the folks who hang out at Wet Canvas seem to be living in their own little world, isolated from the rest of the glass world, at least as far as safety issues are concerned.

The thread starts with a very good question, especially at this time of year:

Every year I have the same problem in the heat of summer. Obviously I want my exhaust to work well, but the window unit in my studio can not cool the replacement air fast enough. I thought I remembered a thread about this years ago, but I can’t find it? Opposite problem in the dead of winter with heating. Any ideas/solutions?

And immediately goes south:

What you need is what air conditioning and heating folks call “make-up air” That is to say, you need another fan to deliver fresh air just outside the capture zone of your hood. This air needs to be deliverd in a volume slightly lower (fewer cfm) than your existing ventilation fan. As it is temperature extreme you are combatting, you need to be sure to deliver this air in a fashion such that it does not blow on you directly. The source for the make-up air needs to be far removed from the area where the current fan discharges, so as not to recirculate any of the nastiness. If you have a competent a/c service professional in your phone list, they should be able to assist you. The setup of the system is not that difficult, but getting the balance correct between the two fans can be tricky.

The underlined/bold parts that I’ve marked for emphasis are the wrong answers. First, makeup air should always be passive – that is, unpowered. The exhaust fan will pull the air it needs from the fresh air source. Additionally, and perhaps most importantly, fresh air should be available in a GREATER volume than that exhausted to prevent the formation of negative air pressure inside the structure you are working. This can be done in any number of ways, but the best is to use ducting one full size larger than you are using for exhaust. For example, if you are using 8″ duct for your exhaust system, use 10″ ducting for your fresh air supply.

Lastly, there is this masterpiece, from someone who claims to an AutoCAD user and working with engineers for 10 years (evidently she didn’t learn a damn thing), there is this:

I add air at the back of my bench. this assists the air going out, as well as reduces the amount of make up air I need. that way my air stays more conditioned to the temp I need. I also adjust my furnace damper to bring in air from outside and heat it/condition it.

a fan in a duct pushing air from far away, coming up, at the back of my bench, toward my exhaust.

one important note-
I always suggest making sure you are not creating negative pressure in your house. MAKE sure your furnace exhaust is not being pulled into the home. AND do not have candles lit when your exhaust fan is on.
(smoldering can happen)

and! If your fan is pulling 600 cfm.
your room is 3,200 cubic feet (like mine)
every 5.5 min. all of the air is going to leave the room.
The rest of the home is 12000 CF …so, It will take approx. 20 min.
before my fan starts pulling onto the furnace exhaust.
hm….interesting to think about huh?

So…if you add air at the back of the bench, it gives you more time to condition your air.

Okay, who am I to know this? after auto cad drawing and working with engineeres for 10 years
on science lab systems…
many variables…
good luck!
and keep it safe and HOT!

This post is wrong in just so many ways…adding air at the back of your workspace is the best way to avoid having to temper/condition your air so it does not affect your general room air temperature. And once again, it should never be powered.

Adjusting a furnace damper should NEVER be used to provide fresh air for an exhaust system. A furnace damper is there to adjust between summer and winter settings for fresh air requirements for THE HEATING SYSTEM ONLY. It is NOT DESIGNED to provide a source of fresh air for a local ventilation system. Local ventilation systems should ALWAYS have a separate source of fresh air.

Doing this will create negative pressure in the house if no other source of fresh air is provided.

The next lines that she’s written I’ve highlighted in italics. These, for the most part are correct, but in my mind, especially the latter paragraph, scary. The fact that she’s discovered that a lit candle will smolder when her exhaust system is turned on should be telling her that she has a very strong negative pressure in the house, and, most likely, is drawing carbon monoxide back into the house through the furnace exhaust!!

The first part of that paragraph is fairly accurate, EXCEPT that the draw on the furnace will be IMMEDIATE, not after 20 minutes. As soon as the fan is turned on, if there is not enough fresh air supplied, the pressure in the house will go negative, and air will start to be drawn in through any way possible, including furnace exhausts, plumbing vents, etc.

Very, very scary.


5 thoughts on “More on makeup air issues

  1. Mike,
    I have been meaning to ask you a question on makeup air. I have set up a glass studio in my basement. I have a 1000 cfm squirrel cage fan for ventilation but my problem is makeup air. I open the door completely to the basement and I open up a large window on the first floor for makeup air. I have a couple of windows in the basement but they are basement windows that are very small and they only open about 45 degrees and they have no screens so a mass of mosquitos come in the house. Do you think I have adequate makeup air and if not is there anything I can do to improve it? The basement is the only place I can work because the second floor is too far up for me to have a propane tank on the outside and I do not want to have propane inside the house of course.


  2. Michelle — can you give me the measurements of the actual window opening itself? Once I have that I can give you a better idea if you have enough fresh air with that one window opening.

  3. The window opening I am currently using is 35 by 31 inches. There is another window right next to it which I have the option of using. The only problem is when winter comes I won’t be able to use those windows. I will have to replace the basement windows with screened windows or something. All the window options are more than 10 feet from the fan exhaust.


  4. Ok — then I suggest you start planning on venting your fresh air from one of the basement windows. Most likely, you will need to remove the window (the rotating part) from its frame, and build a framework to support the duct attachment, then duct from that direct to your bench.

    I suggest you run the duct up under the your bench and through the bench top inside your work area as I’ve described elsewhere in the blog.

    You will need to insulate the duct run as cold air entering into the duct will cause condensation on the surface of the duct which will drip onto your floors.

  5. Thank you Mike for the advice, I have already been planning what I am going to do for winter. I am also going to read all your entries thoroughly on the subject. This blog is really helpful. 🙂

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