Spray Booth Makeup Air

Pulling makeup air through an auxiliary space such as an attic for tempering offers only a marginal benefit. July 3, 2008

I have an open face 10' wide spray booth located in one corner of a 104 x 54 shop. I currently draw outside air from a couple of windows that are 50' from the booth. The problem I encounter is keeping enough heat in the building. I am a small custom shop and don't want to invest in a makeup furnace, and don't want to pay to operate one either.

I am thinking about rigging up a duct to pull the makeup air from the attic. This would temper the air a bit before it enters the shop. I am located in a climate that gets cold in the dead of winter (10-20 deg), but there are many days late fall and early winter that the outdoor temps are around 40 deg. The building is a metal pole building with a steel roof. It is very well insulated.

Has anyone tried such an arrangement? Dust in the finish area is really not an issue as I work alone or with one helper. I was thinking of placing the air inlet from the attic at the far end of the shop to reduce any turbulence to a minimum. The ceilings are 15', so I could bring the duct or chase down low enough that it could be reached from the floor to open and close. Does anyone have any thoughts?

Forum Responses
(Finishing Forum)
From contributor M:
We put an adjustable digital switch on our fan so after spraying we could turn it down to a low speed. I live in Maine and it’s worked well. It still gets a little cold when we have a lot to spray but not as bad as before.

From contributor R:
You will need to make that duct big enough so air will move through it with little restriction. A booth that size may have a fan that moves at 10,000 CFM. I would size the duct at least twice as big as your exhaust duct. A passive makeup like this needs to be sized right so the air will move from the attic and not be pulled through every crack in the building from outside. Air will still come in an open door.

From contributor P:

I don't think the attic will make much difference. If you're exhausting 8,000 cubic feet per minute or more, which equates to a booth that's 8' tall and 10' wide at the face, then it won't take very long to suck all the warm air out of the building, including the attic. And, you'll need to let 8,000 CFM of air back into the building to replace the air you're exhausting.

From contributor D:
I see this question all the time from people living in cold winter states, and I have yet to see a usable, cost-effective response. I think the real answer is that there really is no way to make sure the shop stays warm in the winter when the booth is running without spending lots of money on some kind of furnace to heat the incoming air. If somebody has a better idea on how they are doing it without spending massive money I'd love to hear it, because it gets really cold here in PA in Jan-Feb!

From contributor K:
I took the advice of an old head finisher a few years ago and it has served me well. First, draw your makeup air from as far away from the booth as possible - your warm shop will serve to heat the air up on the way to the spray area. Next consider the airflow you really need. If you are using low solids and conventional spray equip. you will be putting a huge cloud in the air - and that needs a lot of air to clear.

If you use an AAA and high solids finish you will find you need to move very little air to keep the environment safe from exposure and explosion. I throttle my air input (windows) to account for outside temp and my comfort. Bottom line for me was to invest in better spray equipment and use higher solid finishes to minimize the clouds in the shop. When you get these under control you can get by with a much smaller fan. I also rigged up a radio control remote power switch to be able to turn the fans off when I'm not spraying (one man shop).