Heating a New Shop and Spray Booth

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Radiant heat could be ideal, but the spray room will need filtered fans. February 25, 2005

We are in the planning stages of a new shop with showroom and (finally!) enough room to comfortably build, finish, stage and display. I am in east Tennessee and most of our winter is fairly mild. Coldest days are usually a low of 25 degrees. But the average winter temp is probably 40-45 degrees. I have searched the Knowledge Base and read a lot of posts about shop heating and I'm confused. I want to maintain a shop temp of about 60, a little higher in the spray room - probably 70-75 degrees.

There will be a small (I'm thinking 10'x7') area outside the main spray room, separated from the rest of the shop by a door, for the finish to flash off before being rolled back into the staging/hardware area. I want to control the shop heat, spray room heat and humidity, and dust. This building will probably be the last location and I want to make it as user-friendly as possible from the start. What are your considerations and suggestions for the most efficient means to heat this space? Should I use one means of heat for the main shop floor (maybe radiant in-floor) and something else for the spray room?

I'm not very schooled in air exchange issues, etc. Our spray room now is vented only with an explosion-proof fan and the spray area is open to the rest of the shop, so you can imagine the dust issues. I want to keep it separate as well as have space for the finish to flash off to eliminate dust problems. I have also thought about an open front type spray booth that would be in the back part of the spray room with a partition wall and opening to get the cabinets in and out of, then the whole thing partitioned off from the rest of the shop. Get me started, guys - I'm really lost on this.

Forum Responses
(Cabinetmaking Forum)
From contributor D:
I put in a Munchkin boiler in my shop last year. It replaced a forced air unit and it cut my heating bills in half over that 15-year-old furnace. I like it for a few reasons. It has a sealed burner, so it draws in fresh air from outside and exhausts outside, so there is no chance of explosion from inside vapors or dust, plus no filters to change.

It also has a variable output so that on warm days it may run with as little as 66000 btu and on those cold January Minnesota days, this model can go up to 199000 btu. There are also smaller units. I had no good way of doing radiant heat due to the existing shop configuration, so I found some old cast iron radiators for free and used them, but if I were building, I would not do it any way other than radiant heat.

In the booth, you can use the radiant heat too, and if you are spraying a lot, put in a make-up air unit. I have so far gotten by with just the boiler. When I know I am going to be spraying, I bump up the temp to 80 degrees in the morning and that gives me a buffer that allows the boiler to keep up.

From the original questioner:
Thanks, that looks like the direction I was leaning towards. Here's the part where I get confused. What exactly would a make-up air system consist of? Couldn't contact them from their website for some reason. Would you be able to give me some kind of price idea?

From contributor G:
I've had two shop with radiant heat, and as far as I'm concerned, nothing beats it. I put in both systems myself and for 1600 square feet it cost about $2000. Never had any problems. No noise, just warm concrete floors and no blowing dust.

From contributor D:
You would size the make up air unit to the cfm rating of your exhaust fan in your spray booth. Cost varies according to the size. The person you get your booth from should be able to help find the answers to that one.

From contributor P:
A fan is nothing more than an air pump. Air is drawn into the suction (negative pressure) side of the spray booth fan and moved to the outdoors on the discharge (positive pressure) side of the fan. If the space you were venting, the finish area, were airtight, the fan would eventually pull the space into a vacuum. With the space in a vacuum, positive air pressure from the surrounding space would attempt to push into the spray area, dirt and dust included.

A make up air fan pumps outside air into the space to make up the air exhausted by the spray booth exhaust fan. If the capacity (cfm), of both fans were exact, the pressure in the spray area would be neutral. By that I mean neither positive nor negative, the same as all other areas around the shop.

Your biggest concern along with venting the spray area is to keep dust and dirt from entering the finish area. To accomplish this, you need a slight positive pressure in the finish area. The make up fan needs to bring in more air than you are exhausting. (This air also needs to be filtered). When doors to the finish area are opened, air will flow out. The same applies to leakage of air around windows or other small openings to the surrounding space. If the finish area were in a neutral or negative pressure, dirt and dust would enter the area.

Fan pressures are measured in very small amounts. A pressure of 0.3 psi is generally sufficient to provide a positive pressure and keep out dirt and dust.

Just think about positive and negative pressures, and remember that air at a higher pressure will always try to flow to a lower pressure area, equalize, so pressures are equal. Like air in a balloon, the air inside is pushing against all sides of the balloon, attempting to get to the lower pressure outside. Also, remember that as air flows, dust and dirt flow with it.

This is pretty general information. I hope it helps!

From contributor R:
My shop is heated, cooled and dehumidified. My spray room is off of my main shop's controlled environment, so I only have to heat and cool it when I need to. My fan is a 2500 cfm seal motor (not explosion-proof) that is installed in the controlled environment of the shop. Clean, dry air is sucked into the fan, blown through a filter, then into the spray room, where I blow the fumes to the exterior (blowing them out rather than sucking them out), or with a slide of the hand, I can circulate air back into my shop. This way, one cheap fan and filter serves as paint room exhaust, dust filtration plus heat and air handler. I spray nitro-lacquer.

From contributor P:
The purpose of any fan type or style is to create a pressure difference. Because of this difference and the fact that air will move from an area of high pressure to low pressure, we are able to circulate air.

Air should always be filtered before it enters a fan. Filters placed on a fan discharge create a resistance to air velocity and quantity, making the fan motor work harder. Depending on other factors, cfm can be reduced by amounts of 20% or greater. Also, this resistance to air flow overheats fan motors and causes a fan motor to draw more current, (power), resulting in more energy usage. Unfiltered air moving over the fan blades or squirrel cage over time will deposit dirt on the blades, causing further load on the motor and reduced fan performance. All of these conditions shorten motor life. For proper and maximum fan performance, the space being ventilated should be on the negative, (suction) side of the fan. Spray room air should move through the filters, into the negative side of the fan, then out the positive side of the fan to the outdoors.

From contributor R:
I hope you don't really believe "air should always be filtered before it enters a fan." My post read "clean, dry, air is sucked into the fan." There are many situations where it should not be. A rotary turbine will only push or pull an equal amount of air. An explosion proof fan drawing on the negative side would need to be filtered, and it would pass the EPA regs. Screw the EPA - I am feeding my kids with the same trade my daddy did, and he didn't blow himself up. Rotary turbine, positive and negative air pressure sound almost as good as static pressure or inch pounds of vacuum.

From the original questioner:
Contributor P, I should ideally have a slightly larger fan feeding air into the spray room and the exhaust fan should be slightly smaller (both filtered), right? Any rough cfm numbers per foot of space and filter size?

Contributor R, I had thought about dehumidifying but never about cooling. We just always toughed out the hot weather. How do you accomplish this? Also, how do you control humidity in a heated environment? Just relying on the colder weather to be dryer?

From contributor P:
I wouldn't attempt to give you any numbers or information about a system needed for your shop. There are many considerations when designing and installing a good ventilation system. Also, local codes could be a consideration. Obviously, I can't see your shop or know your complete situation.

Based on your first post, it seemed you might need some background and facts about fans and ventilation. My purpose was to give you a few basic and general facts about fans, fan performance, and make up air. Since retiring from a thirty-four year career as a mechanical engineer in HVAC, I turned my woodworking hobby into a small business. There isn't much I have to contribute here as a finish carpenter. You all know a lot more about that than I do. So, I was motivated at the opportunity to make a contribution in an area where I had much experience.

From the original questioner:
And very useful and much appreciated info!