How long can paint booth vent hose be?

I'm getting ready to set up a spray paint booth in my basement. Because my layout is around the walls, the booth will end up having to be in the middle somewhere. This means the dryer hose I set up to vent it could end up being very long by the time I can get it up to the ceiling and over to an outside wall to vent.

Is there a practical limit to how long that should be? I haven't done any measurement yet but I think it could be 30' or so. Is that ridiculous?

Reply to
Vince Guarna
Loading thread data ...

You should probably consult with someone in the HVAC field (Heating Ventilation Air Conditioning). I do know that the plastic hose often used for dryer vents will cause a lot of friction and become less effective in longer lengths. Paint spray dust also will collect in the corrugations in the hose. It would probably be a better idea to use metal ducting. The fan could be sized to be able to provide enough air exchange to effectively vent the booth.

I use plastic dryer vent material in my booth, but it only runs about 3 feet.

Bob Boudreau Canada

Reply to

Doesn't matter how long it is, how many CFM can you EFFECTIVELY move through it? Longer run, higher capacity blower needed. Long enough and you might want more than one blower.


(Keep it short, it just works better that way.)

Reply to

Mine is about 6 feet and I have ZERO problems. As long as it's sufficiently larger than the area of the opening in the booth, you're OK.

Reply to
Brian Paul Ehni

And also it must be at least the same, but preferably a larger area in cross section than the opening in the booth.

Reply to
Brian Paul Ehni

The answer is "A", as long as you change the wording to "fan outlet".

This is the simple answer, but it is all interrelated. The opening in the front of the booth should be of a size sufficient for you to be able to make use of the booth for its intended purpose. For example, you don't need the same size opening for N scale cars as you do for HO. Obviously, there are other factors involved (your hands have to fit, etc). Let's assume the opening is 24" wide x 15" high. That's 2.5 square feet. To be safe, you must have a fan capable of moving air at 200 feet per minute (FPM) into the opening (regardless of opening size). Model Railroader's article in the Jan

88 issue used the formula "Fan Cubic Feet per Minute (CFM) = W X H X FPM", where width and height are in feet. They came up with 525 CFM required. Now you got get a fan capable of 525 CFM.

Once you've got that, you have to match the output opening of that fan to some kind of hose.

In my case, I used the same Dayton 4C445A fan they did, and matched it up to

6" dia air ducting. The fan outlet was about 5" X 4", so I felt pretty safe using the longer hose, since it was much larger than the outlet.

I can email you the MR article, plus photos of my setup if you like.

Reply to
Brian Paul Ehni

How many bends will you have in this exhaust duct? The more, the greater resistance to airflow and the higher the fan pressure needed. Also, have you any filters in the system, or will you just exhaust the paint laden fumes out over the neighbour's cat, dog or (Horror! Horror!) his car! Filters will in turn add more resistance to the system. Certainly something that an Air Con. or heating and vent. engineer should look at. Remember, too, the higher the fan pressure the greater the fan noise, more annoyance to the neighbours. Regards,



Reply to
William Pearce


Before determining required air flow, an analysis of spray booth's purposes should be considered.

  1. To retain over spray.

  1. to remove noxious fumes.

  2. To prevent overspray from settling on painted surfaces without effecting spray pattern.

A wind tunnel is not required nor desired. Excessive air velocity will definitely effect the latter. A fan speed control is helpful. Improving the efficiency of the air flow through the booth will reduce the air volume requirements. The shape and exhaust placement are critical. An easily replaceable filter will keep overspray from the fan and exhaust ducting. Experimental results are discussed on my site.

The cross sectional area of the exhaust is by necessity much less than the front opening which creates a funnel A truncated pyramid with exhaust vent on rear wall is the optimum. Sharp corners create eddies, which may swirl overspray back to painted surfaces.

Yes, corrugations, smaller diameter, longer length and sharp curves increase back pressure. By using reduction fittings, a larger diameter duct may be used for the long run with smaller flexible hose at the ends to ease installation.

For more details with methods and extensive discussion of problems and solutions, see first site below in methods/spray booth.

Hope this helps.

Thank you,


Author of:

MODELRAILROAD TECHNICAL > I'm getting ready to set up a spray paint booth in my basement.

Because my


Reply to

That's too long a run for the common flexible dryer vent tubing, and the typical small blower used in most paint hoods. Most of these hoods are underpowered anyway. A proper hood will provide 100 feet per minute of air flow over the entire OPEN FACE of the work area. That translates to MANY times that velocity in the exhaust duct. The smaller the duct, the higher is the needed velocity.

I should add that ANY venting is probably better than NO venting, but shoot for a good air flow. This is especially true if you intend to spray solvent based (other than water based acryic) paints like Testor's, Floquil, Humbrol, etc. The concentrated fumes from these can be a real health hazard.

Normal plastic 'dryer vent' hose has a very rough interior wall (it usually has a big coil spring inside it) that restricts flow badly. Flexible metal dryer vent hose is a little better in that regard, but not a lot (though it is more flame resistant). Any long hose, even smooth walled, creates a lot of backpressure, and reduces air flow. The smoother, and larger, the tubing the less the restriction.

Also, avoid a lot of sharp turns in the duct. They also cause backpressure and restrict flow.

For long runs consider all sheet metal duct pipe, like 'stove' pipe, but usually made of aluminum or thin sheet steel (not the flexible aluminum foil kind). This is much smoother, and should cause less flow restriction. Consider 6" diameter instead of 4" if space permits. A short section of flex tube can be used at the hood end if necessary.

For a run as long as 30' you may need to consider a more powerful blower to better deal with the backpressure. A second blower could also be installed near the exterior vent, as a 'booster'.

A far better solution is to move the hood or reconfigure something else to get the duct shorter. More than ten feet and you're inviting problems.

Dan Mitchell ============

Reply to
Daniel A. Mitchell

I THINK he meant the size of the exhaust opening in the booth.

However, the statement that the duct need only be larger than this is

*NOT* true. The booth exhaust opening in ONE restriction, yes, but the hose is also a restriction, and they are CUMULATIVE. Its *NOT* a case where the 'weakest link' defines the system.

Dan Mitchell ============

Reply to
Daniel A. Mitchell

Let's see. It's a limp duct, so it make about a 90 degree down from the fan output, then a 180 back up to the garage door, then another 90 into my outlet in the door.

Reply to
Brian Paul Ehni

This is true, and is pointed out in the MR article. It's one reason I went with a 6" diameter duct when the output opening of the fan is about 5" X 4". I didn't want the duct to be a restriction.

As I've stated, I can spray Floquil, Scalecoat, etc., without any smell. I don't even use my respirator mask anymore.

Reply to
Brian Paul Ehni

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.