All Replies on Spray room ducting question

  • Advertise with us
View AandCstyle's profile

Spray room ducting question

by AandCstyle
posted 08-19-2016 11:42 PM

17 replies so far

View a1Jim's profile


118134 posts in 4424 days

#1 posted 08-20-2016 12:03 AM

I think the fan should be fine if you stick to WB finishes but having an opening for the return without some kind of filter could pull dust and dirt into your makeshift spray booth.


View nightguy's profile


213 posts in 1509 days

#2 posted 08-20-2016 12:07 AM

well you will have your air in and out, I think the furnace fan can/ move more air then the 8” duct can handle, maybe go 10-12” duct, which be more air then needed for WB finish. But that may screw up the spray gun atimization. I would put in a adjustable damper in it, like they do for home HVAC systems, and experiment with the air flow/exhaust when spraying some scrap.

View clin's profile


1121 posts in 1843 days

#3 posted 08-20-2016 12:55 AM

I think the idea is sound. Furnace blower motors often have multiple taps for different speeds. You probably want as low a speed as you can get.

I do like the idea of having the fan suck air out. That should eliminate any issue of the ventilation moving air too quickly and interfering with spraying.

Aside from venting fumes and humidity from the drying finishes, bringing the air in through a filter will really help with getting dust on the finishes. The little bit of finishing I’ve done lately, I find that having my Jet room filter going and blowing towards the work (I’m not spraying by the way), leaves almost no dust. It’s sort of against common sense to be blowing air at the work, but since this air is very clean, it actual helps to keep any dust from settling on the finish.

The only other thing I would want to be clear about is where is this return air coming from. Since you are blowing the exhaust outside, you are creating negative pressure in the spray room and wherever the return air is coming from.

Is that just generally from the basement which just means from the house? This has the potential to cause back flow of your gas appliances like furnace and water heater. That’s bad as it can draw in harmful combustion products. At the extreme you will kill yourself or others in the house.

Bottom line is make sure that return air ultimately has an outside source, like a window on the other side of the basement or a window or open door near the top of the basement stairs.

I realize you are not likely to run this for hours on end, but best be aware of how you might be affecting combustion gases. One way or the other you will be drawing air from outside and pumping out inside air.

Again depending on where the return air is coming from, you may be pumping out expensive heated or cooled air. Keep in mind there are heat exchangers that allow you to draw in outside air but warm or cool it with the outgoing inside air. Resulting in a lot less loss of air conditioned air. Of course you need to keep the guzinta away from the guzouta to keep from sucking fumes back in.

I think in the end, having that dedicated finishing area and running lots of filtered air is going to be a great thing. There’s a reason the big boys do it that way.

-- Clin

View JimYoung's profile


389 posts in 2434 days

#4 posted 08-20-2016 11:45 AM

Hi Art,

This is exactly how my friend setup his spray booth. He had a dedicated room and modified the door to use a pair of furnace filters. It was like a filtered screen door.

Another friend who needed to paint a home built airplane setup a 1×2 frame that could fold up in his garage with plastic stapled to it. In the back he had a plain old window fan with a furnace filter on it blowing air into the booth and the front was open down his driveway. Worked great, just park the cars across the street to avoid the overspray!

-- -Jim, "Nothing says poor craftsmanship more than wrinkles in your duck tape"

View Fred Hargis's profile

Fred Hargis

6415 posts in 3340 days

#5 posted 08-20-2016 12:27 PM

I also think the idea is good. The only thing I would watch is for any build up of finish on the fan. It may all be dried after travrling that distance, but it would still be worth an occasional check to see.

-- Our village hasn't lost it's idiot, he was elected to congress.

View OSU55's profile


2657 posts in 2836 days

#6 posted 08-20-2016 02:38 PM

It will work just fine. I have a similar setup. 8” duct should be large enough. I built a box for the blower (wood frame covered by flat sheet metal), with 2 filters, one on each side, and the box sits in the spray room. Maximize filter area best you can. My “room” is a 10’x10’ 3 wall area in the middle of the garage. I recirculate the air while spraying (use a respirator), then open the back door and garage door to blow everything out after spraying. It it’s good weather, I’ll open the doors a little to let in some cross flow, and I have an 8” duct I connect to the box outlet and stuff under the garage door. I don’t use an inlet filter. I turn the blower on for 30 min or so before spraying. Seems to clean the air well – don’t have much issue with contamination in the finish. Consider some disposable paint booth filters as prefilters. Mine are held in place on the inlet side of the framed filters just by suction. All the filters can be cleaned by tapping the dried finish dust out of them. I haven’t had any issue with finish build up in the blower. I spray mostly WB finish. When spraying shellac I do ventilate while spraying.

View Grumpymike's profile


2466 posts in 3162 days

#7 posted 08-20-2016 07:23 PM

Gotta tell you what I did with a 20” box fan … we had very heavy texture on the walls of a 70’s house and SWMBO wanted wall paper. ... I put the box fan in the open window of the bathroom and went to work with the belt sander grinding all that texture off of the wall …

Just about the time I was done I heard this god awful screech from the bedroom. ... It seems that the slider door in the master bedroom was open and guess where all the return air came from … Laden with drywall dust of course. ... it looked as if it had snowed in there, and 6 months later we were still cleaning up patches of dust.

Oh, by the way, after a couple of months SWMBO did start calling me Mike again.

Moral of this story? be aware of where your return air is coming from.

I think you have a solid plan Art, and I agree with what these other folks have added.

-- Grumpy old guy, and lookin' good Doin' it. ... Surprise Az.

View Neil's profile


25 posts in 1496 days

#8 posted 08-20-2016 08:14 PM

Should work fine in my opinion.

Not sure if someone mentioned this already, you should use paint arrestor filters to catch the overspray in the air. If not the finish will clog up and destroy your furnace blower in no time.

I get mine here:

View TTF's profile


154 posts in 4024 days

#9 posted 08-20-2016 08:38 PM

Makes good sense to me as long as you stick to WB spraying. I would still always wear a respirator when spraying.
Post pictures when you get it hooked up.

-- Troy | | The more I see nature, the more I am amazed at the Creator. - Louis Pasteur

View Jim Finn's profile

Jim Finn

2848 posts in 3769 days

#10 posted 08-20-2016 10:25 PM

8” is not large enough. 18” would be more like it.

-- No PHD just a DD214 Lubbock Texas

View AandCstyle's profile


3286 posts in 3104 days

#11 posted 08-20-2016 11:12 PM

I thank everyone for the responses. I always wear a respirator when spraying because I have no desire to paint the insides of my lungs. I will open a window in the basement to provide return air. The two windows are far enough apart that I should not be recycling the exhaust back into the spray room. I bought most of the needed supplies today-air filters, 8” duct, a damper, etc. I also ordered some paint arrestor filters.

Furnace blower motors often have multiple taps for different speeds. You probably want as low a speed as you can get.
- clin

Clin, my motor does have a number of different colored leads in addition to the black, white and green. How can I use those to minimize the blower speed?

Thanks, again.

-- Art

View clin's profile


1121 posts in 1843 days

#12 posted 08-21-2016 12:20 AM

Clin, my motor does have a number of different colored leads in addition to the black, white and green. How can I use those to minimize the blower speed?

Thanks, again.

- AandCstyle

I’m not sure if there is a standard or not. Few times I messed with a furnace motor, I think it had a sticker on it saying which was which.

By here’s something I found on the interwebs, though if you have a label on the motor, you should be able to get manufacturer’s specs. Makes sense that black, white, and green would be the typically things (hot, neutral, ground). Capacitor start is important of course. Others would appear to be difference speeds you would connect instead of the black.

From the interwebs:

One common “standard” is as follows:

White = grounded (neutral) (clockwise).
Black = High speed.
Blue = Medium speed (medium low).
Red = Low speed.
Brown = Run capacitor.
Brown w/ white = Run capacitor.
Green = Ground.
Yellow = Medium speed.
Purple = Grounded (neutral) (anticlockwise).

-- Clin

View AandCstyle's profile


3286 posts in 3104 days

#13 posted 08-21-2016 10:07 PM

Clin, thank you! This is most helpful. I checked and I have blue and red leads available, so I will try connecting the red one tomorrow. :)

-- Art

View Cooler's profile


299 posts in 1690 days

#14 posted 08-22-2016 03:19 PM

I converted my old dark room (photography) into a spray booth. It was set up as a “clean room” to minimize dust (an even more worrisome issue in photography than spraying.)

To accomplish that I have filtered air blowing into the room. I have outlets into the shop. There are several reasons that this is a better setup than running an exhaust fan.

1. Minimize explosion hazard. If you run an exhaust fan then the fumes will pass directly over the motor and sparks could ignite the fumes. You have to use a very expensive shielded fan for this application. But by blowing air into the room the fumes do not pass over the motor and it is much safer.

You will have to run ducts (clothes dryer vent material will be fine) to the exterior.

I simply cut a 14” x 14” opening in the wall and using bungee cords (2) I mounted a 20” box fan with a furnace filter between the wall and the box fan. The filter (a 3M very fine filter) cost as much as the fan, but about $30.00 total.

2. The second reason you don’t want an exhaust fan is that it tries to suck dust into the room. Exactly what you don’t want. With the positive atmospheric pressure caused by the filtered air forced into the room it will push the dust out, instead of sucking the dust in. I always turn on the fan before opening the room’s door.

Also, because housekeeping is important to keeping dust down, I ran a shop vac hose through the wall of the room. I turn on the intake fan first and I keep the vacuum cleaner outside the room. The hose inside the room is used to suck up any dust or debris. It would be an actual disaster to run the vacuum in a clean room environment.

-- This post is a hand-crafted natural product. Slight variations in spelling and grammar should not be viewed as flaws or defects, but rather as an integral characteristic of the creative process.

View AandCstyle's profile


3286 posts in 3104 days

#15 posted 08-22-2016 10:38 PM

Cooler, that sounds like really good advice, but I completed my installation today.

Troy, here are the pix. However, I went for quick and dirty, not beautiful. The butt joints are sealer with caulk to prevent finish from getting inside the boxes I made.

The filters for the replacement air. There is one on each side of the wall.

The box containing the blower. This is directly across from the inlet filters so I will have a good air stream at my back as I spray. There is a furnace filter behind the paint arrestor filters.

The flexible duct going from the finish room to the window.

This is a close up of the box exiting the finishing room. I caulked the opening between the wall and the box.

Finally, the duct exiting through the window, which can be opened and closed behind the plywood without moving the plywood.

If anyone missed it earlier in my original post, I only spray WB products.

Again, I thank everyone for all the assistance.

-- Art

View splintergroup's profile


3978 posts in 2069 days

#16 posted 08-23-2016 06:06 PM

I see what is missing….SAWDUST!

Clean shop = idle hands 8^)

I think you dealt with a tricky installation very well, if anything you might want/need an increase in air volume.

The air source looks like it is at a good height. When I was dealing with a home-made spray booth in a previous life ,I found getting the intake located properly was tricky. For a ‘pull’ type system, with a small intake area (as opposed to a ‘wall-of-filters’) you can get eddys that increase the time it takes to clear out the fumes.

Is this the project you mentioned where you shunned any usage off QSWO???

View AandCstyle's profile


3286 posts in 3104 days

#17 posted 08-23-2016 09:25 PM

Splinter, I am still working on the oak free project, but I had to get my finishing room ready for spraying. I cleaned a bit while I was waiting for the inspiration on how to make the set up you see in the pix.

I tried it out today and discovered a couple issues that needed to be fixed: It works much better with the damper open. :D I needed to seal around where the collar comes through the wooden box in the shop and joins the duct. Even without those two fixes complete, LOML said she could smell the fumes on the back deck which is 15-20 feet from the exhaust window well.

-- Art

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics