HVAC duct work advice needed.

[Steven Smith]

I'm hoping someone out there can give me some advice on some duct work questions. I'm putting a small spray booth in my shop and I've found a 1/4 hp Dayton 12" explosion proof axial fan to ventilate the booth. The little building is brick so I need to ventilate through the roof.

My first question: Should I use 12" ducting or would 10" work? Of course the 10" is cheaper and easier to find. The duct will probably be about 15 ft to 20 ft long with probably three 90 degree turns.

The second question: Can I use flexible ducting or should I use round metal ducting? I learned to work with metal ducting ok when I installed my dust collection system but the flex is sure a lot faster and cheaper.

Last question: What kind of vent should I use on the roof? It's a standard shingled roof with a pitch of about 4.

Thanks in advance for any help !!

[Louie Atienza]

You'd have to find out how many CFM your fan pushes... My friend sells HVAC supplies and he gave me some formulas for calculating stuff like this. Got to give him a call anyway. My gut tells me you need 12"...

I would not use flex duct as the spray can collect in the folds, and it won't be as efficient as straight duct.

You could probably find the correct exhaust boot at your local building/roofing supply. Though with a 4 pitch, you might have to remove some shingles above the boot, get some ice shield or such about 2 feet along the top and sides, on top of the flange, then replace the felt paper and shingles. I would dub the corners of the shingles by the boot also so that water doesn't run under the shingles.

[Steven Smith]

Louie, thanks for the feedback.

The fan is not marked with CFM but a similar fan (same mfg) with a different blade pushes about 1150 CFM with 0 static pressure. My gut says 12" too but I was hoping maybe ...

I understand the normal HVAC flex duct from the big box stores is not very efficient so I will stay away from that even though the price is right. This morning I found some 12" flexible aluminum ducting from Grainger that looks good. Probably not quite as good as straight pipe but should be much better than the normal flex duct.

I also found a suitable roof vent at Grainger but it was expensive. My local guys (small town) didn't have anything on hand for 12", they only went up to 10" without doing a special order. I could have gone into an industrial supply place in Knoxville but I've learned that it doesn't cost any more to order from Grainger or McMaster Carr and saves me time and gas.

[Barry Daniels]

Be aware that these kind of fans are moving air at a very fast velocity. Obstructions (including a bend in the exhast pipe or a restrictive vent) could cause serious problems. I installed one vented through the roof and I had to decrease the diameter of the exhast pipe to get between the joists and rafters. This caused a restriction which seriously cut into my fans capacity.

[Louie Atienza]

I talked to my HVAC friend. He says that you'd be better off using 12" ducting, and recommends using smooth ducting over the flex ducting. A straight run would be best, but any bend or restiction will increase the static pressure. Not only will you lower the effective CFM of the fan, but it may draw more current and tax the fan because it has to work harder to push that air. Every bend or elbow you put will increase the static pressure as well as lower the CFM.

[Steven Smith]

Guys, I appreciate the advice. I'll stick with 12" smooth duct. I have room in the attic area so I can keep it 12" all the way. I can't do a perfectly straight run but I should be able to avoid 90 degree elbows.

[Dick (DT) Trottier]

Hi Steven,

The little building is brick so I need to ventilate through the roof.

I'd start by getting a couple of estimates from masons to put a hole through the brick. By the time you add the time involved and cost of extra ducting, a roof vent and roof repairs, having your vent installed by professionals may not be a bad option.

Good luck...

[Steven Smith]

Dick, that's good advice. I am fortunate enough to have the skills and tools necessary to do pretty much anything to the building. My thoughts were that if I sell the house I don't want to have to fix a 12" hole through the brick wall. Roofs and ceilings are relatively easy to fix. I also would prefer to have the noxious gases vented higher above ground level.

[David King]

You can always strap a bigger motor to the fan. I have the same one and it came with a 3/4 HP and pushes 2250 CFM.

[Steven Smith]

This fan has a cast aluminum fan blade that is a bit different than what I've seen for sale. A larger motor should be no problem and is a good idea if I need better performance.

[Barry Daniels]

Also, be cautious of restricting the flow with your outlet cap. Most vent covers have a full cap on top which would create a terrible restriction for your system. On my system I built a cone shaped hat out of sheet metal. The cap is on a hinge. When I turn on my blower, the cone opens up to allow unrestricted flow. When I turn the blower off, I pull on a small wire cable that pulls the cap closed and I hook the wire to an anchor to keep the cap secure. This takes a little bit of extra effort every time I turn the blower on, but it has worked well for the last 20 years.

[Steven Smith]

Barry, the vent cover that I picked does have a 90 degree turn with a flapper-type rain shield. This was the least restrictive vent I could find but obviously not near as good a flow as yours would provide. My booth is going to be so small that I'm hoping the flow will be adequate in spite of some of the ducting limitations. If I'm wrong then it sounds like the vent cover will be a good place to start.