Shop-vac Physics question

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Forum topic by 12strings posted 08-15-2013 02:51 PM 11454 views 0 times favorited 19 replies Add to Favorites Watch
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434 posts in 2381 days

08-15-2013 02:51 PM

Topic tags/keywords: shop vac

I have a 6-gallon, 3hp shop-vac from lowes. It came with the 1.25” hose.

My question is: Would I get more, or less suction and dust-collecting capability if I went to the bigger 2.5 inch hose? That’s the size of the hole on the vac…it just came with smaller hose for some reason.

I assume that with a certain hp engine, there would be a law of diminishing returns such that a huge hose would not really help…but I don’t really understand the physics of it.

Any tips?

-- I'm strictly hand-tool only...unless the power tool is faster and easier!

19 replies so far

View REO's profile


928 posts in 2071 days

#1 posted 08-15-2013 03:14 PM

the vac will only develop a certain volume of air movement. this will be effected by pressure or vacuum. smaller lines will give you a higher transport speed but increase the pressure drop. larger lines will require less pressure but the velocity will be reduced. so… to big a line and the material will settle out to small a line and the volume is restricted reducing available volume for capture.

just a point to ponder. a plugged system takes LESS power to run most shop vacs use the air for cooling the motors so if the line is clogged and it continues to run there is not enough air to cool the motor and thats why they burn out not because they are working harder to suck. The motor uses the most power when it is moving the most air. With out the proper hose attached (to large a dia) it is possible to burn out some shop vacs because there is not enough pressure drop and the motor is working to hard to move the air!

View Sanding2day's profile


1013 posts in 1843 days

#2 posted 08-15-2013 03:19 PM

Not able to answer the exact physics question very effectively, but can say with a fair amount of certainty that increasing the hose size will decreasey the overall suction.

That being said I believe it likely that you would maintain a fair degree of suction moving from the 1.25 to the 2.5 but would have to try it out to see if it works for you and your setup.

-- Dan

View 12strings's profile


434 posts in 2381 days

#3 posted 08-15-2013 03:40 PM

So if I am willing to live with slower-moving air in order to gain a wider area of dust pick-up…I will be ok as long as the vac is still sucking? I”m mainly thinking of when I attach it to my router table fence…It seems like it is simply to small a hole, so that it doesn’t take in as much dust as I would like. Am I thinking correctly?

Also, in case I wasn’t clear, the Holes (input and output) on the shopvac are big (2.5”), but it came with a smaller hose that has one end that elbows out to fit the larger hole…So I would not be going any bigger than the actual ports on the vac itself. (ie, I’m not trying to attach a 2.5” inch hose to a 1.25” port.)

-- I'm strictly hand-tool only...unless the power tool is faster and easier!

View Sanding2day's profile


1013 posts in 1843 days

#4 posted 08-15-2013 03:49 PM

For that application I believe I would simply buy the 2.5” adapter to hook into the router fence and run the 1.25” to it… Should accomplish the desired goal without attaining the full line.

-- Dan

View pintodeluxe's profile


5657 posts in 2810 days

#5 posted 08-15-2013 04:19 PM

I tried collecting dust from my miter saw with a 1.25” hose, and it was hopeless. I switched to the 2.5” hose and it works well.

The smaller diameter hose does work well with a random orbit sander.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Fred Hargis's profile

Fred Hargis

4984 posts in 2490 days

#6 posted 08-15-2013 04:37 PM

Many vacs some set up to use either size hose anymore. Any differences you see will be tough to measure without proper instruments….I’d go ahead and get the larger hose and not worry about the suction part.

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

View dschlic1's profile


428 posts in 1966 days

#7 posted 08-15-2013 05:24 PM

Most wet-dry vacs that I have seen (which includes Shop Vac) have separate cooling for the motor. You definitely do not want wet air running past electrical windings!

View crank49's profile


4030 posts in 2968 days

#8 posted 08-15-2013 05:49 PM

Your vaccum came with the smaller hose because that was a cheaper accessory to include with the original package. Allowed the manufacturer to make more profit on the sale until people start noticing what they did.

The physics of fan performance is called the “Fan or Affinity Laws”. These laws state that volume is directly proportional to fan speed. Pressure varies as the square of the speed and horsepower to run a fan (a vacuum cleaner is just a fan with a hose and filter) varies as the cube of the fan speed.

What this means is since the speed is approximately constant the increase in pressure caused by the smaller hose is killing more capacity than the increase in volume that would be conveyed by the larger hose.

So, you will gain considerably more flow and better performance by changing to the larger hose and the increase in volume will, as stated by Reo above, increase the amperage draw of the motor. You have to assume that the manufacturer designed the motor to handle this, or they would not have provided the larger connection port.

I have a Ridgid vacuum and purchased their premium hose. It costs about $29 bucks, but is 14 ft long, has a smooth interior that does not clog easily and comes with a bunch of adapters. I have been very happy with the way this works.

View Redoak49's profile


3242 posts in 1985 days

#9 posted 08-15-2013 09:53 PM

A lot really does depends on the motor. One thing to think about is that the area of the 1.25” hose is 1/4 the area of the 2.5” hose. This suggests that the velocity could drop by as much as 4 times. However, if the 1.25” hose is really too small and is the limiting factor for air movement, the drop off going to the 2.5” hose will be less.

As noted above the condition of the inside of the hose will make a big difference in the air flow. The smoother the better.

Please let us know how it works when you try the larger hose.

Note that the comparison between a 2.5” and 4” hose is that the 4” hose has 2.5 times the area of the 2.5” hose.

View gfadvm's profile


14940 posts in 2687 days

#10 posted 08-16-2013 02:14 AM

Crank49, Did you get the “premium hose kit” at Home Depot? I can’t find it.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View Shawn Masterson's profile

Shawn Masterson

1325 posts in 1945 days

#11 posted 08-16-2013 02:55 AM

I switched mine over to the bigger hose and it made a world of difference. For the better. I believe they ship them with the smaller hose to get more money from you later.

View crank49's profile


4030 posts in 2968 days

#12 posted 08-16-2013 01:33 PM

gfadvm, Yes, I got it at HD. I just checked on-line and it appears the price has gone up to $36.97. Part number is VT2570.

I need to point out this is really a 2” hose. Some people in their reviews were not happy with the size. For me it was a good compromise between the 1 1/4” living room vacuum and the 2 1/2” shop vac. I would buy it again.

View debianlinux's profile


53 posts in 1761 days

#13 posted 08-16-2013 02:20 PM

This thread prompted me to order a 2.5” kit for my vac. It never occurred to me that the hose fitting on the machine is obviously capable of accepting a greater diameter than was included in the box. My only regret at this point is the money I spent on the 1.25” to 2.5” dust port adapter for my router table.

View gfadvm's profile


14940 posts in 2687 days

#14 posted 08-16-2013 11:59 PM

Ahhh, I saw that but thought yours was 2 1/2”. Thanks for the reply.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View lumberingjoe's profile


22 posts in 1944 days

#15 posted 08-17-2013 06:25 AM

I can’t imagine Lowe’s would have much happiness if they sold a vacuum that would burn up or even trip the breaker (from being overloaded) every time someone let the vacuum run without a hose. I think you can safely use as big a hose as you wish.

You can feel the increase limit in air volume at the exhaust when you completely take the hose off. Your new air volume will likely be somewhere between what you have with the 1.25” hose connected and no hose connected.

Good luck.

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