# Shop-vac Physics question



## 12strings

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?


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## REO

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!


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## Sanding2day

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.


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## 12strings

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.)


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## Sanding2day

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.


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## pintodeluxe

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.


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## wapakfred

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.


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## dschlic1

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!


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## crank49

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.


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## Redoak49

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.


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## gfadvm

Crank49, Did you get the "premium hose kit" at Home Depot? I can't find it.


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## shawnmasterson

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.


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## crank49

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.


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## debianlinux

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.


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## gfadvm

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


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## lumberingjoe

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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## Danpaddles

3 hp- Impressive! runs on 220V I presume? ;^)


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## crank49

Dan, that's nothing, I've got a Ridgid 16 gallon vac with a* 6 HP* motor that runs on 110 volts;. . . . and fairy dust I guess.


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## djg

The amount of air your shop vac can move will depend on several factors. First the impeller diameter determines how much air your shop vac will move for some given air resistance. The air resistance is basically the static pressure imposed by the diameter of the orifice the vacuum is sucking air through. Both CFM and static pressure map out a fan curve that show how much air the impeller is moving as a function of hose diameter. Unlike a dust collector, the maximum static pressure for a vacuum is much higher. Like 90"+ H20 compared to s the long answer to your question.

DJG


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## djg

The amount of air your shop vac can move will depend on several factors. First the impeller diameter determines how much air your shop vac will move for some given air resistance. The air resistance is basically the static pressure imposed by the diameter of the orifice the vacuum is sucking air through. Both CFM and static pressure map out a fan curve that show how much air the impeller is moving as a function of hose diameter. Unlike a dust collector, the maximum static pressure for a vacuum is much higher. Like 90"+ H20 compared to 10" H20 static pressure. This means that vacuums are good at moving small volumes of air at high speed under high static pressures. THis is the reason your vacuum can suck a nail of the floor but your dust collector hose can not since the air velocity is much higher. So…will your 2"+ hose work? The answer is probably…The static pressure will be less because the hose diameter is larger meaning that your shop vac will have less resistance to move the air. What most people don't understand is that with a larger hose, your motor does more "Work" as in the physics term or expends more energy because the air has mass, the impeller uses more energy to move that mass which translates to power to move that air. So the same situation in dust collection arises all the time. Can you increase the inlet diameter on your dust collector? The only way to tell is using a clamp on ammeter. you would have to test the amperage draw by the motor if the amperage draw is less than the nameplate amperage you will probably be fine…as long as the power conversion efficiency is high enough for the motor…otherwise your motor will heat to the point where it burns it's self up! and your investment ends up in a puff of smoke…. Not sure how you would measure this draw on the vacuum….but I am pretty sure that's the long answer to your question.

DJG


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## mrstan

Hi guys…

I am looking through this thread, and have a very basic question on the hose sizes. My shopvac has a big hole for the hose, but the hose is small. I measure the hole as 2.25" diameter. The hose is 1.5". Will a 2" hose fit in my shopvac hose connector port.. or is the 2" hose a little bigger than my hose hole? I am attaching some pictures of my hose and connections.. Sorry for the crazy questions, but I do not want to put out money for a 2" hose if it will not fit in my hole. LOL


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