How do you debu low suction in a central vacuum system?

I just need to debug WHY a central vacuum cleaner has low vacuum suction at the hose.
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I snaked the hose itself, and it's not clogged:
https://c4.staticflickr.com/8/7573/15965995228_9afb4ff8bd_c.jpg
Strangely, it seems that the vent to the outside has plenty of outward force, yet the vacuum hose has almost no suction.
If there is a hole or a clog in the wall hoses, I don't know how to find them. Is it normal, for example, to snake the wall holes?
The container, in the basement, is not clogged, and seems (by sound) to be running at speed (no abnormal noises).
I haven't taken it apart yet (it's in a nasty cramped space), so, I ask you guys if you have any experience debugging why a central vacuum would have: 1. Very low (almost zero) flow at the wall, but, 2. Decent air flow to the outside of the house.
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Danny D. wrote:

Hi, How many outlets do you have? Tried them all? No utility outlet on the unit?
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Tony Hwang wrote, on Tue, 30 Dec 2014 23:00:20 -0700:

There are at least a half dozen, and, I've tried most of them. I can try more, but, I don't think it's the outlet itself. They're all closed though (but I'm not sure what you're getting at).

If you mean electricity, every single one has an electrical outlet within a foot (since the vacuum beater brush runs off of electricity); but the vacuum itself doesn't need electrical outlets, so I'm not sure what you're getting at.
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On Wednesday, December 31, 2014 2:43:39 AM UTC-5, Danny D. wrote:

What he's getting at is are you sure one of them isn't stuck open?
You can also get a feel for the amount of suction at each by turning the system on manually and then trying to lift each of the outlet covers. If it's working properly there should be lots of suction, it should be difficult to lift open the cover. IDK how your central unit works, but on mine there is switch at the unit that I can use to turn it on. If not, the systems typically use a Xformer with low voltage and rely on the hose shorting the contacts at the outlet to turn it on. I'd also be listening and looking at all the exposed plumbing when it's running. Since you have good airflow out, I agree it sounds like a break.
Ultimately it may come down to just cutting at select points to isolate off major sections until you track it down. It's just plastic PVC pipe and you can glue it back together. Hopefully it's not in a wall somewhere. If there are any easy targets, like maybe the unit connects with a clamp, you could disconnect there to verify that you have vacuum.
Someone mentioned a vacuum relief valve possibly being installed somewhere. Forget about that, they don't exist.

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trader_4 wrote:

Hi, +1 If blockage is very near the unit, all outlets will have problem but if blockage is else where away from the unit, some outlet might normal or there is leak some where. Our system is Beam and at critical points of the plumbing it is transparent so I can visually inspect. Vacuum utility outlet usually mounted on the unit. Our unit is in the garage, I use that utility outlet to clean car interior, etc. If motor gets overloaded, there is a pop switch to turn off the motor B4 it burns out. Only problem I had with ours was replacing brushes on the Lamb blower motor.
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On 12/30/2014 9:56 PM, Danny D. wrote:

If you have decent outside flow with ports blocked it's leakage.

When disconnected, the ports should be blocked. Make sure you don't have a leaky port behind the furniture somewhere...with the system turned on. Stick the hose to your vacuum gauge into the hose and seal it up with duct tape. Use it to measure the vacuum at each port. Those numbers and a map of the piping should give you an indication. If the vacuum is high, but the flow is low, it's likely a blockage. If the vacuum is low, it's likely a leak somewhere upstream of the lowest vacuum reading.
If you don't have a vacuum gauge, you can block the hose except for a small hole and listen to the air flow to get some idea whether it's a leak or blockage problem.
I have never seen an actual system, but I surmise that there may be a vacuum relief valve somewhere so the motor gets cooling air if you leave it on with all the ports closed. That might confuse your diagnosis.
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mike wrote:

Hmm, How can you leave the motor running with all the ports closed? Plugging the hose into port starts the motor. You mean plug the hose?
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On Wednesday, December 31, 2014 10:51:42 AM UTC-5, Tony Hwang wrote:

Mine has a manual switch on the unit itself that you can use to turn it on with all the ports closed. The intent is so that you can use the built-in port right on the unit. That and maybe to test if it works, etc.
The relief valve is a red herring. No such thing exists. I'm sure the units are perfectly capable of running like that. It's like blocking a shop vac, once it ceases to move air, the current levels out, the design parameters aren't exceeded.
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On 12/31/2014 7:51 AM, Tony Hwang wrote:

Hmm, yourself. I said I'd never seen an actual system. That misconception has been corrected. But plugging the hose would do it.
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mike wrote, on Tue, 30 Dec 2014 23:48:14 -0800:

That's interesting. I have an old vacuum gauge that I used in the olden days to test my vehicle.
I wonder if I can adapt it for the hose.
If I understand you correctly, you're suggesting I set up a vacuum test at each of the six wall ports. The wall port with the least air flow would be closest to the leak.
I sure hope there isn't a leak in the walls though, as that would be nearly impossible to find.
Do you know what *material* the pipes are in the wall? Can they be snaked (in case it's a blockage)?
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mike wrote, on Tue, 30 Dec 2014 23:48:14 -0800:

That makes sense, but, (a) I have never seen a vacuum relief valve (but I don't know what it would look like), and, (b) there is no suction unless/until I open a spring-loaded hinged port, and press in the hose.
When I press in the hose, an electrical contact is made, which turns on the vacuum in the basement.
I don't know *any* other way to turn on vacuum, other than to press in a hose. So, when all ports are closed, the vacuum doesn't work. When any one port is opened, the vacuum doesn't work.
It's only when electrical contact is made that the vacuum turns on (but it's far too low to be useful).
It's night now, but I'll snap a picture of the vacuum itself, in the basement, in the morning, and post.
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On 12/31/2014 12:40 AM, Danny D. wrote:

You probably have a rubber hose on your auto vacuum gauge. Poke it down the suction hose and seal it with a rag or duct tape. Doesn't have to be a perfect seal. Stuff the other end of the suction hose into one of the ports to turn on the system. What you want to know is whether the vacuum goes up when the volume goes down near zero. Based on your statement that there's air coming out the exit side of the central system, You probably don't have a blockage. While you're in that mode, go around listening for air leaks.
Another thing you can try is to duct tape the suction hose to a canister vac hose and plug that into the port. Pull the fuse so the central vac doesn't start. Turn on the canister and go to the central unit and listen for air leaks.
Is the piping separate branches or taps of one branch? You may be able to put the canister on one port and stuff a rag up from the central sucker and learn which branch has the leak and doesn't suck. If it's tapped, you may be able to tie a rag on a string and learn which two ports straddle the leak. Just let the vacuum suck the rag into the port. If the leak is beyond the port, you'll be able to tell that the rag gets sucked hard as it hits the tee. Tie the rag securely, or you'll have a blockage to go with your leak. Get help. You may need to pull the motor fuse to get the rag back out without damaging something. Paper towel on a string would be less risky. There's probably screws sticking thru the pipe just where you don't want them.
If I had to bet money, I'd bet that the central system is built from sections held together by something that's no longer holding and they slipped apart. Snaking it probably didn't help that problem. I recently crawled around in the mud to fix my neighbor's dryer vent that had just slipped apart when the tape failed.
The above is "stream of consciousness". Might be better to do the easiest stuff first from the bottom up.
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On Wednesday, December 31, 2014 7:13:44 AM UTC-5, mike wrote:

It doesn't have to be perfect, but it has to be consistent and not change. Otherwise the reading from one outlet to another, where he's looking for a difference in pressure to track down where the leak is closest too, would be meaningless. Personally, I don't think a rag or some tape is going to be easy to do. He has to take an auto size hose, which is ~1/4" and get it connected to something that is ~1 1/14".

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On 12/31/2014 12:56 AM, Danny D. wrote:

This is my 2nd central unit. The 1st one, for over 35 years, never had a problem like low suction. This one, seems to be having similar problems to yours. I have found that there is a screen protecting the fans and it gets plugged with very fine dirt. So, I have to take another vacuum cleaner and clean the central cleaner. So much for convenience. But, it was a used machine in this case. My old unit didn't have any filters and I guess that's why the fan bearings went. I understand that new units have self cleaning pop filters. Now that cash is no longer a problem, I am going to look into something better.
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Danny D.-
You have a leak, but you know that. How could there be a leak? Could a rodent have eaten a hole in a vacuum hose inside the wall?
Lets assume that you do not have rodents in the wall. Then one end of a vacuum hose has come loose. You have checked the end(s) at the main unit, so the leak is most likely where the hose connects to an outlet.
If you can disassemble the outlets, you may find it to be an easy fix.
Are outlets daisy-chained, or is there a distribution manifold that feeds each outlet (or group of outlets) separately? That is another place a hose could come loose.
Fred
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On 12/31/2014 12:56 AM, Danny D. wrote:

That discharge air has to come from some where. As such, I suspect a cracked or disconnected tube on the vacuum side, or maybe a leak around the canister itself. Lot of leg work and taking stuff apart to find the actual leak. There may be more than one leak.
This thread could last a while.
- . Christopher A. Young Learn about Jesus www.lds.org .
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Use a plumbers video snake to inspect the pipes.
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