I'm a new member here. I'm here to learn how to do DIY plumbing work. 

How does my home's main water supply valve, backflow preventer, and Potter switch work? 

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Image 1. The Main water line enters the house.  There is a valve with the red handle.  Then It splits into two lines.  

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Image 2. The Water meter is in the top one. The Bottom one has two Blue valves. I suppose backflow prevention system is right here. Then it has the Potter switch. 

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Image 3 Top line has valve with green handle down the line from the Water meter.  Bottom one got Potter switch and pressure gauge along with white plastic tube going down from the potter switch. 

Couple of questions. 

  1. Which one is the main shutoff valve here?  Red one in the first image. This is before the meter.  There are two Blue valves in the second image.  I could easily turn them off. Green one in the third image.  It makes sense this would be the main valve.  This is down the line from the meter. However this is positioned in a way I cant turn it.   It is right next to the wall.  Can I shut off both blue valves to cut the water supply? Will it affect the backflow prevention system? 

 2 It appears both the top and bottom line go into the unit. Bottom one comes out before the water meter. How do the City calculate the usage for the bottom line then? 

3 Why the Backflow prevention and Potter switch are in the different bottom line, not in the same line top where the water meter is? How does it work? 

  1. What is the purpose of that white plastic tube which comes out from the Potter switch?  Since I am an amateur, obviously I have no clue about this.  Thank you for your time.


Thank you very mcuh for the response.

"Potterswitch" is the Water flow Alarm Switch. I do have sprinkler system.

It makes sense the city does not meter meter.

I'm pretty sure I can't pull away the green valve.  I have no clue why it was installed in such a way. 

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We have to do annual backflow testing as per the local requirement. I'm positive that the plumber fit some devices in the marked area in the image and close one or two blue valves and get the readings.

Our village installed the new meter back in May. He must have turned off the red one.

Why would they install a backflow testing device and valve in the sprinkler line, not in the mail line after or before the meter? It doesn't make sense? What am I missing here?  

@ ThreePhaseEel

Thank you for explaining. You are awesome!

Backflow will prevent sprinkler water backing up to the main water supply. I suppose I do not need the backflow device for the top line as the water won't get stagnant in the domestic line. That's why the backflow device has been installed in the sprinkler line, not the top line. Am I correct?

  • 1
    close up of the label on the item between the blue handles might be diagnostic for what it really is. All those little fittings do make me think it might be air/water separation for sprinkler freeze protection.
    – Ecnerwal
    Commented Sep 5, 2022 at 12:47
  • Use the edit link underneath your question if you want to update it with additional information. Do not post the information as answers. Thanks and welcome to the site.
    – Niall C.
    Commented Sep 5, 2022 at 17:47

2 Answers 2


You want the green valve for normal water shutoff purposes

If you're just seeking to shut off the household (domestic) water to work on the plumbing, you want the green valve, since that will shut off your plumbing without shutting off your fire sprinklers.

The two blue valves are part of your fire sprinkler's backflow preventer

From the arrangement of parts there, we can see that you have a classical fire sprinkler riser/drain installed, with a waterflow switch (Potter is one of the major manufacturers of such parts) that sets off some sort of local alarm (such as your smoke alarms, or even a local bell/horn) and/or interfaces to an alarm panel that can call whoever's monitoring your alarms if the sprinklers ever start flowing water, presumably because a fire caused one of the heads to open.

The white pipe is a drain pipe from the test drain on the system; opening the valve connected to it (located below the waterflow switch) will set the waterflow switch off, and should be done as part of the normal annual checks of the system. Of course, if you have an alarm panel and monitoring company, they'll need to be informed of what you're doing beforehand so they don't dispatch the fire department to a nothingburger!

Going back to the blue valves, though, those are the shutoff valves for the backflow preventer your water utility/local jurisdiction requires to keep sprinkler water (which can get very stagnant, unlike your normal house water, which is used on a regular basis and thus doesn't need any protection beyond the normal measures of air gaps at fixtures and vacuum breakers on hose bibbs and irrigation systems) from backing up into the domestic water system in case of a water main break or water pressure shortage. These need to stay open at all times, as the sprinklers won't have any water to do their job when they're closed. In fact, they shouldn't be left unsupervised as they are now; instead, they either need to be chained and locked open with a good quality lock, or have tamper switches hooked up to them so that they can generate a supervisory indication to your alarm panel when shut, provided you have an alarm panel that's capable of that.

The reason why they're there, and the primary reason why they need to be shut, is because the backflow preventer itself must be tested on a regular basis, and the test procedure requires the use of those valves for short periods of time. (This is what you've been witnessing the plumber doing -- the gadgets they use for this purpose are merely a set of pressure gauges that let them "see" what is going on within the backflow preventer, since it runs entirely on water pressures.) They can also be used to shut down the sprinkler system so that a fire protection contractor can drain it to do work on it, or in case a sprinkler head gets whacked or activated by a fire that's since been extinguished and overhauled, and thus needs to be replaced with a fresh head.

The red valve is a master shutoff, and should also be locked open or supervised

Going upstream, past the water meter on the domestic side, we then come to the red valve. This is in the incoming water main before it splits into domestic and fire branches, and thus shuts off both your domestic plumbing and your fire sprinklers when closed. As a result, unlike the blue valves, you'd at least stand a chance of noticing if someone shut it off out from under you, but there's still a cogent argument to be made for locking it open or electrically supervising it in the same fashion as the blue valves.

Furthermore, since the meter is in the branch feeding your household plumbing, your fire sprinkler supply is unmetered. Given that it'll only ever flow a non-trivial quantity of water in case of a leak (damaged sprinkler head) or a live working fire in your house, most water purveyors don't care to meter fire sprinklers, instead treating it like the fire department using water from a hydrant to fight a fire, and at most charging a fixed fee to you for having a sprinkler system.

Other notes for a fire sprinkler owner

Since you have a fire sprinkler system, there are a few other things you'll need to keep in mind. First, whole-house water softeners or filters should only be cut into the domestic branch of the system; fire sprinklers will run on pretty much whatever water the utility supplies, and softeners and filters can have very negative impacts on the ability of the system to supply enough water to successfully fight a fire until the FD can get there and finish the job.

Second, there should be a cabinet not far from here that has a funny looking wrench/socket and a set of sprinkler heads in it. The wrench is a sprinkler wrench that's used to remove and install heads, and the heads in that cabinet themselves are spares. This is important since a sprinkler head is a one-shot device akin to the fuse in your Christmas lights -- once it goes, you can't put it back together again, and must replace it instead. There should also be a set of plans in that cabinet -- these will be the hydraulic plans your sprinkler system has been custom-installed to, and are very important to provide to your fire sprinkler contractor if you are having major remodeling done on your house so they can recalculate the plans and thus ensure that the new work is as well-protected as the old work is.

Finally, whenever you're painting the walls and/or ceilings (whichever surface your heads are mounted on), you'll want to mask off the sprinkler heads with tissue paper bags, as paint splattering onto a sprinkler head essentially ruins it in the eyes of the fire code -- there's no telling what impact the paint could have on how the head behaves in a fire. (The tissue paper will cleanly burn away if there's an actual fire, which is why it's used in this instance -- it's a trick taken straight from the fire sprinkler code itself, where the bags are used to protect sprinkler heads in paint spraying booths from the inevitable paint overspray found there. See NFPA 13 paragraph for details.)


Never heard of a "Potter switch" but I would guess you have a fire sprinkler system, given the "before the meter" and red paint. Also the much larger diameter line for that purpose than the metered line. I guess the "switch" is to send a signal to an alarm system indicating that the sprinklers have actuated.

The city does not meter the fire sprinklers. Not setting the rest of the city on fire is considered an adequate payment for that specific, hopefully rare, use.

Main shutoff (everything) is the red handle in the first picture. Green handle is "everything that's not the fire sprinklers" and should pull away from the wall so it can be operated if it wasn't installed by an idiot. If it was installed by an idiot you might be able to remove a nut and change the handle position 180 degrees (and replace the nut) so that it can be operated. However, looking at the picture closely, it appears to be installed correctly.

Edit upon the OP doubling down on "I can't operate the green valve" update:

Close up from OP's picture

The arrangement of the stop lug on the bronze valve body (upper middle) and the stop lug on the galvanized steel handle (below that, still middle) show that you can pull the green handle 90 degrees out from the wall to stop the flow. If you have a hard time grabbing it, slip something behind the handle to pull on, or slip a hollow tube over the handle to extend it.

Blue handles are to allow replacement/service of the item between them (backflow preventer, you think - or perhaps a special fire sprinkler thing to allow the sprinkler lines to be pressurized with air until a fire - which permits operation in an unoccupied, freezing house, so long as the basement water entrance hasn't frozen.)

White tube appears to be a drain, but without seeing the other end, unclear. Drain is one of the two things it says on the casting the "Potter switch" is screwed into where that connects. Probably to drain the sprinkler section.

The red, or either one of the blues, will cause your sprinkler system to be disabled, which is likely a Bad Thing® and depending on how sensitive or fussy the switch is may also set off your alarm. So, for your plumbing needs, you should use the green.


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