I was going to build a cistern for my water supply (up here in the Ozarks) with backup from a small (1/2″ or 3/4″) existing rural water line now servicing the cabin I’m living in whose roof was to be the source of water for proposed cistern to be located there. The cistern to new homesite would have about 100 foot drop over about 1000 feet of line. I have already laid a 2 inch line over that distance, which would then be stepped down inside the new home. The backup line via a couple of valves (plus an anti-siphon) could be shut off completely or made to flow into the cistern during dry spells or directly to the house in case of cistern problems. Now my wife is balking at the cistern idea so the backup is about to become the main source (with at least some hope for the possibility of a cistern in the future…). I’m worried about stepping up the diameter of the pipe (schedule 40 PVC). I don’t know if their could be a vacuum problem if the demand at the new house (much bigger than the cabin) started draining the ample supply from the 2 inch line faster than the smaller rural water line could keep that line full. Would an air intake at the start of the 2 inch pipe at the cabin allow better flow in this case? Also, I was planning on installing a pressure regulator where the line enters the new house due to the standard pressure of the rural water line plus that added by the drop in elevation to the house. Is that a good idea? Thanks for any ideas on this… it’s giving me headaches.
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Replies
If I am following you correctly you have the 3/4" supply from the RWD and it is transistion to a 2" line from the cabin to the new house and then back to 3/4" for whatever at the house.
And you are concerned about what will happen if the demand is too high at the house.
The only thing that will happen is that with flow the pressure drops do to friction in the pipes. Too much of a friction lose and you will only get a trickle.
However, with the 2" run that section of pipe will be loseless, for all practical purpose, due to the relative large size and and low velocity.
And you are gaining 43 psi with the drop in elevation, so you would still have pressure even if the pressure at the end of the 3/4" pipe was near zero.
But your probably need a pressure reducer at the house with the 43 psi plus whatever static pressure there is in the main.
Thanks Bill. I had figured that much drop would produce 40+ psi so I wouldn't have to use a pump with the system. I left a 2" pipe sticking up out of the ground where the cistern was going to be built so I could test the 2" line (and find it later!). I used a garden hose at the cabin to check it, and everything seems to work fine. But, the upper end where the hose was in was open to the air so no vacuum was produced when I opened the valve at the homesite to drain it. So, when I'm tied and sealed in with the 3/4" RWD line and demand at the new house outstrips what the RWD can keep up with, I will have that full 2" line "reservoir" to take up the slack during that peak demand while the 3/4" line recharges it at its lesser rate. But, I don't know if being sealed off will create a vacuum problem as the 2" line "reserve" starts dropping. I could put in a tee at the junction of the RWD & the 2" lines and bring a "breather" pipe with a "shepard's crook" and screen (or something like that) attached to the end sticking above ground but didn't know if that was necessary or would produce even more problems.
Rick,
Even though you get a 40+ psi pressure increase from the drop, you can only get a 14.7 psi vacuum, which your pipe can handle easily. That vacuum will be felt at the end of the 1/2" main and will help to increase the flow thru it. This is a good thing.
SamT
Thanks Sam. What a relief! Makes sense when you say it....
There are two issues here.
Sam is right, in that there is no problem with the vaccum hurting the pipe.
But if you want to use the 2" pipe as "storage" and the source is "shut off" you won't get any water out of the pipe unless you add a vaccum break. But 2 problems with this is that
A) you will have limited storage before it get down and the pressure drops (assuming a gradual drop)
B) everytime this happens you will get air in the line and then you will get sputtering and sperting out the faucetts at the house and an upset Mrs Rick.
If this is a real problem (and I suspect that it won't be) the only solution is a storage tank. One that either is open to the air (your cistern ?) and uses gravity or a pump depending where it is in the system or a pressuized tank.
BTW what is the static pressure at the cabin. And here is a simple test. You can get a pressure/flow gauge.
http://tinyurl.com/5qr2w
You can try and borrow/rent one from a place that does landscape irragation. I think that some of the home horror stores that sell irrigation equipement will rent them.
But you can do some educated guess by getting a pressure gauge, hose bib, 50 ft of hose (maybe also some short and long for more ragne) and a 5 gal bucket.
Measure the water pressure and the time to fill the bucket at full open, 3/4, 1/2 and 1/4 open and static pressure.
That will tell you what the flow will be at high demand.
Bill, thanks again for the info. I bookmarked the site you gave but think I'll skip buying the $100 guage; there's no existing hose bib to hook into anyway. (The hose feed I mentioned in testing the line was snaked around from the water heater!) The existing pressure at the cabin seems to be good. I think I could work the flow test though. You're right in assuming the line has a gradual drop in elevation (more or less). I got a pressure reducer for the end of the 2" line in any case. I think I'll use a 2" Tee at the upper end and cap it off in case I ever do need a vacuum breaker, but I hope to never have to resort to it!
Oops, I forgot a couple of things.... I was only going to shut off the RWD source if I had the cistern. Without the cistern there should be (hopefully) no reason to shut it off. You're right in thinking the original plan would have used the cistern system as a vaccum break.
I'm also hoping with this new "no cistern system" in which the RWD line will always be open to never run down the 2" line completely out of water. Would that keep the spitting you spoke of (along with, as you also mentioned, the wrath of you know who!) from occuring?
Problem is, a vacuum break valve will let air in but not out. So once the line gets loaded with air you'll need to bleed it somehow.
OK, I think I've got it straight now. Thanks Dan.
"I'm also hoping with this new "no cistern system" in which the RWD line will always be open to never run down the 2" line completely out of water. Would that keep the spitting you spoke of (along with, as you also mentioned, the wrath of you know who!) from occuring?"
It is not running "out the water in the 2" line - it is if you end up with a vacumm breaker and the RWD supply does not keep up to the demand then the vacumm breaker will open and allow air into the line so that you can draw the water out of the 2" line. That allows air in the line and the air will come out with the RWD starts resupplying the line and the air them become compressed.
Like I told Dan, I think I see what you and he are trying to tell me -- no vaccum breaker on the new system plan. I really do appreciate yourall's help.
I'd suggest you use some sort of storage tank -- 50-100 gallons. Have it fill from the rural line, and gravity or pump feed the house. A Texas-style rooftop hot water tank would do the job.
Thanks for your reply Dan... maybe I can use your message in trying to persuade my wife that a buried tank of some sort isn't such a bad idea. (Perhaps it could even be a stepping stone to that cistern!)