Gravity Cistern

[spence]

I am installing a cistern up the hill from my mountain cabin. Since the property is off the grid, gravity will take the place of a pump. I'll fill the cistern a couple times a year from the well using a generator.
My questions:
What kind of valve can I get to shut off the water supply at the bottom of the cistern? it will be several feet underground to protect from freezing. Does someone make a long stem valve?

What should I use for an underground supply line? I used to use copper for this but now that there is pex I thought it would be more freeze resistant, which will be important in the crawl space where it comes out of the ground and into the cabin. The soil is rocky and I intend to bury it at least 4 feet deep. Should I sleeve it in black poly tubing or something to protect it?

[Rich]

With rocky soil I would probably sleeve it in something.. lessen the risk of puncture that's hidden. There are freeze proof valves you can put down that far with open valve handle - you can then make up an extension to reach it to turn it off / on. Kind of redneck but it works.

[VALENT]

Spence, how far above your cabin is the cistern? And how big is the cistern? I am just curious.

[spence]

The 1700 gallon cistern will be about 40 feet (elevation) above the cabin floor. That should give me just minimal water pressure....about 20 psi.

[VALENT]

Well, thats pretty large. Why do you think it would only be 20 psi? Heck my grandparents house was serviced by a cistern above a well house(about 14 foot to lowest level) filled by a windmill and it the pressure was alright--not good but alright. It seems yours would have good pressure. What size pipe is going from the cistern to the cabin? This would definitely influence volume.

[spence]

You get 14.7 PSI out of every 33 feet, so your grandparents only had about 7 PSI. That's enough to take a drizzing shower. Most homes are serviced with about 35 PSI and I think 30 is about minimum to operate reverse osmosis water filters. I was thinking 3/4 would be enough since it's just a cabin and only one person will run the water at a time. Maybe I need 1"..(?)
Come to think of it 1" pex is only about 3/4 ID.

[VALENT]

I "think" there would be a significant difference if you used the larger size. I think you get additional pressure based on the weight of the water above the water actually leaving the cistern. In your case I think you will have really decent pressure. By the way, how are you constructing your cistern?

[spence]

By the way, how are you constructing your cistern?I'll buy a ready made poly tank designed for burial.

[VALENT]

Have you looked into the price of these that are for potable water?

[spence]

Have you looked into the price of these that are for potable water?Anywhere from one to two dollars per gallon of capacity.
BTW, I spoke to the contractor that will probably install it and he said that concrete tanks are usually cheaper than poly, delivery included. Also he said I should use one inch polypropelene for supply. He said all around Missoula this is being used for underground water supply and it is heavy wall and black, suitable for burial in rocky soils without sleeve. This is not the same as the cheap black poly being used widely for irrigation.

[VALENT]

How much more expensive would using pvc be? We used the poly for some long runs to water troughs etc and they cracked. We also had trouble with roots at the connections(these were clamp connections) I am not certain how long the poly lasted (I am thinking about 15 years before lots of problems) but I would ask the contractor about his specific experience in your area(and check on cost differences).

[spence]

The contractor said the PVC would not work well in that rocky soil and in that cold climate....that it would crack, but that it would be OK for the septic line. I take this to mean freeze damage is the major issue, perhaps aided by upheaval.
Did you use the thick wall poly for the water troughs and was it buried? Another contractor told me that underground water lines only have to be buried below the frost line if the snow will get disturbed, but if it's covered by snow the ground right under the snow will be soft enough to dig with a shovel. The snow acts as insulation.

[VALENT]

yeah, it was the better poly and all of it buried. Why is this poly better than pvc as far as rocks and cold are concerned? I dont know about the other contractor's statements. Sorry.

[spence]

I'm from Texas and am building in Montana, so I'm no expert on the effects of freezing, but my understanding is that the poly is more freeze resistant and since it is more flexible, frost upheaval is less likely to cause it to crack. Has anyone used the PEX tubing for burial? Seem like it would be better, especially if sleeved in the thin black poly.

[bkrahmer]

I definitely wouldn't use PVC. That stuff is way too brittle, not to mention having joints every 20 feet. Over here in N ID, most everybody uses 160psi poly for their water supply connections (usually to the well.) You can easily get it in 300' rolls, and the correct fitting is a 'Ford' fitting. They're about $60, IIRC. I would think the poly is fairly freeze resistant, but I would always put it a safe depth anyway. 3-4'. Another contractor told me that underground water lines only have to be buried below the frost line if the snow will get disturbed, but if it's covered by snow the ground right under the snow will be soft enough to dig with a shovel. The snow acts as insulation. That guy is absolutely full of shee-ite. Snow insulates a little bit, but will only reduce the frost line a little bit.

I have read that PEX is freeze-proof, and does come in 1000' rolls, so that might be a good option. Sleeving it would certainly be a good idea. PVC would be a good one for that job...