Cistern and gutter ...
 

Cistern and gutter design  

 

EngRMP
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February 13, 2011 12:00 am  

Hi folks,

I'm still designing my house for STX. I'm looking more carefully at the cistern and gutter system and have a few questions:

- does your gutter system enter your cistern at multiple locations, or only at one location (ie, is there an issue with having multiple entry points to the cistern)?

- are the gutter downspouts run such that there is easy access (ie, do you find that you have to clean out the downspouts or access them for other reasons)?

- I'm guessing that the downspouts eventually have to connect to horizontal pipes to get to the cistern. Do you have clean-outs at various locations along these pipes? Is the slope of these pipes standard plumbing slope, or are there other considerations?

Thanks, in advance...


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SunnyCaribe
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February 13, 2011 2:36 am  

A downspout is typically 3" PVC and you use as many as needed to clear your roof. Multiple down spouts are common. Depending on your architecture the best method for cleanouts is to have a 3" swept tee where the d/s changes direction from vert to horiz. Install so the cross bar of the tee remains vertical and the stem of the tee goes into the house. Here's the important part: the sweep must point DOWNWARD and the bottom part of the tee gets a threaded cap. This is so that when a storm comes you simply unscrew the cap and the water from the roof, often highly salty and dirty, will pass straight thru onto the ground without contaminating your cistern. Screens in your downspouts are a necessity and should prevent the need for cleaning out. Also, 3" dwv pvc fits neatly inside 8" block cells. Good luck.


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EngRMP
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February 13, 2011 1:07 pm  

Thanks SunnyCaribe,

That helps a lot. I get what you're saying.

So, do your horizontals all join together before entering the cistern, to limit the number of openings to the cistern? Or, is this not an issue?

Also, is a manhole cover sufficient for maintenance access to the cistern?


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SunnyCaribe
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February 13, 2011 4:05 pm  

The overflows should be below the level of the inlets so the number isn't important. Put them where they need to be. It is more important that you have unrestricted flow into the cistern. Remember that the x section of a 4" pipe is twice that of a 3" pipe. If you join too many downspouts before they enter the cistern you could have a constriction that would be problematic.

Cistern covers are 2'x2' and are cast in the floor. Several companies make nice frames to neaten up the opening.

Remember to plumb your foot valve within easy reach of the cover so you won't have to go swimming to perform any maintenance on it. Use a threaded union.


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EngRMP
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February 13, 2011 5:17 pm  

Excellent information.... thanks much!


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sloop jones
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February 16, 2011 9:28 am  

We installed valves and T's on all our " in" pipes. During a hurricane we shut off water entry into the cistern and connect pipes to the T' to divert water away from our house.

Our roof collects 2000 gallons per inch of rain. Typically a hurricane drops over 10 inches of rain. Letting 20.000 gallons of rain drop thru the pipe onto the ground around our house would be a disaster.

sloop


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EngRMP
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February 16, 2011 1:24 pm  

Thanks sloop jones. Those are good points:
- T's at d/s are good directors during storms, but for added safety from salt water contamination, shutoff valves at the input would be nice (I think SunnyCaribe refered to this as his/her "foot valve").
- And, yes, all that diverted water has to go somewhere. A way to keep that from cutting ruts near the house foundation would be nice.

On another note, I saw an interesting design for a roof washing system. It was basically a section of tubing that formed a U-shape. It was sized to hold the amount of water desired to "wash" the roof. It was T'd into each d/s so that they had to fill before water would enter the cistern. A small drain hole would empty the U-tube after the storm. But, it seems that the problems with this design are:
- once the U-tube is filled, you get some splashing of wash-water into the cistern
- I can imagine the drain hole getting clogged. So, maybe a larger drain hole that has a valve that has to be "managed" (if you forget to close this then you'd get no water during the next rainfall).
- you need a U-tube for each DS


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SunnyCaribe
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February 16, 2011 2:21 pm  

Ummm, no, a foot valve is the valve on the bottom of the intake pipe for your pump which allows the pump to build pressure in the pressure tank and retain that pressure once the pump turns off. It is a check-valve with a strainer. It should be installed so that it hangs several inches from the bottom of the cistern so that the inevitable muck can settle out below the level of the intake. Also, when your pump starts sucking air you will still have some water in your cistern in case of emergency.

Sloop's point is valid especially if landscaping and terrain prohibit you from conducting water away from your foundation. Still, I would avoid valves because of their expense and their relatively short life span. Why introduce moving parts if you can avoid them?


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EngRMP
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February 16, 2011 10:41 pm  

Thanks for the correction SunnyCaribe.

Yes, I worry about moving parts and complexity as well. But, it seems that maintenance issues and different operational situations (hurricanes vs rain vs water delivery) may require some optional configurations. I'm still studying the issues. Lots to consider.


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sloop jones
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February 18, 2011 8:51 am  

re. valves. is their an alternative? my site has to have some way to divert the water away from the house. Is there a way to tieinto the bottom of the clean out ?

sloop


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Bombi
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February 18, 2011 11:41 am  

http://www.rainharvesting.com/content/firstflush.asp

http://www.scribd.com/doc/29626577/040810-Roof-Water-Harvesting-3mb

When I last researched this roof water diverter issue I saw some really cool diverters made as a unit of PVC so if you look at rain water collection or roof washer diverters and language like that maybe you can find them. I'll look when I have more time

optimist with low expectations on STX


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EngRMP
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February 18, 2011 1:40 pm  

Thanks Bombi, those are good links...


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EngRMP
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February 20, 2011 1:07 pm  

Folks,

What kind of maintenance have you required on the plumbing to the cistern? And how often have you had to do "surgery" on this plumbing to perform maintenance? I'm becoming concerned about the whole roof washing issue. It just seems that the chance of debris getting into valves and clogging drains is really high. Therefore, I'm thinking that it'd be important to design the plumbing such that it: 1) can be taken apart easily; 2) places valves and drains at easily accessible locations.

I think the issue of cleaning sediment from the cistern has been discussed before... I'll search/review that info...


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SunnyCaribe
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February 20, 2011 1:29 pm  

You need to make a visit and look into designs yourself. You are obsessing on nonissues. People seldom wash their roofs and they simply disconnect their downspouts when they do. Debris is caught in downspout screens. If you live among a canopy of trees you will become accustomed to more frequent cleaning outs and filter changes. Downspout-sized ball or gate valves are expensive and prone to breakage. Make your system as simple as possible and observe the boat-builder's maxim, "if you don't have it, it can't break on you."


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