# Rainwater Cistern: Input please



## Aohtee (Aug 26, 2003)

I want to install a rainwater cistern in the house I'm building.
The house will have a basement and I want to place the cistern under the basement floor. The cistern would have a hatch large enough for ingress for maintenance. I would use the water for showers,laundry, hand washing, and plant watering.

Household consists of one adult with lots of friends.
I live in zone 5.

I'm estimating my water needs at 50 gallons per day or about 20,000 gallons per year. 

Questions:
Tank size: would you build the cistern large enough to hold a years worth of water at any time, or one that holds 1/2 year?

Feed system: pump water to holding tank and use gravity feed or use a second pump and pressure tank to supply water to bathroom and sink?

Pouring basement floor over cistern: any problems with this?

Cistern material: cement or poly?

I'm still in the planning stage and would appreciate a second pair of eyes.


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## TnAndy (Sep 15, 2005)

1. Bet you're low on the daily use......I'd figure couple hundred/day to be safe.....better to figure too much than too little.

2. Size of tank. Unless you live in a climate where it rains only during a certain season, why plan on more than a couple months ?.....say 4-6 at the very outside.

3. Tank. Poly is the quickest route, without a doubt....dig a hole, put some sand on the bottom, put tank in, fill tank and backfill sides at the same time with sand, then pour your floor over the top....but you're talking SERIOUS bucks for a tank the size you're thinking unless you happen up on a good used one......you're probably looking a 50 cents/gallon based on the price of local 1500-2000 gallon jobs at the farm supply store. 

You might find is a lot cheaper to go with a EDM rubber liner ( like pond liner )....depending on your soil type, you might not even have to put in any concrete except a "ring" right near the top to lap the liner over before you pour the floor ( would have to have a "temporary" wood form under neath, that you could go in the hatchway and take out from below after the floor pour ).


4. Feed system: IF you can get enough elevation, the holding tank and gravity feed IS the way to go.....you need a foot of elevation for each 1/2 lb of pressure, basically....and the minimum pressure is about 20lbs....so you'd need 40' of elevation ( a 50-60 would be better ). We have a gravity fed spring and love it. Using this method, you could use a very small pump with a float switch, and run it off a single solar panel....you could eliminate the float switch even by letting the pump simply run anytime the panel gets sun, and use a return line to the tank for the excess.

Your other choice is, as you mention, a pressure system....but you need a larger pump, and then you ARE dependent on power AT the time when you need water.....


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## CamM (Dec 6, 2008)

A concrete tank under the house would be expensive. The top of the tank would need a good amount of reinforcement. Concrete here is over $100 a cubic yard. I've wondered about capacity too, as I'm trying to design a cistern for a college project. I would take the rainfall month by month and measure it against your needs. Here's what my book says: cistern capacity=gpd*days of dry period. So if you have 90 days of low rainfall*50gpd that's 4500 gallons. 

How are you going to filter the water? Those tasks are potable grade. Also, if this is under a basement, that would be deep excavation. I also vote poly.


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## artificer (Feb 26, 2007)

Take a look at the Texas Rainwater Harvesting Manual. Lots of good information there.

The only reason to build a tank with one years storage is if you plan on being besieged, and don't have a in-house water source. :viking:

My first question is: WHERE are you? If you're in the north west, 500 gallons would be more than enough. 

Why do you want this? To save money? To save rainwater? How big is your collection area? What's your average rainfall?

How do you plan on emptying the cistern when it gets funky? Sanitize it?

About the cheapest tanks you can get are used IBC's (internation bulk container) These hold 275 gallons, and cost anywhere from $25-$150 each. You can stack them 2 high for more storage. Not as much storage as a single tank, for the size, but you can isolate them, seal them, and make sure your water supply is potable. Standard poly tanks run about $1/gal as a rough estimate.

For concrete floors/walls, you can use a waterproof coating, like the one's from Sani Tred

There's lots of options, but I think you have a bit more research to do.

Michael


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## Harry Chickpea (Dec 19, 2008)

Poly tanks don't have the wall strength of concrete and if you are thinking along the lines of "stock tank" those should NOT be buried or backfilled unless they will CONSTANTLY be full of water, which kinda defeats the purpose.

50 gal a day is low. The two of us use about 500 gal per week with water efficient toilets and appliances. 

The IBC idea is good, as it is modular. However, I have no idea what the lifespan of these is. 

If you are using some of the water for plant watering, than a cheap above ground swimming pool is adequate for storage. In a pinch, you could line one with poly sheeting and use it for drinking water storage - although you would get some minor plastics contamination.


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## SolarGary (Sep 8, 2005)

Hi,
There are some construction details on tanks here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Storage

I believe that the one that links to the Canada Plan Service includes a buried concrete tank. The Canada Plan Service ones are good detailed designs that have been engineer sized.

More stuff on tanks for rainwater collection here:
http://www.builditsolar.com/Projects/Water/Water.htm

I guess if it were me, I'd think about the advantages of above ground cisterns.

The EarthShip design is nice, and works in Taos NM with 8 inches of rain -- they harvest rain water for all of the house needs, with a little bit left over for landscape irrigation. 
http://www.builditsolar.com/Projects/SolarHomes/Earthship/Earthship.htm
Part of the reason this works well for them is that they reuse the water 3 times before it goes to be used for plant watering.

Gary


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## Aohtee (Aug 26, 2003)

Thanks everyone for your comments. I'll try to answer your questions:

Location:
I live in upstate New York. The winter temperatures drop into the single digits most nights from late December thru to March. As an extreme example, the winter of 1981 saw daytime temperatures at the freezing mark for more than 3 weeks. Having an above ground cistern would be out of the question unless I ran a heat tape thru the collection gutters and into the tank. Frost line is approximately 4 foot deep.

Amount of rainfall: 
average is 46 inches per year. Several years ago we had no rain from May thru September. This past year was very wet and we may have doubled the annual figure.

Daily use: 
I have a 300 foot well and while it has never been a problem during the 10 years I have lived here, I try to be conservative with my water use. 5 minute showers:lather up first, then rinse. Full loads in the washing machine and I recycle the rinse water back to the machine for the next wash load.
Never let it run to become cool. I have several rain barrels I use during the summer for watering and toilet flushing. This may explain my low daily use.

Why install the cistern: 
A) Anticipation: It would be a lot easier to manually pump water out of the cistern than the 300 foot well.
B) I think it wrong to waste 3 gallons of perfectly good drinking water flushing my toilet. Capturing and recycling rain water seems a better way. Why not a composting toilet? I will when I can figure a way around the local ordinances against them.

Size of collection area:
Right now the house I'm designing is 20' x 40'. I can increase the roof overhang to increase the collection area.

Feed system: One story house. No way to go up 40' to 60' unless I build a water tower. Although I would like to see how many shades of red the building inspector could go thru when I handed him the design:nono:.

Gary: thank you for the links. I am a long time reader of your site. 

Artificer: I have a copy of the Texas Rainwater Harvesting Manual along with the Art Ludwig's âWater Storageâ book.

TnAndy: I hadn't thought of the edm liners. I'll look into them.

Any ideas on how to winterize an above ground cistern?


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## TnAndy (Sep 15, 2005)

*Any ideas on how to winterize an above ground cistern? *

Here is how I did it, and we have the close to the same situation.....sometimes it can go weeks below freezing here.

Built a 10x20' block building back into a bank below my spring collection box. Put two 1500 gallon tanks in it. Building is insulated inside with 1" of pink foam board. Never had a problem with freezing. Water line is 4' deep all the way down to the house.


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## Aohtee (Aug 26, 2003)

Nice set-up and a great looking building. I do have a bank I can build into but it's downhill from the house site.

I'll have to go back and look at tank types again. Maybe I could bury the cistern close enough to the house to use a cistern pump.


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## painterswife (Jun 7, 2004)

Our house will have an above ground concrete cistern between the house and the garage( length wise). Both shed roofs will feed to the cistern. It will be long and narrow.
Sort of like a butterfly roof but the roof will not meet.

I don't know what type of buildings you will have but maybe you could do something like that. The buildings should keep it from freezing but we will insulate the outside ends as well.


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## Michael Kawalek (Jun 21, 2007)

It might be far easier to build a small cabin over an exterior tank as Andy has. Our homestead has two 5000 gallon tanks at 5000 feet elevation and it gets about 10-15F in the winter here. The mass of the water is what keeps it from freezing even though the tanks are exposed to the weather. In your area, even though it gets much lower winter temps, being inside will allow the water to collect a lot of solar gain.

If you can go several months without rain, you'd better have a several thousand gallon tank. I'm in the process of finding additional tanks right now; I'm looking at 3000 gallon tanks as the best option for what is availble in this area right now.


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## SolarGary (Sep 8, 2005)

Aohtee said:


> Thanks everyone for your comments. I'll try to answer your questions:
> 
> Location:
> I live in upstate New York. The winter temperatures drop into the single digits most nights from late December thru to March. As an extreme example, the winter of 1981 saw daytime temperatures at the freezing mark for more than 3 weeks. Having an above ground cistern would be out of the question unless I ran a heat tape thru the collection gutters and into the tank. Frost line is approximately 4 foot deep.
> ...


Hi,
An above ground cistern with some insulation might be pretty resistant to freezing.

For example..
A 10 ft diameter by 10 ft high cistern holds about 6000 gal or 50,000 lbs of water. 

In rough terms, if you insulate to R14 (this would be a 2 inch layer of the polyiso rigid insulation), and you look at a 15F temperature difference between the water and the outdoors (for example 40F water and 25F average air temp), then the heat loss is:

Loss = (450 sqft)(15F)/(R14) = 480 BTU/hr or 11500 BTU/day

If the cistern is full, then the temperature drop per day would be:

Drop per day = (11500 BTU/day)/(50000 lbs) (1 BTU/lb-F) = 0.23 F per day.

So, if the water starts at (say)42F it would take 43 days to get down to 32F where it would just start to freeze. Since it takes so much energy to actually turn water at 32F into ice, it would be very slow to freeze even if it stayed at 32F for a long time.

These solar livestock tanks manage to keep water nearly ice free during our very cold (MT and Alberta) winters:
http://www.builditsolar.com/Projects/WaterHeating/SolarHorseTank/SolarHorseTank.htm
This is (I think) a tougher problem because the tanks have a water surface that is open to the air, and a bad ratio of tank surface area to tank volume -- a big cistern should be easier. So, maybe a bit of simple solar assist could also help (if needed).

The cisterns in the Earthships in Taos (which has cold winters) don't seem to be a freezing problem. They have no insulation, but they do share a wall with the house.

Just a thought.

Gary


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## Aohtee (Aug 26, 2003)

Gary, 
Lets take your 10 x10 tank and bury it just below frost line, say 5 foot deep. Insulate the sides and fill in. Now cover the exposed sides and insulation with the finish they use on inflatable dome homes. Painted black I would get some solar gain during the winter. I could train vines or something across it during the summer to reduce the solar gain and slow down the beasties that inhibit the tank.

I think this would work.


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## littlebitfarm (Mar 21, 2005)

FWIW my cistern is 8,000 gallons, in 18 years I have run out of water twice (once due to caretaker leaving barn water on while I was gone, other time due to broken line to barn). Household of one and multiple barn critters (up to 15 horses). Most of the time the cistern is at least half full. 

When I had it poured the top was at ground level and the served as my back patio. Freezing was not a problem for the 10 years that it was that way. 

HTH,

Kathie


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## Aohtee (Aug 26, 2003)

Gary,

I checked out the Fat Boy water tanks on your site and I think they may be the answer. Upright tanks standing just under 8 feet high and 2.5 feet wide, they would fit along the walls of the basement. They can be connected in series. Easy to put a drain line in while pouring the basement. They only hold 650 gallons so I'll monitor my usage for several months to see how many I would need.

So much good information on your site, thank you.


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## SolarGary (Sep 8, 2005)

Aohtee said:


> Gary,
> 
> I checked out the Fat Boy water tanks on your site and I think they may be the answer. Upright tanks standing just under 8 feet high and 2.5 feet wide, they would fit along the walls of the basement. They can be connected in series. Easy to put a drain line in while pouring the basement. They only hold 650 gallons so I'll monitor my usage for several months to see how many I would need.
> 
> So much good information on your site, thank you.


They are pretty interesting tanks -- I'd like to hear how that works out for you.

Gary


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## meanwhile (Dec 13, 2007)

We used two regular cement septic tanks. We put some type of painted on thick...gray stuff....it looked like soupy cement. The man who was helping me said it would water tight the inside of the tanks. We buried them but left the tops open to the air so we can get into the hatches when need be. We drilled a hole in one and set a cheap hand pump on it and we use that when our power is out. We do have to "prime" it each time but we keep a bucket of water beside it for that. 

We also have two above ground plastic type tanks but they get all green and slimy with glop and we have none of that in the underground cement tanks.


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