Wood Cistern

[davestreck]

I've been playing around with different ideas for rainwater collection and storage for our place in Nova Scotia. I need above-ground water storage, since bedrock is so close to the surface at our site and there is no way to get heavy equipment out to the island. Overwinter freezing is my main concern, although the fact that it is an oceanside location moderates the winter temps to some degree. My first thought was plastic rainbarrels, and I have found some that are rated for freezing temperatures, but they're pricey and the larger sizes are too big to fit in my boat.

Here is my latest idea, run up in SketchUp:



5'x5'x3' crib-style water cistern, built of PT 4x4s, with threaded rod run down through all 4 corners and bolted top and bottom. Inner surface lined with T&G spruce (same stuff I used on my cabin walls). 2 inches of filtered sand laid down as a base inside, then lined with a custom-fit, potable-water-certified, vinyl tank liner (best quote I got on this was about $300 plus delivery with a 5 year guarantee). Simple roof with a hinged center section for access, then sheathe the outside with more vertical T&G, slap a couple coats of paint on it and run the down spout through a hole in the gable end. Total water storage will be about 400 gallons.

What I like about this concept is the fact that all the materials are relatively cheap (except for the liner, of course) and will be easy to get to the building site. We should be able to get it put together in a weekend, and be out no more than $500 or so.

My concern is still freezing. The liner is supposed to stay flexible even below freezing, and my gut says that the crib design will flex a little if required without sustaining any serious damage, but I'm not sure. Any opinions on this concept would be helpful. Short of hiring someone to go put the plug back in the tank every spring so that its full by the time we arrive in late summer, I need a system that will hold water over the winter without damage. Our friend on the mainland has a similar setup for his garden, basically an open-topped log cabin of rough spruce logs with a tarp arranged inside and spiked to the top edge, and he claims not to have ever had a problem with freezing.

Thanks in advance for any thoughts or advice.

[Squirl]

Two thoughts. 
First, a round cistern with bent threaded rod might be cheaper, easier, and stronger.  You might want to look into it.


For your current design, have you considered some type of passive solar insulation design?  Like, nice light, rigid foam insulation on all sides except the south facing (lots of it), and maybe a trombe wall for the south side?  You are only trying to keep it above 32 degrees and water has great thermal mass.

http://en.wikipedia.org/wiki/Trombe_wall

[NM_Shooter]

Pretty neat idea.  Does the liner include a top, so that it is essentially a bladder, or is the top open?

I ask because you can get an epdm potable 45mil sheet that is 15' x 10' for about $100.  If you change the dimensions a little and make your crib rectangular that sheet would work great...http://www.relaxingdecor.com/firestone-pondgard-epdm-pond-liner.htm

Don't sweat the strength.  The ground holds the weight of the water.  The only pressure pushing outward is proportionate to the height of the tank.  Roughly 0.5 psi per foot of vertical height.  If you have a 5' (not including the corners) 4x4 with the center of the wood 36" below the top of the water, the total outward force on that 4x4 due to water pressure is :

3.5" x 60" = 210 in^2

3' x 0.5 psi/ft = 1.5psi

Total outward pressure = 1.5 x 210 = 315lbs.  Rebar down through the corners is a great idea. 

[davestreck]

Two thoughts. 
First, a round cistern with bent threaded rod might be cheaper, easier, and stronger.  You might want to look into it.


For your current design, have you considered some type of passive solar insulation design?  Like, nice light, rigid foam insulation on all sides except the south facing (lots of it), and maybe a trombe wall for the south side?  You are only trying to keep it above 32 degrees and water has great thermal mass.

http://en.wikipedia.org/wiki/Trombe_wall

Incorporating passive solar is a good idea. Maybe something as simple as insulated glass "skylights" on the south facing roof, plus insulation for the crib walls.

[davestreck]

Pretty neat idea.  Does the liner include a top, so that it is essentially a bladder, or is the top open?

I ask because you can get an epdm potable 45mil sheet that is 15' x 10' for about $100.  If you change the dimensions a little and make your crib rectangular that sheet would work great...http://www.relaxingdecor.com/firestone-pondgard-epdm-pond-liner.htm

The liner I was quoted is an open topped design, with a rolled edge designed to be snapped into a track run around the top edge of the tank. Basically its an above-ground pool liner made of potable-water-grade material. The problem with a simple tarp is that it would be tough to make a smooth inner surface, which would make cleaning the tank a pain.

In researching pond liner materials, I found the kits used to join multiple liners together. I suppose I could custom cut a 15'x15' sheet into the shape needed to fit a rectangular cistern, using the adhesive backed repair tape to seal the joints, but I have more confidence in a factory cut and welded liner than a home-made one.

Don't sweat the strength.  The ground holds the weight of the water.  The only pressure pushing outward is proportionate to the height of the tank.  Roughly 0.5 psi per foot of vertical height.  If you have a 5' (not including the corners) 4x4 with the center of the wood 36" below the top of the water, the total outward force on that 4x4 due to water pressure is :

3.5" x 60" = 210 in^2

3' x 0.5 psi/ft = 1.5psi

Total outward pressure = 1.5 x 210 = 315lbs.  Rebar down through the corners is a great idea.

Thanks for those calcs.

[Dave Sparks]

Can you circulate sea water to warm the tank exterior?

[Don_P]

If the tank gets wider at the top than at the bottom... if it has draft, as water freezes and expands it will be able to hopefully rise to a wider section of the tank rather than pushing against vertical sidewalls.