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I have an 8 foot wide by about 12 foot tall water cistern (5,000 gallons) about 8 feet under ground.

I’m concerned about the water freezing during this winter as the temperatures drop significantly here.

Im also trying to study the effect the 2/3 of the tank that’s underground has on the 1/3rd above ground.

Any insights welcome. 🙏

Mike
 

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I am not sure what climate you live in, but I have two ideas. (1) Circulate the water within the cistern and (2) investigate the possible use of a septic tank heater.
 
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I'm probably in a warmer climate but...what color is the tank? Black tanks can absorb some heat in winter (when and if the sun shines) and freeze a little less easy than white tanks do
 

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Is the tank new? Does it have a history of over-wintering?

Obviously your location matters. The part below the frostline wont freeze, and that may have enough thermal mass to keep layers immediately above it from freezing too.

If the volume of water/dimensions of tank is large enough, only the top and sides exposed to the cold tank sides will freeze solid. The water under/inside will remain liquid.
 

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When I put in the new septic tank I put 4" of foam on the tank with a 2' overlap. Then I covered it with dirt. We get -40 F here in some winters and -20 F is common. The top of the tank is less than 2' deep.

If you want to keep it at it's present elevation I would put 4' of foam around the top 8' and top and then put some kind of sun block over the foam. You could also bury the tank in sawdust.

Granted wastewater produces heat whereas the drinking water will get it's heat from the earth.
 

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I lived with a cistern and septic in the Manitoba prairie winter for 12 years, so I can relate to your issue. I won't bore you with the details of my set up, but the concrete septic tank was about 20% above grade, which gave rise to some similar issues. Based on my experience, what I would suggest to you is:

1. Your cistern won't freeze below the frost line depth, but the top portion will freeze. If you are using a float switch in the tank, one inch of ice can not only immobilize your float switch, it can cause you to bust the switch trying to chip it out....

2. You can insulate the above grade part in several ways (I used a combination of square bales and earthen planters) and that definitely helps. But you will still need a heat source.

3. Because my situation was a septic tank and not fresh water, I used a suspended heat lamp instead of a livestock submersible heater. If I was in your situation, I would look into the latter.

4. The amount of insulation and the strength of the heat source will be influenced by how quickly you are using and replenishing the water. When I had a 5000 gallon cistern, we could live comfortably on 90 gallons per day. So I could go 2 months before I had to truck more water in. If your cistern is being fed by a well on a float switch, you will get more regular water movement which will help.

Good luck.
 

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You should also have a Vacuum Breaker valve on the line so that any part of the line that could freeze drains back into the tank - if it is not heat trace wrapped.
 

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Sure will help to know your location. Kansas or northern MN will make a difference...

how deep does the frost go where you are if you don’t want to give a location?

depending on frost depth, you are looking at about half the tank below freezing, about half the tank at 50 or so.

to keep from freezing the water will naturally circulate and not freeze in a mild climate as the 50 degree stuff will mix with the below 32 degree stuff.

in a colder climate you will need to use enough water to also be adding more water than what wants to freezeup. If you use the water up every 2 days and replace with more, the water will be flowing, and keep itself Warmed some with fresh water.

or you need to insulate the above ground with snow, straw, or dirt. Be careful with dirt, you are adding weight and crushing force tot he tank.....

In a really cold climate where the frost can go 7 feet deep, you are gonna need some heat most likely.
 

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I have an 8 foot wide by about 12 foot tall water cistern (5,000 gallons) about 8 feet under ground.

I’m concerned about the water freezing during this winter as the temperatures drop significantly here.

Im also trying to study the effect the 2/3 of the tank that’s underground has on the 1/3rd above ground.

Any insights welcome. 🙏

Mike
If the tank is eight feet underground you are good to go. You couldn't make it freeze if you wanted it to. The ground temp eight feet under ground is going to be around 60 degrees. If your temp goes to twenty below for a week, you might get an inch of ice on the surface.
 
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