Hello everyone. I am currently working on planning for a large cistern system (thinking somewhere around 10k galloms). I want to plan far enough ahead in sizing to see if I will be able to use this as a heat sink for a future Geothermal HVAC system. I read a post on here one time before of someone sinking copper tubing in a cistern for this purpose. Can't find the thread now though. I live in South Carolina. System will need to supply heat & AC. I currently have 2 air/air HVAC systems. 1) 4.5 ton (a bit oversized now, will probably go with a 4ton or even a 3.5 2) 1.5 ton unit so how do I determin how many gallons of water, amount of copper for heat sink, etc to be able to work with 5-6ton of Geo HVAC? Thank you all in advance!

10K gallons unlikely to support this. Why use a cistern when you can use the ground which can transport the heat away?

To heat 1 gallon of water by 1 degree will take 8.33 BTUs. 83.300 BTUs for your 10,000 gallon. the heat the water from 65F ground temp in georgia to 90F, which should be your maximum design temp, takes 2,082,500 BTUs.

Lets say you have a 5-ton system, if it runs continuously for 24 hours:
60,000 BTUs +compressor heat (12,000 BTUs) = 72,000/h to reject, x 29 hours = 2,078,000. So in roughly 29 hours your cistern is up to 90F, assuming it does not loose heat to the surrounding.

Yes, it might not run that long all the time, but you max out that heat sink within a few days.

The important think to understand in the geo exchange concept is that ground transfers the heat to and from the pipes and can transfer an enormous amount of heat, something a cistern cannot do.

Multi thousand gallons of water are good for storing heat/cool from night to/from daytime. That can be useful, but don't expect much more from it.

Posted By docjenser on 23 May 2013 12:28 PM
10K gallons unlikely to support this. Why use a cistern when you can use the ground which can transport the heat away?

To heat 1 gallon of water by 1 degree will take 8.33 BTUs. 83.300 BTUs for your 10,000 gallon. the heat the water from 65F ground temp in georgia to 90F, which should be your maximum design temp, takes 2,082,500 BTUs.

Lets say you have a 5-ton system, if it runs continuously for 24 hours:
60,000 BTUs +compressor heat (12,000 BTUs) = 72,000/h to reject, x 29 hours = 2,078,000. So in roughly 29 hours your cistern is up to 90F, assuming it does not loose heat to the surrounding.

Yes, it might not run that long all the time, but you max out that heat sink within a few days.

The important think to understand in the geo exchange concept is that ground transfers the heat to and from the pipes and can transfer an enormous amount of heat, something a cistern cannot do.

Thank you for your input. A few things;

I forgot to specify but the cistern will be below ground. Therefor it will be able to transfer heat to/from the surrounding soil. Can someone please tell me how to calculate the amount/rate of heat transfer per sq/ft of ground contact?

Apparently I had bad info on the btu/ton. I see you are right that 1 ton=12k btu.
So I need 60,000btu + 12,000btu of compressor = 72,000btu X 8hr/day actual usage = 576,000 btu

It takes 83,333 btu's to heat 10,000 gallons 1 degree F
So my system using 576,000 btu /day will increase the water temp  6.9 degree F / day

My current cistern plan will be 5ft deep, 20 feet wide circle. (11780 gallons)
Excluding the top, it will have a 628sq/ft of ground contact. How do I determine how much heat this can transfer to the surrounding ground?

Thank you all again.

If you really want to do this you could have a few solar panels and pump the heat into the cistern during the summer, for part of the winter use. How much depends on your home heat loss. Look up Okotokes, Alberta for a big version of this.

As nice as this might sound, the cistern will not able keep up with your heat pump.

Posted By dcb1101 on 24 May 2013 05:30 PM

Posted By docjenser on 23 May 2013 12:28 PM
10K gallons unlikely to support this. Why use a cistern when you can use the ground which can transport the heat away?

To heat 1 gallon of water by 1 degree will take 8.33 BTUs. 83.300 BTUs for your 10,000 gallon. the heat the water from 65F ground temp in georgia to 90F, which should be your maximum design temp, takes 2,082,500 BTUs.

Lets say you have a 5-ton system, if it runs continuously for 24 hours:
60,000 BTUs +compressor heat (12,000 BTUs) = 72,000/h to reject, x 29 hours = 2,078,000. So in roughly 29 hours your cistern is up to 90F, assuming it does not loose heat to the surrounding.

Yes, it might not run that long all the time, but you max out that heat sink within a few days.

The important think to understand in the geo exchange concept is that ground transfers the heat to and from the pipes and can transfer an enormous amount of heat, something a cistern cannot do.

Thank you for your input. A few things;

I forgot to specify but the cistern will be below ground. Therefor it will be able to transfer heat to/from the surrounding soil. Can someone please tell me how to calculate the amount/rate of heat transfer per sq/ft of ground contact?

Apparently I had bad info on the btu/ton. I see you are right that 1 ton=12k btu.
So I need 60,000btu + 12,000btu of compressor = 72,000btu X 8hr/day actual usage = 576,000 btu

It takes 83,333 btu's to heat 10,000 gallons 1 degree F
So my system using 576,000 btu /day will increase the water temp  6.9 degree F / day

My current cistern plan will be 5ft deep, 20 feet wide circle. (11780 gallons)
Excluding the top, it will have a 628sq/ft of ground contact. How do I determine how much heat this can transfer to the surrounding ground?

Thank you all again.

It is a bit more complex to calculate heat transfer, since your parameter will be rapidly changing. It depends very much for example on the temperature difference between the cistern and the surrounding ground. Lets say you run it only for 8 hours a day max, which is very unlikely, and it raises the water temp by 7F/day, you don't get hardly any heat transfer out of it unless you raised the water in your cistern to 65F-70F, and a few days later to 90F.
By now your heatpump will work very inefficient, since it has to dispense heat against 90F water, so you are adding more compressor heat to the equation. Which means it will take longer to cool a certain load in your house, which means that the heatpump will run longer, putting more BTUs in your Cistern. By now the heat transfer to the ground has decreased because the ground around your cistern has been heated up. Now you need 100F degrees......and this keeps going and going until your heatpump shuts off on high pressure!
I am just guessing here, but your setup might support 1/2 ton of very inefficient heat transfer at max...

The cistern question comes around every now and then, but it flunks math tests unless the cistern is more like an underground lake. At some point, perhaps around 100,000 gallons, the cistern becomes large enough that its walls are in contact with enough ground that enough heat transfers to balance compressor additions / subtractions, but the contact area is the key variable, not the gallonage. Tinkering with various shapes and sizes of cisterns would likely eventually reveal that a very long and thin cistern in contact with lots of ground dirt works best, and a cistern so-shaped bears close resemblance to a loop field.

I've often wondered if a 20,000 gallon cistern, open at the top, sometimes also referred to as an in-ground swimming pool, would support a 2-3 ton water source heat pump in my AO (Jax, Fla.) It should pick up enough solar and ground heat so as not to freeze during what passes for a winter here (a few days with nights in high 20s), and it should lose enough heat to radiation, convection and evaporation, especially at night, to support hot day cooling loads. There would be pool water chemistry / heat exchanger material compatibility to work out, but I think the thermodynamics might pencil out.

Posted By engineer on 24 May 2013 10:37 PM
The cistern question comes around every now and then, but it flunks math tests unless the cistern is more like an underground lake. At some point, perhaps around 100,000 gallons, the cistern becomes large enough that its walls are in contact with enough ground that enough heat transfers to balance compressor additions / subtractions, but the contact area is the key variable, not the gallonage. Tinkering with various shapes and sizes of cisterns would likely eventually reveal that a very long and thin cistern in contact with lots of ground dirt works best, and a cistern so-shaped bears close resemblance to a loop field.

I've often wondered if a 20,000 gallon cistern, open at the top, sometimes also referred to as an in-ground swimming pool, would support a 2-3 ton water source heat pump in my AO (Jax, Fla.) It should pick up enough solar and ground heat so as not to freeze during what passes for a winter here (a few days with nights in high 20s), and it should lose enough heat to radiation, convection and evaporation, especially at night, to support hot day cooling loads. There would be pool water chemistry / heat exchanger material compatibility to work out, but I think the thermodynamics might pencil out.

"...what passes for a winter here (a few days with nights in high 20s) "

That is funny, we call that summer here!

I am thinking the same way. The fact that my 6 ton heatpump heats my 20000 gallon pool up pretty quickly and then only cycles despite the ground being less than 50F right now tells you that that the heat does not transfer away. Yes, if you make the pool 300 ft long, 8ft deep and 3 ft wide, it might pass as 1 circuit for a horizontal loopfield. Build 5 of them, and you have support for 5 tons. There is a reason we use cheap pipe for that.

Is your pool covered when not in use? Even down here, losses from the surface of a pool dominate, particularly if it uncovered.

If I win the lottery (unlikely given that I don't buy tickets) I wanna pool with the underneath bits made of ICF block and a retractable shell pool house. Oughta be able to heat and filter it for pennies per day.

If I ran a 3 ton water source heat pump 12 hours in Y2 (a very long hot day), that would reject perhaps 540,000 Btu, raising a 20,000 gallon pool temperature by 3.24*F, not much, but 10 days of that would be a problem. My limited experience is that an uncovered pool loses several degrees at night, putting matters back where they started. Could also activate a water feature for more cooling if temperature got out of hand.

I guess I've hijacked OPs thread, but a pool is a cistern of sorts.

Would be interesting to see how much cooling one could get by spraying cistern water over a roof at night.

Posted By engineer on 26 May 2013 12:00 PM
Is your pool covered when not in use? Even down here, losses from the surface of a pool dominate, particularly if it uncovered.

If I win the lottery (unlikely given that I don't buy tickets) I wanna pool with the underneath bits made of ICF block and a retractable shell pool house. Oughta be able to heat and filter it for pennies per day.

If I ran a 3 ton water source heat pump 12 hours in Y2 (a very long hot day), that would reject perhaps 540,000 Btu, raising a 20,000 gallon pool temperature by 3.24*F, not much, but 10 days of that would be a problem. My limited experience is that an uncovered pool loses several degrees at night, putting matters back where they started. Could also activate a water feature for more cooling if temperature got out of hand.

I guess I've hijacked OPs thread, but a pool is a cistern of sorts.

Yep, the pool is covered when not in use, since otherwise the heatloss would triple. It is a powered safety cover, so it has to be closed.
A cistern being closed would unlikely have as much evaporation like a pool left open outside.