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I have implemented a heating system for my 500 gallon partially inground tank that may be useful to some of you.  I've learned a lot reading these forums and appreciate when others have shared what they have figured out.

I live in the upstate of SC by the mountains and while we have a moderate climate temperatures often drop to the teens at night. If not heated, the water in the tank would drop to the low 50's which wasn't good.  I'm growing both catfish and tilapia in the pond.  

I debated using electric heaters, but I know how expensive they are to run as I've kept aquariums my entire life, so I came up with this method.  I also know my ability to screw stuff up and I visualized killing myself with a homemade heater.  

I've hacked this system together for this winter as I experimented with ways to make it work correctly and flawlessly.  Now that it's working without a hitch, I intend to rebuild it this summer/fall to get it out of the way and look better.  The current configuration has been working all winter without a single problem.

 I'm a terrible photographer, so I apologize for the bad quality of the pictures.

The key to the system is an Excel Vent-Free gas water heater available from Ebay which you can buy for $149.  They sell models for either Natural Gas or Propane.  Mine is the Natural Gas model.

Here's a picture of mine hooked up on the wall.  They're very small and can be placed directly on a combustible surface.  My greenhouse is a 16 x14 foot lean to against a storage shed at the back of our property.  I covered the wall of the shed with hardiboard sheets for ease of maintenance, so in my case it's not combustible.  

The way I looked at the gas heater was it runs at 100% efficiency.  If it's cold enough to warrant heating the water, any excess heat that doesn't heat the water will warm the greenhouse.  So far, the theory has worked out well.  I heat the greenhouse with a blue flame 20K Btu heater.  Between the blue flame heater and the excess heat from this unit, the greenhouse stays at a minimum of 65 degrees even on the coldest of nights.

At this temperature I've picked 6 tomatoes in the last two days and have bell pepper, green beans, lettuce, kale, and chines cabbage almost ready to pick.  It's a great joy on a cold, gloomy day to pick a fresh, ripe tomato!

Originally I tried to pump clean, filtered water from the pond through the heater and found that no matter how well I filtered the water, there was enough debris to clog the pipes in just a couple of days.  To clean the clog, I would have to connect a regular hose to the unit and blow it out under high pressure.  Therefore I decided on a closed loop system to heat the water.  

Also on Ebay, I found a beer wort cooler for about $75 which is made of 3/8" stainless steel tubing.  The current water container is just a temporary solution and holds about 20 gallons of water for circulating through the heater.  I was originally worried that the water returning via the red hose you can see under the pump would overheat the water in the container, but the cooler is very effective and the returning water is about 72 degrees which is the temperature of the pond water.  This summer I plan on replacing this container with a small, covered drum container and lid and re-position it out of the way near the water heater.

That's a normal sump pump from Lowes in the container.  Get one more powerful than you think you'll need.  The 3/8" piping in the water hoses, the beer wort cooler and in the heater itself really reduce the flow from the pump.  Look for a pump with a high head pressure.  It also takes a certain water pressure to activiate the heating mechanism of the heater.

The hoses you see are just washing machine hoses.  Don't really care for them as some of the 'brass' is rusting when it's submerged in water.  I'll probably replace with some hoses I make myself.  

The next part of this installation is a water heater thermostat controller from  Jehmco is a also a great place to buy Chloram-X for water treatment in bulk.  The controller defaults to Celsius, but you can change to view in Fahrenheit.   Mine is always on Celsius for a reason I'll explain in a minute.  This particular model cost me around $50 and is designed to handle a heater up to 800 watts.  In my case the sump pump is plugged into the control unit.  When the temperature drops, the controller activates the sump pump and pumps water from the container to the Excel heater which then activates the water heater.  The heated water travels through the 50 foot stainless coil in the pond and returns to the container.  The water temperature leaving the heater is about 105-120 degrees (I haven't really ever measured it).  When it returns to the container it has cooled down to the pond temperature.  

The last part of the puzzle is a timer that is set to turn on the controller every 30 minutes for 30 minutes.  The Excel heater has some type of internal safety control (not documented anywhere I could find) that turns if off after about 15 or 20 minutes.  It then will not come on again until you turn off the water supply and reactivate the heater by turning it on again.

Since the water temperature is below what's needed, the pump keeps pumping and the pond actually drops in temperature.  It took a few days to figure out what was happening.  To fix this problem, I installed this timer from home depot that cycles the thermostat on for 30 minutes and off for 30 minutes. I still get up to 12 hours a day of heating from the unit which is way more than sufficient for my situation.  If  you need more heating, you could go with a timer with shorter duration settings.


The problems I ran into that if you install this system you can avoid are:

1. Use 3/4" piping everywhere as it reduces the back pressure on the pump.  I originally used 1/2" pipe because the heater inlet/exits are 1/2", but I couldn't get the heater to reliably fire.  

2. Make your own hoses for connections since washing machine hoses are really 3/8" or 1/2" inch at best.  

3. I had to tinker with the two controls on the heater a whole lot to come up with a happy medium.  One controls the flow of water and the other controls the flow of gas (how hot the exit water is).  Mine is set to fairly low gas (exit water is cooler) with a high flow rate.  Now that it works, I never change the settings.  

4. Buy a powerful pump with a lot of head pressure.  It took me three pumps to finally get one that worked each and every time.

5. Learn Celsius.  The timer when the power resets always goes back to Celsius, so I've learned enough to handle the needed pond temperatures.

6. Plan to do a lot of tinkering to get your unique situation working well.  In my situation, now that it's setup, it runs without me having to touch it.

Hope people find this useful and I look forward to seeing how people can improvise and improve on this setup !



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Well, this is my first year. I have had a smaller system for almost a year but it was inside, I am learning as I go and having a blast doing it.

I have my closed loop tankless natural gas fish tank heater done.

I have about 1,000 gallons of water between my fish tank sump tank DWC and GB.

Mine is similar to yours just a few difference.

My Stainless Steel beer wort cooler developed a small leak so I switched to a DIY Pex 100’ loop heat exchanger.

I used a DIY controller it does Celsius or Fahrenheit. I have its set to Fahrenheit and 72 degrees. It saves these settings even after losing power.

I am using Grundfos 3-Speed Circulator Pump with no holding tank.

I am using an Air Separator

All the Pex I used was 1/2" the orange Pex has o2 barrier but the heat exchanger Pex does not have a O2 barrier I was concerned with O2 barrier and the fish. 

I have only had it running a few days so far but it is working realty well so far.

Thanks so much for posting your setup.

My closed loop system has been working for nine days so far.

I have been running my pump on the medium speed. My tankless heater has a digital display that shows out flow water temp.

The output temp have been running about 52C or 125.6 F This shows the heat exchanger is doing a good job adding the heat to the fish tank.

I have lowered my fish tank to turn off the heater @70F. I have been stopping the pumps water flow at night this has saved natural gas. The front grow bed has expanded shale. That works like a radiator and makes the heater work harder than it needs to.

I have no problem keeping the water at any temp I set it at but it was using more fuel that needed.

The heater just works I don’t need to do anything and I am very happy with it so far.

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