Pond Heater





Introduction: Pond Heater

About: My situation keeps me a home at great deal of the time and as such, I generally have a lot of time on my hands. That said, a while back I swore an oath to myself that I would not be 'that guy' that lay on hi...

Last spring we built an above ground pond using an old billboard sign as a liner [billboardtarps.com] which cost us 1/10th what a 'regular' pond liner would have. In winter, the temp can dip below freezing here & we don't want to loose our fish, some of which have gotten quite large. So being a free energy mad scientist of sorts, I came up with an idea to heat the pond for next to nothing by pumping pond water through a heat exchange in my chimney. This whole project cost me under $50 and is keeping a 750 gallon [2839 liter] open air pond a temperate 70 F [21 C]. 

This project was a smashing success! Every hour that I run this, it saves us 1500 watts of grid electricity per hour which is what our 'old' pond heater consumed. The sky is the limit on this too. I could route part of this to heat our garage, my workshop, etc. Next year, I plan to route a leg of this to heat our green house - fresh veggies all winter! 

Step 1: Aluminum Vs. Copper

I found a 25' [8.2m] coil of 3/8" [0.952cm] Aluminum tubing online, shown in fig1 with a cig lighter as a size reference. I then measured the inside diameter of the stove pipe - my chimney is insulated & has a clean out plug in the bottom. I determined that my actual stove pipe was 6" [15.4cm] wide and I wanted to keep at least 1" [2.54cm] clearance between the heat exchange coil and the stove pipe.

I want to pause here for a moment, in case it isn't evident why I chose to use Aluminum tubing over Copper tubing - the same reason you don't throw copper coins into a fish tank.. using copper tubing will kill the fish.

Step 2: The Coil Becomes a Heat Exchange!

After brushing up on preferred methods of bending tubing online, I found a bit of sprinkler pipe that would give me a total diameter [including the width of the tubing - doubled] that was smaller than the stove pipe & left room for circulation. I set it up in my vice with a bit hanging over the side of the bench; this provided me with plenty of room to work w/o getting all bound up & the unused bit of tubing getting in the way.

Three other points of interest;
1. There is a soft clamp on the far end of the tubing, just to keep it from flopping around.
2. A large 'C' clamp as an axis point to create the 1st bend towards the coil. I found that by pulling the tubing around the pipe, instead of smashing it worked just fine & didn't kink or flatten at all. I took my time, kept tension on the lot & pulled the leading edge until I'd made what I wanted.
3. The edges of the larger sprinkler pipe [that I am winding the aluminum tubing around] are quite sharp, even after filing them down a bit. It is far easier to put on a pair of gloves than it is to rush off to the hospital for stitches because you've laid your fore arm or wrist open on a jagged piece of metal. I have done it myself & it's always the 'oh this will just take a minute' job that does it. Always, be careful. Please!!

Step 3: The Finished Coil

I wanted to make sure I had a good amount of tubing left over to attach the water line & to extend outside of the chimney. A couple of shots here to show the end product. It only took me about ten minutes to do this and it came out perfect. 

Step 4: In Through the Clean Out

Shown here; The whole coil stuffed up in through the clean out plug of my chimney. I cut a bit of scrap wood to the same size as the clean out plug & wedged it in there with a screw. As you can see, I left a good amount of tubing straightened out - which is what is coming out of the wood plug in the bottom of the chimney. 

The 2nd photo is a little closer shot & shows the screw I used to wedge the whole assembly in. I replaced this with a pole which worked out even better. 

NOTE!!! If you are doing this be sure to either have the water running when you insert the coil into the chimney or DO NOT have a fire burning!!! I had the assembly laying on the ground & a siphon was running water through it - I did not realize this & thought the pump was on. Once I slid it up into the stove pipe, it did not take two minutes to blow a plume of steam out the other end of the coil. A few more minutes would have melted the aluminum & if then, the water was turned on with the coil melted - the possibility of a steam explosion/burn/incident/nastiness can occur. If the walls of the coil have sprung a leak, then when the water flow resumes, if any of that goes DOWN the stove pipe & into the fire box - there is no telling how bad it could be so be durn careful.

I purged all the air out of the lines & have both the intake & the discharge well under the surface of the pond water. That way, even if we suffer a power failure the water can rock back & forth without causing any real damage.. hopefully.

Step 5: Inspection Time

Two shots here where I removed the coil for inspection after the 1st and 3rd firing.. the 3rd fire lasted the better part of three days with zero damage to the coils & the results were impressive, I will list those later. On the 2nd photo, you can see where I elongated the coils a bit which seems to help bring up the discharge temperature. I did this with a small block of wood about the size of a carpenter's pencil - to coax apart the coils & keep them even.

The black area on the wood plug is not charred as much as it is stained; a very unexpected thing happened. The cold water running through the coils causes a considerable amount of condensation to form & drip out from the bottom of the assembly. I thought that once the chimney came up to ambient temperature that this would cease but that was not the case. The hotter the fire the more condensation would drip.. it's basically 'liquid smoke' but you won't catch us cooking with it, lol. 

Step 6: Temporary Plumbing

For testing purposes I used PEX tubing which was super cheap [$0.27 per foot at Lowes] and a few odd hose clamps, etc. that were already laying about.

So here's the data:

The other night [test #3] it was 31 F [-0.55 C] outdoors. The water temperature at the intake was 40 F [4.44 C] and at the discharge it was up 12-15 degrees to about 63 F [17.22 C]. By morning, which was about ten hours later, overall the pond temp was up 28 degrees to a balmy (in fish terms) 68 F [20 C]. 

Even without having the PEX tubing insulated, I am only loosing 3-4 degrees between the heat exchange coils and the final pond discharge 25' [7.62 m] away. When spring comes I will decide whether to bury the PEX tubing or to plumb all but the ends in PVC. Regardless, this project was a definite success! Every hour that I run this, I save at least 1500 watts of grid  electricity - that is what our 'old' pond heater used and then it only kept the water just above freezing. 


I have since stopped the experiment; while checking the system, I noted that the coil discharge temperature was WAY higher than what I had come to know as 'normal'. Evidently, the intake sucked up some particulate matter that slowly reduced the water flow to a near critical level. If the water flows too slowly it can turn to steam in the heat exchange & I never intended this system to take that much punishment.

I also noted that as the creosote built up on the coil, the less efficient it became. Fig 1 shows the heat exchange coil after being removed from the stove pipe and Fig 2 after it was pressure washed. Fig 3 is a close up of same to show pitting on the coils. I suspect this is due to the caustic nature of hardwood ash [see 'lye']. This was one of many safety issues that presented itself during this experiment.

I have come to realize that in order to present one idea as an Instructable, I will have to do 2-3 others to explain how I got there in the 1st place. The wood stove we installed is not like the wood stove I grew up with, this one is a bit small & didn't draw well, initially we had to build a small fire to heat it up before we could get real. After speaking with our chimney people [yes, we're inspected yearly] I made some alterations to the stove & it is now an amazing thing! It doesn't burn the wood so much as it incinerates it, the ash left behind from a whole rick of wood can be fit into a shoe box. Also, to the person that said 'new chimneys need to have a temp of 700 deg to operate correctly' I'd really like to know what you base that on because the fire box doesn't even get that hot. After the changes I made to the stove and with no fire burning, I saw the draft from up the chimney blow his ball cap off his head while standing on my roof.

Generally speaking, yes it's true that so long as the water doesn't freeze solid the fish will be fine - I know that. However, not all the fish can handle the drop in temperature - we add two placos every year to help keep the algae down. When the temp gets to 65 F [18.33 C] the placos are the 1st to die and by end of season, they are quite large. This was for a real life experiment and heating the greenhouse 1st would have taken too long to see results, the Instructable wouldn't have been out till this summer or later. Although I did state that 'I could run a leg of this to heat...' that as it turned out, was wishful thinking. Due to the height of the heat exchange in the chimney, the flow at the pond discharge was slight. I have photos but suffice to say, I could fill up a coffee mug at the sink 2x before I could from the pond discharge. Now that said, do not think for a moment that this is our main pump/filter line, that is a totally different system, so let's not get off on a tangent there.

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18 Discussions

A plumber acquaintance made one of these but he wrapped the coil around the flue and then slid the heat safety shield over that - worked okay... But most often we purchase wood fired space heaters with a set of coils built in at the factory, that seems to be the most efficient method of all.

1 reply

Thats what I wanted to do initially; wrap the heat exchange around the stove pipe but then I would have had to punch holes in the wall to run the plumbing & that wasn't an option.

I am working on a follow up that will address most your questions. At this point I am waiting on a break in the weather to get some snaps. In the mean time, keep your questions coming & expect an essay response by next weekend. Thank you!

Second thought. The aluminum would most likely burn in the creosote fire adding to the fuel load thereby intensifying the fire.

I've often wondered about scavenging heat from my wood stove flue and this is a pretty cool application for that heat. I have a little pond too and would like to get the water temps up a bit in the winter.

Have you thought about possible problems around dropping the flue gas temperature? I understand that modern wood stoves and chimneys are designed for a particular exhaust temp to ensure proper draw and to minimize the fouling of the chimney by soot. You'll know if you have a draw problem right-quick but the fouling could be less obvious and has the potential to lead to a chimney fire, something to keep an eye on especially if you don't have the chimney swept regularly.

Just a thought. The temperature in a solid fuel chimney should not drop below 700 degrees F. As the gas cools below this temp the volatile distillates in the smoke condense on the chimney walls accumulating as creosote which if accidently ignited can cause very serious fire. Many home fires are caused by this. How often do you clean your chimney? The coil probably will interfere with removing the creosote.

Nice instructable.. and not knowing how cold it gets where you live.. but most fish don't mind the cold .. and as long as the pond isn't frozen solid they will be fine

This looks amazing. So many uses for winter outdoor heated things. My house doesn't have a modern chimney like yours, but I am inspired.

5 replies

I'm especially pleased to learn about billboardtarps.com - thank you for that one!

one word of caution, having used several of these as sacrificial liners under an actual pond liner after discovering a large number of pinholes in several that I purchased. Get a bright light bulb and inspect it for holes before you install it.

Foobear - Thank you for the comments; it does me good to know I've helped lite the flame of inspiration in you too. This project worked out so easily & so well, it has really got me thinking - all kinds of heat going up that stove pipe.. what else can I plug into that, ya know?

Also on the billboardtarps.com thing; I bought one of their smaller tarps which was still huge but that allowed us to fold it over and double it. Not that we needed to, they are rated for a Class III hurricane, fiber reinforced AND the back is black. The purchase price and shipping was only $35.00 delivered to my door. You can't beat that with a bowling pin in each hand, lol.

I use my wood fireplace one in a while in the winter and I had this same idea but not to use it to heat a pond but to help heat the cold water that goes into the water heater. This way it uses less electric power to heat the water in the tank and save $$..

I checked out that billboard site and comparing to the tarp that I recently bought at my local billboard it's kinda high but ok if you don't have a place near you to get one. I bought a 15'x45' at a billboard place near me for only $30.00 and I can go in and pick out the I want black or white. Check the yellow pages to see what you have near you and ask. Chances are they sell them a lot cheaper.. =)

What an awesome idea! You are so clever! Thank you for sharing!!

One thing though man - as a marine biologist, i would advise you that as a general rule, most fish are happiest when the temperature of their surroundings changes no faster than roughly 2 degrees F per hour. That's not an absolute of course, and maybe fish that have lived outdoors become more tolerant of faster changes in temperature, but its the guideline I used when I worked in a fish-toxicology lab. It seems to me like the heat from the stove MIGHT create pretty fast changes. Pond volume will of course mitigate Delta- T.

Great idea for making use of heat which would normally go to waste, and even more so if you use it to heat a greenhouse too.

I was wondering whether having summer temperatures all year rather than having a semi-dormant period during the winter might have a detrimental effect on your fish, but there again there are places where the temperature gets nowhere near freezing all year, so maybe not.
I suppose a problem might be that they get too hot if you've got the fire going on a warmer day and I can see it would be a problem turning off the pump as the heat exchanger would overheat.  An electronic overheat alarm with the sensor in the pond and the sounder in the house may be an idea.

I have a small ground level pond in the UK and made a simple de-icer with a 25 watt aquarium heater and aquarium bubbler in a wide tube.  This keeps a couple of square feet of surface free of ice and the pond aerated all winter.

1 reply

AndyGadget - it's a wood stove and there's no need to fire it unless it's nippy. I don't see a need to over complicate it adding something that requires more power - kind of defeats the purpose. I do appreciate the input though, thank you.


4 years ago

Awesome! I really want to try this.