Instructables
Picture of Pond Heater
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! 
 
 
Remove these adsRemove these ads by Signing Up

Step 1: Aluminum vs. Copper

Picture of 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.
teddlesruss9 months ago
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.
Pragmatic Man (author)  teddlesruss9 months ago
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.
Pragmatic Man (author) 9 months ago
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!
doughnut439 months ago
Second thought. The aluminum would most likely burn in the creosote fire adding to the fuel load thereby intensifying the fire.
Labyrinth9 months ago
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.
doughnut439 months ago
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.
tvengineer9 months ago
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
foobear9 months ago
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.
foobear foobear9 months ago
I'm especially pleased to learn about billboardtarps.com - thank you for that one!
dcms4 foobear9 months ago
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.
Pragmatic Man (author)  foobear9 months ago
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 $$..
PitStoP foobear9 months ago
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.. =)
doccat59 months ago
What an awesome idea! You are so clever! Thank you for sharing!!
Lt.Greg9 months ago
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.
AndyGadget9 months ago
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.


Pragmatic Man (author)  AndyGadget9 months ago
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.
thimas9 months ago
Awesome! I really want to try this.