Stainless Steel Tube
I'm going to build a firepit pool heater from this to add to the length of our pool season by making a plate steel box to fit this laying down, fill it with sand and tack a cap on it, then put it in a firepit that I can burn practically anything woodwise, like pine and leaves. The pool is on a sloped lot and I can place the fire pit slightly lower than the pool bottom, run a line in from the bottom and out to the top and the heated water will flow on it's own, plus I am gonna add a temp sensor to activate a hot water pump for when the fire is really burning.
I'm guessing 400,000 BTU's with a good fire going which would heat my 30,000 gallons of pool water a degree an hour.
Add the 16 mil thermal cover I have and we probably will get 8 months of use from the pool.