Hi! Sorry this is a little long, but I guessed I needed to describe my situation.

Ok, my winter "to-do" list requres that I make a homemade wood-burning-pool-heater by spring.
What this does is to provide very warm water to the pool to extend the swimming season. As the wood burns it heats up pipes that are carrying pool water by way of the pool pump. Water pressure is about 25 PSI. Water going in would be anywhere from 55 to 75 F and leave about 78 or higher.

As for my question, (I have no idea where else to ask this....)

I have all the plans on paper for this woodburner, what is holding me up is the design of the heat exchanger.

I want to use thin wall pipe as that would transfer the heat the quickest;
I will be using about 100ft of pipe.
I beleive copper is the best common thin wall pipe metal
If I use 100 ft coil, it costs about $260.00.
I'm cheap. I need to be cheap!
So I am trying to figure a cheaper way than use of coil.
If I do a vertical up and down along the heater walls, (which is cheaper cost pipe cause it is straight) that involves I imagine, soldering pipe to u-joints.

A wood fire is about 2,000 degrees so I beleive it would melt the solder. I say "think "because someone said to me the "cool" pool water would prevent the solder from melting.
Is this true?

Is there another way of joining copper pipes and to have the joint "safe" from the heat? I dont need any wood leaks in a stove!!!

I heard brazing or high temp solder, but I'm not sure, this is getting beyond me.
Just wanted your opinions, ......in fact your opinions would be GREATLY apprecaited. Again, I have no idea who else to ask.

If joining copper is impractical, then I will go with a coil pipe.
Or if you have better ideas!