hi Ivan1547, Great Idea!
Now for a long story and my suggestion for a more efficient dual benefit unit. As you might notice, geographically I might use heat 1 or 2 months out of the year. Still I like the idea of efficiency. Not knowing the layout of your homestead the suggestion may or may not be feasible.
The suggestion:
*A long term solution to heat the pool year round and also benefit by heating your garage/shop/nearby building.
*Upgrade the barrel to a wood burning barrel stove and incorporate the water heating element into it.
*Install a safe chimney and locate unit in a safe indoor location.
*Kits are available at reasonable prices, here are some links.
Barrel Stove Kit
Grate for Barrel Stove
Top Barrel Adapter Kit
Hot Plate for Barrel Stove
*Berry and insulate the Hot Water Feed and suction line to protect from freezing when not in use and prevent heat loss during the water cycling.
*Start a fire, turn on the pump enjoy warmer pool and heated building.
*For an even more permanent install use large pipe or have some steel plate rolled for the stove body instead of the thin steel drum.
*Use of copper is a great idea because of the corrosive nature of pool chemicals. (Edit: Nat brought up a point I was unaware of - "Copper and clorinated water don't mix very well.") I stand corrected
*Alternative or Dual Fuel is an easy option. Example: Natural Gas burner with lots of Lava Rocks to help retain and distribute the BTU's.
*Here are a couple of informative links.
Build a deluxe barrel stove
Bob Vila - DIY Wood Burning Pool Heater
-------------------
Man I know it has gotta be Cold Up There !!!
Well, my use for the coiled copper tubing is reversed. I have 50' of 1/2" copper tubing plumed into a refrigerator that is used as my compressed air dryer. Very humid here, and dry compressed air is important for many jobs.
Good Luck, Stay Warm
Rhett