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you would probably have too much head pressure going through that much tubing. the cold water ahead of the heated water acts like a plug pushing down, causing surges and unsafe pressure inside the tubing that could cause it to burst. for a greenhouse i would attempt something similar, but use a large compost pile for the heat source or rocket mass heater.
gas or propane would work, but most of the heating comes from the coal bed sitting on my coil. it would burn a lot of gas or propane to initially heat, the same it does with wood. the flames don't heat it as much as the coals. large masses aren't quick to heat up or cool down, so heating takes time. slow and steady with smaller fires or heat sources giving yourself ample time works the best. with a few small gas flames under the coil and a bit of tinkering you could probably keep it heated for cheaper than using wood.