Extra air

Compressed air has much more stored energy than water or even steam. Watch the Mythbusters episode where they overheat a water heater (w/ relief blocked). Exploding boilers was what led to the ASME pressure vessel codes in the early 1900's. I don't even trust water heaters and have a tankless type (safe and efficient). I sure wouldn't want to stand next to 2" PVC pipe with 150 psi air in it.