Ive always decarbed my concentrates as is over a double boil. You could add a small amount of coconut oil if your hash isnt full melt. It doesnt really matter too much. Basically the cooh carboxyl group leaves as co2 and h2o bubbles. The double boil keeps it a steady temp. You can watch these bubbles slow, like popcorn lol. Thats when you know its done. Now of course a natural decarb over time at room temp is best but not practical for most people. The boiling point of water is pretty good for quickly breaking the bond.
You will likely loose much of the fruity esters and aldehydes with a boiling point 150-170 or so but many of the terpenes have a similar bp or higher than thc, pinene for example as i was discussing earlier in another thread