[@Mr Allen J] [@tsukune]
Actually, how about this variant:

He syphons heat the same way as before, but instead of being stored within him as-is, it is indeed turned into metahuman energy that, in turn, is expended towards giving him the typical "brick-man" powerset of mediocre regeneration and one of superstrength, superspeed or superdurability which he has to switch around. Trick is that if he draws in too much heat to be converted, his power automatically kicks into the ashing-inducing and exhausting "overdrive" and if he draws in too little, gets strongly cooled or remains too long (but much less than a normal person needs to freeze to death) in a very cold enviroment where it is hard for him to syphon any meaningful amount of energy, his own temperature will begin to fall and his body will begin to fail because due to the damage that brought him into coma his brain is unable to regulate his body heat automatically, as it happens with healthy humans. That, and without drawing outside energy in order to convert it, he does not produce enough metahuman energy for anything except syphoning heat. 
Oh, and also due to his power always being on, even if on a very low level, he might be unable to spend a lot of time inactive, sleeping or otherwise resting, because in that case the slowly accumulated metahuman energy will make him ash sooner or later - so the guy has to constantly move, run, lift weights, train or expend his energy on something else to somewhat even out the income and expenditure of energy lest he wants bad things to happen. 
Also i'll change his alias to "Redliner"