Furnace is one of those movies that tries really hard at being a psychological thriller, and only comes out looking like a bad indie attempt. Chief among the problems, is a good bit of badly executed special effects, a story with plot holes big enough you could drive a truck through, and a setting that just doesn't quite make sense.
The special effects could have been much better. They go more for the standard "shock" type of scare, where the creature appears before you when you least expect it. This works only a handful of times, but before long it just starts looking like one of the editors let their twelve year old daughter have a go at it. If they had laid off the fire effect (though this does become important later on) and less of the choppy jumping effect for the ghosts, I think I could have looked past the general cheesy feel that they gave off anyway. One ghost, that of a little girl, looked much like just a dirty little girl. Even though the story established that she was essentially ash. The other ghost, that of an older man, was greyed out and looked more like a photo on some bad film than a ghost. Oddly his "ash" effect was better.
The plot left a good bit to be desired, more than once I found myself thinking "really? I mean, are you really expecting me to buy this load of bull?" One of the biggest problems was that of the "Furnace" for which the movie takes its name. It was obvious, even to the most casual observer, that this furnace would never have worked in any way shape or form, and while it looked ominous enough; that didn't quite make up for the fact that it just simply didn't fit in with the set. Perhaps had they tried to make the furnace itself more realistic, and less looking like something cobbled together by a set crew, it would have been better. Furthermore, the movie takes place (mostly) in a prison. However once again there are a number of plot problems, not the least of which being that outside detectives and prison guards are seen carrying firearms around open cells of inmates. Trouble waiting to happen, yet it never actually comes to that.
The movie doesn't really start coming together until the very end, when a long flashback scene explains everything that's been going on; at least in the past. Even then, when some of the earlier instances start making sense, the movie falls flat on its face. What could have been a very good ending fight, just ends up being this one woman screaming at the detective, while he fights with the spirit of the man that had been killing inmates. That's just it. The movie ends there and we never get to find out any epilogue or what really happened afterward.