What part of a cell would a home furnace be an analogy for?

I would have to say it is the mitochondria. The mitochondria produces energy for the cell by using sugar molecules as its "fuel" along with oxygen in a series of bio-chemical reactions. (I think it is part of the ATP cycle of a cell, but I'm not a 100% sure of that.) Anyway this is similar to a home furnace in that your furnace uses a fuel, such as natural gas, along with oxygen in a combustion reaction that liberates heat energy that is used to heat your home.