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Its a single unit, in some cases two sections. connecting them together is a ten minute operation.
We can go from here without a time limit. Please do not forget to rate my work when you have the answer you need, it is the only way the company pays me for my time, it does not affect your charges, nor close the question.
It is unusual by the way for the heat exchanger to fail, the company that told you it comes in six sections is way off base and if they also did the diagnosis, it is entirely suspect. If you can tell me which direction you are from the center of the nearest city I can get you a short list of well rated companies the will most likely get you a much less costly repair.