In the sentence "Their work involves commercial solar heating systems," should there be a comma after the word "commercial"? These are solar heating systems for commercial use as opposed to private homes, so I see the entire phrase as descriptive of the systems and therefore not needing a comma.
No, the comma is not needed.
Although commas are sometimes used to separate adjectives, they are used in that way only in certain circumstances:
Because the adjectives in your sentence aren't coordinate(check them out), no comma is needed.