Heating Crawl Space with Furnace

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I wanted to run this through the forums to see if I can get any input.  Long story short, the entire left part of my house is not accessible from the crawl space.  There were some water issues from the bathtub drain and drainage outside that made part of the house sink.  I believe I have these water problems taken care of.  I recently had the subfloor of these rooms rebuilt, they sistered up the old floor joist and put 3/4" OSB for my subfloor.  I have waterlines in the bathroom that are not accessible from the crawl space, they are only a few inches below the subfloor and there is old metal ductwork right beside it, so there will be some heat coming from that line.  My concern is that if I were to have tenants in the property and a line freezes, I would need to tare up the floor to get to my lines and replace them, a suggestion was made to run a small 4" T from a line of ductwork and heat that part of the crawl space.  Has anybody heard of this? That is what we are currently doing now.