I am no expert on pool heaters (non-solar), but an exhaust near a crawlspace vent sounds like a deadly dangerous code-violating idea that could spread deadly carbon monoxide through your house. Crawl spaces are not hermetically sealed off from the house (I duck-walked through many as a young carpenter...when I was still young!).

Is it possible to run an exhaust vent from the heater to a safe location that meets code?