We also have an indoor pool and all the equipment is in the basement. Typically a fan is suppose to be running while the heater is on, ours has a 35 foot vertical stack so we don't need a fan to assist, but since you it sounds like you have a horizontal run you likely need it. It is plumbed or piped the whole way out of the basement though the crawl space correct?
Another trick is to open a basement window to allow fresh air to come in. When the pool heater is running it is dumping a LOT of hot exhaust air outside and unless it has an easy source for make up air or fresh air it can back draft or just exhaust slowly allowing some to stay in the basement and crawl space.
I would strongly recommend putting a CO2 detector in the basement, although not right next to the heater or it will go off a lot, but in the nearest living space and not in the pool room. Having said that a working heater should not make CO2, just a lot of heat and water (humidity), but should definitely be vented outside.
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