Right. Pool equipment gets hot and needs fresh air and water, and some equipment may produce noxious or hot gasses. This stuff *can* be used indoors, but special procedures need to be taken to provide adequate airflow and venting of byproducts (hot gasses, chlorine vapors, etc) to the outside.

My heater vent points up so it does get rain in it. The heater guys said that water is bad, but the amount it gets by rain is no big deal. However, he cautioned me about my sprinklers, because it would give a constant shot of water to the heater regularly, which over time may cause corrosion from the water and hot temps and metal used within the heater.