What is the best way to seal a French drain board due to high levels of radon?
If the French drain is between your basement slab and the exterior walls, it may be the source of most of your radon. Sounds like a small suction from the drain sump to the outside is a good plan. A small fan with its own exhaust pipe shouldn't be a problem for the water heater or furnace but a CO detector is never bad for closed rooms with gas appliances.