The levels of Radon are way to high in my house. When installing the Radon Mitigation System, can the power vent be routed into the same vent going through my roof that is coming from my water heater? Currently only the water heater vents here, the furnace was replaced 2 years ago and is now vented out the side of my house.

Thank you,
Paul

Probably not. But some info is missing.

Do you have a conventional gas hot water heater with a lined chimney? A lined chimney is just a flexible pipe.

What size vent is required for the radon system?

You can probably use another flex line in the chimney for the radon system.

Second possibility is to use an concentric vent (it looks nice) with the inlet closed off. There are some minimum distances that have to be met.

FYI: The reason for the "liner: is without the furnace, the gasses could condense on the cold chimney and not make it out. The liner is a smaller diameter pipe which means the gasses have a higher velocity (speed).