Troubleshooting forced air blower

I have a forced air wood burner to heat a shop. It sits outside and has duct to blow the air inside including a return air duct. The return is about 12' long 12" round. The supply is 10" round with about 9' of run and enters the building through a 12" duct. The blower is rated at 1800 cfm, has a 1/2 hp 1075, 3 speed motor. I don't have a real good way to measure the actual air flow but I don't thing there is any way this thing is putting anywhere near that much out.

I have checked rotation...

Troubleshooting forced air blower