Diagnosis
The MVH is designed as a whole-room circulator first, so even its "low" fan speed is relatively powerful to move air. However, if there's truly no audible or airflow difference between settings I and II, the fan speed switch or internal resistor might be faulty.
How to Fix It
First, test it properly. 1) Set the function switch to heat setting I (low heat/fan). Listen closely. 2) Switch it to heat setting II (high heat/fan). You should hear the fan pitch increase slightly and feel a stronger airflow. The difference is subtler than on a regular fan. If you're sure there's zero change, the switch may be defective.
If That Didn't Work
There's a small possibility the internal wiring for the multi-speed function has come loose or a resistor has failed, locking the fan on high speed. This requires internal inspection.
Heads Up
Don't confuse the thermostat dial (outer ring) with the fan/heat selector (center switch). The thermostat controls when it turns on/off to maintain temp, but doesn't change the fan speed itself.