Ditto what Fred HVAC said, it does take15 minutes or more for the wave to get going.
Always leave it on high for about 30 min. before you turn it on low. That gets the pad really cooking.
I always open the door when it is initially starting, giving it about 3-5 min to get burning good.
That way any fumes will be removed.
If the wind is coming in the door then you may have trouble getting the pad to fully ignite!
I will add that it does take a couple of hours running for the smell of the parts etc to out gas...
I ran mine with the door open...
Jim