I might be wrong, but I understand that our Heatmor 200CSS is supposed to have a good 4" or more bed of hot coals - which it does most of the time. So a lot of the time our grates are definitely not exposed, per se, unless I were to let the fire burn WAY down to fine ash or completely out. When I do get to that point, I can sift the ashes around with the flue cleaner tool (doubles as a nice log pusher / ash raker) and, if the ash pan underneath isn't too full, easily push the fine powder down and expose the grates. If I can't, I will check with the auger and make sure it's not full down there.
I think the manual says something about trying - if possible - to rake the ashes around and expose about 25% of the grate area. I don't find that to be possible most of the time and, as I said, the instructions I have from our dealer suggesting that we want a deep coal bed throughout the entire firebox would tend to mean that often isn't possible. Maybe that also means the air is coming up from the grates to a degree, but fueling those coals from underneath.
Most important thing, from what I understand, is that your blower has a clear path, the air vents from the blower aren't blocked, the ash channel underneath the grates isn't packed with ashes and the flapper on the blower is opening when running and sealing well when not.
Ours burns hot as hell and will reduce a good load of wood to coals and ash in the course of a day with no signs of trouble.
Hope this helps!
- Aaron