I am remodeling my fireplace surround and hearth.

The fireplace box is black metal with 6 rows of vents above and below the fireplace opening. There is a fan builtin which I don't use. The fan blows air out the vents. The fireplace is wood burning with a gas log set. I only use the gas logs - I don't burn wood. The fireplace box bumps out from the house. The front of the fireplace is flush with the interior wall. There are glass doors which remain open when the fire is burning, and are closed when it is off.

Originally I wanted to make the hearth flush with the wood floor. However I refinished the wood floor and it came out badly. I will have to redo it, and maintain it after the hearth is tiled with marble. I am concerned about ruining the marble whenever I refinish the floor again. I want to finish the fireplace before I tackle the wood floors again. The wood floors will have to be maintained periodically long after the fireplace is remodeled.

So now I am thinking about raising the hearth. I can raise it about 3.5 inches and then it would cover half (3) of the bottom rows of vents, leaving 3 rows below and 6 rows above the fireplace opening for ventilation. I do not know if the ventilation is necessary. When I chose the box, I wanted the fan, and I was always under the impression that the vents were used for the fan only. Other similar boxes had no vents.

Here are my questions:

(1) Is it safe to cover 3 of the 12 rows of vents?

(2) Would a 3.5" hearth elevation be too short?

(3) If the hearth instead were flush to the floor, how could I isolate the marble tile from the wood floor to make it easy to refinish the wood floor without damaging the marble? (I am considering tung oil or wax for the next refinish but I will still have to sand. Ideally I want to find a finish that I can spot-finish instead of applying poly-plastic to the whole floor since I have dogs. Stain is dark and marble is light, mostly white. Grout is white.)

Advantages to raising the hearth:

(a) I can frame the vertical edge with a removable 1x4 painted moulding so that it acts as a baseboard which will make it easier to refinish the wood floors.

(b) I can build up the hearth base with enough plywood that I don't have to worry about the marble cracking. If I did not elevate the hearth, I have to re-engineer the subfloor under it to install enough plywood and backerboard to make sure the marble doesn't move. This is a pain and risky to me (a novice).

Traffic really isn't an issue since this is mostly a short, decorative wall in the corner of the house.

If this is a resonable solution, I will mock it up to make sure I like the proportions OK. I have avoided the raised hearth in the past because I thought it would be too short. However 3.5" is the width of my patio door moulding so maybe it wouldn't be so bad?