Gravel needed under buried intake vent pipe

I'm installing a 4" PVC pipe for a fresh air intake for my fireplace, which will be buried under my garage slab (slab has not been poured yet). Do I need to put some gravel under the pipe for support during the pour, or is this not necessary? If gravel is recommended, what kind? Crushed? Should I wrap the pipe in 6 mil poly before burying it?

Also, not sure if this is even common practice, but considering the slab will be structural and the soil underneath will settle (along with the pipe), would it be a good idea to somehow brace the pipe to the underside of the slab so it doesn't settle as well? Such as wrapping some coil strap or tie wire around the pipe and securing it to the rebar so the pipe remains against the slab as opposed to settling with the soil?

 

Although a source of intake air is good I have no way of judging the size of that pipe so have never advised anything like that. My initial reaction is "too small". Will the resulting opening of the pipe be, above, below, of level with the fireplace? If it ends up above the fireplace (as in a basement location) then it might do more harm than good. The opening for intake air needs to be as far below the fireplace as possible.

As for the soil under that slab settling, even though you describe it as a structural slab, I would expect sufficient compacting to eliminate or minimize that. Spanning 12' (I'm guessing) and parking a large car on top would take s substantial floor with no support under it.

Bud

 

Code in my area requires a fresh air intake. The existing opening is about 1.5"x8" (12 sq in), so I used a 4" diameter pipe (12.57 sq in). The top of the intake is about 3" below the base of the firebox and will slope downward away from the home and out the far end of the garage foundation wall.

BTW, the slab will be supported by three grade beams and a slab ledge on the wall. The soil base is only to provide a temporary form while the concrete cures.

 

The only other issue I've heard about with fresh air intakes is the potential for hot ashes to get blown back out the intake and PVC would not be good. However yours sound like it will be all underground and I don't know if this relates to any code. never seen it mentioned other than the one time and unknown where.

Bud

 

The code doesn't stipulate what to use, so hopefully it is fine. Guess I'l find out.

 

I can't comment on the fireplace portion of the question - but I believe piping is usually buried in the dirt under the slab, not embedded in the concrete slab. I've never seen piping sitting up in the space where the cement will be poured.

Maybe the plumbers who have more experience with new construction slab piping can comment?

 

Let me clarify. Only a few feet of the top of the pipe (1" at most) will be in the bottom of the slab. As the pipe slopes, it will be out of the slab. I'm sure I am over thinking this and it will be fine.