Firebrick slabs - time to heat up?

[harro]

Gday all,

Planning a new build, my first, and i'm not an expert on firebricks so bear with me.

I was wondering if a 50mm thick firebrick slab would take long to heat up and pass heat through to above when heated by a fire underneath?

The reason is because i'm building a parrilla (argentinean bbq) with a firebox area and hoping to have a small warming oven above the fire. It could be made from the large firebrick slabs that Claypave sell (450 x 300 x 50 thk) spanning from wall to wall. The internal dimensions of the oven would be 370w x 300 h x 300 deep. It would be like an enclosed little cube with firebricks on all sides, with a door on the front. The fire heats up the bottom firebrick slab, but the smoke from the fire passes up behind the back and out the top chimney.

Is this a feasible idea or would it take all day for heat to pass through the 50thk slab? For a standard asado/bbq the fire will be going for a few hours. It would be great to have somewhere to heat up some bread/veg, potatoes etc.

Cheers.

[david s]

Re: Firebrick slabs - time to heat up?

Hi Harro,
Yes, I believe your idea will work quite well, but as the firebrick will not be insulated you can expect that your oven won't retain the heat you pump into it. Additionally, if I understand your concept correctly, you will also probably find that the floor of your oven will be way hotter than the top, although this will be compensated by heats tendency to rise. I suspect that your oven would be ok for a batch of bread or a roast, but never hot enough for pizza. If you got the floor hot enough to cook the base ok the top of the pizza would not cook properly.could you post a drawing?

[harro]

Re: Firebrick slabs - time to heat up?

Hi David,

Thanks for your response. Here's a rough sketch, hopefully you can make out whats in my head!

There will be doors on the front of the oven box and the firebox.

There is an internal wall separating the firebox from the grilling area, which i can position so that the 450 wide slabs are built in and span across between this wall and the end external wall.

Yes I agree that the oven floor will get all the heat, and if I can only do bread, veg, pastry etc, then that will still be a bonus. The standard parrilla simply has the fire on one side of the wall and the grill on the other, and i'm just trying to utilise the heat of the fire for another purpose in addition to creating the coals for under the grill.

Maybe I could put insulation above the top firebrick slab? Would it help?

Hopefully the fire will still vent ok with a solid slab directly above it. the depth of the entire structure is 900, with 150 wall at the back, and 300 for the oven box leaves about 400 -450 clear space for venting. Obviously if i didn't need to heat the oven box i could place the fire closer to the back wall.

[david s]

Re: Firebrick slabs - time to heat up?

I think your arrangement should work quite well, but you will only find out once you've built it. I think you should consider insulating both over the top firebrick slab with plenty of insulation(say 4" of 10:1 vermicrete encased in a 4:1:1 sand,cement ,lime 15mm cap. Also reduce conduction loss between the firebrick sides and the fireplace sidewalls with a 1" gap of vermicrete. That's how I would approach it anyhow. Can't say how it would work,but I think it should be ok.Leave the back and bottom firebricks exposed to the heat as these will be your heat source for the oven. The heat from the fire should be enough ( if maintained a bit) to retain plenty of heat there, but once the fire dies it will work the opposite way and become a major source of heat loss.

[harro]

Re: Firebrick slabs - time to heat up?

Thanks for your info david.
i'm getting closer to building the oven and will take your advice. The blockwork frame of the bbq is up, i poured the benchtop slabs and the hood/flue/cowl has been pressed out of galv sheet to be fixed inside the chimney. Just need to buy the firebricks, flats and slabs to build out the oven. Then get the steel doors made up. First though i was going to insulate between the conc structure and the firebricks with 25mm ceramic insulation board, and also on top and side of oven box as you sugested (well, ceramic instead of vermicrete). will try to post some pics soon.