Peter Smith wrote:Thanks guys, so the heat riser does not need to be round? Fire brick would imply square. Most pictures and drawings I have seen are round.
Hi Peter,
I'd definitely go with clearances like you would use for a woodstove around the barrel. Top third of the barrel can reach as hot as 600-800 degrees, depending on fuels. Normal operation is maybe 400-600 at the top edge.
So we use clearances like for a non-certified woodstove: about 36" to combustibles, which can be reduced to 18" or 12" with good heat shielding.
The lower part of the barrel tends to stay more like 200-300 F or lower. Below that, there is usually at least 4 to 5" of masonry around it (5" would be to code for masonry heaters), you can do the 4" air gap from the outside of the masonry in that area if you need to.
Glowing barrels in home heaters are rare - I think the most I've seen on purpose-tweaked stoves (for cooking) was a small red spot dead center on the top surface.
Heat risers can be round or square, as long as they are
- the right internal cross-sectional area (same as all other channels e.g. exit chimney)
- well insulated (1" rock wool, 2" perlite, 4" vermiculite, or similar)
- leave enough space between riser outside and barrel interior for good flow (minimum same CSA as other channels, can be 2 or 3 times larger. 1.5" to 2" is a practical minimum so it doesn't choke on fly
ash.)
Brick risers are more durable, but sometimes it's worth it to use a cast refractory or cast-in-place riser to save space, so you can use a smaller barrel and reduce the
footprint in the room.
Yours,
Erica W