Double/twin heat riser heater

I'm wanting to build a heater for my workshop - something along the lines of a rocket mass heater, but without the cob bench - so just a rocket heater I suppose?

The biggest issue is keeping it "stealth", to which end I want to remove as much heat from the exhaust gases as possible before allowing them to exit the chimney.

I don't have enough room in the shop to build a cob bench, but I was wondering whether a second heat riser in a second barrel would work.
basically take the horizontal outlet from a normal rocket heater and where it would enter a cob bench, put another "L" bend and run the gases up another insulated chimney and then down a second barrel and then out the roof(wall actually)something akin to the attached image.

Has anyone done this? Did it give a significant reduction in exhaust temperature?did it work at all?
I'm going to give it a go with a 6" system,but there's no point in me duplicating work, if someone's already tried it and it didn't work.

The shop is quite well insulated, so I'm hoping that a fire for an hour or so in the morning would take the chill out of the air until the sun rises (it's quite mild in UK, usually around 25-40F in winter), then again on an evening if I'm working late.

Greg,
Yes it has been done before but leave out the riser etcetera of the second barrel. Just place the barrel behind the first, exhausting half a pipe diameter above the bottom and feeding in a little higher than the exhaust. This way the barrel is going to act like a "bell", the hot gases are rising to the top and the cooler gases will be pushed down and out the exhaust. The height difference between the inlet and outlet are there to avoid bypassing straight through. As long as the cross section area of the bell is 4 times or more the csa of the connection pipe this will work. Since this mechanism is driven by gravity there's no friction to speak of. As long as there's a vertical exhaust to some point above the roof of the workshop.

If you have limited space, you would be better off just replacing the barrel with a hollow masonry box, as tall as you have space for. There are formulas for the correct internal surface area of a "bell" as these are called, to get full heat absorption without taking so much that you lose your draft.

Bells work by letting the hottest gases rise to the top and stratify; the coolest gases after they have given their heat to the masonry fall to the bottom and go out the exhaust.

Glenn Herbert wrote:If you have limited space, you would be better off just replacing the barrel with a hollow masonry box, as tall as you have space for. There are formulas for the correct internal surface area of a "bell" as these are called, to get full heat absorption without taking so much that you lose your draft.

Bells work by letting the hottest gases rise to the top and stratify; the coolest gases after they have given their heat to the masonry fall to the bottom and go out the exhaust.

I was looking at these, but I want to fit the heater into an alcove (which used to be a fireplace) - it is only 44in internally, but 38 inches wide and deep - so enough floor space for the double barrel, but not enough headroom for a tall bell

So your alcove is all masonry? How thick is the masonry on various sides? You may be able to use that as part of the mass that stores your heat, with a bell built as a liner for it.

Glenn Herbert wrote:So your alcove is all masonry? How thick is the masonry on various sides? You may be able to use that as part of the mass that stores your heat, with a bell built as a liner for it.

Interesting...



this would be how the bell would be built?


I've discovered an issue unfortunately - the steel chimney liner which I was going to leave in the chimney to use as the exhaust is only 3.5inches in diameter, so I'm going to have to remove it, and use the original chimney for the exhaust - bit of a pain, because that means i have to climb onto the roof to disconnect the cowl so i can pull the liner out.

If you want to fit the bell(s) inside the masory opening, you probably don't have space for two. If the existing masonry is thick enough and isolated from combustibles properly, I would just build one brick/firebrick bell inside it, and put an exhaust tube well insulated straight down from the chimney to near the bottom of the bell.

You do not make a bell with a big baffle in the bottom directing all airflow upwards; just arrange inlet and outlet so they are not directly in line and the outlet is a bit lower, and you're good, as long as the cross section of the bell is at least 4 times the duct cross section.