Concentric flue pipe

Hello Permies,

I have been experimenting with a rocket wood burner and I am going to get the materials needed to make a RMH sofa + oven and cooktop, design is progressing.
For the flue gas exhaust and intake of fresh air I was thinking of using one double walled concentric flue pipe (CFP) rated to 800°C.
Does anyone else have experience using these kinds of pipes with rocket wood burners?

More specifically, I want to use this CFP to pre-heat the outside air intake for the combustion process and make a semi air-tight RMH.
At the bottom of the CFP there will be T-joint where the flue gas can come in from the end of the RMH and the intake air comes out horizontally and can then be piped to the air intake of the RMH.

For the unavoidable condensation I will place another T-joint below the first one, here the combustion air goes in horizontally and the condensated liquid will be captured in a cap that's put on the bottom of this T-joint.
For a cold start, I will place a bypass using two more T-joints and a butterfly valve in between, so I can bypass the thermal mass, oven and cooktop to start up the rocket and slowly close the valve to let more air into the rest of the system.

Any additional thoughts and ideas about my set-up are welcome.

Best regards,
Dominique

Hi Dominique, I think we at least need a basic hand drawn sketch to understand what you want to achieve.

Fox James wrote:Hi Dominique, I think we at least need a basic hand drawn sketch to understand what you want to achieve.

You are right, ofcourse, plase enjoy my paint skills

Welcome to permies Dominique.

This looks like a counterflow heat exchanger.
You are recovering heat from the exhaust at point where it will usually have very little heat left.
You risk not just condensation but also reducing your draft to nothing.


If you want your combustion air to come from the outdoors and be preheated, running the intake duct through your mass would save on complexity and cost.

William Bronson wrote:Welcome to permies Dominique.

This looks like a counterflow heat exchanger.
You are recovering heat from the exhaust at point where it will usually have very little heat left.
You risk not just condensation but also reducing your draft to nothing.


If you want your combustion air to come from the outdoors and be preheated, running the intake duct through your mass would save on complexity and cost.

Thanks for the heads up.

My main reason for using the CFP is not to preheat the air, but to have one simple hole in the roof for both intake and exhaust.
I would think this would be less costly and also easier to install as opposed to two seperate holes.

Are you sure there is very little heat left? What I know from gas fired water heaters is that they purposefully condensate the water out of the flue gas because the transistion from vapor to liquid releases a lot of energy.
My idea was keeping the flue gas in the thermal mass above the dew point and condense it out in the counter flow heat exchanger, hence the condensation collection point.
Why do you see condensation as a risk?
I see it as a must for any efficient system.

The draft is something that has to be thought of regardless of using a CFP or like you suggest, running it through the mass.
It's all based on duct size, length and bends either way.

Dominique Bouchier wrote:

William Bronson wrote:Welcome to permies Dominique.

This looks like a counterflow heat exchanger.
You are recovering heat from the exhaust at point where it will usually have very little heat left.
You risk not just condensation but also reducing your draft to nothing.


If you want your combustion air to come from the outdoors and be preheated, running the intake duct through your mass would save on complexity and cost.

Thanks for the heads up.

My main reason for using the CFP is not to preheat the air, but to have one simple hole in the roof for both intake and exhaust.
I would think this would be less costly and also easier to install as opposed to two seperate holes.

Are you sure there is very little heat left? What I know from gas fired water heaters is that they purposefully condensate the water out of the flue gas because the transistion from vapor to liquid releases a lot of energy.
My idea was keeping the flue gas in the thermal mass above the dew point and condense it out in the counter flow heat exchanger, hence the condensation collection point.
Why do you see condensation as a risk?
I see it as a must for any efficient system.

The draft is something that has to be thought of regardless of using a CFP or like you suggest, running it through the mass.
It's all based on duct size, length and bends either way.

The condensate will be fairly corrosive to start with, so you'll have to deal with that (even fairly low grade stainless steels like 410 and 304 will hold up to it, so it's not a huge deal), but I think the biggest risk will likely it being getting the fly ash from combustion damp and setting into concrete in your flue, plugging the condensate drain and further decreasing draft.

The counterflow will be an awful lot of restriction against the draft as well (consider that as you're heating your combustion air, the natural tendency will be for the annular space to want to begin to draft as well, working against your flow), and I'd be concerned about killing your draft when you open your fuel hopper.

If you're wanting to go to the nth degree of efficiency and condense flue gas, I'd consider using a heat exchanger on the stack to heat room air, and then drawing combustion air from the room itself (warmer than outside air, but not truly "preheated"). This avoids killing your draft when you open your fuel hopper, will provide WAY less resistance to the draft, and helps exchange room air, improving indoor air quality.

My fear is that the "intake air" will not want to flow down the pipe because the air is getting heated as it moves.

Dominique Bouchier wrote:Are you sure there is very little heat left? What I know from gas fired water heaters is that they purposefully condensate the water out of the flue gas because the transistion from vapor to liquid releases a lot of energy.

If you want to do the same with a wood heater, install a extraction ventilator in the exhaust flue. This way the air inlet will keep working. The ashes need to be separated at all cost from the condensation fluid, gas fired water heaters don't have that type of problem.

I spotted another, potentially major fault in your setup. That's the bypass in the riser part of the contraption. That valve won't live long, it will corrode away in due course being in a hot and carbon frugal environment. Almost all steel will corrode very quickly in such an environment, except the exotic and costly types.

A double walled flue rated for 800° is bound to be more expensive than installing a second opening for outside combustion air.

Rocket mass heaters are generally designed to have exhaust that is roughly 140 to 200° F.
Below 140 you get condensation and that will mean you need to use more expensive materials to avoid corrosion.

Condensing water heaters use fans to create draft and their exhaust tempatures are observed to 110-120.
Fans add cost, complexity and potential points if failure.
Most rocket mass heaters use natural draft.
There are wood gasification stoves that do not, and they might extract more heat from a given amount of wood, at the cost of increased complexity and cost.

If you were to tune your exhaust to a very low tempature, simply  by adding to your mass, you could use cheap exhaust fans to keep the draft moving.
The downside is the need for electricity to drive the exhaust fan and the dangers associated with the fan failing, like backdrafting and exhaust gasses ending up in your living space.