Dominique Bouchier

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.

Best domestic use case is to use thermal solar panels and store the heat in water to use for showering and/or central heating.
For an industrial scale they use molten salt as a thermal storage and spin a steam turbine 24/7.

From my experience working on due dillegence of large scale solar farms, I see one big problem here which is maintainance.
I would flag a huge risk on any project trying to get funding for this because if one panel gets damaged by a bird dropping some stone or big seed, it would be a nightmare to repair.
And because of the (lack of) angle, damage is more likely to happen.
Also, the steel used to raise solar panels makes up only about 10% of the total installation costs.
Ofcourse production per panel is less because of the angle to the sun, therefor much of the land space savings are nullified.
Lastly, ventialltion, solar panels need ventilation to cool down for better efficiency. When angled and access to free air, this happens naturally.

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

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