Question about primary air intake and adding in a cooktop

[earthenshelter]

Hello! 
We are about to starting our Rocket Stove build, and cant find solid examples of good primary air intakes.  We have read all about it, and see several examples of people saying they would't do it the same way again, but I cant seem to find any examples that people are saying work well, and they would repeat.  We will be using a door from Firespeaking, which deos have a small damper, but not enough to be the main primary air.  In the rocket stoves I have built and lived with before, we just left the door a tiny bit ajar, but I realize that is not the most efficient route.  Any links to photos would be so appreciated.  

Our rocket stove is a 6" batch box style, based on Peters design. Heat riser will be built with insulated kiln bricks, and walls of stratification chamber will be built with adobe.  

Our second question is that we are adding in a cooking surface to the unit, and not sure the best place to do so, as the top of the stratification chamber is too high to be very practical.  Our two thoughts are either as a bell off of the stratification chamber, as the flue gasses are on their way to the bench, or using our cast iron cook top as the top of the fire box.  I was thinking that this would reduce efficiency too much, to not have the top of burn chamber insulated, but then noticed that one of Peters builds he used glass on the top, which made me think maybe it would be an okay sacrifice.  

Any thoughts or advise, or links to examples of either of these things would be super helpful.  

Thanks! Sasha 

[josephcrawley]

donkey32.proboards.com/thread/3678/mark-cooktop-oven-side-exit

Not sure if this is what you had in mind.

[peterberg]

Answer to second question: the main chore of the firebox is pyrolysis of the fuel, where the real thing happens is in the riser/afterburner area. Following that, a cooktop on the firebox is a good idea. The firebox need to stick out for that but that wouldn't be a major problem, just a design issue. Efficiency of the 8" batchrocket at permies, the one with the glass top, has been measured at the time and proved to be absolutely top notch.

Regarding the main air inlet: it could be done with the door open just a crack, no problem. Actually, that's what I do when starting our own red bell heater. Temperature in the chimney is the main factor by which to decide it's time to close the door. Of course your heater differs from ours, as are the fuel and outside temperature. But here are the numbers where I go by: 90ºC (194ºF) when ramping up, closing a bit further. Usually the temperature falls down a bit to 80ºC (167ºF), and rising then to say, 100 to 110ºC (212 - 230ºF). When that temperature is reached, I'll close the door completely. The temp will go down to 90ºC (194ºF) again and stays there until the end-of-burn.
All measured in the center of the chimney pipe. Size of the combined (primary and secondary) air inlet: 45 cm² (6.98 sq in). As one can see, it's very well possible to control the burn by following the chimney entrance temperature.

The air inlet isn't governed by opening only, it seems logical to regard the air speed as the other factor to reckon with. It might be that the inlet opening of your door is already good enough for a core that's running full bore. To date, nobody has come up with an automated inlet valve governed by the chimney temperature plus door open or closed. Would be very interesting, don't you think?

[serg247]

2:20
youtu.be/qdD8HcevMdc

[earthenshelter]

Thanks so much everyone!
Peter, I really appreciate your advice on the cooktop being right on top of the fire box. This does feel like the best option. It brings up another question for us about what to do with the P-Channel. We just got it welded, and ready to go on top of the fire box, but now that would be in the way of he cooktop. I see in the video that your P-channel in that glass top rocket stove just gets air from right above the firebox. This feels like a great solution, as long as we will be per-heating the air enough.
Thanks again for your help and generosity of time!
Sasha

[peterberg]

What you could do is using a floor channel, which is on the floor of the firebox, as its name suggest. Sorry for the already welded part, this won't be right for the floor application.

[earthenshelter]

That does seem like one option, but access to the welding is a bit of a challenge for us. Do you think we would be making a mistake doing something similar to yours in the glass top version, using the same P-channel, just cutting the intake end shorter?

[peterberg]

Yes, that could be done, no problem. Be careful though, not to block the opening with a frying pan or something like that while the fire is going on.