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Insulating top of feed tube & other questions ?

Posts: 3
Location: Brussels, Belgium
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I'm a permaculture enthousiast building his first rocket mass heater.
I'm mostly following the standard design for an 8 inch system from Ianto's book.
I made a quick mockup of the combustion chamber/heat riser outside, and it seemed to draw very well, so I started building the real thing inside.

(20 cm (8'') diameter exhaust and heat riser, 20cmx15cm burn tunnel and feed tube)

I've got a few questions :

1) In the book, the feed tube is insulated on all its height.
However, I don't understand the usefulness of insulating the top half of it.
It seems to me it might be even counterproductive, as keeping heat in the top of the feed tube might help the draft going the wrong way ?

Is my understanding wrong ? Any comments ?

2) If possible, I would like to insert a small glass window in one side of the burn tunnel, near the feed, to see the flames... Would that be much of an issue to lose some heat there from lack of insulation ?
What kind of glass would you advise to use (findable cheaply, ie. not industrial grade things) ? Pyrex seems to be holding up 350° C, which seems far too low for the burning chamber...

3) Are there any specific things that were discovered since the book's publication, and that should be taken into consideration ?

Thank you very much for your time and knowledge, I'll certainly have more questions soon


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