A fulltime vandweller from Canada, over at the Vandwellers Yahoo group, is thinking about winter approaching and planning to build a super-efficient gasifier woodstove to heat his van. He also wanted the stove to help ventilate the interior of the van, while drawing most of its air from underneath and outside the van. He asks for suggestions and, although I have no experience with gasifier design, I have built a few woodstoves and at least thought about putting one in a van; so I offered a few thoughts on the matter. It may be of interest to this site's readership as well, so here are my comments on the subject.
I haven't used a gasifier stove, but I have built and used a few more traditional woodstoves, including one for use in a tent. Based on that, I will offer a few suggestions which may or may not help you.
First, flue size. Most tent stoves use a 4" or 5" flue, depending on the size of the stove. Because a stove for a van would probably need to be on the small side of the tent-stove range, I would suggest using a 4" flue. In fact, when I built my tent stove, what I used was galvanized 4" double-wall flue pipe, of the variety intended for gas appliance venting. This worked perfectly on my stove.
Being galvanized, it needs to be protected from becoming hot enough to burn off the zinc coating, because that produces a toxic gas. That doesn't happen until the pipe becomes glowing hot, so it is not difficult to prevent, especially considering that this particular pipe is double-wall. If any part becomes red-hot, it is likely to be confined to the inner wall, in which case any gas produced will be exhausted to the outdoors.
It is best to use a section of thickwall steel or iron pipe at least 8-12 inches long and permanently attached to the stove itself as the initial flue section; this will better withstand the high temperatures that would occur in that area, and will allow the gases to cool before reaching the galvanized pipe. The thickwall section is also the ideal place to put a flue damper.
This thickwall section can also be wrapped with soft copper tubing, with the lower section of tubing being terminated with a spigot and the upper section being plumbed into a water container such as a stainless-steel stock pot, which is placed higher than the stove. This accomplishes three things: a source of domestic hot water, moderating the hottest part of the stove for safety and longevity, and heat is stored in the water, to be released slowly. Oh, and it's a ready source of water for fire-fighting if necessary, right beside the stove.
Where the flue exits the roof of the van, you will probably want to have a removable 2' section to provide a proper draft, with the section removed and replaced with a cap when underway.
For the air intake, perhaps you could add another short section of 4" heavywall pipe drawing through the floor of the van (well away from the fuel tank, etc.) with another draft control, and a 2" or so iron pipe T-ed into it from the side, with a metal gate valve added, to control intake from inside the van. That way you could fine-tune air intake, outside-inside, by fiddling with the two controls.
As for cooking on the stove, why not? I have a gas stove, but as long as the woodstove is hot anyway for heating, I'm gonna cook on it too and save gas.