Chimney exhaust pipe

Hey, I live in Athens and I'd like to build a rocket mass heater which would be my first build. My question is about the chimney exhaust pipe (the part that is outside the house: I live on the 6th floor of an 8 floor building. I just moved here nut at least summer can be windy. In the orientation that I want to install the stove, it's a bit tricky to run a  chimney to a place that I can secure it to the next two floors. I read that to do an horizontal exhaust is not a good idea and could draw drafts from outside.
My floor is 2.90m which I can install the exhaust easily even 3m, either inside the house with exit at the top or probably better (?) exit at the bottom and the hole exhaust is outside.
Would that be enough or do I have to go to the roof? Can't I just use a draft stopper for an horizontal or vertical exhaust?

Some quick thoughts that I have are:

- Are you confident that the floor of your apartment is strong enough to support a RMH? The mass can weigh a considerable amount. You might want to double-check the structure before inadvertently creating a new stairwell!

- There are special cowls (chimney tops) for preventing downdrafts in very windy conditions. Over here the most common is the H-cowl which looks like this:



- Chimneys draw best when they are above the roof of the building. In general, the longer the chimney (vertically), the better the draw will be. Would you be allowed to extend your chimney vertically up to the roof space?

- Insulated chimneys also draw better. This is why chimneys inside buildings work better than those outside. Any cold air inside the chimney can prevent the hot flue gasses from escaping.

Note that I don't have any experience with RMHs, just wood-burning stoves, open fireplaces and clay ovens. I don't see any reason why the above wouldn't apply to RMHs though!

How much should it weight? I'll ask an engineer if it's possible in my apt. It's actualy an industrial building so I think it won't be a problem

Weight for a rocket mass heater could be anywhere from 500 kg to 2000 kg depending on the space and the system, and the climate.

In an apartment in the middle of a building in Athens, you would have a Mediterranean climate so not deep cold, and no roof heat loss, both calling for a smaller system. What is the usual day/night temperature in the winter? How big is your apartment, and how much outside wall do you have? What direction does the biggest wall face, and how much of the walls are window area? I would guess that in your region an industrial building would have uninsulated masonry walls.

I think it is likely that you could use a modest RMH and that your floor could carry the weight. An engineer's opinion would be necessary before proceeding.

The apt is 80sqm. Its one large room except for the bathroom. it’s uninsulated masonry . The biggest wall with large windows (single glazed) directing west. I want to put the RMH perpendicular to the wall directing north (no windows) in the middle of the room with the heat raiser directed to the room and either put the exhaust chimney from this wall ( if its a short chimney because there’s no windows in this wall and no way to secure it all the way to the roof) or going out to the small wall directing south and to the roof.
I think a small system would be great. It’s not really getting cold for long stretches and gets warm a lot (attached the temps) but I’m not sure how it is in that smace.
What manual should I use to make a smallest system? Is it the one with the smaller pipe diameter?
What’s your opinion about the exhaust pipe?