Strawbale cottage sounds like a very cozy project
. Hope it's going well!
I assume you're doing a thick earthen plaster or cob
layer on the inside to store heat, and protect the straw from the immediate heat of your stove.
We've found that 6" is the smallest practical dimensions for the pipes. Too much drag at 4" and even 5" is tricky. (Laminar flow means the effective flow volume drops a lot faster than the actual cross-sectional area).
You could certainly shorten the bench. Although you'd lose more heat, the hottest part is going to be the first 10-15 feet; after 25 or 30 feet you're talking diminishing returns, squeezing out one more degree at a time.
A 5-gallon metal barrel might be too small and lightweight to last well, and not have enough
surface to shed the heat that makes the downdraft work. You might need a skinny heat riser like cast refractory. We used a 35-gallon sized barrel, so it's not super huge. Water heater
tanks may be cut down, for something smoother and narrower yet.
There are a couple of examples on the forums here of 'mini rocket heaters
' that people
have made. Mostly for shops; if they don't have the thermal mass, they won't store heat. I'd recommend searching around a bit. There are some very tiny ones. Watch for smokeless burn; it's not safe to capture exhaust heat if the exhaust has smoke or creosote in it.
In-floor layout? One thing that can work really well if you haven't done the floors yet, is to embed the heater pipe about 4" under an earthen floor. That makes its size 'disappear' into your daily routine; you can put furniture on top of it and everything. You only need to sacrifice the floor space for the combustion unit itself.
I've seen the built-in bench or bed work really well in small spaces, though.
Probably the smallest design I have is one I haven't put up online
yet: the Daybed
6". It fits in an 11x11 room, as a daybed along one side (4' by 11'). It has a bypass too, which you probably don't need but it helps with cold starts. (the room in question was a tiny guest cottage, so it didn't stay occupied all winter).
You can PM or email me if you want to try that plan instead. (It's been proven out for over 2 years but I don't have a good photo of it yet; the bed's unmade and my photo is from before they finished the natural plasters on the walls. I want it to look awesome when I publish it.)
8" fits in a 3' (to 3.5') by 9' floor footprint
, and you could certainly scale it down to 6" and shave off another few inches.
You may want a heat shield with air gap around the barrel, to modulate the direct radiant heat while you're running it. Don't insulate or completely cover the barrel, though, or the draft can go squicky on you. Might work on a very short system but I'd be very cautious.
Hope that helps,