Different diameters on the heat exchange pipe... Urgent advise sought

Hi, I really need some urgent advise.  I am.building my system now,. There are two lengths of steel pipe 20.5cm diameter.  They will be joined by a u- bend fitting but I have just discovered that this is only 20 cm diameter.  Will this effect the draft?   Also. If the vertical chimney was 18cm instead of 20.5cm how would that effect things?

The u- bend was sold as 20.5 cm... It arrived today and I discovered it was a fraction smaller.

If it does effect things, am I right in thinking that fan in the chimney will solve the problems ?

Thanks, any helpful advise received will be greatly appreciated

Steve

If Im reading correctly your pipes are nearly the same diameter. in a short distance a minor diameter change poses negligible effects ,  consider the laminar flow when you build. keep things flowing.

the critical issue you didn't mention--are those pipes in the bench of a RMH,,if so what's the diameter of the heat riser?

The heat riser is the same size as a 20cm tube circumference.  I got the plan directly from the Wisner's book for an 8 inch system.  The pipes are, or rather will be in a (clay/sand) bench.   Since I posted I have completed the system as mentioned with smaller chimney diameter (18cm as opposed to 20.5cm) and it seems to draw ok....getting a bit of smoke occasionally from the feed tube.   Now we have temperatures of -4/5c outside  and the three times I have fired it up it works well so the small changes in diameter of tubing seem to have had no effect.   Thankfully

Sounds like you have it under control.

Ordinarily the exhaust size would be bigger if anything, all the way to the open air, but other factors can affect that, and having something that works is the main objective.

If the smoke back does get too intense then at least you know where to start looking. Since the chimney is probably not going to change, you could maybe line the riser with a piece of 1/2 inch ceramic fiber blanket --a little extra insulation and a quick easy fix That would bring it down to about 17.5 cm

The riser is made from fire bricks.... That insulation you mention is not available in Russia where I live, I am constantly having to compromise on materials here as only the most basic things are available.... This is all new to me so I am hoping for a steep learning curve. On the third burn my passive solar greenhouse got to 14c with a starting temperature of 2c.  And that's with exposed pipes.   But was a 5 hour burn.

If I get into trouble then I can swap out the chimney section for 20cm diameter galvanised pipe as the most obvious potential issue.

One can read all the theory there is, seems to me that there is a practical element of just ' making it work ' with what you have to RMH.

something a lot of people have been doing with RMHs that makes me wonder about is the use of a "stack" or vertical chimney, especially since the inventor--ianto evans used horizontal exhausts almost exclusively.. The horizontal exhaust in an rmh with a good push on the exhaust  internally does not really need the draw of a chimney--in fact, the vertical chimney is working at odds with the principle of extracting as much heat as possible which is exactly counterproductive to the need for extra temperature just to get a stack working--there goes some of that energy wasted.

One of Pauls early demonstrations had a pretty young woman with her face directly  in the exhaust stream talking about how it smelled like a campfire a long way away.

Not something you can easily do with 150 F as a minimum exhaust temp.

In houses with existing stacks and people who don't want to put holes in their walls I get it, but through the roof exhaust systems are downright expensive and troublesome.

My exhaust has to run up a little since the system is underground, but it turns horizontal at ground level and runs slightly downhill from there. If my exhaust is as hot as 130F I start looking for ways to take more heat out of the thing and keep that energy  inside.

I certainly hope someone sees this and reminds me of the reasons a vertical exhaust is worth all that extra heat loss and trouble,  I'd like to see the reasons why everyone seems to be ignoring this feature and embracing vertical stacks as a first choice.

I mention this thinking that if your vertical 18cm stack does prove to be too constricting, a horizontal option might be an easier way to go

Thanks Bob, that's an interesting idea, contrary to everything I have seen before, I am sure the Wisners and Mr Wheaten would have a thing to say on it :-)

The only thing I think I would change would be trying to insulate the heat riser.  I used thick fire bricks thinking that that would be enough, but now I wish I had done more. Hey ho, seems to still be putting out good heat though.

I plan to run it for a week as much as possible to get it settled in and for the moisture in the brickwork etc to clear.. then I will see if tweaking is required.   The draw seems pretty good most of the time though

Cheers

Difficult to say if anyone else is watching this post, but I was sort of hoping that someone might comment on why so many have abandoned the horizontal output. I remember when Paul first moved to his farm, one Rocket they put in the office I believe, had two different exits, one through the roof and one through the wall. So maybe some of those lessons caused people to move away from Ianto's original design premise.

Anyway, yes, Insulation through the burn areas, firebox included is far better than thermal mass. If you are using a square riser of bricks the csa of that works different than that of a circular riser.

The insulation of course gives a hotter, more complete burn with less pollution.

One thing that is cheap and readily available, another of Ianto's trials , is a riser made from a mixture of clay and sawdust. The sawdust burns out leaving a clay riser full of air spaces that is very highly insulative.

The only drawback is that once fired it is very fragile to being handled, so extreme care needs to be used when inspecting and cleaning.

Have fun,

Horizontal exhausts are discouraged because they only work reliably when the prevailing wind can be counted on 100% of the time... which is seldom the case. Ianto is in a coastal valley where the wind always blows in one direction.

I have occasional problems when there is very high wind, but mostly I just wait it out--the old saying --"if you don't like the weather, wait a minute"

I came on an arrangement where the final is an elbow pointed down, and at that point the pipe is a couple feet off the ground, so I used a section of stove pipe that was not yet closed, meaning that the exit from the elbow was almost completely enclosed to the ground except for the seam opening on one side of the open stove pipe support

It's not perfect,   yet,    but I think with a little redirection I can probably get it there. I have also been thinking about venting it underground into a chamber totally out of the wind, but it's not high on my priority list right now.

Maybe I could use a piece of black corrugated plastic pipe inside a mulch pile (or grey water system or...)

Seems like there ought to be a way to squeeze out a little more energy from that exhaust. If I figure it out I'll let you know:-)