RMH exhaust

I'm a bit confused on RMH exhaust. In a typical wood burning stove exhaust is vertical because the fire is hot and wants to rise. With RMH I'm under the impression that all the rise happens in the heat riser and this pushes the "cold" air in whatever direction, and must fight gravity possibly? So, aesthetics aside, should I keep the system horizontal and vent sideways or plug into the chimney and expect it to climb 20 or so feet more efficiently? Some explanation would be awesome also.

Always better to plug into the chimney, due to stack effect.

An RMH will have cooler exhaust than a standard woodstove, but it should still be 100 to 200 degrees, and will create helpful draft in a vertical chimney. Also, the "horizontal exit" idea only works if you have a totally constant prevailing wind, which only happens in a few places. Most of the world needs the flexibility of a vertical chimney with its top at least a few feet above the roof, to draw without backdrafting in any wind direction.

What about 90's in the exhaust to get to the exterior- how does that effect the flow? I have a 4" system and am planning to use 4" stove pipe. I can either elevate my stove to the height of the window I will be mounting it next to (not my favorite idea) or throw two 90's between the stove and wall thimble with a third outside the thimble.

Richard

Richard Wagner : Good luck with your 4'' system, for a 1st build most people find that a 6'' system is enough of a challenge, it is much more forgiving

than a 4'' system . The total length of the horizontal piping you can work with and get a good draft is much smaller with the 4'' vs. a 6''. While we often

think in terms of what is normal for a wood stove, you certainly can extend your horizontal run at its present height and pas it out through your outside

wall and its thimble within inches of the floor if you protect that exposure from extreme heat ! In your case at least one of those 90º elbows will need

to be a " T'' with clean out Cap ! Redesign your system if possible ! For the good of the Craft ! Big AL

try to have most of the chimney inside the house.

So I'm still a bit confused on whether to vent horizontal or vertically through chinney. My friend is telling me it won't come out hot enough to make it up the chimney and it might sink back down. He recommends running along the wall and punching out the wall near the window to keep all possible leaks contained. If venting out the chimney should I plug into the pipe sticking out and close the flue below it or just have an elbow facing up at fireplace level and open the flue? Thanks (:

What size is the existing chimney flue? And how tall? What condition is it in? If it is lots larger than your RMH system size, or if it is in poor condition, you may want to put in a liner (I have heard of stainless ducting used for this). Otherwise, as long as the chimney is at least partially built into the house, you would be better off using it. A cold plug on starting is an issue in some circumstances. How cold are your winters?

Going straight out the wall will not help you; you would then have to run a new metal chimney all the way up above the roof, and it would get even colder than the masonry chimney. A terminal at room level will only work if you have 100% reliable prevailing winds and your exit is on the downwind side. There are only a few places in the world where that is true.

How thick is the masonry on the exterior side of the fireplace? Is there an air gap or any insulation? What does the outside look like? (Is it beautiful or visually prominent?) If it is not visually important outside, you could cover it with some insulation and a new shell. This would let you use the fireplace mass as part of your system, which could save you some space in the room.

I'm not sure if I understand your questions. The chimney is a foot and a half by just over 4 feet wide. Probably 20 feet tall. It seems structurally intact. I live in Detroit so it gets to be as low as -40 sometimes? The chimney is pretty built into the structure. I don't understand your last paragraph. It comes out maybe 1.5 feet to 2 feet outside. I think the whole system is straight brick but I couldn't say for sure. What do you mean by using the fireplace as part of the mass? If I exit outside can't I just put a wind cap around it? Is there a chance that venting into the chimney it might not be able to rise out the top? I just don't want it to sink back down and kill me. Having kickback out the feed tube is one thing, having it not make it out the chimney is another. It would be better functionally if I can vent out the wall because I could make a longer bench. Regardless aesthetics are of no concern. Thanks!

Shadrick Wilsons : Lets start with what i think I see in Your picture of your existing Fireplace/ Chimney ! So you seem to have a Cement block style fireplace / Chimney

with what looks like a fake brick appliqué on its surface, 50s 60s technology you can improve on ! A couple of pictures of the appearance of the outside of your Chimney

from The ground up and then climb up on your roof with some brite and powerful light source to shine a light down into your chimney and take pictures will be useful.

The inside dimensions of the chimney at its peak would be very helpful !

I am expecting that inside of the Concrete blocks that make up your chimney you will find large Terra Cotta flue tile generally in the red hot dog to pale redbrick color.

We have to know if All of this tile is in like new condition !

I am assuming that the stovepipe stuck into the chimney is 8'' pipe - This is a good time to tell you that you can not have a damper in a Rocket Mass Heater RMH and not

avoid future problems ! If that pipe is 8 inch pipe we may be able to use regular stovepipe as a liner inside your present flue tile, you can then fill in around the space

between the pipe and the Concrete Block Chimney by pouring in penlight or zonolite as insulation and not have to worry about temperature drop in the pipe and potential

Back draft !

This would be permanent and require closing the fireplace damper but would allow you to use the fireplace as Thermal mass on your build !

As a last note does all this fireplace /Chimney mass set on a concrete slab or is there a basement or crawl space under it ? If so that will need to be inspected !

Happy hunting. A picture is worth a 1000 words. For the good of the craft ! Big AL

if you want to be sour you on the safest side from the first time, put a new 6" duct into the old big chimney. the duct should go vertically until the upper end of the old chimney.
if you want to try to save money, you can try venting to the old chimney first.
don't listen to the friend.

By "how big is the chimney flue" I meant the inside dimensions. Is it bricks, or terra cotta tiles, or ?

If it's realy big inside, you could also use it as a bell.