Greggery Kunkle wrote:I am brand new here and did look around at some of the posts about RMH's. I am also new to RMH's in general. I have one question about how they are designed and if I can incorporate one into my existing set up. Forgive me if this subject is covered somewhere else on this forum, I did not see it.
I bought my house a year ago. It has a brick chimney with what looks to me to be new stove pipe lining it. It comes into the house in the basement. There is no stove of any kind in the basement, just the pipe stubbed out. I already have a central air/forced hot air furnace(The heat is produced by propane) and a propane fireplace in the living room. Both of these are fed from the same propane tank in the back yard. I was looking at putting a wood stove in the basement for a back up heat source when I was re-introduced to RMH's.
I understand that a certain amount of horizontal distance is required for the exhaust on an RMH before it exits the house. All the designs I have seen show the exhaust leaving at ground level. My question is: Can I build an RMH in my basement to the correct specifications but have the exhaust pop up at the end and connect with the existing pipe coming through the wall at about 4 or 5 feet high and use the existing chimney for the RMH?
Any advice or info is greatly appreciated.
Using an existing, tall chimney is usually preferable to building in the basement with a lower type exhaust.
There is no minimum requirement for a horizontal run on a rocket mass heater (you can hook one directly up to a chimney right at the manifold/barrel outlet, it just draws ridiculously hard and you'd have to crank down the air controls at the feed throughout most of the burn). The horizontal run is where you get your most useful comfort heat for a seating bench/bed, and allows a large storage mass with its load spread out over minimal foundations and its heat output low in the room to mitigate warm-air stratification. If a heated bench doesn't fit the space or intended use, a tower-like bell or a wall-shaped series of horizontal channels stacked on top of each other will also work.
A 2- to 3-story house will definitely have some of the stack effect that Allen so enthusiastically warned you about. You may need make-up air, or intelligent control of the natural draft up the stairwell while running the heater.
I favor make-up air (to the room) over outside air because it's easier to install safely, and easier to adjust as conditions change (you could put a vent on each side of the house, and just open the one on the currently-upwind side, for example). Bringing in outside air directly to the firebox in a basement is non-trivial - the outside air would need to come from that daylight side, not as a possibly-competing chimney trying to snake discreetly down the wall.
Intelligent operation:
If you did end up with negative-pressure problems not amenable to simple fixes, you might shut the stairwell door during sub-optimal draft conditions (while starting the heater / if anyone is running kitchen or bathroom vent fans / if the outdoor weather was creating a disadvantage for your chimney such as gusty or misdirected winds).
This should reduce the "suction" effect on your basement fire room.
You could probably open it again once the draft was working well (once the fire is going strong / mass and chimney are warm and drawing well / outdoor temperature and wind conditions give advantage to the stove).
Because of the mass storage, you would still be able to heat the upper parts of the house using stairwell convection by building heat-exchange channels into the mass, and using the mass as your heat source at most times.
To reduce the hassles of this kind of gamesmanship with your stove, you can configure it to put a bit more heat out the chimney. Less efficient, but also less of a hassle. Build it with a bell with slightly less internal surface area than Peter's recommendations for a given batch box, or a shorter set of pipes (2/3 or less) than the maximum length guideline for that J-style. The more heat you put up that chimney (200 or 300 F is common for masonry heaters), the more it will work for you as a positive, draft-ensuring feature of your rocket heater. The less heat you put out the chimney, the more of the intended fruit of all this effort remains in your house. It's a straight tradeoff, with about 500 years of development behind the current industry practices for all chimney-equipped indoor wood-burners.
Given the basement situation, and that you really do want some heat to those upper floors...
I'd be tempted to suggest a batch box with a bell (if you can get the skill to do a reliably-sealed capping slab or dome of some kind). The bell will not necessarily be lower-mass than the bench, depending how you make either of them, but it can be taller with less floor
footprint.
I'd consider locating the heater where I can cut a hole in the floor, and let the suitably-insulated top of that bell poke up into the living area of the home as a heated floor, bench, or bed.
I suggest this because it sort of combines the functions of building a foundation to put the heater up in the living space, with your original idea of having the heater in the basement. It is more trouble than several other options, but might also give you more tangible rewards.
(While we're at it, the other problem with basements is tending the fire. You could do a periscope or fiber-optic board to let you see how the fire is doing. I'm envisioning one periscope to see the fire itself, and maybe another one that is oriented so it will only flicker if the flames are misbehaving (e.g. coming out the air intake or something).
Though a batch-box does operate on a more predictable schedule the way Peter tells it, so maybe a timer would be fine, along with a stove-operation log that lets you notice what conditions (weather, end of season, etc) affect that timing in the case of your particular stove. Ideally of course, the stove does not operate unattended; you're down there puttering in the workshop most evenings, and the upstairs signalling-board is just for convenience the odd night or two a week.
To pass the heater through a combustible floor, you'd make the cutout bigger than the heater, and re-frame the cutout with double or triple headers. Depending how the floor is laid out, you may need post-and-pier supports for the resulting spans. The heater should not be load-bearing, in case it (or the house) need future work.
The area between the heater and floor would either need a 2" air gap with non-combustible insulation to protect the floor, or a masonry wing-wall to connect the combustible floor and heater masonry. I would definitely consult a heater mason with more experience than me about the technical details, since the above two suggestions are from different parts of the masonry heater documentation (ASTM E-1602), and I'm not sure which would apply.
This is kind of a 'dream solution' that would address some of your goals, but probably cost more in time and expertise than a more standard solution.
A batch box / bell system without the hole in the floor would be simpler to install, and might do just fine (it might even turn the basement into a prime hang-out space, kind of the winter rec-room). A J-type heater with reclaimed materials could be much cheaper to install, and could also work just fine.
The beauty, and the cost, of site-built heaters is enhanced when they fit your situation like a glove. It sometimes takes an experienced expert to make all those tailored changes sing together safely.
Another thought from the cost perspective: If you think of this heater as supplementing your household heat budget, not as the primary heater, you might want to just use it when you're down there and not worry about storing heat in masonry mass. The basement walls are usually massive, and in direct contact with damp or even frozen ground outside. If there is insulation outside your
concrete basement walls, hurrah! You have an ideal heat-storage battery, and even a plain low-mass radiant heater down there will effectively bank mass-storage heat in those walls for you.
Heating up the whole basement to the point where it's a net heat source for the building may not happen without insulation. So you might think in terms of a smaller-mass, clean-burning, wood-fired heater that lets you warm up yourself and your shop by direct radiant heat when you're down there puttering, adds a little bit of benefit upstairs as a token for the missus, and doesn't involve the complexity or expense of the higher-end masonry heating options just discussed. Something like a metal radiant bell on a 6" batch box, or a small barrel on an 8" J-type heater, whatever is closer to the current size of your existing chimney flue.
yours,
Erica W
1
