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6" vs 8" mass heater capacity - how to decide?

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Hi everyone,

I am planning on building an insulated cast core and riser, from clay, perlite, and refractory morter, per broaudio's video on youtube:


My current old inefficient wood stove (Fisher Moma Bear) that I am replacing has a 6 inch tube, which exits to the flue (cinderblock stack, lined with clay tube)

I am trying to decide if I should build a 6 or 8 inch mass heater. If I build to 8 inch, I will need to make the flue hole larger, I think, which means chipping out some of the cinderblock and clay liner. I understand the need to keep the same area of opening thru the whole system.

I have a relatively small house (25 x 35 per floor = 875 sq ft * 2 = 1750 total), and the mass heater will be in a room in the basement. The goal is to have the heat rise up to heat the whole house. We have a few strategically placed vents already, and the bedrooms are generally right above, so this should work. Our house could use better insulation in the walls, but the ceiling insulation is good (was built to 1960s standards). The kitchen cabinets have cold air in them on cold mornings.

We are in SW Virginia mountains. I would say average 15 - 20F for most cold nights, and dips to 5 - 10F on the coldest nights. I have seen sub zero temps only once. Jan, feb, march are the prime heating months.

I guess I am wonder if a 6 inch stove will be powerful enough for this, or if I should go for an 8 inch stove. What criteria should I use to make the decision?

Also, if I go with the 6 inch solution, should I look at a shorter total bench pipe length?

Thanks, Douglas
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Douglas Smith : The broaudio video you refer to was made by Mat W. who often posts to these pages ! Please be Suspicious of what you see on u-tube
there is a lot of crap out there !

While you probably can chip away at the cinder block opening you would have to use great care as a chip or an old glob of mortar used to bind the cinder
blocks together might be in a position to make a point of uneven pressure and break the cinder block !

The same goes double for the Clay Flue tile , as such you may want to consider a 6'' J-Bend or top loading batch box Rocket Mass Heater RMH !

Before finalizing your plans to place your RMH in the basement, you need to ask yourself do you have a reason to be often down in your basement,
a RMH in the basement will tend not to get the attention it deserves, and will only serve you as well as you serve it !

Second, every house breaths, sending out hot moist air out from the top of the house usually from ceiling cracks, voids in insulation,and other
penetrations for electric and drain waste and vent,and drawing in cold air at the bottom of the house often around the space where the Wood of the
house meets the foundation at the sill plate !

Because of this the warm air exiting the house near its top can cause the RMH to Smoke Back !

For this reason i am asking you to do a google search for Stack effect and Whole house Stack Effect ! This is not an intellectual exercise, when you
understand how this works well enuf to explain it to your grandmother, we can decide if you want to attempt to place your rocket in the basement !

for the good of the craft !Big AL
Douglas Smith
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Thank you Allen! These are good items to chew on. I especially had not thought about the house stack effect.

o The RMH would be in the family room in the basement, and the whole family helps currently tend our wood stove there. I am sure the heated bench would be a favorite place to hang out.

o The flue is constructed smack in the middle of the house, and not exposed on the side, so it generally stays warmer, and does not get cold. I opened the stack clean-out door, and lit a match. There is currently a draw up. I think I should also test this on as cold a night as possible. The stack clean-out door also provided a convenient place to light some paper, to get a draw going, if needed.

o I realized the pipe-to flue connection is actually a clay pipe that the 6 in pipe fits into, that I could remove, and looking at it, it might leave about an 8 inch hole. I may be able to remove the clay pipe, and replace with 8 inch stove pipe (and seal), without further damage to the stack.

o I may need to seal air leaks to the attic, to reduce potential for back draft.

So far, I am leaning toward the 8" RMH solution. I don't have a measurement for the pipe length of the bench yet. Will get that shortly.

Does anyone else have any other caveats, best practices or bad experiences, related to items above?

Thank you! - Douglas

allen lumley
Posts: 4154
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
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Douglas S. Caveating away! I have a mental picture of a two story house with a converted rec-room basement ! Is this Close ?

We explain, clearly I hope, to people that the Feed Tube must be much shorter than the Heat Riser to avoid any possibility of
the Feed Tube becoming a competing chimney! with the Rocket Mass Heater RMH located in the basement the following scenario
must be considered as a real world possibility !

Since early morning the RMH has been working for you warming the house from basement to upper stories the heat from the
1st floor rising up an open stairwell. Also,It has been a nice sunny day with the sun streaming into upstairs windows hours
before the lower floor gets any solar gain, as heat rises this is now the warmest room in the house, and a family member wants
to open a window and let in some fresh air.

A column of warm air streams up from the 1st floor and out through the open window, creating a chimney with an exit size say
20x30 to compete with the other skinny chimney! The fire In the RMh was nearly ready to go out as planed and now the larger
diameter stronger chimney pulls cold air down the RMHs chimney causing the last of the fire in your RMH to smoke back into the
house, in this scenario while the lower levels are getting smoked, our family member stays alone and upstairs for along time
before finally smelling the now much deleted smoke, or hopefully the smoke alarm will go off !

I must note here that this long wider, stronger drafting, Whole House Chimney can occur with any other wood stove too !

Other areas of potential problems are any simultaneously running Clothes Drier, vented to the outdoors! Bathroom exhaust fans,
Attic fans, and the fan over the Top of your oven range in the kitchen !

This is why the Chimney that accepts the exhaust from the RMH should end at least 4' above the peak of the house or any other
close object bigger than a TV Antenna. Ideally it should be located on the Lee or downwind side of the House, which yours could
actually be !

Other than that, you have described a near perfect placement for your Rocket Mass Heater ! Good luck ! Big AL
Posts: 1288
Location: Okanogan Highlands, Washington
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I think you're leaning the right direction with the 8" size.

We've lived with a 6" as a heater for a similar-sized house in Portland OR, which is almost a similar climate (but doesn't get quite as low due to maritime effect). It was enough for most of the house (all on one level), but the back room with 2-3 intervening walls didn't ever quite heat up, and we did sleep on the heated sofa for a couple nights during our worst cold snap (-teens).

For our current home in a colder inland climate, we have an 8" heater for again a similar-sized house, which keeps everything up to tolerable temperatures (even the back bathroom, which has 2 intervening walls, does OK if we remember to open the bathroom doors while running the stove so it can get some circulation. Also, we insulated this home better, which helps .) I think you'd have good results in your climate with the 8" heater, and you would not need to run it as long or as often as we do here - probably a couple times a week during the shoulder seasons, building to regular evening fires during the colder parts of winter.

There's more detail here: https://permies.com/t/41461/rocket-stoves/RMH-stick-built-construction

It does sound like you're in reasonably good shape for a basement heater installation. I'd like to keep in touch and find out how it goes for you, especially if you end up using one of our J-type plans. We give the same standard warnings about basements that Allen has summarized, and given that your situation involves the right answers to each concern (good chimney, routinely used space that is already part of the heating budget, heater will not need to burn unattended), I'd love to hear how it goes.

The scenario Allen describes is a real possibility, I've seen more times than I care to count while prototyping or troubleshooting a dysfunctional stove, that people will open a big door or window to clear out the smoke, and it would be on the downwind side of the building or up high, and make the negative pressure problem worse, causing smoke to pour out of the firebox where before it was a fitful trickle now and then. The upstairs-hot-downstairs-cold problem is one that simply requires sensible family training for any household managing their own heat: If the upstairs person is too hot, they shut the stairwell door before opening a window. End of drama.

If there are concerns about negative pressure problems, or cold starts for that matter, you could do a somewhat shorter version of the 8" heater bench and sacrifice a little more heat up the chimney. Or use any of the cold-start tips in our O&M manual (on scubbly.com alongside our plans, as an excerpt from the Builder's Guide). And I'm sure we've discussed cold starts here on permies.com too, several times.

A cold start is any time the mass (or chimney) is colder than outside air - it will tend to want to run backwards until the fire's heat builds up in the right places. You might notice the same thing with your current woodstove; I know my grandma's basement woodstove, and our downstairs fireplace insert growing up, were more susceptible to this kind of cold downdraft than the upstairs versions. It's partly due to the house itself acting as a chimney, as Allen has been emphasizing; if the house is warmer than outside air, but the stove isn't, then the house sucks warm air upwards and the cold stove/chimney sucks cold air downwards. The strongest negative pressure in the house is at the bottom of the house, just as with any warm flue stack.

You can also have an associated chimney stall: where the heater runs properly for a little while but then builds up a cold plug of super-saturated water vapor, due to repeated cycles of condensation in the cold heat-exchange pipes or cold chimney. Finally the cold plug reverses and dumps downward. If you are game enough to keep the fire going after it regurgitates billowing clouds of foggy smoke at you, half-drowning its own fire, it can draft properly for a while and then do it again, until finally the chimney is warm enough to keep the exhaust above the dew point and the draft starts to operate properly. (Pre-heating the chimney is the trick to avoiding this particular mess, and starting the fire when outside air is much colder than indoors can help too.)

If you're using the heater at least a couple times a week, I would not anticipate a problem. But in the shoulder seasons, first fire of the fall, or returning from an extended a winter vacation, you might need to pull out a few cold-start tricks.

The other thing you can consider is making sure there's plenty of fresh air coming into the basement in the first place. Air-to-air heat exchangers for a windowbox can be made pretty easily, and you could do one for a basement lightwell window (south side if possible for some extra heat). Enough warmed, incoming air to the room helps mitigate negative pressure issues.
So does starting any weathersealing improvements near the top of the house, instead of on the ground floor, so the house holds heat in like a cap. Again, you're already in the right zone having the best insulation in the ceiling, just work your way down from there.

Good luck, and please post more updates (with pictures)!

-Erica W
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