Rocket mass heater from Basement to main floor breakfast nook

So, I am planning on building a rocket mass heater in my house basement going upstairs (6' ceiling in the basement) through an old gravity heat floor vent to a breakfast nook bench (the mass) then back through the floor downstairs and out the side of the house.

2 questions:

1. Since the hottest part of the combustion exhaust pipe will be a 6' pipe extending upstairs to the mass, will all the heat be lost before it has a chance to even make it to the mass?
2. Is it fine to change from a 8" pipe to a 6" pipe once I get to the mass (due to the small size of the mass)?

thanks

Jesse Keller

Hi Jesse, welcome to Permies!
I see a number of issues with your idea just in the little you have shared.
First, a RMH in the basement, unless the basement is a living space you will occupy the whole time you are burning, will be hard to attend to properly. It will often burn down while you are not looking, and you will have to run up & down the stairs frequently and maybe wait for feeding time, or have to rebuild a fire that has died down too much. This can be alleviated with a batch box style RMH, which is designed to take a whole load of wood at once and burn unattended for an hour until it is done. The caveat is that a batch box is a more finicky system to build, and should probably not be the first RMH you build unless you follow exactly a proven design.

If you can put the combustion core in the main living area of your house, tending it will become a second nature part of your day. You will need to build a foundation from the basement floor up through the first floor anyway, to support the ton or two of mass a good RMH needs.

I would not risk running a duct up through the old grate opening, as you will not have safe clearances to wood framing. You really need to cut out enough floor joists to give 4" or more of clearance to the masonry, and reframe perhaps with a steel post or two to support the rest of the floor. If you plan a long spread-out bench mass that will be carried on a dozen or more different joists, you may be able to just add some posts and beams under the existing framing and be okay.

If you must have your combustion core in the basement, I would suggest a batch box inside a tall masonry bell (hollow brick box) extending up through the floor; the hottest part of the bell will be the top third, and you can take the exhaust off near the base of the bell if you want to heat the basement, or near the basement ceiling if not.

A RMH system should keep the same cross sectional area throughout, with a few exceptions like a transitional zone between some elements. A bell needs to be at least four times the system cross section area in plan for the heat stratification to take place, so for a 6" batch box system you would want a bell more than a foot square clear inside (not including the insulated riser), and twice that would be good. There are formulas for sizing elements that we can apply when your plans are more firmed up.

You mention running a stovepipe out the basement wall to a chimney; this is not the best plan. If you have any way to run the exhaust straight up through the house and out the roof, your RMH will draft better and more reliably and give you more heat. Failing that, you must have an insulated chimney that goes up to above the peak of your roof, or you will have smokeback issues in windy conditions. You might also get the house itself competing with the chimney to draw air up to attic/roof level, and it may pull smoke back through the chimney and into the house when the RMH draft is not strong enough. Google "whole house stack effect" for explanations of this.

Don't be discouraged - there is usually a good way to get an RMH into a house, we just have to find out what it is for you.

Some information on your climate and your house will help us advise.

The whole house stack effect can hinder starting and worse, it can sneak up on you at the end of a burn when glowing embers don't produce smoke, but could produce carbon monoxide. The odorless mix of co2 and Co can be deadly. The draft reduces near the end, so a reversal may go undetected.

Bumping this.

I want to switch my main energy cost to real renewables, and where I live has a basement.

There are 3 brick supports already in the basement here , one is about 2.5' square, then two 1' square supports near by it, about 6" away north and south of it.  I imagine that is behind the stove upstairs, where the fireplace once was.  And the chimney is above that.  Is that enough to support a RMH(rocket mass heater) too? How heavy is a chimney already? Could it be removed and repurposed to become part of the RMH?

In the city basement space is at a premium. Already a welding space and band rehearsal space have lined up to ask for basement usage--so building a RMH down here is a stretch.

Should I focus instead on electric conductive heat?  Is there a way to improve radiators (we have natural gas furnace and steam) to heat the person more radiantly and less the air? Rumford radiators?

Is there a low-heat replacement for steam that can conduct heat upwards along a pipe into a radiator?

Thanks!

PS other limiting factor is elserly housemate who is also blind.  I thought about just moving the kitchen to the basement but doubt we could really pull that off, mold dust another set of stairs (she practically lives in the kitchen).

Is there a less massive thrmal battery substance?  Antigravity rock?:)