Rocket Mass Heater, Part 2

We want to keep as much heat from the mass heater in the house as possible.  One of the ideas with the rocket stove is that we will be able to dispose of trash wood, prunings, etc more easily than we can now, but another idea is that we want to extract the heat from the wood and let it help heat the house.  This place is amazingly easy to heat with gas-fired radiators throughout, but what’s the harm in improving and adding options for heat?

So, the concrete slab and foundation are almost certainly not insulated from ground.  The basement is about 3 feet below grade.  So even though heat rises I am trying to design and build mass heater that radiates most heat up and out into the basement space and then up through the house from there.  I am trying to avoid losing heat through concrete walls or floors into the ground.  So, we picked up some polyiso foam board off craigslist.  Some of it is a little rough, but I think it will still work fine.  The foam board is 2 inch.  Here is where the design question start to arise:

  1. Should we place the foam board right on the concrete or float it above the floor with 2 inch space between concrete and foam board?

Looking out on the internet is not real helpful.  There are not a lot of people using foam board on top of concrete in a rocket mass heater project.  The consensus view appears to be that there is no problem with putting the foam board right on the concrete.  I think that makes sense because this same kind of faced foam board in put under concrete slabs, so it must be pretty moisture-tolerant.

Here I am just laying out the foam board and starting to think about the footprint of the rocket mass heater. I am going to follow a standard Ernie and Erica layout – rocket stove in the corner, barrel/manifold right behind that, stove pipe and mass heater to the left, then back to the corner and up and out behind the barrel and out through the glass block “window.”

Ernie and Erica think that the heat from barrel radiating to the cooler exhaust pipe helps the heater draw well and I think they are right about that.

I am contacting Ernie and Erica to see if they want to function as consultants on this project.

pic3Here is another view from different angle.

pic4

Found a good deal on fire bricks in Eugene, so our daughter is down visiting friends, family and picking up bricks for us.  Bricks are supposed to arrive tomorrow, Sunday, March 8, 2015.

Questions that I am considering:

  • should I put air spaces under and around the mass heater so that the heat can scoot under from down in front and then rise up and out from the space between the frame walls and the masonry mass?

I think yes on this question.  I don’t see a downside to attempting to create a radiator-like air pattern around the stove.  I think we may get more air movement with this plan and not have to rely quite as much on the radiant heat from the masonry mass.  I am doing some checking on that.  Here is an example of that kind of approach from Ernie and Erica.

Ernie and Erica are my “go to” experts on rocket stoves.

More questions:

  • Will the foam board be crushed or damaged by the weight of the firebrick, the manifold, the mass heater?
  • How much heat protection am I going to need between the rocket stove, the barrel manifold and the framing?
  • Can I build a plywood floor above the foam board, then put the stove pipes in on top of the plywood?  How much cob, gravel, or rubble fill will I need on top of the plywood or foam so that the material under the mass heater does not get over-heated.

More soon on this project.