outdoors RMH with heat exchanger for indoor heat

Hi all,

I've been thinking about putting a rocket mass heater in my surburban house and for a variety of reasons I think that's not really practical at least on the inside

However, doing a pilot project outside my house is quite possible.

So I'm looking at a brick planter outside the back south-facing wall of my home. It's on a patio with protection from rain (Sun-tuff) so exposed to lots of ventilation.

The planter is around 30" wide, 30" tall, currently filled with dirt. There's also a gravel area nearby that could hold the heater.

The planter and the ground nearby is next to a basement crawlspace opening where I could tie into the ventilation system.

So, here's my idea. If I put in a RMS with thermal mass and some form of heat exchanger buried in the RMS, it would be possible to circulate the warm air generated by the RMS through the ventilation system.

I could do this as a relatively low-investment project that doesn't involve renovating the interior. I could move air through the RMH with a small blower outside the building envelope tieing into the existing vent system. I have an existing pellet stove that only heats the front rooms. With the RMH running potentially I could warm up the back rooms as well.

Questions I have: is this practical to do? What kind of heat exchanger would work? Galvanized pipe, duct work? Is it likely I can generate enough heat this way to move it into a vent system and get a useful result?

Even if it doesn't work as an external heat system, it's worth doing just for the experience of setting one up. Is a brick-lined planter a good way to do this? Barrier system against heat loss through the ground?

Thanks for any tips here.

Hi Todd,
I'm not sure a traditional RMH would be a good fit in that area. You could probably knock out the end wall of the planter and add a length of pipe that goes in and out of the window. That would radiate some heat into the basement.

If you are just building it to see if you can build a RMH... I would say go for it... just maintain proper distances to your outside wall as if it was inside.

If you are hoping to get an appreciable amount of heat from an outdoor heater... I might suggest a rocket water heater of some sort to get the heat inside faster, before it goes to heat your back yard.

Hi Matt,

Okay, thanks. That's helpful. I may well do what you suggest and build one outside to see how it goes.

Hi Todd.  I'm really curious if you tried your experiment and what the results were?  I too have been toying with the idea of some way to heat exchange or preferably radiant heat in my crawlspace using an outdoor RMH. I realize that there is the potential for a lot of heat loss at the burn chamber but it seems that this can be captured in another outer chamber and use forced air to move it through another well insulated chase to the return of my central air unit.  I have a 2,000 sq ft home with the floor 18 inches to 4 ft above ground.  I'm in the deep south so we don't have harsh winters but when it is cold, our feet are frozen, and the electric bill is extremely high.  I'm just not sure how effective the different models would be with this building and outside.  I've even considered building multiple RM heaters spaced out to try to radiate the majority of the floorspace.