I have an idea for an outdoor RMH but I don't know if it will work.

I'd like to build a Rocket Mass Heater, but we don't have room in our house. Our current setup is an old Central Boiler outdoor furnace. It has a 200 gallon water sleeve around a 3x3x4 foot firebox covered in a couple inches of foam insulation, then there is an insulated underground pipe running to a radiator in the forced air furnace in our basement. It works beautifully but we burn a wheelbarrow of wood a day just to keep the water hot. On really cold days it can be 2 or even 3 wheelbarrows.

I had the idea of building a gravel style RMH out next to our current furnace with a long water pipe coiled through it, then heavily insulate it and use it in place of the water boiler. Since there is tons of space outdoors I could even make it a 8 or 10 foot cube! It would have to be a batch style one so I don't have to keep going out in a blizzard to feed it.

Is this a reasonable idea? It sounds awesome to me but I'm wondering if you all can see an obvious flaw in it.

Years ago I had plans for a water heater that had a grid of cpvc plastic hot water tolerant tubing buried in sand. I never built it and am trying to remember what it was called and what the stove part was like. The general idea was lots of tubing in a grid pattern buried in sand is lots of surface area to exchange heat energy. Also once a large mass of sand is warm it stays warm for a long time. I would consider mixing sand with the pebbles to fill the voids. But I’ve not built a pebble style rocket mass heater. Since you already have the buried pipes in place and the air handler in the furnace your plan makes a lot of sense. The other way is a batch water heater perhaps similar to what Geoff Lawton and Co built in AU. It is a open system if I recall correctly and perhaps safer. Double and triple check all safety designs! It would be a interesting experiment. I’d consider a pellet burner rocket design or batch design rocket so as not to need to feed it every few minutes. Not sure which design would be applicable but perhaps more knowledgeable people will chime in.

I'm pretty inexperienced in all this, so just have questions. Don't outdoor boilers work because they have thermostats and fans to regulate the burn rate? Is that part of your design?

I know there is a common warning against using RMHs to heat water as they readily become "boom - squish" machines. Wheaton labs does have something working to heat shower water with though...

Hi N ;
Well you might get that to work but I'm not too sure.
How hot is your current boiler getting the water?
Forget gravel style fill, in your application. It works sort of OK for a temporary fast indoor rmh but would not work well at all outdoors.
You would want an 8" batch box with a brick bell.
Your water tank , is it separate from your current wood burner?
Or is it attached?   You might be able to utilize it in your batch build.

Is your water pumped from the current heater to the forced air rad?
If so your counting on the power staying running...   Or perhaps you have a backup genny you can use.

Another though might be to plumb the new rmh hot water over to the existing boiler unit and use it to move the hot water indoors.

Good luck with your plans,
Dragontech  would be happy to supply you with any metal batch parts you need.

Coydon Wallham wrote:Don't outdoor boilers work because they have thermostats and fans to regulate the burn rate? Is that part of your design?

Some of the new outdoor boilers have fancy things like fans and reburner chambers, but ours is just a solenoid powered vent in the door. It opens when the water gets down to 170 and closes again when the water gets to 180. It doesn't seal very well on purpose so the fire just smoulders until the water gets cold again.

My idea is to light a fire every time the mass cools off some, maybe once or twice a day. The indoor portion uses the original thermostat and runs the fan then the house gets cold.

thomas rubino wrote:How hot is your current boiler getting the water?

The manual recommends 185, but we run it at 180.

Forget gravel style fill, in your application. It works sort of OK for a temporary fast indoor rmh but would not work well at all outdoors.

Would sand be ok? I don't like the idea of installing 8+ yards of cob 😆

Your water tank , is it separate from your current wood burner?
Or is it attached?   You might be able to utilize it in your batch build.

Attached. The fire box is inside the water tank. The reason it has 200 gallons of water is to act as a thermal mass. In the RMH design I think I would have a small tank somewhere (maybe the basement?) Just to act as a buffer for evaporation.

Is your water pumped from the current heater to the forced air rad?
If so you're counting on the power staying running...   Or perhaps you have a backup genny you can use.

Yeah, our current setup is dependent on the power staying on. We do have a small generator, but we have never had to use it.

Another though might be to plumb the new rmh hot water over to the existing boiler unit and use it to move the hot water indoors.

The pump is in the basement, so I'm thinking of just extending the pipes to the new furnace.

Good luck with your plans,
Dragontech  would be happy to supply you with any metal batch parts you need.

Thanks! I've seen your website linked somewhere on permies and your parts look really slick. I'll definitely keep it bookmarked!

You say your firebox is 3 x 3 x 4 feet... what is the exact configuration (is the 4 horizontal or vertical?)

That is enough volume to build a batch box inside, and 200 gallons of water is a lot of thermal mass to buffer overheating. A batch box would only be running for an hour or so per loading, and I expect you would have a minuscule risk of boiling the water in a power outage before the fire finished burning. Redundant overpressure vents would be a necessary precaution in any case. How pressurized is the system in its current configuration?

If your firebox is indeed big enough and can be cleared, details including photos would help us give good advice.

I might consider putting a lattice of bricks in the empty space after building a batch core, for additional mass and buffering of instant heat.