Heating water on rocket stove exhaust

Here's something I've been pondering:



Basically water flows through a copper tube that's wrapped around (or inside) the exhaust of a rocket stove, getting heated in the process. Since the water tank is indoors, it would be at or slightly below room temperature (not less than 10C/50F). Do you think there would be enough heat in the exhaust to heat the water to 50C/125F?

There is no thermal mass to siphon off the heat up to that point and, if needed, insulation can be placed on the vertical exhaust to reduce heat loss before the copper pipe.

I suppose it would be more efficient to place the copper tube lower on the exhaust, but having it come out up there makes for easy gravity feeding of warm water to where it's needed.

Also, can the exhaust of a rocket stove go out horizontally through the wall like that?

Aleksander Jankovic : Welcome to Permies.com, our sister site Richsoil.com, and a Big Welcome To the Rocket and Wood Stoves Forum Threads!

With over 35,000 Fellow members world wide you can come here 24 / 7 to talk to someone who wants to talk about what you want to talk about ! Your written

English is very good, please understand that we may make comments in Fahrenheit Temps and Inches, feet It will pay you to double check our conversions !

You are not the only one to 'like' the idea of using water for heat storage - However there are several problems with doing so !

You have omitted all thermal mass from your sketch, lets workout what that means

THe temperature at the top of the barrel - your cooking surface can easily see 600 ºƒ or 330ºÇ the gases just under this can run as high as 1000ºƒ or 520 Ç -

Again without the thermal mass we have to protect the Floors and Walls from these high temps, by the time your hot exhaust gases have reached the location

selected by you for your Stainless steel or Copper coil ( large diameter 3-4 cm.) The temps will not have fallen below the temperature that heated water will

Flash to steam 212ºƒ or 100ºÇ This can happen in a matter of microseconds, the expansion rate at atmospheric pressure of water to steam is 1 :1700 - Here at

Permies we call this the Boom Squish Moment the boom is your coil and the squish is you !

While simply running the hot exhaust gasses through a Thermal mass can and will absorb and store much of the heat load to a point well nearer 140ºƒ-55ºÇ

where a coil can be safely wrapped around the exhaust gas piping - this can only be determined with careful testing and for many many reasons will vary widely

between Rocket Mass Heaters RMHs !

Designing one of these systems for safe operation would require a Heating and Cooling engineer and operated by a boiler certified operator AND Yearly Inspections !

I would start with understanding much more clearly how a regular Rocket Heater with Thermal Mass / Heat Energy Storage works - This would actually require

building one - and then plan on a lengthy hands on period. Perhaps 1st with a solar hot water heater system before a project of this magnitude is considered !


While this sounds much more negative than I intended it was only a brief outline, also it is very likely that a more conventional system will serve you adequately .

For the good of the Crafts ! Big AL

Hello Big Al! Thanks for the welcome and the detailed reply I have a clearer picture now and the dangers involved.

I forgot to mention/draw that the stove would have thermal mass (bricks and cob) around it. Actually a variation of this stove already exists (outdoors) and this is where I got the idea to heat water in an indoor system. The exhaust on it is warm, but not hot so I was "worried" about it not being enough to heat water... but I suppose I should point my worries in the opposite direction for an indoor system

Unfortunately solar water heating won't cut it in the winter, so we'll have to stick to wood or propane. Still, there's about a year to go before we attempt this and we'll be building another RMH in the meantime... so there will be a lot of experimenting and learning in the meantime. Living in the age of infrared thermometers certainly makes this process a little safer

Thanks again!

_ just a minor point - Ir thermometers read surface temps, Not the temperature of the gasses flowing past ! My reference to a solar Water heater

was to use that as a simple way to visualize a safe hookup! The learning curve is steep and leaves little room for error ! For the craft! Big AL

My rocket sauna stove is set up almost exactly as you show in your diagram, so far it works well for me! I briefly experimented with a copper coil on the exhaust for heating water, but without success. I had hoped to have it work as a thermal siphon in bucket on the floor. When my stove is running very hot (heating the room to 200F+), the 6 inch diameter exhaust pipe can be quickly touched without getting burned.