Bruno Marques

Thanks for your replies al of you =)

Glenn Herbert wrote:On one hand, trying to do multiple things with one appliance can result in doing none of them really well. On the other, if you can cook, bake, and heat water, all of that heat will end up warming the house. If you are concerned about having enough heat for all of the immediate tasks, you could make the heater a size larger to give more capacity. Ultimately, you don't need to fire the heater as long to warm the house if it is larger than you need, so you won't overheat the place.

I've had a hard time understanding the other answers until I read yours @Glenn Herbert:
To me it seemed a bit nonsensical to say that I could "draw too much heat" from the rocket stove and thus ending with a stove that doesn't properly work for the given tasks: If my "appliances" require more heat than given by the stove, don't I simply need to burn more wood to "push" more calories into the system?

To me, the hassle of starting two fires, one in a cookstove and another for a heat exchanger/space heating stove, makes little sense. But I understand from your answer that the question is probably more about the firebox capacity than the burning duration, am I getting this right?

But if a well-built 8in RMH duct system can decently warm up 50ft of straight pipe buried in cob without affecting draft, as I read in another post, that's A LOT of mass, much more than what I seem to need, no? Sadly, I can't experiment with different designs currently.. only read, theorize and ask stupid questions...

I have tons of questions so I bought "The Rocket Mass Heater Builder’s Guide".
I'll come back to this thread once I learned all I could get from there

Hi there!

This spring, I will build a RMH in a small single-story, 75m2 house I am renovating in Slovakia (sorry for metric units ) I've heard so much about rocket mass heaters, and here in Slovakia, firewood is (relatively) cheap and abundant, plus I don't have a gas line in this house.. so in terms of both space heating, water heating and cooking, I want to take maximum advantage of renewables.

I thought: RMHs have been used in all three of those areas, so why not creating a kickass, all-in-one RMH, that would let me cook, heat the house and heat my shower & tap water in winter?!

So I came up with this quick draft, of a "rocket mass heater cookstove heat exchanger":


I would really like to have your opinion on this design, because I have some doubts:

1. In my design, the firebox ends up having some sort of a heat riser over it.. that's not standard in the rocket stove designs i've seen so far, although I have seen a similar design during my research (googling)(Himalayan rocket stove).  I don't know how this secondary heat riser could impact the stove's efficiency, or cause trouble with its "rockettiness", cause smoke issues through the feeding chamber etc...?

2. I'm not sure the oven could get enough heat, without even considering the uneven temperature between the right side (against the steel barrel) an the left, by the exhaust, which could make uneven baked goods...

3. related to the 2nd question..: would the heat exchanger still have enough heat to exchange, after the fire has gone through so much heat losses, through the oven, and the two cooking surfaces? how would the placement of the heat exchanger be the smartest? around the exhaust as shown in the drawing? under the oven? inside the barrel, around the vermiculite heat riser (maybe too hot for copper pipes on the long run)?

4. On this design, the firebox must be very long (because of the cooking surface on top), compared to what I usually see in RMHs designs. is that a big issue?

I've read somewhere that when considering the length of the exhaust pipe buried in the cob, a rule of thumb for an 1/1.5/3,  8-inch pipe system, was to count 50ft of pipe, and subtract from it 5ft for every 90° elbows in the system. that sounds like a lot of cooling mass, so it seems like there's quite a lot of heat to extract from a RMH, which keeps me optimistic.. but I'd love to get your opinion!

I'm looking forward to your ideas and answers!
- Bruno

Hey, thanks for the answer =)

Yes, you are right, I'm looking into an integrated central heating system, that would combine thermal solar, Rocket mass heater with a heat exchanger coil, and a cheap electric backup heater for when I'm on holidays for example, to prevent freezing.
My initial thought was indeed to have this system working for hot water heating through radiators, and sanitary hot water (boiler) for showers, kitchen tap etc.

Here is the diagram of my system, as I thought of initially:


So the hot water loop is first pushed by the pump into the 3-way valve, which depending on the needs will redirect the flow to the boiler, the radiators or both. the water cools off (as it warms the sanitary water or the radiators) before going to the RMH, the solar panels and the electric heater to heat up, then back again to the pump for another loop. Bypass loops allows me to cut parts of the circuit (for example to drain and prevent water to go in the solar panels in winter when it might freeze, or to prevent hot water from the solar panels from heating the RMH's mass on hot summer days). I didn't show this idea with anyone yet, so I hope it's not ridiculous, but it seems feasible to me, and I hope it would greatly reduce my electrical bill...

Now my question is mainly regarding that air-to-air heat exchanger in that video, and whether it wouldn't be smarter (in terms of energy efficiency) to directly heat up the air in the rooms with the RMH through a ventilation system, instead of heating water to be sent to the radiators. Obviously the limitation of this system would be that I can't really heat up the sanitary hot water with a air-to-air system, but for the space heating, I wonder which would be the best..?

Hi there! I'm new to this forum, but damn, what a great amount of detailed information we can find here!

So, I am renovating an old house in Slovakia, I am about to be done with the electricity, and I am working on the plumbing designs, especially the heating part.
Winters are cold here, and I don't have access to gas lines.. so my central heating system will be mainly fed with a mass heater, solar thermal panels in the future, and complemented with the cheapest electric heater I will find 2nd hand.

My first thought was to bury a copper pipe inside the mass of a rocket stove, to act as a heat exchanger for the heat-transfer liquid sent to my domestic heaters... but then, I found a video from a guy, who added an extra conduit and a fan, from the top of the mass heater's body to drive hot air released by the oil drum into corrugated pipes, to warm the air inside the rest of the house (a geodesic dome in his case, here's the video: ).

That would be feasible for me too, and air-to-air heat exchanger seems simple and more efficient to me than using a heat exchanger to warm up the water, to warm up the radiators in the rooms...? I am no expert on these questions, so I thought you guys might have better insights! What is your opinion on this technique? What seems the most efficient, knowing that my house is small (single story, 75m2 house... so there souldn't be a lot of heat loss when carrying neither air nor heat-transfer liquid to the different rooms)?

I'm looking forward to your replies!
Cheers