Rocket Mass Heater vs. Kachelofen

Hi there,

I´ve done some research in the last few days about Rocket Mass Heaters and similar types of wood heaters. I am German and RMHs are not realy known here and it is hard to legally run them in your house.
But there is an old type of mass heater here, the Kachelofen, that seems very similar. Lots of old farmhouses still have them.

I am curious if anyone knows if Rocket Mass Heaters are still better than Kachelofen or in what situation they would make more sense. Which type of heater would be more efficient and why?

The main difference I see between the two is that the heat riser in a RMH seems to be taller than in a Kachelofen. Also a Kachelofen is made completely out of firebricks, there´s no metal barrel or pipes. The Kachelofen has a door that can be closed and the fuel feed is not open.
The fire in a Kachelofen burns at around 1000°C and the temperature of the gases leaving the chimney can be as low as 60°C, which is the minimum temperature required by law in Germany. Most people have the fire going for 1,5 hours. The thermal mass gives of heat for 12 to 24 hours (some ressources even say 30 hours).

Is there a big difference that I´m missing? Or are both heaters basically the same?




I am very curious to see what you guys think. Thank you.

In the US, we have similar code in place for masonry heaters.

Many people have built batchbox RMHs that comply with that code.

Hi Katha;
The principle of both stoves is the same.  a super hot fire to store heat in a mass that is then released back into your home over time.

I am going to guess that the big difference between them is efficiency. The RMHs have been extensively tested and refined to burn very cleanly.
The Kachelofen was built to use as little wood as possible and provide heat for many hours, I doubt anyone cared about clean burning in those days.

Here is a thread about the difference between a RMH and a masonry heater.

https://permies.com/t/129524/RMH-masonry-heater-differences

Thanks for your answers!

I found some more info on the legal requirements for a Kachelofen here in Germany:
- the efficiency has to be at least 80%
- up to 1.25 g/m3 of carbon monoxide allowed
- up to 0.04 g/m³ of particulate matter

Is there any data about this for Rocket Mass Heaters? Has anyone ever measured the carbon monoxide or particulate matter content for Rocket Mass Heaters?

When people test their Rocket Mass Heaters, the carbon monoxide usually measures in the thousands of parts per million. Which should be far less than a gram per cubic meter. Here is one test run.

Thank you for that link, Jeremy! Very interesting!

I just looked up the conversion of ppm to g/m3. The formula is: concentration (ppm) = 24.45 x concentration (mg/m3) ÷ molecular weight. The molecular weight of CO is 28.01 g/mol. The conversion factor from mg/m3 to g/m3 is 1000.
So 1.25 g/m3 = 1250 mg/m3 = 1091.128 ppm

1091.128 ppm CO would be the limit value in Germany.
If my conversions are all correct the RMH in that test would not be meeting the german requirements. This in turn would mean that a modern Kachelofen is burning cleaner than this one tested RMH.

Are there more tests like this to get a bigger picture?

If I made any mistakes in the conversion, please correct me.

Peter van den Berg did many tests with his Testo meter while developing the batch box design, and has published some at batchrocket.eu/en/workings#results. There is an optimum one, an average one, and a wonky one from early testing. The wonky one averaged around 1500 ppm CO, the average one around 700 ppm ranging from less than 50 ppm to about 1500 ppm for most of the burn, and the optimum one averaged around 400 ppm CO, with much of the run less than 50 ppm.

There are many more published here on permies and at the donkey32.proboards.net forums.

Katha Meyer wrote:I found some more info on the legal requirements for a Kachelofen here in Germany:
- the efficiency has to be at least 80%
- up to 1.25 g/m3 of carbon monoxide allowed
- up to 0.04 g/m³ of particulate matter

Is there any data about this for Rocket Mass Heaters? Has anyone ever measured the carbon monoxide or particulate matter content for Rocket Mass Heaters?

Usually the numbers are average over the whole burn time. The numbers spike when wood is added and the beginning and end of the burn, but the average should be under your requirements.