4″ Castle Build Data Log

Small masonry heater built with 4" rocket heater

4″ Rocket Heater Castle Build Test

We were impressed with the power of this 4″ Rocket Heater built with a Dragon Burner. It is a little different from the 6″ Castle Build in that it has only one bell. It also uses stove pipe to radiate  heat into the space. The short stack on the right is the heat riser. The stove pipe is on the left. We were trying to minimize the footprint for smaller spaces and provide a mix of immediate and stored heat.

4 inch rocket heaters require very small fuel — less than one inch. However, if that works for you, it is an amazing heater for the size.

How The Heat Is Used

75% of the heat is stored into the Flues for gradual release and 10% is radiated into the room via the stovepipe. The remaining 15% is used to drive the chimney.


Bell Surface Temperatures

Rocket Heater Heat Transfer in a Bell

shown in minutes

The inside of the bell was over 400°F within an hour. It reached 536°F in the second hour. It took about an hour for the heat to show up on the skin of the outside flue liners. You can see at minute 61 the temperature finally starts gradually climbing. Despite some very high bell temps the bell skin stays at a nice sub 130°F surface temperature.  If you skin the flues with stacked stone or similar, it will take even longer for the heat to migrate, however you will be storing more.

Stove Pipe Heat Radiation

Stove Pipe heat loss on 4" Rocket Heater Castle Build

Heat loss from 9ft of Stove Pipe

Although the exit temperature into the stove pipe was higher, the stove pipe  up to the roof reduced the exit temperatures by over 140°F when the temperatures were at their highest.  9 feet of stove pipe were able to keep the exit temperatures under 210°F. You will also notice the greatest amount of heat was radiated into the room at the highest temperatures.

Heat Dissipation

4 Hour Rocket Heater Castle Build Heat Dissipation

4+ Hours

In this chart, you can see how the heat dissipated after we stopped feeding the fire. The blue line represents the inside of the bell, which still has some heat over 4 hours after the burn.

Option to store more heat

If you want to put more heat in thermal mass rather than in the stove pipe for immediate radiation you could do anyone of the following.

  • Add a second bell
  • Add a bench
  • Line with firebrick anywhere in the bell starting at the top. Firebrick will store twice is much heat 4 times as fast.

Click here to view the 4″ and other size rocket heater castle build kits

Copyright Dragon Heaters 2013  All rights reserved




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  1. Sandy, when measuring exit temps, are you just measuring the surface temp of the stove pipe, or do you use a probe to the center of the gas flow?

  2. This looks like a very interesting build. What are you using to joint everything and cap the flue liners?

    • Fireclay, caps are 3000F cast refractory, but appropriate stone can also be used. Cement will not work.

      • Thank you for the reply.

        I’ve only just found out about this style of heater so could you let me know if this is built straight onto a concrete foundation? Any need to provide insulation around the base of the bell?

        Thank you!

  3. I love what you are all doing with RMH…do you have data on how these chimney liners hold up? Sorry if you have already written about this, but I have not yet found that information.

    I think this is an amazing accomplishment for a 4″ RMH. Great design!

    • Protected from thermal shock they last decades in homes. The designs we employ always protect the liners from thermal shock, so they should last a very, very long time.