Weather’s getting cold again, so this is on my mind once more. I’ve revised my requirements slightly, in that I don’t think I want a cooktop in the masonry stove. We’ll be better off with an outdoor kitchen in summer and possibly a normal wood cookstove for winter use. That said, a black oven for baking would be easy to add, and should provide lots of fun during heating season, making pizzas and baking bread etc.
I’ve studied through the existing literature on DIY masonry builds, and it looks like most folks are using one of several designs by Peter van den Berg. He is a genius as far as I can see, basically the Wayne Keith of masonry stoves. There are three main designs, the Batch Box Rocket (BBR), the Double Shoebox Rocket 2 & 3 (DSR2 & DSR3). This combined with the concept of “bells” (large open chambers for heat absorption) gives me somewhere to start. For my usage, the DSR2 will work best. The upper chamber will serve as a black oven, which is desirable. This is the cross section of a DSR2 (from Peter’s site here: Batchrocket.eu - Designs)
I still need to incorporate hydronics as not every room of the house will be able to “see” the heater, and therefore I’ll wind up with cold spots. I really like the heater/boiler that Dan Allard linked to: Lopez Labs - Masonry Hydronic Heating. This shows me that a properly designed heater will be able to radiate directly as well as heat plenty of water for distribution. The important design feature here is that the cold water is AWAY from the combustion zone. Putting cold water jackets in the firebox is asking for trouble, as several folks have experienced. Instead, I will use the DSR2 core to create hot gases, which will circulate around the heat exchanger as well as the masonry for heating.
Here’s a first draft of the heater design. Not 100% to scale, just looking for feedback on the flow path:
The small two-chamber unit in the center is the DSR2 core. Hot gases exit the top front slot, into the first bell. Gases circulate in the bell, giving up heat to the top masonry first, as they cool they will fall to the bottom near the rear exit. The water heat exchanger is in this section to pick up the most heat, yet be outside the combustion area and so not influence the burn at all. This exchanger is plumbed in as a thermosyphon to the storage tank above it.
Cooler gases reaching the bottom will flow through the rear gap and up behind the first bell into the second larger bell. This space is much larger to extend the residence time of the gases. It also contains a large uninsulated water storage tank. Gases will transfer heat to the masonry first, then to the tank - but only if the water is cold. When the water is warm, it will not absorb heat, but won’t lose any either. If the water becomes too hot, it should begin to shed heat to the masonry. I will of course have safety valves etc. This water will then circulate out to the radiant flooring system.
The gas continues down the side of the second bell, into the third bell, which is a bench. Flow once again has reduced, increasing residence time. It is a dead end, meaning gases will flow in the top left, cool, then back out the bottom left. The bench should get warm but not too hot to sit on. Gases once cooled enough will drop to the bottom, to exit via a slot near the base. (Dead end bench idea from here: MHA News - 2015 Meeting at Wildacres)
Behind the stove, you can see the slot admits gasses into a manifold, which goes to the final flue pipe. You can also see a bypass valve, which connects back to the first bell. This should improve cold starts if necessary, by shortening the path and drawing directly from the flue.
Here’s the Sketchup model if anyone wants to get a better view:
