There's a fine line between too much thermal mass that it cools the combustion chamber and results in a dirty burn, and takes forever to warm up. On the other side, if you don't have enough you'll get too much radiant heat - and no residual heat after the fire goes down. Here's a rough sketch of what I've always wanted to build for a hearth. Kinda of an insert / freestanding hybrid. Use a steel stove that cranks radiant heat into the stone and let it slowly dissipate as the stove cools down, but leaves enough air gap around it for convection without the use of a blower. (Not to scale, 3 minute sketchup drawing)
I found some huge old fire brick in a building supply salvage yard 2 years ago for 50 cents a piece (!!!). They are 18”x9”x4.5” and weigh 55lbs a piece. I have about 40 of them. They are going to be used for a 6’ masonry bench with 2’x2’x2” cement sidewalk pavers on top for a 6” rocket mass heater I’m building in front of the fireplace in my basement family room. The 6’ bench will be 2000lbs with these firebrick. These came out of a local brick factory kiln that closed in the early 1960s. They were in storage then in a pile open to weather in the back of the salvage lot for 30 years. I’m cleaning off the old refractory mortar right now. It’s some of the hardest mortar I’ve ever seen. I’m tapping the mortar with an 8 lb wood splitting maul’s splitting edge just to chip and break it off the firebricks!
about 1500lbs thermal-mass ... 2 stainless 3 gal tanks ... 3 “heat exchangers” ... have a fan connected to a 30min timer that cycles on / off ... temp gage in right tank ... I use the water temp to run the stove
I have about 1200lbs of concrete paver and bricks on and around my hearth. It’s really made a difference this year. I think I was losing a lot of heat through the logs since they’re exposed to the outside weather. The radiant heat is much more pronounced and seems to even out the heat better with smaller heat spikes and valleys. The concrete heats up to as high as 160 then after 12 hours they are still around 100. I’ve modified the stove to make it more radiant by taking off the rear and bottom heat shields. And also blocking the side convection cast panels. I’ve only burned through 1 cord so far this year but it’s been pretty mild. Still I’ve noticed that I can extend my 12 hour burning schedule in colder weather than I did previously. Got an arctic blast coming soon it will be interesting to see how it works then.
We have a barrel stove in the shop at work. I’ve been wanting to try something for several years. Barrel heats up fast and cools down fast. Not nearly enough stove for the size of the shop but can get it above 60f for the day. Often times there mass in the shop in the form of a payloader or truck. But something closer to the stove would help a lot I think
Been there done that. Didn’t draw for chit, but the stove pipe was had a long almost horizontal run back then. It’s straight up now, probably would work better
