Good morning. I'm new to rocket mass heaters as well. I just finished my first build and rebuild. My system is similar to Ben's, except that my core and riser are made of fire brick. My stove features the traditional J tube, but like Ben's is a 4 inch system. My feed tube is 6", burn tunnel is 12" and riser is 32" (all measured at their longest points). The whole thing is inside 2 16 gallon barrels put together.
I am also concerned about temperature. My highest so far has been 375F. It usually hovers between 300 and 350. I have some ideas as to what causes this.
First, my riser is insulated with fiberglass. I have an 8" metal pipe around the riser, and the fiberglass is in there. My burn tunnel is not insulated. Would covering the burn tunnel and riser in perlite help?
Second, I have a small port in the front of the feed tube. This serves as a clean out. I have a small brick that fits in the port. After my mortar cured, the gap around the plug increased a bit. I can slide paper around 3 sides of it. Is it possible this is causing problems? Seems to draft well enough.
Third, my exhaust duct ties into an existing chimney. I have about 8' of 4" duct, then an adapter to 6" where the old chimney was. Could this create too much draft and suck a lot of the heat out? After an hour's burn, the exhaust is too hot to touch, but there is only steam coming out from the stack outside.
My last target is the exhaust port in the stove. Is it possible that much of the heat is just blowing through to the outside? I feel like that may be the case, which might be caused by the change in duct size. Either that, or I need some sort of baffle over the exhaust port to slow the flow.
This stove lights and burns pretty well, I just wish I knew how to make it burn hotter. I have seen where other folks made sub 6" and claimed to have good results. Any help is appreciated. Of course, I will report back any findings to help someone else who may be in the same boat.