5" steel rocket heater

hey all new to the forum and hope you can help me out.  I know its not recommended to use steel but I have it and am going to use it.  I put the heater together using 5" square tubing for a j tube and using a 55 gallon barrel over it with a 1/4"  top and bottom plate.  I put 2 1/2 inches of perlite around the riser and burn chamber. my feed tube is 10" tall at 22 degree angle, the burn chamber is 14 1/4" and the riser is 32 7/8" tall with 1 3/4 gap at the top. the exhaust is 6".  upon the first few burns it draws great but doesn't heat up very much.  are my dimensions  off or do I just have an insulation issue? hope you can help me out and thanks for you time.

You give dimensions of 10", 14 1/4" and 32 7/8"; are these from the inside corners or outside?

1 3/4" is nominally a decent minimum top gap dimension for a 5" system, but could the barrel top be warping in? Or is the 1/4" plate the whole top? How concentrated is the heat at the barrel top? A small spot, or the whole top evenly hot? The smaller the gap, the more the heat will concentrate, but if it is too small, the heat may not be able to build up in the first place.

That amount of perlite should give sufficient insulation where it covers, though the steel will conduct heat away to the feed, cooling the burn tunnel some. Does the insulation extend to the feed end of the burn tunnel and around it? That junction is possibly the hottest spot in the combustion path (someone else who built a steel J-tube has photos of the glowing areas), and leaving it bare might critically reduce combustion effectiveness.

What did you use to contain the perlite?

As your combustion zone is distinctly smaller than your exhaust, it might be that the draft is pulling air through too fast and cooling the fire too much. What kind of chimney do you have on it? What dimensions, and what enclosure?

sorry for the lack of details and thank you very much for your help.  the dimensions are from the top of burn chamber to top of feed tube, inside the burn chamber from feed tube(barrel side) to riser and inside of riser.  the top of the barrel is one whole piece of plate that bows up a little.  the top does heat up more in the middle maybe 50 degrees difference toward the edges.  the insulation goes to the front of the feed tube (side facing you standing in front) on the sides and bottom but nothing on top.  the perlite is contained in sheet metal made into a box around everything.  as of right now I just have the exhaust running along the rafters and then out through the wall then 90 up for 6 feet and a t with 2 elbows on it.  thanks again for you time.

So about 15 x 24 x 38 along outside edges, which is the most common form of measurement (along with centerline distances). By that metric, you have about 1:1.6:2.5 proportions, which is distinctly less than the minimum suggested of 1:2:3 or 1:1.5:3. However, this may not be the issue.

A major factor may simply be that a 5" system will have a much smaller heat output than an 8" or even 6" system. My guess would be that 5" would give you around 1/4 to 1/3 the power of an 8" system, and the steel core is likely to reduce that further relative to the standard refractory core. An 8" system with a 55 gallon drum can often make the barrel top 700 to 1000F, but a quarter of the power will give a distinctly smaller temperature rise on the barrel top.

ok I thank you very much for your help. I have been testing and tonight I got my 750 square foot garage to 59 degrees and it is 21 outside and only used the amount of wood I would have starting a fire in a barrel stove so I guess I'm headed in the right direction.  I might try insulating with pearlite and mortar mix but not sure as of now. thanks again for your help

Insulating the top of the burn tunnel would likely get you some more heat output.

ok I will try that tomorrow and let you know the effects thanks again have a good one.

hey just wanted to give an update on my heater. I redid the feed tube putting it straight up and down and probably 3/4" closer to the barrel and surrounded it with more perlite. I also raised the barrel up around a 1/4" and it is working way better.  it heats the top plate more evenly and it gets hotter faster than before.  it is drawing great and burning clean only thing I notice is a decent amount of moisture.  thanks again for your help